Epidemiology

Gastrointestinal dysfunction is frequently seen in persons with neurologic diseases who require rehabilitation. Besides the direct effects of neurologic disease on gut function, other factors can also play a huge role in the development of enteric problems, including debility, insufficient fluid intake, and the use of anticholinergics and other medications. Digestive tract problems in stroke, brain injury, multiple sclerosis, Parkinson’s disease, neuromuscular diseases, dysautonomias, peripheral nerve injuries, SCIs, and other neurologic disorders have been shown to be difficult and challenging to manage. Neurogenic bowel difficulties can be a primary disabling and handicapping feature for patients with SCI, stroke, amyotrophic lateral sclerosis, multiple sclerosis, diabetes mellitus, myelomeningocele, and muscular dystrophy.^∗

Neurogenic bowel dysfunction results from autonomic and somatic denervation, and produces FI, constipation, and difficulty with evacuation (DWE). These symptoms are common. The prevalence of FI and fecal impaction ranges from 0.3% to 5.0% in the general population. The prevalence of DWE ranges from 10% to 50% among the hospitalized or institutionalized elderly.^100,115 Although many gastrointestinal disorders can contribute to FI or DWE, disorders that impair the extrinsic (sympathetic, parasympathetic, or somatic) nervous control of the bowel and anorectal mechanisms are more common among the patient populations seen by physiatrists.

Impact

Living with nausea, vomiting, bloating, abdominal pain, constipation, diarrhea, or FI profoundly affects the quality of life. It significantly affects nutrition, overall health, and sense of well-being. Loss of voluntary control over bowel function alters every facet of a person’s life at home, at work, and in the community. The ability to spontaneously regulate and direct bowel function is essential for participation in everyday activities at home, work, or school. It plays a major role in one’s ability to create and maintain relationships. It has a significant effect on such activities as recreation or travel. Problems with constipation and FI can create considerable psychologic, social, and emotional trauma.

FI decreases the return-to-home rates for stroke patients.⁴⁹ Almost one third of persons with SCI report or exhibit worsening of bowel function 5 years beyond their injury, with 33% developing megacolon, suggesting inadequate long-term management.^52,110 Recent evidence has shown some improvement in outcomes for SCI bowel management.⁶⁷ When restoring normal defecation is not possible, social continence becomes the goal. Social continence is defined as predictable, scheduled, adequate defecations without incontinence at other times. It is often achievable by persons with neurogenic bowel dysfunction. Embarrassment and humiliation from FI frequently result in extreme vocational and social disability. Vocational disability and excessive institutionalization add substantial costs to the care related to neurogenic bowel dysfunction. Nursing home costs are higher for patients with FI.¹¹⁵ A 1983 report estimated that $8 billion per year is spent in the United States for the care of fecally incontinent institutionalized patients.¹⁰⁰

Neuroanatomy and Physiology of the Gastrointestinal Tract

In the last two decades, there has been significant advancement in the science of neurogastroenterology. This includes new discoveries in the basic physiology of the gastrointestinal tract and interactions with the brain and spinal cord, autonomic and enteric nervous systems, and somatomotor system (pharyngeal muscles and swallowing, pelvic floor muscles and defecation, continence and pelvic pain).

Normal functioning of the stomach and intestines entails coordination of muscle contraction, digestion and absorption of nutrients, and regulation of blood flow. The neural schema of the gastrointestinal tract is much more complex than previously thought. Neural control of the gastrointestinal tract is an extremely organized and integrated hierarchy of mechanisms that involve the central nervous system (CNS; brain and spinal cord), autonomic nervous system (sympathetic and parasympathetic), and enteric nervous system (ENS) (Figures 29-1 and 29-2).^123,126

FIGURE 29-1 The bowel has three nervous systems: somatic, autonomic, and enteric. ES, External sphincter; IS, internal sphincter.

FIGURE 29-2 Neurologic levels and pathways for the sympathetic, parasympathetic, and somatic nervous system innervation of the colon and anorectum. Not shown is the enteric nervous system, which travels along the bowel wall from esophagus to internal anal sphincter and forms the final common pathway to control the bowel wall smooth muscle.

Enteric Nervous System

The ENS is a distinct system that has its own set of neurons that coordinate sensory and motor functions. In the ENS the ganglia are interconnected, which allows for integration and processing of data (as opposed to autonomic ganglia, which only serve as relay centers for stimuli transmitted from the CNS). There are three different types of neurons in the ENS based on function: sensory neurons, interneurons, and motor neurons.^123,126 Sensory neurons perceive thermal, chemical, or mechanical stimuli and transform these sensations into action potentials that are conducted to the nervous system. Interneurons serve as conduits between the sensory and motor neurons. The numerous synapses between interneurons create a highly organized circuitry that processes sensory input from the gut and other parts of the nervous system, and integrates and generates reflex responses to these stimuli. Motor neurons are the final common pathway. They receive and translate signals to the gut (mucosa, muscle, vasculature) that affect digestive, interdigestive, and emetic functions based on the transmitters released.^123,126

Automatic feedback control is present in the ENS, with the neurons being in close proximity to the stomach and intestines. This can be manifested as reflex circuits that systematize reflex responses to sensory signals, as integrative circuits that coordinate motor patterns (migrating motor complex, digestive activity, giant migratory contractions),^123,126 or as a pattern-generating activity occurring when a “command neuron” is actuated with a resulting rhythmic, repetitive behavior.^11,126

The ENS is the key to proper functioning of the entire gastrointestinal tract. This collection of highly organized neurons is situated in two primary layers: the submucosal (Meissner’s) plexus and the intramuscular myenteric (Auerbach’s) plexus. These plexi have an estimated 10 to 100 million neurons, plus two to three glial cells per neuron. The ENS glial cells resemble CNS astrocytes and are much less abundant than the 20 to 50 glial cells per neuron in the CNS.⁴⁶ The coordination of segment-to-segment function is largely regulated by the ENS.¹²¹ The ENS also has its own blood-nerve barrier, similar to the blood-brain barrier of the CNS.²⁴

Gastrointestinal Neurosensory System

Enteric Nervous System Relationship to the Spinal Cord and Brain

Sensory information from vagal afferents in the ENS is relayed to the nodose ganglia (caudal ganglion of the vagus) and consequently to the nucleus tractus solitarius (NTS) and area postrema in the medullary area of the brainstem. The NTS and the area postrema send signals to the rostral centers in the brain.^123,126 The brain processes the data and projects descending connections to the dorsal vagal complex. The brain also participates in vago-vagal reflex circuits that continuously monitor and promptly modify responses to changes in the chemical, thermal, and mechanical environment in the whole gut (mostly in the esophagus, stomach, duodenum, gallbladder, and pancreas). Sensory stimulation through this vagal circuit does not appear to reach the level of consciousness.^123,126

Besides making connections with the NTS and the area postrema in the brainstem, vagal afferents synapse with the dorsal motor nucleus of the vagus and the nucleus ambiguus, creating the dorsal vagal complex. The incoming sensory information is integrated and shared with the forebrain and brainstem by the dorsal vagal complex. Brain coordination and influences (conscious and nonconscious) are translated to the dorsal vagal complex, where the dorsal motor nucleus of the vagus and nucleus ambiguus represent the efferent arm of the reflex pathway. It is the final common pathway from the brain responsible for precise control of muscular, glandular and circulatory responses of the gastrointestinal tract.^123,124,126 Multiple neurotransmitters are involved in the intricate conduction of impulses between the neuronal circuits in the dorsal vagal complex. Approximately 30 are identified and include acetylcholine, biogenic amines, amino acids, nitric oxide, and peptides.^123,126

Spinal afferents (splanchnic and pelvic) send sensory impulses to the dorsal root ganglia (or prevertebral sympathetic ganglia) that are then conducted to the dorsal horn (laminae I, II, V, X) of the spinal cord and the dorsal column nuclei.^47,88 The dorsal column is thought to play a greater role in nociceptive transmission of messages from the gut than the spinothalamic or spinoreticular tracts. The conscious perception of pain in the digestive tract is largely mediated through spinal afferents. The spinal cord modulates the conduction of neural messages (both nociceptive and nonnociceptive) to the higher brain.⁴⁷ Other somatic and visceral sensory stimuli from the vagina, uterus, bladder, colon, and rectum are conveyed through the dorsal horn and dorsal column in the spinal cord as well. Likewise, somatic afferents supply sensory input from the muscles of the pelvic floor through the pudendal nerve to the sacral region of the spinal cord.^10,47,88,89

Brain centers modulate the nociceptive impulses from the bowel that are relayed to the dorsal horn of the spinal cord. These descending pathways are facilitatory, inhibitory, or both, based on the visceral stimulus, and can alter the perception of pain in the digestive tract.^47,97 The neurotransmitters serotonin, noradrenalin, and dopamine are released by these descending pathways as they synapse with the spinal cord.¹²⁶

The secondary somatosensory cortex and, to a lesser extent, the paralimbic and limbic areas (anterior insular, anterior and posterior cingulate, prefrontal and orbitofrontal cortices) mediate emotional, volitional, and psychologic responses to sensory input from the gut. These are manifested by abdominal pain, anorexia, nausea, vomiting, hyperphagia, constipation, or diarrhea.^47,73,104 The somatosensory cortex regulates the awareness and recognition of pain, and the paralimbic and limbic areas contribute to the cognitive and affective aspects of pain.^47,104,126

Data from the brain and descending vagal pathways are conducted to the spinal cord, and then with the preganglionic neurons in the thoracolumbar area (modulates the sympathetic response) and the sacral area (modulates the parasympathetic response). Sympathetic or parasympathetic output is translated to the neurons in the ENS circuitry, which can be excitatory or inhibitory to motor neurons, gastric or digestive glands, and secretory mechanisms.^47,126

Gastrointestinal Neuromotor System

The muscles of the gastrointestinal tract carry out essential functions throughout the gut, including propulsion, grinding, mixing, absorption, storage, and disposal. These muscles are composed of “self-excitable” smooth muscles that contract in an all-in-one manner. These smooth muscles spontaneously responds to stretch and can be independent of neural or endocrine control. The interstitial cells of Cajal act like a pacemaker and allow propagation of electrical slow waves into the circular muscle layer, which generates spreading muscle contraction. These smooth muscles act like an electrical syncytium where action potentials are conducted in three dimensions from one smooth muscle fiber to another through gap junctions.^47,126

The gastrointestinal tract muscles respond to influences of the vagal efferents and the ENS microcircuitry based on excitatory or inhibitory innervation of motor neurons. Contraction is mediated by the release of excitatory neurotransmitters by vagal afferents at the neuromuscular junctions, acetylcholine (at the muscarinic receptor), and substance P (at the neurokinin-1 receptor).^26,47,82,126 Conversely, the release of nitric oxide, ATP, and vasoactive intestinal peptide from the inhibitory motor neurons (which express the serotonergic 5-HT₁ receptor) impedes contractile activity and facilitates relaxation. The aboral direction of propulsive activity throughout the digestive tract is achieved by segmental inactivation of inhibitory motor neurons distally.^{15,16,47,58,126} Contraction can only occur in the segments where the inhibitory motor neurons are inactivated. With passing of the food bolus or stool, the esophageal and internal anal sphincters (smooth muscle sphincters), and inhibitory motor neurons are usually shut off and are inactivated. During vomiting, however, the inhibitory motor neurons are deactivated in the opposite direction.^47,126

The sympathetic and parasympathetic nervous systems seem to modulate the ENS, rather than directly controlling the smooth muscles of the bowel.¹²¹ The smooth muscles of the bowel also have their own electromechanical automaticity, which is directly modulated by the inhibitory control of the ENS.^24,46 Sympathetic nervous system stimulation tends to promote the storage function by enhancing anal tone and inhibiting colonic contractions, although little clinical deficit occurs from bilateral sympathectomy.³² Parasympathetic activity enhances colonic motility, and its loss is often associated with DWE, including impactions and functional obstructions, such as Ogilvie’s pseudoobstructive syndrome.³²

The ENS and sympathetic postganglionic neurons transmit excitatory or inhibitory messages to secretomotor neurons. These secretomotor neurons release acetylcholine and vasoactive intestinal polypeptide when there is excitatory activation from paracrine stimulation by mucosal and submucosal cells such as enterochromaffin cells, mast cells, and other immune and inflammatory cells. Acetylcholine and vasoactive intestinal polypeptide are released at the neuroepithelial and neurovascular junctions. This promotes secretion into the gut of water, sodium chloride, bicarbonate, and mucus drawn from the intestinal glands. In addition, dilatation and an increase in blood flow occur with the release of nitric oxide from the vascular endothelium.^6,13,47 The inhibitory influence reduces neuronal firing from secretomotor neurons by hyperpolarization of membranes. Sympathetic release of norepinephrine from nerve endings of the α₂-noradrenergic receptors inhibits secretomotor actuation, preventing the release of excitatory neurotransmitters. As a result, there is a decrease in secretion of water and electrolytes into the lumen and a congruent shunting of blood from the splanchnic to systemic circulation.^47,76,91

Physiology of Normal Defecation

The colon is a reservoir for food waste until it is convenient for elimination. It also acts as a storage device as long as the colonic pressure is less than that of the anal sphincter mechanism. Fecal elimination occurs when colonic pressure exceeds that of the anal sphincter mechanism. Other functions of the colon are to reabsorb fluids (up to 30 L/day can be reabsorbed from the large and small bowel walls, with typically only 100 mL of water loss in feces) and gases (90% of the 7 to 10 L of gases produced by intracolonic fermentation is absorbed rather than expelled). The colon also provides an environment for the growth of bacteria needed to assist in digestion, and serves to absorb certain bacterial breakdown products as well.⁴⁸ The layers of the colon wall are depicted in Figure 29-3.

FIGURE 29-3 A transverse section of the gut, showing the enteric plexus and the distribution of adrenergic neurons. Note the ganglionic plexuses of Auerbach and Meissner. The deep muscular plexus contains a few ganglia, the subserosal contains an occasional ganglion, and the mucosal plexus shows none. The adrenergic fibers are all extrinsic and arise from the prevertebral sympathetic ganglia. The adrenergic fibers are distributed largely to the mesenteric, submucous, and mucosal plexus and to blood vessels.

(From Goyal RK, Crist, JR: Neurology of the gut. In: Sleisenger MH, Fordtram JS, editors: Gastrointestinal disease, Philadelphia, 1989, WB Saunders, with permission.)

The rectum is usually empty until just before defecation. Perception of rectal contents and pressures¹⁰⁵ is essential for signaling voluntary contraction of the anal sphincter. Normal defecation begins with reflexes triggered by rectosigmoid distention produced by approximately 200 mL of feces (Figure 29-4).⁹⁴ A rectorectal reflex occurs in which the bowel proximal to the distending bolus contracts and the bowel wall distally relaxes, serving to propel the bolus further caudad. Reflex relaxation of the IAS also occurs, which is enhanced by, but does not require, an extrinsic nerve supply. This relaxation, called the rectoanal inhibitory reflex, correlates with the urge labeled “the call to stool.”¹²¹ One can then volitionally contract the levator ani to open the proximal anal canal and relax the EAS and puborectalis muscles. This allows a straighter, shorter, and more open anorectal passage (see Figure 29-4), which permits the bolus to pass. Increasing the intraabdominal pressure by squatting and by Valsalva’s maneuver assists bolus elimination. For 90% of normal individuals, only the contents of the rectum are expulsed, whereas 10% will clear the entire contents of the left side of the colon from the splenic flexure distally.³³

FIGURE 29-4 A, Defecation is prevented by a statically increased tone of the internal anal sphincter (IAS) and puborectalis, as well as by the mechanical effects of the acute anorectal angle. Dynamic responses of the external anal sphincter (EAS) and puborectalis to rectal distention reflexes or increased intraabdominal pressures further impede defecation. B, To initiate defecation, the puborectalis muscle and EAS relax while intraabdominal pressure is increased by Valsalva’s maneuver, which is facilitated by squatting. The levator ani helps reduce the acute anorectal angle to open the distal anal canal to receive the stool bolus. C, Intrarectal reflexes result in continued IAS relaxation and rectal propulsive contractions, which help expel the bolus through the open canal.

(Modified from Schiller LR: Fecal incontinence. In: Sleisenger MH, Fordtran JS, editors: Gastrointestinal disease, Philadelphia, 1993, WB Saunders.)

One can elect to defer defecation, however, by volitionally contracting the puborectalis muscle and EAS. The reflexive IAS relaxation subsequently fades, usually within 15 seconds, and the urge resolves until the IAS relaxation is again triggered. The rectal wall accommodates to the bolus by decreasing its wall tension with time, resulting in less sensory input and less reflex triggering from that particular accommodated bolus. This continence and reflex process is somewhat analogous to the function of the striated external urethral sphincter in volitional control of urinary voiding (see Chapter 28).

The EAS generally tenses in response to small rectal distentions via a spinal reflex, although reflexive relaxation of the EAS occurs in the presence of greater distentions.¹⁰⁵ These spinal cord reflexes are centered in the conus medullaris and are augmented and modulated by higher cortical influences. When cortical control is disrupted, as by SCI, the EAS reflexes usually persist and allow spontaneous defection. During sleep the colonic activity, anal tone, and protective responses to abdominal pressure elevations are all decreased, while rectal tone increases.^24,121

The gastrocolonic response or gastrocolic reflex refers to the increased colonic activity (GMCs and mass movements) in the first 30 to 60 minutes after a meal. This increased colonic activity appears to be modulated both by hormonal effects, from release of peptides from the upper gastrointestinal tract (gastrin, motilin, cholecystokinin) that increase the contractility of the colonic smooth musculature, and by a reduction in the threshold for spinal cord–mediated vesicovesical reflexes.²⁴ Upper gastrointestinal receptor stimulation also results in increased activity in the colon, possibly because of reflexively increased parasympathetic efferent activity to the colon. The possibility of a purely ENS-mediated activation exists, although the small bowel and colon motor activities do not seem to be synchronized. In persons with SCI the measured increase in colonic activity after a meal is blunted as compared with that in normal subjects.²⁴ The gastrocolonic response is often used therapeutically, even in patients with SCI, to enhance bowel evacuation during this 30- to 60-minute postprandial time frame.^1,34 Occasionally certain foods can serve as trigger foods that are especially likely to induce bowel evacuation shortly after consumption.

The resting anal canal pressure is largely determined by the angulation and pressure at the anorectal junction by the puborectalis sling and smooth muscle IAS tone. Continence is maintained by the anal sphincter mechanism,^81,107 which consists of the IAS, EAS, and puborectalis muscle.⁸¹ Only about 20% of the anal canal pressure is due to the static contraction of the somatically innervated striated EAS.⁹ The EAS and puborectalis muscle are the only striated skeletal muscles whose normal resting state is tonic contraction, and these muscles consist mainly of slow-twitch, fatigue-resistant type I fibers (unlike the situation in nonupright animals such as the cat or dog, in which these muscles consist of predominately type II fibers).⁹ Anal pressure can be increased volitionally by contracting the EAS and puborectalis muscles. Maximum volitional squeeze pressures, however, are not as high as can be generated reflexively against Valsalva pressure. The EAS is physically larger than the IAS, and its contraction is under both reflex and volitional control. The volitional control is learned during the course of normal maturation. Normal baseline reflex action of the anorectal mechanism allows spontaneous stool elimination.¹⁰² The EAS is innervated by the S2 through S4 nerve roots via the pudendal nerve, and the puborectalis muscle is innervated by direct branches from the S1 to S5 roots (see Figures 29-1 and 29-2).⁹² The remarkable degree of learned EAS coordination allows the selective discrete passage of gas while juggling a variable mixture of solids, liquids, and gases.

Pathophysiology of Gastrointestinal Dysfunction

A whole range of neurologic diseases affecting central, peripheral, and intrinsic enteric nervous innervation can cause disorders that affect various segments of the bowel. These are predominantly characterized by disturbances in gastroesophageal, small or large intestinal motility and by disturbances in sensation. Symptoms of dysphagia, vomiting, bloating, abdominal discomfort and pain, constipation, and incontinence have been described in individuals with neurologic ailments.^5,18,19,93

Exact and detailed pathophysiologic mechanisms of gastrointestinal dysfunction in the various neurologic diseases are not very well understood. Recent studies, however, have been able to identify various mechanisms of symptom production that might facilitate therapeutic strategies.