The Enteric Nervous System

The enteric nervous system (ENS) is an elaborate network of neurons and neural connections in the gastrointestinal tract. It modulates motility, microcirculation, sphincter tone and activity, hormone secretion, and immune and inflammatory responses of the gastrointestinal tract.

The ENS is closely connected to the CNS and there is bidirectional communication between them. The ENS relays information collected in the periphery (bowel) to the CNS through motor and sensory sympathetic and parasympathetic connections [Burns and Pachnis, 2009]. There are many different classes of enteric neurons that differ in their neurotransmitters’electrophysiological properties, morphology, inputs, channel and receptor expression, targets (muscle, other neurons, etc.), and the direction in which their axons project. Neural circuits underlying motility reflexes consist of inhibitory motor neurons, excitatory (cholinergic) motor neurons, interneurons, and intrinsic sensory neurons [Young, 2008]. Excitatory and inhibitory molecules include acetylcholine, biogenic amines, and the opioid system. Intermediate cells include neuroendocrine cells, interstitial cells of Cajal, and cells of the immune system [Altaf and Sood, 2008]. The parasympathetic innervation via the vagus nerves influences both the motor and secretomotor function of the upper gastrointestinal tract, while the sympathetic adrenergic fibers from the prevertebral ganglia innervate only the secretomotor neurons.

The brain–gut axis plays a prominent role in the modulation of gut functions. Signals from different sources (sound, sight, smell, somatic and visceral sensations, pain) reach the brain. These inputs are modified by memory, cognition, and affective mechanisms, and are integrated within the neural circuits of the CNS, spinal cord, autonomic nervous system, and ENS. These inputs can have physiologic effects, such as changes in motility, secretion, immune function, and blood flow to the gastrointestinal tract. One of the most important neurotransmitters is serotonin, which plays a key role in the pathogenesis of the most common chronic functional gastrointestinal disorder, irritable bowel syndrome.

Similarly, stimuli affecting receptors in the gut mucosa or the oral cavity can be transmitted to the CNS. For example, functional magnetic resonance imaging (fMRI) has revealed activation of several forebrain regions in response to intragastric infusion of solutions that activate the sensation of taste [Kondoh et al., 2009]. Glucose activates the nucleus accumbens; monosodium glutamate (MSG) activates the medial preoptic area, dorsomedial nucleus of the hypothalamus, and habenular nucleus. Both glucose and MSG activate the amygdala. Glucose-induced brain activation develops slowly and persists for a long time, whereas activation by MSG develops rapidly during infusion and is reduced rapidly after ending the infusion. Vagal afferents may be playing an important role in this process.

The ENS arises from progenitors, neural crest cells, which migrate from the caudal hindbrain, enter the foregut, and then move caudally through the gut mesenchyme to colonize the entire gastrointestinal tract. Contact between enteric nerves and effectors are developed at 26 weeks’ gestational age. Motility patterns mature with age. Neurotrophin-3 is required for the differentiation, maintenance, and proper physiological function of late-developing enteric neurons that are important for the control of gut peristalsis [Chalazonitis, 2004].

Studies in humans with Hirschsprung’s disease (distal aganglionosis), and of animal models of Hirschsprung’s disease, have led to the identification of many of the genetic, molecular, and cellular mechanisms responsible for the colonization of the gut by enteric neuron precursors [Young, 2008]. Genetic defects, including those related to the neurogenic receptor with tyrosine kinase activity (RET), can interfere with migration and development of enteric ganglia.

In the mature ENS, there is complex synaptic circuitry within myenteric ganglia that underlies motility reflexes. Developmental disorders involving the enteric nervous system in children include abnormalities such as dysphagia, feeding intolerance, gastroesophageal reflux, abdominal pain, and constipation. Alteration in bowel motility has been described in most of these disorders and can result from a defect in the enteric neurons of the gut or their central modulation.

Dysphagia

Swallowing is a complex dynamic process that requires the recruitment and coordination of the muscles of the lips, tongue, palate, pharynx, larynx, and esophagus. Swallowing occurs in three main anatomic regions: oral, pharyngeal, and esophageal. Therefore, swallowing disorders may be due to impairment of any or all of these regions. Difficulty swallowing, or dysphagia, can be present in children. Certain groups of infants with specific developmental or medical conditions are at higher risk for developing dysphagia, including those with neurologic conditions or structural deficits (e.g., cleft lip or palate). Left untreated, these problems can lead to failure to thrive, aspiration pneumonias, gastroesophageal reflux, or the inability to establish and maintain proper nutrition and hydration. Individuals with dysphasia also are more likely to develop aspiration pneumonia, and pneumonia has the highest mortality rate of all the different types of nosocomial infections [Prasse and Kikano, 2008].

Inhibition of gastric emptying and stimulation of colonic transit is the most consistent pattern in the motility response of the gastrointestinal tract to acute short-term stress. Endogenous corticotropin-releasing factor (CRF) plays a significant role in CNS mediation of stress-induced inhibition of upper gastrointestinal motor function and stimulation of lower gastrointestinal motor function. CRF is involved in the central mechanisms by which stress inhibits gastric emptying while stimulating colonic motor function. Systemic or central administration of interleukin-1β has the same functional response through activation of CRF release. The paraventricular nucleus of the hypothalamus and the dorsal vagal complex are sites of action for CRF that inhibit gastric function, while the paraventricular nucleus and the locus ceruleus complex are sites of action for CRF to stimulate colonic motor function. The central actions of CRF are conveyed by autonomic pathways and are different than pathways that stimulate pituitary hormone secretion. CRF also acts on the locus ceruleus to induce selective stimulation of colonic transit without influencing gastric emptying. These findings help our understanding of the pathophysiology of the irritable bowel syndrome (IBS) [Tache et al., 1993, 1999] and provide evidence that stress affects visceral sensitivity in humans [Monnikes et al., 2001].

Cyclic Vomiting Syndrome and Recurrent Abdominal Pain

Cyclic vomiting syndrome (CVS), described first by Gee [1882], is manifested by repeated bouts of vomiting lasting for hours or days, which can cause dehydration, hypochloremic alkalosis, and ketosis in an otherwise healthy child [Abu-Arafeh and Russell, 1995]. Abdominal pain and fever may be present, and the patient may become pale, drowsy, and lethargic [Li and Balint, 2000]. The time of onset and duration of the episodes tend to be stereotypic for the individual patient. CVS usually occurs in infancy but can start later childhood and usually resolves during adolescence. In some children, recurrent abdominal pain may last for hours, with or without headaches, nausea, and vomiting. Pallor, weakness, fever, skin blotching, and other autonomic findings are common.

A 2005 epidemiologic study done in Ireland found that the annual incidence of CVS was 3.15 per 100,000 children per year [Fitzpatrick et al., 2008]. The median age at onset of CVS was 4 years (range 0.5–14 years), with 46 percent of the children having an onset at or before the age of 3 years, and the median age of diagnosis was 7.42 years (range 1.8–15 years). The median duration of an episode was 24 hours, but the range was up to 5 days. Of school-age children, 85 percent had missed a substantial amount of school in the year prior to the study due to CVS. Another study of school-age children with CVS in Turkey found that the mean-onset age was 7 ± 3.4 years. Episodes were accompanied by pallor and anorexia in all children, and by headache and abdominal pain in the majority of individuals [Ertekin et al., 2006].

The pathophysiology of CVS is not understood clearly. A brain “generator” for vomiting is present in the brainstem’s nucleus tractus solitarius. It receives efferent innervation from chemoreceptors in the area postrema in the floor of the fourth ventricle, which are sensitive to different stimuli from vagal gastrointestinal fibers, the oculovestibular system, and the cortex. Vomiting, therefore, can be a result of gastrointestinal, autonomic, or cerebral dysfunction secondary to tumors, hydrocephalus, toxins, hormonal changes, familial dysautonomia, or migraine [Johns, 1995].

CVS and migraine share comorbid dysautonomias, including orthostatic intolerance and syncope [Chelimsky et al., 2009]. Autonomic abnormalities, primarily sympathetic autonomic dysfunction, affecting the vasomotor and sudomotor systems were reported in 6 children with CVS [Chelimsky and Chelimsky, 2007]. Both CVS and recurrent abdominal pain are associated with migraine, although, in at least half of all patients, headaches are not present. Recurrent abdominal pain has been referred to as abdominal migraine [Bentley et al., 1995]. A history of migraine headache in first-degree relatives is common [Symon and Russell, 1995].

Mitochondrial dysfunction is a hypothesized component in the multifactorial pathogenesis of common migraine and CVS. A mitochondrial component in migraine is supported by the finding that subjects with migraine have lactic acidosis accumulation in skeletal muscle mitochondria and decreased respiratory chain complex. Mitochondrial DNA sequence variations are suspected of being a risk factor in the pathogenesis of CVS. Mitochondrial DNA polymorphisms have been associated with CVS and migraine [Zaki et al., 2009]. In one study, the 16519C polymorphism was detected in 21 of 30 CVS subjects (70 percent) and 58 of 112 migraineurs (52 percent), compared to 1 of 63 (1.6 percent) controls. These patients had, in addition to CVS, multiple neuromuscular abnormalities meeting diagnostic criteria for Kearns–Sayre syndrome, and severe symmetrical growth retardation [Boles et al., 2007]. Other suggested causes of CVS include metabolic abnormalities, ion channelopathies, excessive hypothalamic-pituitary-adrenal axis activation, or heightened autonomic activity [Li and Balint, 2000; Li and Misiewicz, 2003]. Epileptiform abnormalities also have been observed, suggesting that the mechanism of CVS involves spontaneous spreading depression, similar to that seen in migraine patients. The differential diagnosis of CVS includes gastrointestinal obstruction, metabolic disorders, renal disease, and neurologic space-occupying lesions [Forbes, 1995]. It is important to evaluate individuals with CVS symptoms thoroughly, even if they are asymptomatic and well between vomiting episodes, as this does not assure the absence of another disease [Catto-Smith and Ranuh, 2003].

Treatment of cyclic vomiting is by fluid and electrolyte replacement and treatment of esophagitis, which may be caused by the continued vomiting. Antimigraine, antiemetic, and antiepileptic agents have been used [Li and Misiewicz, 2003]. Lorazepam has antiemetic and anxiolytic properties. Phenothiazines, butyrophenones, metoclopramide (Reglan), or trimethobenzamide (Tigan) may be effective as antiemetics, but may cause extrapyramidal side effects. Ondansetron, a newer antiemetic, also may be considered. Prophylactic therapy with propranolol [Forbes and Withers, 1995] and a beneficial effect of oral or nasal sumatriptan have been described [Benson et al., 1995; Kowalczyk et al., 2010; Kakisaka et al., 2009]. Antiepileptic drugs, such as phenobarbital or valproic acid, have been suggested as ameliorating symptoms [Hikita et al., 2009]. Prophylactic therapy with valproate at a dose of 10–40 mg/day had a beneficial effect on some children with CVS [Hikita et al., 2009]. A beneficial effect of amitriptyline also has been described [Ghosh et al., 2009]. Flunarizine also was found to be effective as a prophylactic agent in a study of a small number of children with CVS and abdominal migraine [Kothare, 2005].

Gastroesophageal Reflux

Gastroesophageal reflux (GER) is present when reflux of gastric contents causes troublesome symptoms or complications [Sherman et al., 2009]. While infant regurgitation was found in 12 percent of Italian children followed for 2 years, 88 percent of these infants improved by age 12 months, and only 1 of 210 had gastroesophageal reflux disease (GERD) [Campanozzi et al., 2009]. GER occurs in about one-third of all children with severe psychomotor retardation, as well as in other neurologic conditions, including myopathies and cerebral palsy. It occurs in children with sequelae of birth asphyxia and lower esophageal sphincter tone abnormalities [Pensabene et al., 2008].

There may be incompetent lower esophageal sphincter tone, which allows entry of gastric contents and acid into the esophagus, with possible aspiration and esophagitis [Hillemeier, 1996]. Some patients also may have a hiatal hernia [Mittal and Balaban, 1997]. Factors related to the frequent occurrence of GER with neurologic conditions include esophageal muscle weakness, hypotonia, incoordination of the swallowing mechanism, scoliosis, and immobility. Cases of “dysphagia-GER complex” have been reported in patients who suffered from perinatal asphyxia, cerebellar hemorrhage, or congenital dysphagia of unknown origin and had lower brainstem dysfunction, with different combinations of facial nerve palsy, inspiratory stridor, central sleep apnea, and dysphagia [Saito et al., 2006].

Symptoms of GER include pain, discomfort, irritability, and vomiting. Anemia and recurrent aspiration pneumonia also may occur. Respiratory symptoms resulting from reflux, aspiration, and laryngospasm that cause apnea, cyanosis, and stiffening may be confused with seizures. A relation between hiatal hernia, GER, and dystonic neck posture, described by Sandifer, was reported by Kinsbourne in 1964. The dystonic posture consists of extension and rotation of the neck. Others have suggested that the dystonic posture allows for better peristalsis and clearance of acid from the distal esophagus. It has been suggested that, in children with severe neurologic impairment, self-injurious behaviors or considerable agitation may be a marker for GER [Goessler et al., 2007b].

Esophageal pH monitoring demonstrates intermittent periods of reduced pH that indicate reflux of acid gastric contents into the distal esophagus [Hillemeier, 1996]. Esophageal manometry may show decreased sphincter pressure and reduced peristaltic activity. A barium swallow of the upper intestinal tract also should be performed to determine the presence of other anatomic or functional abnormalities that may mimic reflux. As reflux can be acidic (pH <4) or nonacidic (pH >4), the use of combined pH measurements and multiple intraluminal impedance monitors is recommended [Del Buono et al., 2006]. In children in whom the etiology of GER is not established, evaluation for a neurologic cause should be considered. The diagnosis of GER in children with neurologic impairment is more difficult because symptoms frequently are less specific.

Medications that enhance gastrointestinal motility (e.g., metoclopramide, cisapride, ranitidine, sucralfate) usually are beneficial in children with GERD [Cucchiaraa, 1996; Hillemeier, 1996; Horvarth et al., 2008]. The effects of therapy in decreasing respiratory symptoms such as apnea are less likely in children with underlying neurologic disorders [Bagwell, 1995]. Children with GERD who do not respond to medical therapy may benefit from gastrostomy and Nissen fundoplication, which can be done laparoscopically [Iwanaka et al., 2010]. The procedure may improve quality of life, have a positive effect on growth, and reduce proton pump inhibitory use and frequency of respiratory symptoms [Engelmann et al., 2009]. Surgical treatment is usually more effective for the resolution of digestive than of respiratory symptoms [Tannuri et al., 2008]. Major postoperative complications or recurrent GERD occur more frequently in neurologically impaired children [Bagwell, 1995; Mathei et al., 2008; Goessler et al., 2006; Goessler et al., 2007b].

Intestinal Pseudo-Obstruction

Chronic intestinal pseudo-obstruction is one of a number of esophageal and gastrointestinal motility disorders, which include esophageal achalasia, pre- and post-fundoplication motility disorders, gastroparesis, motility disorders occurring after repair of congenital atresias, motility disorders associated with gastroschisis, motility after intestinal transplantation, motility disorders after colonic resection for Hirschsprung’s disease, chronic functional constipation, and motility disorders associated with imperforate anus [Gariepy and Mousa, 2009].

Intestinal pseudo-obstruction is manifested by intestinal obstruction in the absence of mechanical blockage [Scott, 1996]. Box 103-1 lists some of the more common causes of intestinal pseudo-obstruction in children. Acute pseudo-obstruction (paralytic ileus) usually occurs postoperatively or secondary to the use of medications, including anticholinergic agents, phenothiazines, tricyclic antidepressants, narcotics, and certain anticonvulsants, particularly benzodiazepines and barbiturates. Infant botulism can present as colonic ileus, mimicking Hirschsprung megacolon. Infant botulism should be considered in infants with constipation and neurologic abnormalities. Cases of vincristine-related pseudo-obstruction have been described. Most have been attributed to a drug interaction with itraconazole, accidental vincristine overdose, or liver failure [Diezi et al., 2010]. Abnormal motor function may also contribute to the overall intestinal dysfunction of children who have other underlying gastrointestinal diseases, such as Hirschsprung’s disease or gastroschisis.

Chronic pseudointestinal obstruction (CIPO) may be due to a variety of abnormalities involving the intestinal musculature (chronic myopathic intestinal pseudo-obstruction) or intestinal innervation (chronic neuropathic intestinal pseudo-obstruction). Pathologies associated with CIPO may be due to abnormalities of neural innervation or to intestinal smooth muscle structure. CIPO also can occur in association with systemic neurologic, endocrine, and connective tissue diseases or malignancy. When no recognizable etiology is found, CIPO is referred to as idiopathic (CIIPO) [Millar et al., 2009; De Giorgio et al., 2010]. Chronic pseudo-obstruction not related to a clearly identified cause probably involves degenerative and inflammatory abnormalities, which compromise the ENS and lead to impairment in gastrointestinal transit [Di Nardo et al., 2008].

Clinical manifestations typically include recurrent episodes of bowel obstruction without a mechanical obstructive cause. Symptoms include poor feeding, vomiting, abdominal distention, megacolon, and constipation.

The ability to study gastrointestinal motility has improved greatly in the past few years, with the development of less invasive diagnostic testing methods. Evaluation for pseudo-obstruction requires exclusion of structural, mechanical, and pharmacologic etiologies, followed by additional studies to evaluate motor activity and motility, including breath hydrogen determination, scintigraphy, and manometric studies [Scott, 1996]. A mitochondrial neurointestinal encephalopathy may cause chronic pseudo-obstruction in addition to cranial nerve abnormalities, ataxia, ophthalmoplegia, facial and limb muscle weakness, and pigmentary retinopathy. Brain MRI demonstrates diffuse leukoencephalopathy. Muscle biopsy reveals neuropathic and myopathic changes, scattered ragged red fibers, and focal cytochrome oxidase deficiency [Zimmer et al., 2009].

Medical and surgical therapies often are unsatisfactory and long-term outcome frequently is poor [Stanghellini et al., 2007]. Optimal treatment of children with intestinal motility disorders relies on a multidisciplinary approach, which focuses on optimizing nutrition, improving gastrointestinal motility, and reducing psychosocial disability [Di Lorenzo and Youssef, 2010]. Intestinal transplantation has become an accepted treatment for intestinal failure and its complications. Disorders of bowel motility may represent up to 25 percent of patients on waiting lists for intestinal transplantation.

Intussusception

Intestinal intussusception, with invagination of the gut into its distal segment, is the most common cause of bowel obstruction in children between 2 months and 5 years of age. Presenting signs and symptoms typically are abdominal, but encephalopathy with irritability, lethargy, obtundation, or coma has been described as an early or late feature of the condition.

Brainstem reflexes are usually normal, but extreme miosis may occur. Muscle tone and deep tendon reflexes may be decreased [Hoisington et al., 1993]. Neurologic symptoms resolve after surgical reduction.

The pathophysiology of this disorder is unknown but, based on an apparent response to naloxone, it has been suggested that pain may cause endorphin release that accounts for miosis, hypotonia, hyporeflexia, and a generalized cortical depression that also could mask the abdominal pain. Alternatively, endotoxin release from intestinal mucosa may act as a CNS depressant [Conway, 1993].

Celiac Disease

Celiac disease is an immune-mediated malabsorption disorder characterized by permanent gluten intolerance. Exposure to gluten perpetuates an enteropathy leading to malabsorption with chronic diarrhea, weight loss, and abdominal distention. The small intestine mucosa is abnormal, and jejunal biopsy demonstrates villous atrophy, absence of surface mucosa, and crypt hyperplasia. The disease can present with mild symptoms, such as irritable bowel syndrome, anemia, slight weight loss, and fatigue. In many individuals, it may be clinically silent, despite manifest small-bowel mucosal lesions, accounting for underdiagnosis of the disease [Feigbery, 1999; Maki and Collins, 1997; Catassi et al., 1996; Kolho et al., 1998; Holmes, 1996].

Celiac disease affects nearly 1 percent of most populations but remains largely unrecognized. Using combined determination of serum immunoglobulin G (IgG) and IgA antigliadin antibody testing [Catassi et al., 1996], the prevalence of the disease in young Italians aged 6–15 years was 1 in 184. Using IgA antiendomysium antibody determination among healthy Finnish adults, the prevalence was as high as 1 in 130 individuals [Kolho et al., 1998]. A prevalence rate of 4 percent was reported by Not et al. [1998].

Neurologic symptoms infrequently are the presenting manifestation of celiac disease [Luostarinen, 2001]. Also, in some patients, gluten sensitivity can be present with neurologic abnormalities without meeting the strictly defined criteria for celiac disease [Fasano and Catassi, 2001; Hadjivassiliou et al., 1998]. Neuropathy, manifested by paresthesias, numbness, reduced tendon reflexes, sensory ataxia, and weakness, occurs late in the disease and usually in adults with severe steatorrhea [Kaplan et al., 1988]. Nerve conduction velocity studies and nerve biopsies show evidence of distal axonopathy. The neuropathy is believed to result from vitamin deficiency. CT scans in celiac disease patients may demonstrate cerebral calcifications, as well as cerebral and cerebellar atrophy and white-matter lesions. If undetected or neglected, celiac disease may cause considerable late complications from malabsorption or secondary autoimmune diseases [Feigbery, 1999; Maki and Collins, 1997; Holmes, 1996]. Therapy consists of permanently excluding gluten from the diet, which allows healing of the mucosal intestinal lesions and improved absorption of nutrients.

Neurologic and psychiatric abnormalities are more common in adults and children with celiac disease compared with nonceliac disease subjects [Holmes, 1996; Hadjivassiliou et al., 1998; Volta et al., 2002; Zelnick et al., 2004; Bushara, 2005]. Approximately 25–50 percent of patients develop symptoms, including depression, seizures (some with cerebral calcifications), ataxia, migraine, dementia, and, rarely, peripheral neuropathy or myopathy [Collin and Maki, 1994; Holmes, 1996]. The most common neurologic problem is ataxia [Luostarinen et al., 2001; Kieslich et al., 2001]. Celiac disease is found in 9 percent of patients with idiopathic cerebellar ataxia, in comparison with 2 percent of the healthy Finnish population. Antigliadin antibodies are more prevalent in patients with ataxia than in the general population. In studies of patients with cerebellar ataxia of unknown cause, the reported percentage of biopsy-proven celiac disease has been variable (i.e., 1.9–16 percent) [Burk et al., 2001]. Seizures occur in 3.5–5.5 percent of patients with celiac disease [Holmes, 1996]. Bruzelius et al. [2001] found a significantly increased frequency of epilepsy and dementia in patients with celiac disease. Specific syndromes of occipital calcifications and epilepsy, and cerebellar degeneration with or without epilepsy, also have been described in children with celiac disease [Arroyo et al., 2002; Labate et al., 2001]. A significant percentage of patients with Down syndrome have celiac disease or are positive for antiendomysial antibodies [Zachor et al., 2000].

Psychiatric abnormalities seen in celiac disease include anxious behavior, depressive syndromes, and schizophrenia [Addolorato et al., 1996; Pynnonen et al., 2004; Ludviggson et al., 2007]. Common symptoms include apathy, anxiety, and irritability [Bushara, 2005; Carta et al., 2002]. Psychiatric symptoms do not always disappear after the start of a gluten-free diet [Ciacci et al., 1998; Fera et al., 2003; Cakir et al., 2007; Addolorato et al., 2008a; Carta et al., 2002, Corvaglia et al., 1999]. While state anxiety disappears after a gluten-free diet is started, depression persists in treated patients. Cases of psychosis or schizophrenia with celiac disease have been reported [De Santis et al., 1997]. There are reports of immunopathological changes in the intestinal mucosa of autistic children, but a direct association between autism and celiac disease has not been shown. Reports of behavioral improvement in autistic children on a gluten-free diet and casein-free diet have not been substantiated.

The mechanisms involved in the etiology and pathogenesis of psychiatric symptoms related to celiac disease remain unclear. Similarly, the role of a gluten-free diet in the improvement of symptoms is uncertain. It is possible that the antigliadin antibodies may be markers of autoimmune activity with an unidentified neurotoxic antibody [Hadjvassiliou, 1996]. Abnormalities of tryptophan availability and serotonin metabolism were suggested [Pynnonen et al., 2004]. Low tryptophan levels were found in 4 of 11 patients with celiac disease [Holmes, 1996] and were lowest in children with behavioral abnormalities. Mood and mental ability improved in some of the children after the start of a gluten-free diet.

Untreated patients with celiac disease can have signs of attention-deficit hyperactivity disorder (ADHD). Niederhofer and Pittschieler [2006] investigated a cohort of 78 patients with celiac disease. The patients had positive serum for endomysium antibodies and elevated tissue transglutaminase antibodies, as well as total or partial villous atrophy and the typical jejunal or duodenal mucosal inflammatory features associated with celiac disease. After at least 6 months of gluten-free diet, patients or their parents reported a significant improvement in their behavior and functioning compared to the period immediately before diagnosis and dietary treatment. The authors suggest that ADHD should be included in the list of neurobehavioral conditions associated with celiac disease, and that celiac disease should be included in the differential diagnosis of ADHD [Taddeucci et al., 2005].

The pathogenesis of central or peripheral nervous system dysfunction in gluten enteropathy remains unclear. It has been suggested that neurologic dysfunction in celiac disease is the result of malabsorption causing folic acid, vitamin B₁₂, or vitamin E deficiencies. Hypocalcemia and hypokalemia also have been implicated [Ventura et al., 1991]. Although some patients have malabsorption, the majority of patients with neurologic symptoms do not, and there is no evidence to indicate that the neurologic complications are secondary to this problem.

Abnormalities of humoral and cell-mediated immunity suggest that celiac disease is an immunologic disorder [Walker-Smith, 1996]. It is caused by an inappropriate immune response to the gliadin component in dietary gluten [Dietrich et al., 1997]. There is a genetic susceptibility and 90 percent of patients have the human leukocyte antigen (HLA) DRG 3 DQ-2 haplotype. There is a close relation between the biochemical properties of tissue transglutaminase and the basic molecular mechanisms responsible for celiac disease [Gentile et al., 2002]. Antiendomysial, antigliadin, and antireticulin antibodies also are associated with the disease. Anti-tissue transglutaminase antibody assay has been used as a serologic screening test for celiac disease but clinical symptoms, histologic abnormalities, and presence of antiendomysial antibodies do not always coexist. New genetic loci and candidate genes that may contribute to the risk of celiac disease and its overlap with type 1 diabetes mellitus have been identified. Novel deamidated gliadin peptides antibodies have better diagnostic accuracy over native gliadin-based tests [Rubio-Tapia and Murray, 2009].

The exact relation between gluten sensitivity, as judged by high antigliadin antibodies, intestinal pathology, and the antibodies that correlate with mucosal damage (antiendomysium and anti-tissue transglutaminase antibodies) and neurologic disease, is hard to define. It is not clear whether gluten sensitivity can cause cerebellar degeneration or other neurologic symptoms. Antigliadin antibodies are not specific for neurologic symptoms associated with celiac disease and also have been described in a variety of CNS and autonomic nervous system diseases, including neuropathy, and cerebellar ataxia. Their presence in patients with neurologic disease (i.e., ataxia) may not necessarily prove the presence of gluten sensitivity [Hadjivassiliou et al., 2003; Wills and Unsworth, 2002].

CNS antineuronal antibodies are found more frequently in celiac disease patients with neurologic symptoms, suggesting their etiological importance. In a large series of adult patients (n = 160), CNS antineuronal antibodies were found in 8 of 13 (61 percent) patients with neurologic manifestations, compared to only 1 of 20 (5 percent) patients without neurologic disease and none of the controls [Volta et al., 2002]. A study by Pratesi et al. [1998] found serum antibodies reacting with human brain vessels in untreated celiac disease patients with neurologic involvement. This interaction decreased significantly after a gluten-free diet. These findings support the hypothesis that neurologic involvement in celiac disease is the consequence of an autoimmune mechanism mediated by antineuronal antibodies [Hadjivassiliou, 1996].

Neuropathologic findings in celiac disease patients, including cerebellar lymphocytic infiltration, Purkinje cell loss, and posterior column damage, support an autoimmune etiology for the neurologic abnormalities [Wills, 2000; Hadjivassiliou et al., 1998]. Neuropathology in an adult with a pancerebellar syndrome consisted of cerebellar and basal ganglia cell loss and fibrillary gliosis. Abnormalities also have been described in the cortex, spinal cord, and muscle [Steinberg and Frank, 1993]. In a study of patients with celiac disease and neuropathy, antiganglioside antibodies were found in 65 percent of 20 patients. These antibodies bind to the Schwann cell surface, nodes of Ranvier, and axons in the peripheral nerves [Chin et al., 2003]. This suggests that autoimmune mechanisms are likely responsible for the neuromuscular symptoms described in this condition.

It also has been suggested that the neurologic abnormalities in celiac disease are due to vasculitis involving the vascular endothelium, possibly mediated by tissue transglutaminase, an enzyme considered important to the maintenance of vascular endothelial integrity [Kim et al., 2002]. This enzyme is present in the brain, being most abundant in the caudate, putamen, and substantia nigra, and antiendomysial antibody has been reported to immunofluoresce with cerebral vasculature [Pratesi et al., 1998]. The neurologic abnormalities therefore may be the result of vascular or inflammatory disease. A few cases of biopsy-proven CNS vasculitis have been described [Ozge et al., 2001], as well as cerebrospinal fluid (CSF) oligoclonal bands, myelin loss, and lymphocytic infiltration [Ghezzi and Zaffaroni, 2001].

Patients with celiac disease and villous atrophy improve after withdrawal of gluten from their diet. Neurologic abnormalities can be present before the appearance of celiac disease symptoms and may progress despite a gluten-free diet. In general, there is no evidence that a gluten-free diet or vitamin supplementation reverses the neurologic abnormalities, although, in an adult series by Volta et al. [2002], neurologic symptoms improved or disappeared in 7 of 13 individuals who started a gluten-free diet within 6 months of onset and in none of the 4 patients who started later. Similarly, in a follow-up study of children with the disease, there appeared to be a greater frequency of symptoms in the group following an unrestricted diet [Bardella et al., 1994]. Neurologic complications, specifically epilepsy with cerebral calcifications, occurred only in patients who were not treated. Improvement may follow gluten restriction in some patients with peripheral neuropathy or myopathy.

Short Bowel Syndrome

Short bowel syndrome is a malabsorption syndrome that follows resection of a significant part of the small bowel, usually because of a congenital anomaly, necrotizing enterocolitis, inflammatory bowel disease, or ischemic injury. Such patients invariably need chronic parenteral nutrition, although an adaptation response of the remaining bowel may enable some children to grow without parenteral nutrition.

An unusual complication of this syndrome is an encephalopathy that affects infants with hyperchloremic acidosis and increased blood and urine lactate. The encephalopathy has been attributed to elevated blood and presumably cerebral lactate concentrations [Gurevitch et al., 1993]. Early recognition, treatment with an intestinal antibiotic, and discontinuation of enteral feeding facilitate correction of the hyperchloremic acidosis and recovery.

Escherichia coli

An outbreak of Escherichia coli hemorrhagic colitis in 1990 in Japan was associated with neurologic manifestations, including generalized seizures, impaired consciousness, urinary incontinence, nystagmus, phrenic nerve palsy, action tremor, and vertigo. Neurologic symptoms were believed to be secondary to bacterial endotoxins [Hamano et al., 1993]. Additional similar cases have been described [Ephros et al., 1996].

Campylobacter jejuni

Campylobacter jejuni, a frequently identified bacterial cause of gastroenteritis, has been associated with Guillain-Barré syndrome and encephalopathy [Duret et al., 1991; Nasaralla et al., 1993]. There is some evidence that Guillain-Barré syndrome in these patients may be a more severe variant of the disease with predominantly axonal degeneration. Cross-reactivity between neural antigens and C. jejuni may be one of the mechanisms by which Guillain-Barré syndrome is triggered [Rees et al., 1993].

Nipah Virus

Nipah virus infection can produce a rapidly progressive, severe illness affecting the central nervous and respiratory systems [Hossain et al., 2008]. Neurologic sequelae are frequent following Nipah encephalitis. CNS disease frequently results in high case-fatality rates. Patients with encephalitis (32 percent overall) had persistent neurologic dysfunction, including static encephalopathy, oculomotor palsies, cervical dystonia, focal weakness, and facial paralysis. MRI abnormalities include multifocal hyperintensities, cerebral atrophy, and confluent cortical and subcortical signal changes. Behavioral abnormalities were reported by caregivers in over 50 percent of subjects under the age of 16 years. The onset of neurologic abnormalities can be delayed and new neurologic dysfunction may develop after the acute illness. Survivors of Nipah virus infection may experience substantial long-term neurologic and functional morbidity [Sejvar et al., 2007].

Infant Botulism

Infant botulism is a disease of young infants caused by the release of toxins (either type A, B, or E) produced by several anaerobic, Gram-positive bacteria, including Clostridium botulinum, Clostridium baratii, or Clostridium butyricum [Fox et al., 2005]. The botulinum toxins (BoNTs) are among the most poisonous biological substances. In a recent clinical report reviewing all cases of infant botulism poisoning over the past 30 years in a major U.S. city [Underwood et al., 2007], the two principal pathogenic serotypes identified were BoNT/A (40 percent of cases affecting mostly older infants) and BoNT/B (60 percent of cases affecting mostly younger infants). The neurotoxic effects of the disease are produced because of presynaptic blockage of the neuron’s neuroexocytosis of acetylcholine and disruption of acetylcholine transmission. The toxin blocks acetylcholine release by impairing calcium influx associated with membrane depolarization and causes an acute reversible motor unit disease. The toxin effect can last many weeks or months, depending on the BoNT’s serotype. At postsynaptic sites, BoNTs can also block transmission of action potentials by affecting the motor end-plate nicotinic receptors.

Infant BoNT infections typically occur between 2 weeks and 1 year of life (median 10 weeks). Initial symptoms can be severe, including constipation, cranial nerve deficits, pupillary involvement, generalized weakness and hypotonia, loss of deep tendon reflexes, and flaccid paralysis of striated skeletal muscles, including the diaphragm, which can lead to respiratory failure and death [Arnon, 1992; Cochran and Appleton, 1995; Hatheway, 1995]. Autonomic dysfunction also is common, and may cause constipation, dysphagia, reduced lacrimal and salivary secretions, impaired visual accommodation, and heart rate dysregulation [Patural et al., 2009].

Affected infants have a history of lethargy, irritability, poor feeding, constipation, and generalized weakness of 12 hours’ to 7 days’ duration. Examination reveals ophthalmoplegia with sluggish pupillary responses, as well as cranial nerve VII, IX, X, and XI abnormalities. Severe cases can present with areflexive flaccid coma or apnea, requiring prolonged ventilation. Botulinum neurotoxin can be detected in serum, and confirms the diagnosis. Progressive respiratory failure occurs in many patients, requiring prolonged periods of mechanically assisted ventilation, and is the cause of potential major morbidity. Other symptoms of autonomic dysfunction include delayed gastric emptying and a paralytic ileus. There usually are no major cardiovascular symptoms. However, there can be cardiac dysautonomia, manifested by a major decrease in all heart-rate variability indices, which may persist for weeks beyond observable physical recovery. The recovery of gastrointestinal motility is progressive and usually complete.

Repetitive nerve stimulation studies document an unusual incremental response, and electromyography demonstrates a distinctive pattern consisting of brief, small, motor unit potentials. Treatment is supportive, focusing primarily on the need to maintain adequate ventilation and nutrition in a critical care setting. Evaluation and treatment of botulism is further reviewed in Chapter 91.

Shigellosis

Shigellosis is an acute bacterial enteritis caused by Shigella bacteria, and is characterized by intestinal and extraintestinal symptoms. Shigellosis has a worldwide distribution, but the majority of cases occur in developing countries. Sixty-nine percent of all episodes and 61 percent of all Shigella-related deaths involve children younger than 5 years of age. The mainstay of treatment is correction of fluid and electrolyte loss. Appropriate antibiotic therapy shortens the duration of clinical symptoms and fecal excretion of the pathogen. However, the increasing antimicrobial resistance of shigellae worldwide constitutes an international health problem [Ashkenazi, 2004].

Neurologic manifestations of dysentery caused by Shigella organisms are common and include convulsions, meningismus, headache, and, infrequently, an encephalopathy, with lethargy, confusion, hallucinations, delirium, or mutism [Selimoglu et al., 1995]. The Klüver–Bucy syndrome was described in a child suffering from Shigella encephalopathy [Guedalia et al., 1993]. Septic shock is a very unusual presentation of Shigella infection and can occur in young children, with symptoms including a severe encephalopathy [Beigelman et al., 2002]. Neurologic manifestations may appear before the onset of diarrhea and other gastrointestinal symptoms. Although seizures and encephalopathy usually are benign and rarely recur or have permanent sequelae [Ozturk et al., 1996], complications of shigellosis encephalopathy [van Dongen et al., 1993] and cases of severe fatal encephalopathy have been described. CT may demonstrate cerebral edema. Hemorrhage, demyelination, and brain necrosis have been reported in patients with Shigella-related fatal encephalopathy [Perles et al., 1995].

The mechanisms responsible for the neurologic complications of shigellosis have not been defined clearly. Some manifestations, especially seizures, may be related to fever. In the past, neurologic symptoms were attributed to the neurotoxicity of the shiga-toxin but it is thought that the role of this toxin is not essential and that other toxic products may be involved [Ashkenazi et al., 1990].

Seizures are the most common neurologic complication of shigellosis, occurring in 12–45 percent of hospitalized patients with culture-proven shigellosis; they are most common in children between 6 months and 4 years of age [Hiranrattana et al., 2005; Daoud et al., 1990]. The high incidence of seizures was found in studies that usually included children with severe shigellosis. By contrast, data collected from a well-defined pediatric population and over a prolonged period suggest that seizures are less common [Galanakis et al., 2002]. About 25 percent of children with seizures during shigellosis infection had a history of previous febrile seizures. A study of 55 children with Shigella organism-associated convulsions, who were followed for 6.9–14.1 years, found that none had subsequent nonfebrile seizures and only two had subsequent febrile seizures [Lahat et al., 1990]. Therefore, no chronic antiepileptic drug therapy is indicated [Ozturk et al., 1996]. It was suggested that corticotropin-releasing hormone plays a role in the increased susceptibility to seizures following exposure to Shigella dysenteriae [Yuhas et al., 2004]. Tumor necrosis factor-α, at high concentrations, may exert a suppressive effect on Shigella-mediated seizures [Yuhas et al., 2003].

Rotavirus Infection

Enteric rotavirus infection causing gastroenteritis with concomitant afebrile seizures has been described in infants [Lin et al., 1996; Tsai and Cho, 1996]. Transient electroencephalography (EEG) abnormalities also have been reported. Seizures did not recur, and no long-term anticonvulsant therapy was prescribed in these patients. Direct inoculation of three strains of rotavirus, including human rotavirus strain 2, into monkey brain caused a neurologic syndrome including transient paresis, with pathologic evidence of viral invasion of the CNS. The fact that rotavirus also causes exanthems in 4 percent of patients suggests that infection causes generalized viremia. It is possible that the virus invades the CNS hematogenously [Contino et al., 1994].

Whipple’s Disease

Whipple’s disease is a chronic, multisystemic, bacterial, inflammatory systemic disorder. The disease is named after George H Whipple who, in 1907, was the first to describe an intestinal “lipodystrophy.” It is caused by a soil-borne, Gram-positive bacillus – Tropheryma whipplei [Panegyres, 2008]. It is a rare disease that is misdiagnosed frequently. It is described primarily in adults and is manifested clinically by intestinal features that include loss of weight, diarrhea, malabsorption, and abdominal pain, as well as polyarthritis, lymphadenopathy, cardiopathy, hyperpigmentation, hypotension, and cardiovascular, pulmonary, and neurologic dysfunction [Jovic and Jovic, 1996].

Jejunal biopsy reveals macrophages filled with glycoprotein granules that are positive with periodic acid–Schiff (PAS) staining. PAS-positive macrophages also have been identified in other body tissues, including the CNS. The presence of PAS-positive macrophages in biopsy specimens and demonstration of T. whippeli by electron microscopy are diagnostic signs of active Whipple’s disease.

Although Whipple’s disease generally is recognized as a multisystem chronic granulomatous disease, primarily involving the digestive system, up to one-third of patients demonstrate signs of neurologic involvement. On occasion, Whipple’s disease also can present as a primary neurologic disorder. Neurologic manifestations include progressive dementia, external ophthalmoplegia, myoclonus, seizures, ataxia, hypothalamic dysfunction (sleep disorders, hyperphagia, polydipsia), and meningitis [Alba et al., 1995; Jovic and Jovic, 1996; Perkin and Muray-Lyon, 1998]. Oculomasticatory myorhythmia, possibly caused by brainstem tegmental abnormalities, is a movement disorder nearly unique to patients with Whipple’s disease. It consists of rhythmic convergence of the eyes and synchronous contractions of the eyelids, jaw, face, and neck, with characteristics resembling spinal myoclonus. A review of 84 cases of CNS Whipple’s disease revealed that 80 percent of patients had systemic signs. Cognitive changes were frequent (71 percent), and 47 percent of the patients with cognitive changes also had psychiatric signs. Oculomasticatory myorhythmia and oculo-facial-skeletal myorhythmia, pathognomonic for CNS Whipple’s disease, were present in 20 percent of patients and were always accompanied by a supranuclear vertical gaze palsy. Tissue biopsy was a sensitive technique; 89 percent of those who had biopsies had positive biopsy results.

Brain CT and MRI may be normal, but may show cortical or subcortical atrophy, hydrocephalus, and focal or multiple nodular enhancing lesions, including low-density focal lesions in the pontine tegmentum and temporal lobe [Schnider et al., 1995].

Increased protein and lymphocytic pleocytosis are detected in the CSF of patients with the disease. PAS-positive cells have been found in brain biopsy material.

Diagnosis can be confirmed by the finding of a positive polymerase chain reaction (PCR) test for T. whipplei in blood or CSF [Panegyres et al., 2006]. The CSF can sometimes contain PAS-positive macrophages. The diagnosis and treatment of definite CNS Whipple’s disease should be based on the presence of pathognomonic signs (oculomasticatory myorhythmia or oculo-facial-skeletal myorhythmia) and a positive biopsy or PCR result [Louis et al., 1996]. In the presence of clinical suspicion of CNS Whipple’s disease, PCR should be performed on the CSF and blood, even if the brain MRI is normal. Sometimes, brain biopsy may be necessary if the diagnosis is still suspected and the PCR is negative.

The disorder responds to antibiotic therapy. Without extended antibiotic treatment, the course of Whipple’s disease can be fatal. Medications include cotrimoxazole (trimethoprim-sulfamethoxazole). Third-generation cephalosporins, as well as penicillin and streptomycin, can be effective in patients who do not respond to trimethoprim-sulfamethoxazole [Panegyres, 2008; Alba et al., 1995]. Appropriate therapy, instituted early in the course of the disease and for an adequate time period, is associated with a better neurologic outcome. CNS relapse may be resistant to treatment. In cases with high clinical suspicion, in which Whipple’s disease cannot be diagnosed with procedures such as jejunal biopsy, antibiotic therapy is recommended. Recovery of established neurologic deficit may occur rarely [Jovic and Jovic, 1996].

Turcot’s Syndrome

Turcot’s syndrome is the rare association of polyposis of the colon and malignant tumors of the CNS. Most reported CNS tumors are glioblastomas, medulloblastomas, and anaplastic astrocytomas. It is inherited as an autosomal-recessive trait. Two genetic subtypes have been described. Disease can result from a mutation in the adenomatous polyposis coli gene or the mismatch repair genes underlying the syndrome of hereditary nonpolyposis colon cancer [Agostini et al., 2005]. Survival of Turcot’s syndrome patients with glioblastoma may be longer than expected [Lusis et al., 2010] . The diagnosis of polyposis can precede or follow detection of the CNS tumor.

Porphyria

The porphyrias are a heterogenous group of metabolic disorders characterized by abnormal heme biosynthesis. The abnormalities involve porphyrin metabolism in the metabolic pathway that converts glycine and succinyl-CoA to heme [Tefferi et al., 1994]. Every porphyria subtype is caused by abnormal function of a separate enzymatic step, resulting in a specific accumulation of heme precursors. The critical enzyme is a mitochondrial enzyme, gamma-aminolevulinic acid synthetase. Accumulation of the heme precursor, delta-aminolevulinic acid, is associated with the neurologic manifestations, and accumulation of photoreactive byproducts, the porphyrins, causes cutaneous photosensitivity and dermatopathic manifestations [Bont et al., 1996; Ellefson and Ford, 1996; Lockwood, 1995]. The hepatic porphyrias are transmitted genetically as autosomal-dominant disorders with variable expression. Although all subtypes are rare, acute intermittent porphyria (AIP) is the most common form of the neuroporphyrias. It manifests with occasional neurovisceral crises due to overproduction of porphyrin precursors, such as aminolevulinic acid (ALA), which is released from the liver into the circulation. More than 60 mutations causing AIP have been found [Schreiber, 1995]. The degrees of expression range from mild to severe, and acute episodes of neuropathic porphyrias can progress to paralysis and life-threatening respiratory failure. Although porphyria patients carry genetic defects and mutations, the manifestation of neuropsychiatric symptoms in hepatic porphyria patients is triggered by precipitating factors, including various drugs, hormones, fasting, and other agents. All precipitating factors can increase the demand for hepatic heme and induce the production of δ-aminolevulinic acid synthase (ALAs), which is the rate-limiting enzyme in the heme biosynthetic pathway. Depletion of free heme and the accumulation of heme precursors, such as δ-ALA and porphobilinogen, are responsible for neuropsychiatric manifestations seen acutely [Anderson et al., 2005; Satoh et al., 2008].

Acute porphyrias present with acute attacks, typically consisting of severe abdominal pain, nausea, constipation, polyneuropathy mental symptoms, confusion, and seizures, and can be life-threatening. Porphyrias are rare and sometimes are misdiagnosed because various symptoms and signs mimic other diseases. Screening of porphyrin metabolites in neurologic patients with acute polyneuropathy or encephalopathy associated with pain or dysautonomia reveals the presence of urinary porphyrins and their precursors in 21 percent of cases. Cutaneous porphyrias present with either acute painful photosensitivity or skin fragility and blisters.

In general, presentation in childhood is uncommon [Parsons et al., 1994]. Rare recessive porphyrias usually manifest in early childhood with either severe cutaneous photosensitivity and chronic hemolysis, or chronic neurologic symptoms with or without photosensitivity.

Symptoms include features of autonomic neuropathy, including severe abdominal pain, vomiting, constipation, hypertension, tachycardia, and bladder dysfunction [Gupta and Dolwani, 1996]. Other symptoms are those of peripheral axonal neuropathy, including motor weakness and sensory involvement that correlate with peripheral axonal neuropathy, and neuropsychiatric symptoms. Acute porphyric neuropathy predominantly involves the motor system and can be manifested by symptoms, signs and CSF abnormalities resembling acute Guillain-Barré syndrome [Suarez et al., 1997; Barohn et al., 1994; Albers and Fink, 2004], causing confusion with this order.

The pathogenesis or porphyric neuropathy is complex, but overproduction of ALA via direct neurotoxicity, oxidative damage, and modification of glutamatergic release may initiate the neuronal damage.

Acute encephalopathy in patients with hepatic porphyria is manifested as a combination of neuropsychiatric symptoms, including confusion, anxiety, depression, and insomnia, with seizures, the syndrome of inappropriate antidiuretic hormone secretion (SIADH), and, rarely, focal CNS deficits, such as aphasia, hemiparesis, and visual field abnormalities [Crimlisk, 1997; Bylesjo et al., 1996], along with abdominal pain and peripheral neuropathy. In rare instances, patients have presented with acute cortical blindness [Wessels et al., 2005]. Psychotic symptoms during an AIP attack have been reported [Altintoprak et al., 2009]. Structural brain abnormalities, demonstrated on neuroimaging, are uncommon. MRI ischemic lesions also have been reported [Gurses et al., 2008]. Posterior reversible encephalopathy syndrome has been found in patients with severe encephalopathy on MRI acquired during an acute attack, and could explain the pathogenesis of encephalopathy and seizures in AIP.

In general, the underlying mechanisms causing symptoms have remained poorly understood, partly due to lack of a suitable animal model [Satoh et al., 2008]. Two hypotheses, the possible neurotoxicity of δ-ALA and heme deficiency in the nervous tissue, have been suggested. The present evidence suggests that multiple mechanisms interact in causing the varied symptoms, including ALA interaction with gamma-aminobutyric acid (GABA) receptors, altered tryptophan metabolism, and, possibly, heme depletion in nerve cells [Meyer et al., 1998]. Neurologic manifestations are nonspecific, and careful interpretation of abnormal excretion of porphyrin precursors should be done before symptoms can be related to the inherited acute porphyrias.

Currently, the prognosis of neuropathy and encephalopathy in AIP is good, even in severe attacks, but the variability of symptoms may interfere with the diagnosis of AIP. Physicians should be aware of a potentially fatal outcome of the disease or long-term permanent neurologic damage [Pischik and Kauppinen, 2009].

Diagnosis is essential to enable specific treatments to be started as soon as possible. Biochemical analyses detect porphyrins and their precursors from blood, urine, or feces.

Measurement of urinary porphobilinogen is the best biochemical test for AIP, although it is nonspecific and does not distinguish AIP from other acute porphyrias. Therefore, identification of acute porphyria also requires heightened clinical awareness, a good medical history, and a careful evaluation of signs and symptoms during an acute attack, along with access to urinary porphobilinogen measurement [Elder and Sandberg, 2008]. Measurement of urinary porphobilinogen can also identify AIP patients studied during remission. As biochemical analyses often remain indeterminate in remission, and since the correct diagnosis of a hereditary disease is essential, mutation analysis is needed to confirm the diagnosis [Kauppinen and von und zu Fraunberg, 2002; Pischik et al., 2008]. Early diagnosis and information about precipitating factors can diminish mortality and prevent subsequent attacks, so mutation screening also is recommended for family members [Kauppinen, 2005; Puy et al., 2010]. Acute inherited porphyria is not infrequent among a selected group of neurologic patients, and screening of urinary porphobilinogen is cost-effective and potentially beneficial.

Treatment is mainly symptomatic and includes supportive treatment with analgesia, monitoring for and treating complications such as hypertension, prevention and treatment of water and electrolyte disorders that may result from inappropriate antidiuretic hormone production, and a high-carbohydrate diet. Long-term management consists of genetic counseling and should include advice on lifestyle modification, involving avoidance of alcohol, smoking, and known porphyrogenic drugs; on diet; and on and avoidance of medications (e.g., barbiturates, phenytoin, sulfonamides, chloroquine) that precipitate acute attacks. Of the new anticonvulsants, lamotrigine has been reported to precipitate porphyric symptoms, whereas gabapentin has been used successfully to treat seizures in patients with AIP [Gregersen et al., 1996; Krauss et al., 1995; Tatum and Zachariah, 1995]. Specific treatment with heme preparations needs to be instituted as soon as possible, following confirmation of increased excretion of porphobilinogen in the urine. All cutaneous porphyrias can be alleviated by avoidance of sunlight. Treatment of erythropoietic protoporphyria involves administering large doses of beta-carotene, which may improve tolerance to sunlight. Congenital erythropoietic porphyria is a rare, early-onset, severe, photomutilating condition, for which bone marrow transplantation has been shown to be successful [Badminton and Elder, 2002].

Neurologic Abnormalities

Neurologic examination may demonstrate mental status changes; motor abnormalities with pyramidal tract dysfunction including hypertonia, later changing to hypotonia when coma develops; hyperreflexia and extensor plantar responses; and early parkinsonian syndrome, which may include hypomimia, muscular rigidity, bradykinesia, monotony of speech, dyskinesia, motor clumsiness, and tremor [Spahr et al., 2000]. Other signs include ataxia, action tremor, dysarthria, and asterixis, a characteristic flapping tremor that consists of frequent involuntary flexion–extension movements of the hand, associated electrophysiologically with periods of complete electrical silence in muscles. Asterixis can be elicited by having the patient extend his or her arms with dorsiflexion of the wrist, which elicits a “flap” [Rio et al., 1995]. Seizures are relatively uncommon, occurring in 10–30 percent of patients [Decell et al., 1994). Focal neurologic abnormalities, as well as cerebral and retinal visual abnormalities, may occur. Patients with minimal hepatic encephalopathy usually have no overt neurologic abnormalities.

Cognitive and behavioral abnormalities

The earliest changes of hepatic encephalopathy are subtle behavioral changes and mild impairments of intellectual function apparent to the patient’s family and close friends, which reflect bilateral forebrain, parietal, and temporal lobe dysfunction [Jones and Weisborn, 1997]. The history, therefore, should focus on changes in activities of daily living, impairment of sleep–wake status, attentiveness, cognition, consciousness, and motor functions. Sleep disturbances are a common early sign [Cordoba et al., 1998]. Verbal ability is relatively preserved initially. Later, performance at school deteriorates, motor functions become impaired, and consciousness decreases. Neuropsychiatric evaluation reveals abnormalities in mental and motor status, including an abnormal level of consciousness, reduced attention, slow reaction speed, defects in orientation, memory, affect, perception and judgment, and psychomotor slowing. Symptomatic hepatic encephalopathy traditionally is graded into four stages, with derangement of consciousness progressing from drowsiness to stupor and coma. These stages are:

grade I: altered sleep cycle, altered affect (euphoria or belligerence), mild confusion, loss of spatial orientation

grade II: drowsy, responding to simple commands

grade III: stuporous, responding only to painful stimuli

grade IV: unresponsive to pain; decorticate or decerebrate posturing.

Laboratory Tests

Liver function tests usually are abnormal, but derangements may not be as drastic as expected, based on the clinical examination. Arterial blood ammonia may be elevated, and correlates somewhat with the clinical state and rate of ammonia uptake and metabolism by the brain. However, most studies have felt that the ammonia level does not correlate with the severity of encephalopathy. CSF glutamine concentration is more specific than blood ammonia and correlates better with the stage of hepatic encephalopathy. The diagnosis of hepatic encephalopathy is, therefore, based primarily on clinical findings. There is no consistent correlation between any one laboratory test and the severity of symptoms. Neuropsychologic tests provide useful information, especially in patients with subclinical hepatic encephalopathy, which would otherwise be overlooked.

The EEG may assist in the clinical staging of hepatic encephalopathy. In the early stages, it shows progressive reduction of the alpha rhythm, which is mixed with theta activity. With a more advanced encephalopathy and further clouding of consciousness, slower delta frequencies appear. Triphasic waves are characteristic but not specific, may not be found in young children, and, as in other metabolic conditions, may be present in later stages; they carry a poor prognosis. Occasionally, triphasic waves are seen before encephalopathy is clinically evident and may be diagnostically useful. Subsequently, there is decreasing amplitude of the EEG waveforms and, finally, absent cerebral activity. Latencies of evoked brain potentials are delayed [Kullmann et al., 1995; Saxena et al., 2001], although the role of evoked potentials in determining the level of hepatic encephalopathy remains unclear.

CT may be normal early in the course of hepatic encephalopathy. Cortical atrophy may be seen later. Cerebral edema sometimes is demonstrated in acute cases. Radiologic abnormalities detected by MRI in adults with non-Wilsonian chronic hepatic failure include increased signal in basal ganglia regions on T1-weighted images, with no corresponding abnormalities on T2-weighted images [Ballauff et al., 1994; Weissenborn et al., 1995; Lockwood et al., 1997]. The abnormal MRI signal detected in the basal ganglia (in particular, the globus pallidus) in patients with cirrhosis may be caused by depositions of manganese [Butterworth et al., 1995; Klos et al., 2005].

Three neurologic syndromes were detected in 15 patients with chronic liver failure and basal ganglia T1 hyperintensity:

All but one patient had elevated blood manganese levels. The etiology of the liver disease was primary biliary cirrhosis in 6 patients, and other non-Wilsonian forms of liver disease in the remaining patients. Ammonia levels were checked and were elevated in 4 of 9 patients. Cognitive impairment consisted of difficulty with sustained attention and concentration, immediate and delayed recall, and executive function, with visuospatial dysfunction. Psychiatric symptoms ranged from a major depressive disorder to anxiety symptoms and insomnia.

Single-photon emission computed tomography (SPECT) scans in patients with cirrhosis reveal bilateral hypermetabolism in the basal ganglia. Significant abnormalities have not been demonstrated in the cerebral cortex or cerebellum. MRI does not correlate with laboratory indices of hepatic function, with histologic liver diagnosis, or with the neurologic status at the time of MRI acquisition. MR spectroscopy studies demonstrate depletion of myoinositol, preservation of N-acetylaspartate, and elevation of glutamine [Laubenberger et al., 1997]. Spahr et al. [2000] evaluated cirrhotic patients with subclinical or grade 1 hepatic encephalopathy for parkinsonian signs and neuroanatomical abnormalities using MRI and MR spectroscopy, and reported hyperintensity in the occipital white matter and basal ganglia with reduced myoinositol/creatinine and choline/creatine ratios. Parkinsonian signs correlated with basal ganglia abnormalities. MR spectroscopy changes are not related to the stage of hepatic encephalopathy, can be demonstrated in the earlier stages of this condition, and were interpreted as suggesting the development of cerebral osmotic abnormalities in patients with cirrhosis [Cordoba et al., 2002].

Positron emission tomography (PET) studies suggested a rise in the cerebral ammonia metabolic rate, coupled with an increase in the permeability surface area of the blood–brain barrier to ammonia. A PET study examining glucose metabolism has shown reduced metabolism in the anterior cingulate gyrus, an area that is a part of attentional systems [Lockwood et al., 1997].

Neuropathology and Pathophysiology

The pathologic hallmark of hepatic encephalopathy in patients with cirrhosis and with portal-systemic shunts, is the Alzheimer type II astrocyte found in many brain locations, including the cortex, lenticular nucleus, lateral thalamus and dentate, and red nuclei [Lockwood, 2002]. Although not specific to hepatic encephalopathy, these hypertrophic astrocytes are commonly found in the brains of patients who die with cirrhosis and portosystemic shunting, develop shortly after the onset of hepatic encephalopathy, are related to the duration of coma, and are reversible. There are no other significant parenchymal neuropathologic abnormalities. The major finding in acute fulminant hepatic failure is cerebral edema.

One of the most important factors in the pathophysiology of hepatic encephalopathy is an increase in the level of substances toxic to the brain, which occurs in the presence of parenchymal liver disease or portosystemic shunting. The metabolic effects of these substances are important etiological factors in hepatic encephalopathy [Treem, 1996]. These products, including those of bacterial action on protein in the large intestine, normally are metabolized by the liver. In the presence of parenchymal liver disease or portosystemic shunting, their level in the systemic circulation rises; they accumulate in the extracellular fluid and reach the brain. In addition, the blood–brain barrier may be damaged in patients with liver failure, resulting in an increased permeability and increased access of such neurotoxins to the brain. Ammonia is regarded as the most likely toxin [Butterworth et al., 1987]. Ammonia is produced in the intestinal tract by bacterial breakdown of ingested protein and transported to the liver via the hepatic portal vein; it is converted to urea by the urea cycle enzymes, and then is excreted by the kidneys. Ammonia concentrations in blood, CSF, and brain increase in patients with hepatic encephalopathy, with brain ammonia concentrations sometimes rising to more than 20 times the normal level. This may be due to increased permeability of the blood–brain barrier, allowing ammonia to diffuse more freely into the brain parenchyma. This results in an ammonia-induced encephalopathy, even though arterial ammonia levels may be near-normal or normal [Lockwood et al., 1991]. In the brain, ammonia is detoxified by astrocytes, combining with alpha-ketoglutarate to form glutamic acid, which then forms glutamine, leading to elevated glutamine levels in the CSF [Fraser and Arieff, 1985].

Several mechanisms have been proposed to explain the neurotoxic action of ammonia, including modification of blood–brain barrier transport, effects on glucose metabolism and brain energy production, and disruption of amino-acid profile in the brain (particularly glutamate by decreasing glutaminergic neurotransmission). Ammonia also inhibits the cellular chloride pump and normal neuronal membrane physiology, which contributes to CNS depression.

Convergence of research studies confirms a major role for ammonia in the pathogenesis of hepatic encephalopathy:

However, hyperammonemia alone cannot explain the severity of hepatic encephalopathy because correlation between the arterial and CSF ammonia concentrations and the stage of hepatic encephalopathy is poor, and grade 4 encephalopathy (i.e., coma) may be found with normal blood ammonia levels. Also, ammonia toxicity affects the cortex but not the brainstem centers that are involved in hepatic encephalopathy. Other factors, including alterations in monoaminergic, glutaminergic, GABA, serotoninergic, and endogenous opioid neurotransmitter mechanisms, were demonstrated in the brains of animal models of hepatic encephalopathy and in postmortem brain tissue from patients with liver disease, and may play a role in hepatic encephalopathy [Butterworth, 1996; Felipo and Butterworth, 2002].

GABA, a principal neurotransmitter, is synthesized by intestinal bacteria and is the major inhibitory neurotransmitter in the human brain. Increased GABA-mediated neurotransmission is known to cause impaired consciousness and psychomotor dysfunction. There are few lines of evidence, some from studies in animal models, that support the hypothesis that increased GABA-mediated neurotransmission contributes to the manifestations of hepatic encephalopathy. GABA levels are markedly elevated in patients with fulminant hepatic failure and correlate well with the stage of encephalopathy. Both increased GABA release and enhanced activation of the GABA receptor complex have been demonstrated [Albrecht and Jones, 1999]. Elevated plasma levels may be due to defective hepatic clearance by GABA transaminase. With impaired blood–brain barrier function, GABA accumulates postsynaptically and results in neuronal membrane inhibition. In a rabbit model, development of hepatic encephalopathy was associated with increased plasma levels of GABA, increased blood–brain barrier permeability, increased brain GABA and benzodiazepine binding sites, and a pattern of neuronal activity similar to that induced by drugs that activate the GABA neurotransmitter system [Schafer and Jones, 1982]. Similarly, there is an increased density of brain GABA receptors in patients who died with chronic liver disease and encephalopathy, suggesting that symptoms were related to increased sensitivity of brain to GABAergic neural inhibition. It was postulated that a combination of chronic low-grade glial edema and potentiation of the effect of GABA on the CNS by ammonia may be responsible for many of the symptoms of hepatic encephalopathy [Lewis and Howdle, 2003; Haussinger et al., 2002; Basile and Jones, 1997].

A possible role of natural benzodiazepine-like substances in the pathogenesis of hepatic encephalopathy has been suggested [O’Caroll, 2007]. Benzodiazepines can act at the GABA_A receptor complex. Elevated levels of benzodiazepines have been found in the brain in hepatic failure [Basile et al., 1994]. Later studies found elevated benzodiazepine levels in patients with cirrhosis with and without hepatic encephalopathy [Hernandez-Avila et al., 1998; Avallone et al., 1998]. In patients with severe hepatic encephalopathy, greater binding of ligands to benzodiazepine receptors in CSF also has been observed [Muyllen et al., 1990]. In addition, benzodiazepine receptor ligands ameliorated the behavioral depression and visual-evoked response abnormalities associated in the thioacetamide-induced rat model of hepatic encephalopathy, suggesting involvement of the GABA/benzodiazepine receptor complex [Gammal et al., 1990]. Increased sensitivity of the brain of patients with cirrhosis to an exogenously administered benzodiazepine has been demonstrated [Jones and Weisborn, 1997]. It is possible that, in some of these patients, elevated benzodiazepine levels were the result of exogenous benzodiazepines given to the patients as procedural sedation. Some patients with hepatic encephalopathy improve following administration of the GABA_A receptor antagonist Flumazenil, although there was no effect on mortality or recovery [Pomeir-Layrargues et al., 1994].

Other factors with a possible role in the pathophysiology of hepatic encephalopathy include neurotransmitters other than GABA, including glutamate, dopamine, serotonin, and opioid systems. Elevated CSF glutamine levels may be more specific than blood ammonia levels for hepatic encephalopathy. Plasma levels of amino acids, short-chain free fatty acids, and substance P also are elevated in patients with hepatic encephalopathy. False neurotransmitters, such as octopamine, are elevated in the brain and CSF. Brain cholinergic impairment also has been reported in hepatic encephalopathy. A 30 percent increase in cortical acetylcholinesterase (AChE, the acetylcholine hydrolyzing enzyme) activity was found in patients who died from hepatic coma. AChE activity in brain cortical extracts of bile duct-ligated rats was reduced similarly. The reduction of acetylcholine in the brains of bile duct-ligated rats was associated with reduced learning and memory functions. A hyperammonemic diet did not change the brain AChE activity [Garcia-Ayllon et al., 2008].

Thus, it is very likely that the pathogenesis of hepatic encephalopathy is multifactorial and involves the synergistic actions of a number of agents, including ammonia, GABA, and others [Treem, 1996].

Treatment

The treatment of HE is directed towards removal or correction of any precipitating factors, diminishing the absorption of nitrogenous substances from the intestinal tract, and minimizing portosystemic shunting [Jones and Weisborn, 1997]. Most effective therapies are directed to reducing the ammonia concentration and treating various complications, including infections, renal and cardiovascular dysfunction, and bleeding secondary to ruptured varices, hypersplenism, or clotting factor deficiencies [Riordan and Williams, 1997]. Therapy is more effective for portosystemic encephalopathy and much less so for fulminant hepatic failure.

Ammonia production can be reduced by decreasing dietary intake of protein, treating constipation, controlling gastrointestinal tract bleeding, or reducing bacterial intestinal growth. Ammonia also can be removed from the intestines by the osmotic action of nonabsorbable disaccharides, such as lactulose. Lactulose is metabolized in the lower intestinal tract to lactic acid, reduces local pH (to 5.5), and inhibits growth of colonic organisms and the formation of ammonia. Ammonia is converted to the ammonium ion (NH₄) and then is excreted in the stool. The resulting decrease in ammonia concentration is associated with improvements in mental status and in EEG abnormalities.

Complications of lactulose therapy include dehydration and hypernatremia. Reduction of gut ammonia also can be achieved using antibiotics such as neomycin, which are active against urease-producing bacteria. The efficacy of neomycin is similar to that of lactulose. An alternative strategy for lowering of blood ammonia is the stimulation of ammonia fixation – for instance, by using l-ornithine-l-aspartate. Treatments aimed at reversing the neurotransmitter defects include the use of the imidazobenzodiazepine, flumazenil, a selective high-affinity competitive antagonist of central GABA_A benzodiazepine receptors. It competes with high specificity with benzodiazepine receptor agonist ligands (e.g., diazepam) for binding to those receptors. It treats that part of hepatic encephalopathy that is related to the effects of natural benzodiazepine receptor agonists. Flumazenil, given as an intravenous bolus injection, improved clinical and electrophysiological signs and reduced the severity of hepatic encephalopathy [Gooday et al., 1995].

Management of acute severe cerebral edema involves restriction of intravenous fluids, use of hyperosmolar agents (e.g., mannitol, hypertonic saline), and, in selected patients, intubation and hyperventilation. Cerebral edema is best controlled with mannitol (0.25–0.5 g/kg), infused rapidly over 20 minutes every 4–6 hours. Higher doses may cause a hyperosmolar state and result in decreased cerebral perfusion. Usually, 3–6 doses are sufficient. Prolonged use is not recommended. Intracranial pressure monitoring, although involving some risk, may be of value, in certain clinical situations, in monitoring the effects of treatment and progression of the disease. Mannitol is contraindicated in patients with hepatorenal syndrome. Increasingly, hypertonic saline is being used for treatment of cerebral edema in patients with liver failure [Gasco et al., 2010].

Prognosis

Prognosis of hepatic encephalopathy is guarded, in spite of improvement in intensive medical support [Bustamente et al., 1999]. One-year survival is 40 percent and 3-year survival is 15 percent. Fulminant hepatic failure has a mortality rate of 75 percent, and severe hepatic coma carries a substantial risk of permanent neurologic disability. On the other hand, the encephalopathy in patients who recover from acute liver failure can resolve without residual neurologic sequelae because, in most individuals, there are no irreversible structural brain lesions.

Pathogenesis

The basic defect in Wilson’s disease is the impaired biliary excretion of copper, resulting in the accumulation of copper in various organs, including the liver, cornea, and brain. Copper’s unique electron structure causes ionic copper to be very toxic, participating in reactions that allow the synthesis of damaging reactive oxygen species, adversely affecting mitochondrial respiration, among other things [Ferenci, 2005]. The consequence of copper accumulation is the development of severe hepatic and neurologic disease. Serum ceruloplasmin levels are low (<20 mg/dL), serum copper levels are decreased (<20 μg/dL), and 24-hour urinary excretion is increased (>100 μg/24 hours) [Pleskow and Grand, 1996].

Liver disease can occur in the form of hepatitis and cirrhosis. Neurologic manifestations are gradual in onset and include cognitive abnormalities; dementia; movement disorders, including tremors of the hand or head, whole-body dystonia and rigidity, or other Parkinsonian features; ataxia; and, occasionally, seizures [Lockwood, 1995; Steinberg and Frank, 1993]. In children, deterioration in school achievement and the development of behavioral and conduct disorders begin insidiously and progress slowly. Neurologic manifestations have been reported in children as young as 6 years of age, although, traditionally, children younger than age 10 years have the hepatic form of the disease, whereas children older than 10 years have neurologic involvement [Walshe and Yealland, 1992]. Psychiatric disorders are common and include mania, psychosis, depression, and even schizophrenia. Almost all patients with neurologic and psychiatric manifestations develop corneal Kayser–Fleischer rings, a granular brown pigmentation of the cornea caused by copper deposition in Descemet’s membrane, which can be visualized by slit-lamp examination of the cornea. Kayser–Fleischer rings are present in 98 percent of patients with neurologic Wilson’s disease, while just 50 percent of patients with liver disease only have Kayser–Fleischer rings.

As noted, neurologic manifestations of the disease are rare before age 10 years, and dystonia is the most common feature. Dysarthria, tremors, and psychiatric manifestations start more commonly during the second decade of life. Three subsets of patients with Wilson’s disease have been recognized [Oder et al., 1993]:

Psychiatric symptoms often correlate with the severity of the neurologic disturbances [Dening and Berrios, 1990; Akil et al., 1991].

Neuropathologic examination reveals diffuse copper deposition, atrophy of the caudate and putamen, neuronal loss, and Alzheimer type II astrocytes with cortical and basal ganglia demyelination [Yarze et al., 1992]. Abnormalities of the white matter and cerebral cortex occur in about 10 percent of cases [Brewer and Yuzbusiyan Gukan, 1992]. Copper concentrations in affected and unaffected regions of the brain are similar.

CT scan abnormalities reported in patients with Wilson’s disease include cortical atrophy, brainstem atrophy, ventriculomegaly, hypodensities in the basal ganglia, and posterior fossa atrophy. More recent studies have confirmed these CT observations [van Wassenaer-van Hall et al., 1996] and extended them to MRI [King et al., 1996]. MRI is more sensitive than CT in detecting abnormalities [Nazer et al., 1993] and has demonstrated repeatedly the presence of abnormalities in the corpus striatum [Thomas et al., 1993]. Abnormal MRI findings are common in Wilson’s disease patients with neurologic abnormalities. MRI studies show bilateral high-signal, T2-weighted abnormalities in the basal ganglia (particularly the putamen, the caudate head, the ventral thalamus, and the dentate nucleus), the brainstem, and the cerebellum, and atrophy of the cortex and cerebral white matter [Page et al., 2004; Alanen et al., 1999; Das et al., 2007; Srinivas et al., 2008; Machado et al., 2008]. SPECT studies demonstrated diffuse or focal hypoperfusion in superior frontal, prefrontal, parietal, occipital, and temporal areas, and in the caudate and putamen. SPECT may reveal brain abnormalities earlier then MRI [Piga et al., 2008].

Neuroimaging findings in Wilson’s disease suggest that the pathogenesis involves efferent and afferent dopaminergic projections and dopaminergic transport mechanisms [Westermark et al., 1995; Snow et al., 1991; Jeon et al., 1998]. After administration of a radioligand that binds to presynaptic dopamine transporter, and a radioligand that binds to postsynaptic striatal dopamine D2 receptors, SPECT studies in patients with neurologic Wilson’s disease revealed a concordant pre- and postsynaptic dopaminergic deficit [Barthel et al., 2003]. Earlier PET studies demonstrated reduction of glucose metabolism in all brain regions, except the thalamus. Later PET studies demonstrated changes in dopaminergic metabolic markers, which were correlated with structural MRI changes but not with clinical symptoms [Westermark et al., 1995]. These and other studies suggest impairment of dopaminergic, frontal and parietal subcortical circuits in Wilson’s disease [Portala et al., 2001]. Proton MR spectroscopy in neurologically symptomatic patients with Wilson’s disease demonstrates reduced N-acetylaspartate/creatine ratios and can determine the degree of neuronal loss in these patients [Van Den Heuvel et al., 1997; Lucato et al., 2005].

Mental changes were observed by Samuel Wilson in 1912, when he described the disease. Some form of mental change was referred to in 8 of his 12 cases [Wilson, 1912]. In children, deterioration in school achievement and the development of abnormal behavioral and conduct disorders progress slowly. Neuropsychological testing reveals abnormalities in the patients who have neurologic deficits. The major areas of deficit are motor and memory functioning. Studies revealed cognitive dysfunction, which includes motor deficits [Portala et al., 2001], and deficits in visuospatial abilities, memory, and abstract reasoning, as well as subcortical dementia [Collie, 2005]. Memory deficits are similar to those found in other frontal dysfunctions: difficulties in searching for and retrieving information. Executive function deficits, including working memory deficits, attention deficits, and impulsivity, have been reported [Portala et al., 2001]. A study of 19 symptomatic (combination of hepatic and neurologic) and 2 asymptomatic young adult Wilson’s disease patients, using an Automated Psychological Test system, a comprehensive computerized neuropsychological test battery, revealed that the symptomatic Wilson’s disease patients had significantly lower performance on finger tapping tasks, reaction time tasks, short-term memory test, the index of word decoding speed, the grammatical reasoning test, and the perceptual maze test. They made more impulsive errors [Portala et al., 2001].

There was no significant correlation between the level of copper toxicity and the degree or nature of associated neurologic deficits [Rathbun, 1996]. Neuropsychological deficits may precede the neurologic symptoms.

Seniow et al. [2003] examined the pattern of cognitive deficits with different types and degrees of neurologic involvement in 67 Wilson’s disease patients, who were divided into three groups: neurologically asymptomatic, neurologic involvement limited to the basal ganglia, and more extensive neurologic involvement. Neurologically involved patients showed mild impairment in all cognitive functions, including frontal-executive ability, memory, and visuospatial processing (but usually were still within the normal limit). The neurologically asymptomatic patients showed no cognitive deficit. Multifocal pathology was associated with more severe cognitive deficits, but did not contribute significantly to memory deficit. Antisocial behavior, impulsivity, and decreased intellectual performance have been described [El-Youseff, 2003]. There are reports of Wilson’s disease with initial ADHD manifestations without hepatic disease [Jainn-Jim et al., 2006]. Atypical presentations of Wilson’s disease in children have included presentation of symptoms suggestive of a frontal lobe syndrome [Carlson et al., 2004], as well as with neglect dysgraphia without evidence of spatial neglect [Auclair et al., 2008]. It is possible that the type of cognitive abnormalities observed in Wilson’s disease is similar to that seen in Parkinson’s disease, and may result from the abnormal connections between the basal ganglia and association areas of the prefrontal cortex [Dubois and Pillon, 1997].

Psychiatric disorders are common in Wilson’s disease, and include personality changes such as irritability and a low anger threshold [Akil and Brewer, 1995]; psychosis; affective disorder spectrum, including depression and mania; and, less frequently, schizophrenic illness [Keller et al., 1999; Srinivas et al., 2008]. The severity of psychiatric and behavioral symptoms varies from mild personality and psychological disturbances to severe psychiatric illness, including psychosis and depressive syndromes [Oder et al., 1991]. These disorders may start early or late in the course of the disease, or may precede neurologic signs and symptoms [Srinivas et al., 2008]. Increased sexual preoccupation and reduced sexual inhibition have been described [Akil and Brewer, 1995]. Mania as the first manifestation of Wilson’s disease has been reported [Machado et al., 2008]. In this case, involving a young man, the first manifestation was mania, followed by depression. The neurologic manifestation of upper-extremity tremor started after the onset of psychiatric manifestation. MRI revealed increased T2-weighted signal intensity bilaterally, involving the cerebral peduncles. The patient was asymptomatic 2 years after starting treatment with penicillamine, and remained asymptomatic without any psychiatric medications. Repeat MRI 4 years after starting therapy for Wilson’s disease showed bilateral and symmetrical low signal on T2-weighted images in the lentiform nuclei and substantia nigra. Proton spectroscopy revealed decreased N-acetylaspartate levels in the basal ganglia, frontal white matter, and parieto-occipital cortex.

Psychiatric symptoms often occur with neurologic symptoms but not with hepatic manifestations. Presentation may be in adolescence or young adulthood. The pathophysiology for psychiatric symptomatology in Wilson’s disease is related to involvement of the basal ganglia and other cerebral structures. Psychopathology may be related to the genotype of the disease, although specific genotype–phenotype correlations have not yet been substantiated.

It is estimated that more than 50 percent of patients with Wilson’s disease exhibit psychiatric disorders and a large proportion (one-third) of patients initially are seen for psychiatric assessment and treatment before the diagnosis of Wilson’s disease is made [Akil et al., 1991]. Some authors suggest that every patient with Wilson’s disease has “dementia, psychosis, psychoneurosis or behavior disturbances characterized by impulsivity at some point during the disease” [Scheinberg and Sternlieb, 1995]. In a series of 101 Brazilian Wilson’s disease patients with neurologic manifestations, 47 percent had associated psychiatric abnormalities, including depression, irritability, impulsivity, and emotional lability [Machado et al., 2003]. Other studies report a much smaller percentage of patients having psychiatric manifestations as dominant symptoms [Srinivas et al., 2008]. Such differences may be the result of differences in terminology or lack of long-term patient follow-up.

Therapy

Treatment is directed at reducing systemic copper levels with a low-copper diet and with several anticopper medications. The four drugs currently being used include:

Zinc is considered by some to be the treatment of choice for maintenance therapy, for treatment of the presymptomatic patient, and for treatment of the pregnant patient because of its efficacy and lack of toxicity [Brewer, 1995; Czlonkowska et al., 1996]. For patients with mild liver failure, combined treatment with trientine and zinc is effective [Brewer, 1995; Dahlman et al., 1995]. Trientine gives a strong, fast, negative copper balance, and zinc induces hepatic metallothionein, which sequesters hepatic copper. For patients with neurologic disease, tetrathiomolybdate is preferred to penicillamine now by some groups [Brewer et al., 1994; Schilsky, 1996]. Tetrathiomolybdate provides rapid, safe control of copper. Patients treated with penicillamine are at great risk of serious permanent neurologic worsening [Brewer, 1995] or side effects of this drug, which include rash, bone marrow suppression, immune complex disorders, nephrotic syndrome, optic neuropathy, and loss of taste. Pyridoxine should be given concurrently in those patients receiving penicillamine.

A reduction of clinical symptoms, including neurologic abnormalities, usually is achieved with therapy, but the degree of recovery is variable and depends on the subtype of the disease, its severity, the time elapsed since onset, and the appropriate management. Similarly, most of the psychiatric manifestations and cognitive abnormalities usually respond to copper chelation therapy [Dening and Berrios, 1990; Lang et al., 1990; Prashanth et al., 2005], but some continue to need symptomatic psychiatric medications [Srinivas et al., 2008]. There have been reports of the appearance of psychiatric disease after the start of penicillamine [McDonald and Lake, 1995]. MRI abnormalities also may improve after treatment [Takahashi et al., 1996]. When patients with neurologic and psychiatric manifestations are diagnosed late in the course of the disease, some psychiatric dysfunction may remain. Cranial CT and MRI abnormalities, other than brain atrophy, can be reversed with chelation therapy [Rohe et al., 1994].

The psychiatric manifestations of Wilson’s disease are treated with various medications, including lithium for mania [Loganathan et al., 2008]. Mood stabilizers are effective in resolving the mood symptoms. Some patients continue to need symptomatic treatment after therapy that is directed at reducing the levels or effects of copper. Some Wilson’s disease patients are sensitive to neuroleptics. Neuroleptics may worsen parkinsonian features and tardive dyskinesia.

Neurologic and psychiatric symptoms may improve after transplantation. In selected patients, primarily those with fulminant hepatitis or chronic hepatic insufficiency with or without neurologic symptoms, liver transplantation may be successful [Bellary et al., 1995; Chen et al., 1997; Schilsky et al., 1994]. Patients with neurologic Wilson’s disease in the absence of hepatic insufficiency also may show clinical improvement [Bax et al., 1998]. Case reports of patients who underwent liver transplantation for Wilson’s disease demonstrated an increase in ceruloplasmin level, disappearance of the Kayser–Fleischer ring, and normalization of urinary copper excretion.

Reports on the neurologic, cognitive, and psychiatric effects of liver transplantation in patients with Wilson’s disease vary. Neuropsychiatric abnormalities, as well as MRI abnormalities, tend to improve [Geissler et al., 2003; Stracciari et al., 2000], although not always [Guarino et al., 1995; Kassam et al., 1998]. Stracciari et al. [2000] reported a case of liver transplantation in a patient with Wilson’s disease, followed by a dramatic improvement in motor functions, normalization of copper excretion, and disappearance of Kayser–Fleischer rings, and later, by reversal of the abnormalities seen on MRI. A similar outcome in a patient with Wilson’s disease was described by Wu et al. [2000].

Anatomic and Biochemical Effects of Undernutrition

Early studies at the beginning of the last century demonstrated that undernutrition early in life has dramatic effects on brain growth [Dauncey and Bicknell, 1999]. Undernourished animals have increased brain water content, decreased myelination and brain lipid content, and decreased cerebral cortical volume, but no significant reductions in the total number of cells compared with age-matched control animals. These and later studies suggest that the primary effect of protein-energy deprivation is on the replication and growth of cells (rather than as a destructive process) and involves those elements most actively proliferating during the insult. These studies (based on rat models of malnutrition) suggested that the mammalian brain is most vulnerable to malnutrition during the period when the brain is growing most rapidly; consequently, those structures that develop postnatally, such as the cerebrum, hippocampus, and cerebellum, would be the most susceptible to permanent morphologic changes [Georgieff, 2007]. An alternative hypothesis is that the maximum period of critical vulnerability occurs when specific neurons are undergoing organizational development earlier in life, implying greater susceptibility prenatally than postnatally [Levitsky and Strupp, 1995]. Table 103-1 summarizes the most important nutrients required for brain growth during late fetal and neonatal brain development, and the particular brain structure or function that they regulate.

Table 103-1 Important Nutrients During Late Fetal and Neonatal Brain Development

LC-PUFAs, long-chain polyunsaturated fatty acids.

(Adapted from Georgieff MK: Nutrition and the developing brain: nutrient priorities and measurement, Am J Clin Nutr 85:614S–620S, 2007.)

Box 103-2 lists some of the major neuropathologic abnormalities resulting from malnutrition described in animal experimental studies (see Georgieff [2007] and Levitsky and Strupp [1995] for comprehensive reviews). Morphometric analysis of specific areas of cerebral cortex and of selected subcortical nuclei by Diaz-Cintra et al. [1990] indicates that there are specific differences in the responses of selected neurons to undernutrition. For example, in the visual cortex of the protein-deprived rat, changes in the developmental pattern of the pyramidal cells of layer V differ from those of layers II and III [Diaz-Cintra et al., 1990]. Results of these anatomic studies and gross measurements of brain water and lipid have been confirmed by more elegant biochemical investigations. DNA has been used to measure cell size. White-matter lipid, particularly cerebroside, has been used to measure myelination. Several recent reviews compare the effects of malnutrition in animal models to that occurring in humans [Rice and Barone, 2000; Wainwright and Columbo, 2006].

Box 103-2 Nutrition and Brain Development

Effects of Malnutrition on the Developing Brain

Reduction in brain volume

Sparing of cortical neurons

Increased cell packing

Disruption of cortical pyramidal cells

Reduction in cortical dendritic spines

Decrease in width of cortical neurons

Decreased dendritic branching in cortex

Reduced number of cortical glial cells

Reduced number of cortical synapses

Reduced number of synaptic reactive zones

Effects of Restoring Nutrition on Reversibility of Brain Lesions

Increase in brain weight and volume: “catch-up” head growth

Prolongation of period of mitotic activity

Prolongation of period of protein synthesis

Reversal of cell packing

Reduced cortical glial cell density

Persistence of reduced number of cortical dendrites and synaptic spines

Persistence of reduced myelination

Increase in number of mitochondria in neurons

Increase in synaptic density

(Modified from Levitsky DA, Strupp BJ: Malnutrition and the brain: Changing concepts, changing concerns, J Nutr 125:2212S, 1995.)

Pregnant rats subjected to severe protein restriction from the fourth day of gestation give birth to litters that average a 15 percent decrease in the number of brain cells [Patel et al., 1973]. The reduction is most marked in the areas adjacent to the lateral ventricles and in the cerebellum, and similar decreases in cerebral protein have been observed. Reductions in protein and caloric intake during the suckling period produced a 10–30 percent reduction in brain weight, with a corresponding decrease in DNA [Fishman et al., 1971]. The major cellular change appears to involve oligodendroglial cells. These cells are reduced in number in both the cortex and white matter, and their maturation is delayed. There probably is no significant reduction in the number of neurons in the cerebral cortex, although neurons in the cerebellum, particularly granule cells, may be reduced [Clos et al., 1977]. In undernourished pre-weanling animals, the reduction of DNA content in the cerebellum is considerably greater than in the cerebrum. This difference probably is caused by the rapid proliferation of cerebellar neurons after birth in the rat. The effects of undernutrition on cell number are greatest when a nutritional insult is prolonged from early intrauterine life through the period of lactation, which results in a 60 percent reduction in cell number in human infants by the time of weaning. Studies in Rhesus monkeys disclosed no effect on brain growth when the mother was nourished adequately during gestation and the infant protein-restricted at birth [Portman et al., 1987].

Myelination is another parameter of brain development that is measured easily in undernourished animals. Cholesterol, which is found in all membranes, is decreased in proportion to total lipids [Dobbing, 1990]; however, greater reductions occur in lipids, such as cerebrosides, which are found predominantly in myelin. The chemical composition of myelin from brains of undernourished animals does not differ markedly from that of control animals [Fishman et al., 1971]. Even with prolonged starvation in postnatal life, changes in the amount of myelin are not large. For example, in animals starved from birth to 21 days of age, total myelin quantity was reduced to only 86.5 percent of control animals. In animals starved from birth to 53 days of age, myelin quantity was 71 percent of age-matched control animals [Fishman et al., 1971].

Other studies on the effects of maternal undernutrition, as it relates to myelination, gliogenesis, and glial maturation, have shown that delays in myelination appear, in part, to be related to delays in oligodendroglial cell proliferation and differentiation [Lai and Lewis, 1980].

Reductions in RNA and protein in the cerebral cortex of undernourished rats are greater than reductions in DNA [Dauncey and Bicknell, 1999]. This finding suggests that there is impairment in cell growth that exceeds the reduction in cell number. It also has been observed that the number of cortical synapses in undernourished rats is decreased. However, this reduction does not correspond to the general reduction in synaptic transmitters. For example, in the undernourished rat brain, serotonin is increased [Resnick and Morgane, 1984], acetylcholine is unchanged [Wiggins et al., 1984], and area-to-area variability exists in norepinephrine [Wiggins et al., 1984].

The major aspects of the anatomic and biochemical changes associated with severe PEM in animals have been observed in malnourished children. Brain weights are reduced, water content is increased, and a definite reduction in myelin is present that correlates with the length of starvation [Dauncey and Bicknell, 1999]. Cerebral and cerebellar DNA content also is reduced.

Animals may recover from periods of undernutrition, as evidenced by changes in brain weight and biochemical indices [Gressens et al., 1997]. However, if the period of undernutrition persists during the entire period of cell replication, there is a persisting deficit in cell numbers, irrespective of the diet provided thereafter. This deficit is indicated by reduced brain DNA in later life. Briefer insults ending before 21 days of age in the rat (a period in which cell division is still programmed to occur in the rat brain) produce no permanent defects once the animal is re-fed [Levitsky and Strupp, 1995; Dauncey and Bicknell, 1999].

Evidence in humans is less direct, but measurements of somatic growth and head size suggest that the length of the insult during development is an important factor in the production of permanent effects in humans, as well [Georgieff, 2007]. Physical growth also is decreased after long periods of kwashiorkor later in childhood. At age 1 year, human premature infants who were malnourished in utero, born small for gestational age, and undernourished in early extrauterine life had smaller head circumferences and poorer performances on the Bayley Infant Development scales than did children of the same gestational age with normal birth weights, who were undernourished in early extrauterine life [Georgieff et al., 1985]. This finding suggests that, in animals and humans, a combination of intrauterine and extrauterine malnutrition produces the greatest effect on brain weight, chemical composition, and function.

There probably is no single common pathway by which the lack of protein and energy affects growth of the nervous system. It is certain, however, that, for there to be a significant effect on brain development, the insult occurs during a period of rapid nervous system growth or at a point when cell–cell synaptic interconnections are developing. An in-depth discussion of these issues is beyond the scope of this chapter and several reviews discuss historical, as well as other, aspects of the development of current insights into how nutritional impairments affect the developing nervous system [Benton, 2008; Georgieff, 2007; Wainwright and Columbo, 2006; Black, 2003; Dauncey and Bicknell, 1999; Gorman, 1995; Grantham-McGregor, 1995; Dobbing, 1990].

Acute Effects on Behavior and Cognition

Severely malnourished children demonstrate marked behavioral changes during the acute stages of malnutrition [Grantham-McGregor, 1995; Wainwright and Columbo, 2006]. Apathy, reduced activity, decreased interest and exploration of the environment, reduced stress responses, and impaired cognition are all well characterized. Brief postnatal insults lasting less than 4–5 months do not appear to have a permanent effect on mental function. Starvation over a considerable period does have an effect on behavior and intelligence. However, separating the effects of undernutrition from other variables has made it difficult to categorize and quantify the nature and magnitude of deficits attributable to undernutrition. Numerous studies have demonstrated that the intellectual function of populations malnourished for long periods differs from age-matched control subjects or from siblings in the same family [Dauncey and Bicknell, 1999]. The environments in which malnourished children are reared, however, almost always are suboptimal [Singh, 2004]. Overcrowding, poor education, lack of parental stimulation, and poverty are variables that cannot be evaluated confidently, even in sibling studies.

Treatment

The only treatment of undernutrition is adequate food intake. An adequate protein and calorie intake certainly ends the acute insult, but it is not clear whether caloric supplements always improve growth and function, as genetic and environmental factors play a role in the potential for recovery [Dauncey and Bicknell, 1999]. The use of dietary supplements, both in utero and during the suckling period, usually contributes to cognitive recovery but the results may be variable. The reasons for this variable recovery are difficult to isolate, as, typically, many factors contribute to the composite effect on brain development. Many supplements are not used, or are used improperly. Likewise, improved nutrition cannot compensate for other problems, such as poverty, lack of parental interest, overcrowding, other environmental factors (e.g., industrial toxin exposure), and also factors related to genetic susceptibility. It also is uncertain when improvement ceases, or if differences between supplemented and control populations are less important later in life. For example, in one study of undernourished children, which used nutritional supplementation and environmental stimulation in different treatment groups, nutritionally supplemented children at 18 months of age benefited in all tested developmental domains, except language function. However, at 36 months, language skills had improved compared with age-matched nonsupplemented children. In contrast, other studies have suggested that the effects of early environmental stimulation may lessen with age [Waber et al., 1981].

All facets of neurologic function may not improve in parallel and there appear to be regional differences in the effects of micronutrient supplementation on structural and functional recovery [Dauncey and Bicknell, 1999]. Studies also have shown that there is a positive effect of environmental stimulation or relatively well-structured home environments during the recovery period [McEwen, 2003].

Vitamin A (Retinol)

Vitamin A is the generic term for a group of fat-soluble compounds that possess the biologic activity of retinol (vitamin A₁) [Olson and Mello, 2010; Kumar, 2010]. Although the basic function of vitamin A is the formation of the retinal pigments of the eye, vitamin A and its biological active derivatives, the retinoids, regulate key processes such as growth and differentiation of epithelial tissue; bone growth; reproduction; embryonic development, including inhibition of cell proliferation, differentiation, apoptosis, shaping of the embryo, and organogenesis; and enhancement of the immune system. Retinoids regulate expression of genes and proteins of different tissues, including genes that control neural differentiation, neurite outgrowth, and the patterning of the neural tube, and play an important role in neuromodulation [Mey and McCaffery, 2004; Olson and Mello, 2010]. Retinoic acid, a bioactive metabolite of vitamin A, is a signaling molecule in the brain of growing and adult animals; it regulates gene products and modulates neurogenesis, neural survival, and synaptic plasticity [Lane and Bailey, 2005; Tafti and Ghyselinck, 2007; Olsen and Mello, 2010]. Dietary vitamin A supplementation improves learning and memory in vitamin A-deficient rodents. Experimental rodent work reveals that retinoid signaling has postembryonic neuromodulation on hippocampal long-term depression and potentiation – measures of long-term synaptic plasticity [Malaspina and Michael-Titus, 2008; Lane and Bailey, 2005]. Retinoic acid also has restored hippocampal neurogenesis and reversed spatial memory deficits in vitamin-deprived adult rats [Bonnet et al., 2008].

Physiologic functions of vitamin A are mediated through different forms of the compound. Retinol, the alcohol, serves as the transport molecule; retinal, the aldehyde, is active in the formation of visual pigments; and retinoic acid may be the active metabolite in the growth, maintenance, and differentiation of body tissues. Retinol esters function as storage material. The actions of vitamin A are mediated by nuclear retinoid receptor proteins termed retinoic acid receptors (RAR-alpha, beta, and gamma). Retinoid receptors may regulate specific neuronal phenomena, including movements and sleep [Maret et al., 2005; Sei, 2008].

Vitamin A is necessary for the adaptation of the retinal rods and cones to dim light. Rhodopsin, formed from the combination of the protein opsin and 11-cis-retinal, is the photosensitive pigment of the rods [Perrotta et al., 2003]. After absorption of a photon of light, rhodopsin undergoes transformational changes leading through a series of intermediary steps that, ultimately, results in the development of a retinal action potential. Retinol also has an important role in maintaining epithelial cell integrity by stimulating mucus production. Deprived of adequate amounts of retinol, goblet mucus cells disappear and epidermal basal cells proliferate, resulting in keratinization. The absence of normal mucus secretions promotes irritation and infection.

Stored retinol esters are a dietary source of vitamin A. Carotenoids, pigmentary compounds present in all photosynthetic plant tissue, provide another major dietary source. These retinol esters are hydrolyzed to retinol in the intestinal lumen and within the brush border of intestinal cells. Absorption mediated by cellular retinol-binding proteins depends on the presence of absorbable fat and bile, and is reduced considerably in conditions associated with steatorrhea and other chronic diarrheas. Vitamin A is stored in the liver as retinol ester. In the plasma, retinol is bound to a specific retinol-binding protein that binds to specific cell-surface sites. The absorption, distribution, and metabolic fate of retinoic acid differ from those of retinol.

The recommended daily requirement for vitamin A is 375 retinol equivalent units for infants younger than 1 year of age, and 700 retinol equivalent units for children and adolescents. Under normal conditions, the liver concentration of retinol ester approximates 100–300 μg/g, and the normal plasma retinol concentration is 30–70 μg/dL.

Thiamine (Vitamin B₁)

Thiamine functions as a co-factor in oxidative decarboxylation and transketolation reactions, and functions as a coenzyme in the metabolism of carbohydrates, lipids, and branched-chain amino acids [Kumar, 2010]. Thiamine pyrophosphate, the physiologic active form, is an essential co-factor in the oxidative decarboxylation of pyruvate to acetyl-CoA; alpha-ketoglutarate to succinyl-CoA; and the alpha-keto derivatives of isoleucine, leucine, and valine to their corresponding branched-chain CoA derivatives (see Chapters 32 and 34). Thiamine pyrophosphate is also the co-factor for the transketolase reaction in the hexose monophosphate shunt that provides pentose for nucleotide synthesis. In addition, thiamine has roles in cerebral metabolism, where it is present both as a diphosphate and a triphosphate. Thiamine is important in fatty acid synthesis, energy production via adenosine triphosphate (ATP) synthesis, myelin sheath maintenance, neurotransmitter production, neuronal membrane transport, neuromuscular transmission, and axonal conduction [Kumar, 2010].

Thiamine is readily available in meats, grains, and vegetables but is destroyed by heat. It is transported actively across the small intestine by a saturable process, limiting the amount that can be absorbed. In the blood, thiamine is bound to protein, predominantly albumin. Its transport into the CNS and CSF is controlled by a rate-limiting process involving a specific membrane-bound phosphatase. Excesses of the vitamin are excreted in the urine.

The requirement for thiamine depends on the metabolic rate and is increased when carbohydrates are the major energy source. Thiamine requirements also are increased in children, during pregnancy and lactation, and with vigorous exercise. The recommended daily dietary intake is 0.3–0.4 mg in infants younger than 1 year of age, and 0.7–1.5 mg in older children and adolescents, amounts readily available in normal diets.

Riboflavin (Vitamin B₂)

Two coenzymes, flavin mononucleotide and flavin adenine dinucleotide, are the physiologically active forms of riboflavin [Powers 2003; Hoey et al., 2009]. These nucleotides play a vital role in a variety of mitochondrial oxidation-reduction reactions involving flavoproteins, including amino acid oxidase, xanthine oxidase, glutathione reductase, and nitric oxide synthase [Hoey et al., 2009]. Riboflavin is widely distributed in plants and animal tissues. Phosphorylation of riboflavin to flavin mononucleotide occurs in the intestinal mucosa through a reaction catalyzed by the cytosolic enzyme, flavokinase, and both riboflavin and flavin mononucleotide are absorbed into the circulation. The upper limit of absorption is approximately 25 mg/dose. Only small amounts of riboflavin are stored, and unused riboflavin is excreted in the urine and feces. In the plasma, riboflavin and flavin mononucleotide are bound to protein, predominantly albumin. In tissues, riboflavin is converted to flavin mononucleotide in a reaction catalyzed by flavokinase. Flavin mononucleotide subsequently is converted to flavin adenine dinucleotide in a reaction catalyzed by flavin adenine dinucleotide pyrophosphorylase, and then transported into the mitochondria. The recommended daily requirement for riboflavin is 0.4–0.5 mg in infants younger than 1 year of age, and 1.0–1.8 mg in older children and adolescents [Hoey et al., 2009].

Niacin (Vitamin B₃)

The pyridine nucleotides, nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate, are the physiologically active forms of niacin (nicotinic acid), which is present in nucleotides in the form of an amide (nicotinamide) [Bourgeois et al., 2006]. These nucleotides function in a variety of oxidation-reduction reactions. The reduced nucleotides are reoxidized subsequently by flavoproteins.

Cellular nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate, found in a variety of foods, function as the dietary source of niacin. After ingestion, the nucleotides are hydrolyzed in the small intestine to niacin and nicotinamide by the mucosal enzyme nicotinamide adenine dinucleotide-glycohydrolase, a rate-limiting step in absorption. The released niacin and nicotinamide then are transported across the intestine by passive diffusion. The absorbed niacin is converted rapidly to nicotinamide adenine dinucleotide in erythrocytes, and in the liver, nicotinamide adenine dinucleotide glycohydrolases subsequently release nicotinamide for transport to other tissues, where it is reconverted into nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate.

Tryptophan is an important secondary dietary source of nicotinic acid. Nicotinic acid is synthesized from tryptophan in a series of reactions through kynurenine, 3-hydroxy anthranilate, and quinolinate. Approximately 1 mg of nicotinic acid is derived from 60 mg of dietary tryptophan. The fact that symptomatic niacin deficiency (pellagra) occurs predominantly in populations in which corn, which has a low content of tryptophan, serves as the major dietary staple attests to the importance of the substance as a dietary source of niacin. Biotransformation of tryptophan to nicotinic acid requires several vitamins and minerals, such as B₂, B₆, iron, and copper. As tryptophan is necessary for niacin synthesis, vitamin B₆ deficiency can result in secondary niacin deficiency [Kumar, 2010].

The minimum amount of dietary niacin required to prevent symptomatic deficiency is 4.4 mg/1000 kcal. The recommended daily dietary allowance for niacin is 5–6 niacin equivalents in children younger than 1 year of age, and 9–20 niacin equivalents in older children and adolescents. (A niacin equivalent is equal to 1 mg of niacin or 60 mg of dietary tryptophan.) There is now concern that niacin overload could be contributing the high prevalence of obesity seen in American children [Li et al., 2010].

Pyridoxine (Vitamin B₆)

Vitamin B₆ is the generic term for three naturally occurring pyridine derivatives – pyridoxine (an alcohol), pyridoxal (an aldehyde), and pyridoxamine (an amine) [Gospe, 2006]. Pyridoxal phosphate (PLP), the physiologically active form, functions as a co-factor for more than 100 enzymatic reactions, including the decarboxylation, transamination, and racemization of amino acids, and reactions in the metabolism of tryptophan, sulfur-containing amino acids, and hydroxy amino acids [Clayton, 2006]. The interconversion and metabolism of B₆ are dependent on riboflavin, niacin, and zinc. Niacin, carnitine, and folate require B₆ for their metabolism [Kumar, 2010]. There also are several important interactions between pyridoxine and therapeutic drugs [Lheureux et al., 2005]. Vitamin B₆ enhances the peripheral decarboxylation of l-DOPA, thus reducing its therapeutic effectiveness. Isonicotinic acid hydrazide (isoniazid) acts as a potent inhibitor of pyridoxal kinase by combining with PLP. Pyridoxine also interacts with cycloserine and hydralazine, and penicillamine promotes urinary pyridoxine excretion.

Humans are unable to synthesize B₆ and must obtain it from exogenous sources by intestinal absorption. Pyridoxine, pyridoxal, and pyridoxamine are present in meats, liver, cereals, soybeans, and vegetables [Bender, 2003]. As all three compounds are degraded by heat, ultraviolet light, and oxidation, considerable losses may occur during food preparation. All three compounds are absorbed from the intestine by passive diffusion. In the blood, they are bound to proteins and hemoglobin. After passive uptake by the liver, they are converted to PLP by the hepatic enzyme, pyridoxal kinase. Plasma concentrations reflect concentrations in the liver. The principal excretory product is pyridoxic acid, which is excreted in the urine after its formation in a reaction catalyzed by the hepatic enzyme, aldehyde oxidase. Some unmetabolized pyridoxal is also excreted in the urine.

The requirement for pyridoxine increases with the amount of protein in the diet. The recommended daily dietary allowance of pyridoxine is 0.3–0.6 mg in infants younger than 1 year of age, and 1–2 mg in older children and adolescents. There are several different mechanisms that lead to an increased requirement for pyridoxine and/or PLP [Clayton, 2006]. These include: