Medium-Chain Acyl CoA Dehydrogenase (MCAD) Deficiency

MCAD deficiency is the most common fatty acid oxidation disorder. The disorder shows a strong founder effect; most patients have a northwestern European ancestry, and the majority of patients are homozygous for a single common missense mutation, an A-G transition at cDNA position 985 that changes a lysine to glutamic acid at residue 329 (K329E).

Very Long Chain Acyl CoA Dehydrogenase (VLCAD) Deficiency

VLCAD deficiency is the second most commonly diagnosed disorder of fatty acid oxidation. It was originally termed LCAD deficiency before the existence of the inner mitochondrial membrane-bound VLCAD was known. All patients previously diagnosed as having LCAD deficiency have VLCAD enzyme deficiency. Patients with VLCAD deficiency are usually more severely affected than those with MCAD deficiency, presenting earlier in infancy and having more chronic problems with muscle weakness or episodes of muscle pain and rhabdomyolysis. Cardiomyopathy may be present during acute attacks associated with fasting. The left ventricle may be hypertrophic or dilated and show poor contractility on echocardiography. Sudden unexpected death has occurred in several patients, but most who survived the initial episode showed improvement, including normalization of cardiac function. Other physical and routine laboratory features are similar to those of MCAD deficiency, including secondary carnitine deficiency. The urinary organic acid profile shows a nonketotic dicarboxylic aciduria. Increased levels of C_6-12 dicarboxylic acids may be noted in the urine. Diagnosis may be suggested by an abnormal acylcarnitine profile with plasma or blood spot C_14:1,14:0 acylcarnitine species, but the specific diagnosis requires assay of enzyme activities of VLCAD in cultured fibroblasts or direct mutational analysis of the VLCAD gene. Treatment is based primarily on avoidance of fasts for >10-12 hr. Continuous intragastric feeding is useful in some patients.

Short-Chain Acyl CoA Dehydrogenase (SCAD) Deficiency

A small number of patients with two clear null mutations in the SCAD gene have been described with variable phenotype. Most individuals classified as being SCAD deficient have been shown to have polymorphic DNA changes in the SCAD gene. The two common polymorphisms are G185S and R147W, which are present in 7% of the population. Some investigators argue that these may be susceptibility changes, which require a second, as yet unknown, genetic mutation to express a clinical phenotype; while others believe that SCAD deficiency is a harmless biochemical condition. This autosomal recessive disorder presents with neonatal hypoglycemia and may have normal levels of ketone bodies. The diagnosis is indicated by elevated levels of butyrylcarnitine on blood spots or plasma and increased excretion of urinary ethylmalonic acid and butyrylglycine. These metabolic abnormalities are most pronounced in patients with null mutations and variably present in patients who are homozygous for the polymorphisms.

The necessity for treatment in SCAD deficiency has not yet been established. It has been proposed that long-term evaluation of symptomless individuals is necessary to determine whether this is or is not a real disease.

Long-Chain 3-Hydroxyacyl CoA Dehydrogenase (LCHAD)/Mitochondrial Trifunctional Protein (TFP) Deficiency

The LCHAD enzyme is part of a mitochondrial trifunctional protein (TFP), which also contains two other steps in β-oxidation, long-chain enoyl CoA hydratase and long-chain β-ketothiolase. It is a hetero-octameric protein composed of 4 α and 4 β chains that derive from distinct contiguous genes with a common promoter region. In some patients, only the LCHAD activity of the TFP is affected (LCHAD deficiency), whereas others have deficiencies of all 3 activities (TFP deficiency).

Clinical manifestations include attacks of acute hypoketotic hypoglycemia similar to MCAD deficiency; patients often show evidence of more severe disease, including cardiomyopathy, muscle cramps and weakness, and abnormal liver function (cholestasis). Toxic effects of fatty acid metabolites may produce pigmented retinopathy, progressive liver failure, peripheral neuropathy, and rhabdomyolysis. Life-threatening obstetric complications; acute fatty liver of pregnancy; and hemolysis, elevated liver enzymes, low platelets (HELLP) syndrome are observed in heterozygous mothers carrying homozygotic fetuses affected with LCHAD/TFP deficiency. Sudden unexpected infant death may occur. The diagnosis is indicated by elevated levels of blood spot or plasma 3-hydroxy acylcarnitines of chain lengths C₁₆-C₁₈. Urinary organic acid profile in patients may show increases in levels of 3-hydroxydicarboxylic acids of chain lengths C₆-C₁₄. Secondary carnitine deficiency is common. A common mutation in the α subunit, E474Q, is seen in >60% of LCHAD deficient patients. This mutation in the fetus is significantly associated with the obstetric complications, but other mutations in either subunit may also be associated with maternal illness.

Treatment is similar to that for MCAD or VLCAD deficiency; that is, avoiding fasting stress. Some investigators have suggested that dietary supplements with medium-chain triglyceride oil to by-pass the long-chain fatty acid oxidation process for long-chain defects and docosahexaenoic acid (DHA, for protection against the retinal changes) may be useful. Liver transplantation does not ameliorate the metabolic abnormalities.

Short-Chain 3-Hydroxyacyl-CoA Dehydrogenase (SCHAD) Deficiency

Very few patients with this inborn error have been described. Only 5 patients with proven mutations of SCHAD have been reported although a few additional unpublished cases are known to the authors. Four cases in 3 families with recessive mutations of SCHAD presented with episodes of hypoketotic hypoglycemia that was shown to be due to hyperinsulinism. In contrast to patients with other forms of fatty acid oxidation disorders, these cases required specific therapy with diazoxide for hyperinsulinism to avoid recurrent hypoglycemia. The 5th child with a mutation was compound heterozygous for two different SCHAD mutations and presented with fulminant hepatic failure at age 10 mo. Other reports of cases not proven to have mutations include a child with attacks of fasting hypoglycemia and myoglobinuria associated with deficiency of SCHAD in muscle but not in cultured fibroblasts, 3 children with fatal liver disease, and an infant who died suddenly and unexpectedly. Specific metabolic markers for SCHAD deficiency include elevated C4-hydroxy acylcarnitine and urine 3-hydroxyglutaric acid.

Treatment of SCHAD deficient patients with hyperinsulinism is with diazoxide.

Plasma Membrane Carnitine Transport Defect (Primary Carnitine Deficiency)

Primary carnitine deficiency is the only genetic defect in which carnitine deficiency is the cause, rather than the consequence, of impaired fatty acid oxidation. The most common presentation is progressive cardiomyopathy with or without skeletal muscle weakness beginning at 1-4 yr of age. A smaller number of patients may present with fasting hypoketotic hypoglycemia in the 1st yr of life before the cardiomyopathy becomes symptomatic. The underlying defect involves the plasma membrane sodium gradient-dependent carnitine transporter that is present in heart, muscle, and kidney. This transporter is responsible both for maintaining intracellular carnitine concentrations 20- to 50-fold higher than plasma concentrations and for renal conservation of carnitine.

Diagnosis of the carnitine transporter defect is aided by the fact that patients have extremely reduced carnitine levels in plasma and muscle (1-2% of normal). Heterozygote parents have plasma carnitine levels approximately 50% of normal. Fasting ketogenesis may be normal because liver carnitine transport is normal, but it may be impaired if dietary carnitine intake is interrupted. The fasting urinary organic acid profile may show a hypoketotic dicarboxylic aciduria pattern if hepatic fatty acid oxidation is impaired, but it is otherwise unremarkable. The defect in carnitine transport can be demonstrated clinically by severe reduction in renal carnitine threshold or in vitro by assay of carnitine uptake using cultured fibroblasts or lymphoblasts. Mutations in the organic cation/carnitine transporter (OCTN2) underlie this disorder. Treatment of this disorder with pharmacologic doses of oral carnitine (100-200 mg/kg/day) is highly effective in correcting the cardiomyopathy and muscle weakness as well as any impairment in fasting ketogenesis. Muscle total carnitine concentrations remain <5% of normal on treatment.

Carnitine Palmitoyltransferase-IA (CPT-IA) Deficiency

Several dozen infants and children have been described with a deficiency of the liver and kidney isozyme of CPT-IA. Clinical manifestations include fasting hypoketotic hypoglycemia, occasionally with markedly abnormal liver function tests and, rarely, with renal tubular acidosis. The heart and skeletal muscle are not involved because the muscle isozyme is unaffected. Fasting urinary organic acid profile shows a hypoketotic C₆-C₁₂ dicarboxylic aciduria but may be normal. Plasma acylcarnitine analysis demonstrates mostly free carnitine with very little acylated carnitine. This observation has been used to establish CPT-IA diagnosis on newborn screening by tandem mass spectrometry. CPT-IA deficiency is the only fatty acid oxidation disorder in which plasma total carnitine levels are elevated to 150-200% of normal. This may be explained by the fact that the inhibitory effects of long-chain acylcarnitines on the renal tubular carnitine transporter are absent in CPT-IA deficiency. The enzyme defect can be demonstrated in cultured fibroblasts or lymphoblasts. CPT-IA deficiency in the fetus has been associated with acute fatty liver of pregnancy in the mother in a single case report. A common variant in the CPT1A gene has been identified in individuals of Inuit background in the USA and First Nations tribes in Canada. The variant results in a positive newborn screen and 20% residual enzyme activity which is unregulated. It has not been established if this is a pathological DNA variant or an adaptive process to ancient Inuit and First Nations lifestyles. Treatment for the severe CPT1A deficiency is similar to that for MCAD deficiency with avoidance of situations where fasting ketogenesis is necessary.

Carnitine-Acylcarnitine Translocase (CACT) Deficiency

This defect of the inner mitochondrial membrane carrier protein for fatty acylcarnitines blocks the entry of long-chain fatty acids into the mitochondria for oxidation. The clinical phenotype of this disorder is characterized by a severe and generalized impairment of fatty acid oxidation. Most newborn patients present with attacks of fasting-induced hypoglycemia, hyperammonemia, and cardiorespiratory collapse. All symptomatic newborns have had evidence of cardiomyopathy and muscle weakness. Several patients with a partial translocase deficiency and milder disease without cardiac involvement have also been identified. No distinctive urinary or plasma organic acids are noted, although increased levels of plasma long-chain acylcarnitines are reported. Diagnosis can be made using cultured fibroblasts or lymphoblasts. The human gene has been cloned, and mutations have been identified in affected patients. Treatment is similar to that of other long-chain fatty acid oxidation disorders.

Carnitine Palmitoyltransferase-II (CPT-II) Deficiency

Three forms of CPT-II deficiency have been described. The antenatal presentation of this disorder is associated with a profound enzyme deficiency, and neonatal death has been reported in several newborns with dysplastic kidneys, cerebral malformations, and mild facial anomalies. A severe deficiency of enzyme activity is associated with an infantile-onset form. This form shares all the clinical and laboratory features of CACT deficiency. A milder defect is associated with an adult presentation of episodic rhabdomyolysis. The 1st episode usually does not occur until late childhood or early adulthood. Attacks may be precipitated by prolonged exercise. There is aching muscle pain and myoglobinuria that may be severe enough to cause renal failure. Serum levels of creatine kinase are elevated to 5,000-100,000 U/L. Fasting hypoglycemia has not been described, but fasting may contribute to attacks of myoglobinuria. Muscle biopsy shows increased deposition of neutral fat. The myopathic presentation of CPT-II deficiency is associated with a common mutation S113L. This mutation produces a heat-labile protein that is unstable to increased muscle temperature due to exercise resulting in the myopathic presentation. An intermediate form of CPT-II deficiency presents in infancy/early childhood with fasting-induced hepatic failure, cardiomyopathy, and skeletal myopathy with hypoketotic hypoglycemia but does not have the severe developmental changes seen in the neonatal presentation. This pattern is more like that seen in VLCAD deficiency and management is identical. Patients are generally heterozygous for one of the severe mutations and one of the milder mutations.

Diagnosis of all forms of CPT-II deficiency can be made by demonstrating deficient enzyme activity in muscle or other tissues and in cultured fibroblasts. Mutation analysis is available.

Electron Transfer Flavoprotein (ETF) and Electron Transfer Flavoprotein Dehydrogenase (ETF-DH) Deficiencies (GLUTARIC Aciduria Type 2, Multiple Acyl CoA Dehydrogenation Deficiencies)

ETF and ETF-DH function to transfer electrons into the mitochondrial electron transport chain from dehydrogenation reactions catalyzed by VLCAD, MCAD, and SCAD, as well as glutaryl CoA dehydrogenase and at least four enzymes involved in branch-chain amino acid oxidation. Deficiencies of ETF or ETF-DH produce illness that combines the features of impaired fatty acid oxidation and impaired oxidation of several amino acids. Complete deficiencies of either protein are associated with severe illness in the newborn period, characterized by acidosis, hypoglycemia, coma, hypotonia, cardiomyopathy, and an unusual odor of sweaty feet due to isovaleryl CoA dehydrogenase inhibition. Some affected neonates have had facial dysmorphism and polycystic kidneys similar to that seen in severe CPT-II deficiency, which suggests that toxic effects of accumulated metabolites may occur in utero.

Diagnosis can be made from the urinary organic acid profile, which shows abnormalities corresponding to blocks in oxidation of fatty acids (ethylmalonate and C₆-C₁₀ dicarboxylic acids), lysine (glutarate), and branched-chain amino acids (isovaleryl-, isobutyryl-, and α-methylbutyryl-glycine). Most severely affected infants do not survive the neonatal period.

Partial deficiencies of ETF and ETF-DH produce a disorder that may mimic MCAD deficiency or other milder fatty acid oxidation defects. These patients have attacks of fasting hypoketotic coma. The urinary organic acid profile reveals primarily elevations of dicarboxylic acids and ethylmalonate, derived from short-chain fatty acid intermediates. Secondary carnitine deficiency is present. Some patients with mild forms of ETF/ETF-DH deficiency benefit from treatment with high doses of riboflavin, which is a cofactor for the pathway of electron transfer.

β-Hydroxy-β-Methylglutaryl CoA (HMG CoA) Synthase Deficiency (Chapter 79.6)

HMG CoA synthase is the rate-limiting step in the conversion of acetyl CoA derived from fatty acid β-oxidation in the liver to ketones. Several patients with this defect have recently been identified. The presentation is one of fasting hypoketotic hypoglycemia without evidence of impaired cardiac or skeletal muscle function. Urinary organic acid profile showed only a hypoketotic dicarboxylic aciduria. Plasma and tissue carnitine levels are normal, in contrast to all the other disorders of fatty acid oxidation. A separate synthase enzyme, present in cytosol for cholesterol biosynthesis, is not affected. The HMG CoA synthase defect is expressed only in the liver and cannot be demonstrated in cultured fibroblasts. The gene has been cloned, and mutations in the affected patients have been characterized. Avoiding fasting is usually a successful treatment.

β-Hydroxy-β-Methylglutaryl CoA Lyase Deficiency

See Chapter 79.6.

Succinyl-CoA : 3-Ketoacid CoA Transferase (SCOT) Deficiency (Chapter 79.6)

Several patients with SCOT deficiency have been reported. Characteristic presentation is an infant with recurrent episodes of severe ketoacidosis induced by fasting. Plasma acylcarnitine and urine organic acid abnormalities do not distinguish from other causes of ketoacidosis. Treatment of episodes requires infusion of glucose and large amounts of bicarbonate until metabolically stable. All patients exhibit inappropriate hyperketonemia even between catabolic episodes. SCOT is responsible for activating acetoacetate in peripheral tissues using succinyl CoA as a donor to form acetoacetyl CoA. Deficient activity can be demonstrated in brain, muscle, and fibroblasts from affected patients. The gene has been cloned, and numerous mutations have been characterized.

β-Ketothiolase Deficiency

See Chapter 79.6.

Etiology

Peroxisomal disorders are subdivided into 2 major categories (Table 80-2).

In category A, the peroxisomal biogenesis disorders (PBD), the basic defect is the failure to import one or more proteins into the organelle. In category B, defects affect a single peroxisomal protein. The peroxisome is present in all cells except mature erythrocytes and is a subcellular organelle surrounded by a single membrane; >50 peroxisomal enzymes are identified. Some enzymes are involved in the production and decomposition of hydrogen peroxide; others are concerned with lipid and amino acid metabolism. Most peroxisomal enzymes are first synthesized in their mature form on free polyribosomes and enter the cytoplasm. Proteins that are destined for the peroxisome contain specific peroxisome targeting sequences (PTS). Most peroxisomal matrix proteins contain PTS1, a 3-amino acid sequence at the carboxyl terminus. PTS2 is an amino-terminal sequence that is critical for the import of enzymes involved in plasmalogen and branched-chain fatty acid metabolism. Import of proteins involves a complex series of reactions that involves at least 23 distinct proteins. These proteins are referred to as peroxins encoded by PEX genes. Table 80-3 summarizes the PEX genes that are defective in human disease states.

Epidemiology

Except for X-linked adrenoleukodystrophy (X-ALD), all the peroxisomal disorders in Table 80-2 are autosomal recessive traits. X-ALD is the most common peroxisomal disorder, with an estimated incidence of 1/17,000. The combined incidence of the other peroxisomal disorders is estimated to be 1/50,000.

Pathology

Absence or reduction in the number of peroxisomes is pathognomonic for disorders of peroxisome biogenesis. In most disorders, there are membranous sacs that contain peroxisomal integral membrane proteins, which lack the normal complement of matrix proteins; these are peroxisome “ghosts.” Pathologic changes are observed in many organs and include profound and characteristic defects in neuronal migration; micronodular cirrhosis of the liver; renal cysts; chondrodysplasia punctata; corneal clouding, congenital cataracts, glaucoma, and retinopathy; congenital heart disease; and dysmorphic features.

Pathogenesis

It is likely that all pathologic changes are secondary to the peroxisome defect. Multiple peroxisomal enzymes fail to function in the PBD (Table 80-4). The enzymes that are diminished or absent are synthesized but are degraded abnormally fast because they may be unprotected outside of the peroxisome. It is not clear how defective peroxisome functions lead to the widespread pathologic manifestations.

The PBD are associated with genetically determined import defects. The PBD have been subdivided into 12 complementation groups. The molecular defects have been defined in 10 of these groups (see Table 80-3). The pattern and severity of pathologic features vary with the nature of the import defects and the degree to which import is impaired. These gene defects lead to disorders that were named before their relationship to the peroxisome was recognized, namely, Zellweger syndrome (ZS), neonatal adrenoleukodystrophy (NALD), infantile Refsum disease (IRD), and rhizomelic chondrodysplasia punctata (RCDP). The first 3 disorders are considered to form a clinical continuum, with ZS the most severe, IRD the least severe, and NALD intermediate. They can be caused by 11 different gene defects, which involve mainly the import of proteins that contain the PTS1 targeting signal; the gene defects cannot be distinguished on the basis of clinical features. The clinical severity varies with the degree to which protein import is impaired. Mutations that abolish import completely are often associated with the ZS phenotype, whereas a missense mutation, in which some degree of import function is retained, leads to the somewhat milder phenotypes. A defect in PEX7, which involves the import of proteins that utilize PTS2, is associated with RCDP. PEX7 defects that leave import partially intact are associated with milder phenotypes, some of which resemble classic Refsum disease.

The genetic disorders that involve single peroxisomal enzymes usually have clinical manifestations that are more restricted and present subsequent to the neonatal period and not infrequently in adolescents or adults. The clinical manifestations may be related to the biochemical defect. The primary adrenal insufficiency of X-ALD is caused by accumulation of very long chain fatty acids (VLCFA) in the adrenal cortex, and the peripheral neuropathy in Refsum disease is caused by the accumulation of phytanic acid in Schwann cells and myelin.

PBD With Milder or Atypical Phenotypes

Newborn infants with Zellweger syndrome show striking and consistent, recognizable abnormalities. Of central diagnostic importance are the typical facial appearance (high forehead, unslanting palpebral fissures, hypoplastic supraorbital ridges, and epicanthal folds; Fig. 80-3), severe weakness and hypotonia, neonatal seizures, and eye abnormalities (cataracts, glaucoma, corneal clouding, Brushfield spots, pigmentary retinopathy, and nerve dysplasia). Because of the hypotonia and “mongoloid” appearance, Down syndrome may be suspected. Infants with Zellweger syndrome rarely live more than a few months. More than 90% show postnatal growth failure. Table 80-5 lists the main clinical abnormalities.

Figure 80-3 Four patients with Zellweger cerebrohepatorenal syndrome. Note the high forehead, epicanthal folds, and hypoplasia of supraorbital ridges and midface.

(Courtesy of Hans Zellweger, MD.)

Table 80-5 MAIN CLINICAL ABNORMALITIES IN ZELLWEGER SYNDROME

Patients with neonatal ALD show fewer and, occasionally, no dysmorphic features. Neonatal seizures occur frequently. Some degree of psychomotor development is present; function remains in the severely or profoundly retarded range, and development may regress after 3-5 yr of age, probably from a progressive leukodystrophy. Several patients are now in a stable, albeit disabled, state in their 3rd or 4th decade. Hepatomegaly, impaired liver function, pigmentary degeneration of the retina, and severely impaired hearing are invariably present. Adrenocortical function is usually impaired, but overt Addison disease is rare. Chondrodysplasia punctata and renal cysts are absent.

Patients with infantile Refsum disease have survived to the 2nd decade or longer. They are able to walk, although gait may be ataxic and broad based. Cognitive function is in the severely retarded range. All have sensorineural hearing loss and pigmentary degeneration of the retina. They have moderately dysmorphic features that may include epicanthal folds, a flat bridge of the nose, and low-set ears. Early hypotonia and hepatomegaly with impaired function are common. Levels of plasma cholesterol and high- and low-density lipoprotein are often moderately reduced. Chondrodysplasia punctata and renal cortical cysts are absent. Postmortem study in infantile Refsum disease reveals micronodular liver cirrhosis and small hypoplastic adrenals. The brain shows no malformations, except for severe hypoplasia of the cerebellar granule layer and ectopic locations of the Purkinje cells in the molecular layer. The mode of inheritance is autosomal recessive.

Some patients with PBD disorders have milder and atypical phenotypes. They may present with peripheral neuropathy or with retinopathy, impaired vision, or cataracts in childhood, adolescence, or adulthood and have been diagnosed to have Charcot-Marie-Tooth disease or Usher syndrome. Some patients have survived to the 5th decade. Defects in PEX7, which most commonly lead to the RCDP phenotype, may also lead to a milder phenotype with clinical manifestations similar to those of classical Refsum disease (phytanoyl CoA hydroxylase deficiency).

Rhizomelic Chondrodysplasia Punctata (Rcdp)

This disorder is characterized by the presence of stippled foci of calcification within the hyaline cartilage and is associated with dwarfing, cataracts (72%), and multiple malformations due to contractures. Vertebral bodies have a coronal cleft filled by cartilage that is a result of an embryonic arrest. Disproportionate short stature affects the proximal parts of the extremities (Fig. 80-4A). Radiologic abnormalities consist of shortening of the proximal limb bones, metaphyseal cupping, and disturbed ossification (Fig. 80-4B). Height, weight, and head circumference are less than the 3rd percentile, and these children are severely retarded mentally. Skin changes such as those observed in ichthyosiform erythroderma are present in about 25% of patients.

Figure 80-4 A, Newborn infant with rhizomelic chondrodysplasia punctata (RCDP). Note the severe shortening of the proximal limbs, the depressed bridge of the nose, hypertelorism, and widespread scaling skin lesions. B, Note the marked shortening of the humerus and epiphyseal stippling at the shoulder and elbow joints.

(Courtesy of John P. Dorst, MD.)

Laboratory Findings

Laboratory tests for peroxisomal disorders can be viewed at three levels of complexity.

Diagnosis

There are several noninvasive laboratory tests that permit precise and early diagnosis of peroxisomal disorders (see Table 80-6). The challenge in PBD is to differentiate them from the large variety of other conditions that can cause hypotonia, seizures, failure to thrive, or dysmorphic features. Experienced clinicians can readily recognize classic Zellweger syndrome by its clinical manifestations. PBD patients often do not show the full clinical spectrum of disease and may be identifiable only by laboratory assays. Clinical features that may serve as indications for these diagnostic assays include severe psychomotor retardation; weakness and hypotonia; dysmorphic features; neonatal seizures; retinopathy, glaucoma, or cataracts; hearing deficits; enlarged liver and impaired liver function; and chondrodysplasia punctata. The presence of one or more of these abnormalities increases the likelihood of this diagnosis. Atypical milder forms presenting as peripheral neuropathy have also been described.

Some patients with the isolated defects of peroxisomal fatty acid oxidation (group B) resemble those with group A disorders and can be detected by the demonstration of abnormally high levels of VLCFA.

Patients with RCDP must be distinguished from patients with other causes of chondrodysplasia punctata. In addition to warfarin embryopathy and Zellweger syndrome, these disorders include the milder autosomal dominant form of chondrodysplasia punctata (Conradi-Hünermann syndrome), which is characterized by longer survival, absence of severe limb shortening, and usually intact intellect; an X-linked dominant form; and an X-linked recessive form associated with a deletion of the terminal portion of the short arm of the X chromosome. RCDP is suspected clinically because of the shortness of limbs, psychomotor retardation, and ichthyosis. The most decisive laboratory test is the demonstration of abnormally low plasmalogen levels in red blood cells and an impaired capacity to synthesize plasmalogens in cultured skin fibroblasts. These biochemical defects are not present in other types of chondrodysplasia punctata. Chondrodysplasia punctata may also be associated with a defect of 3β-hydroxysteroid-Δ⁸,Δ⁷-isomerase, an enzyme involved in biosynthesis of cholesterol.

Complications

Patients with Zellweger cerebrohepatorenal syndrome have multiple disabilities involving muscle tone, swallowing, cardiac abnormalities, liver disease, and seizures. These conditions are treated symptomatically, but the prognosis is poor, and most patients succumb in the 1st few months of life. Patients with RCDP may develop quadriparesis owing to compression at the base of the brain.

Treatment

The most effective therapy is the dietary treatment of classic Refsum disease with a phytanic acid-restricted diet.

For patients with the somewhat milder variants of the peroxisome import disorders, success has been achieved with multidisciplinary early intervention, including physical and occupational therapy, hearing aids, alternative communication, nutrition, and support for the parents. Although most patients continue to function in the profoundly or severely retarded range, some make significant gains in self-help skills, and several are in stable condition in their teens or even early 20s.

Studies to mitigate some of the secondary biochemical abnormalities include the oral administration of docosahexaenoic acid in a dosage of 50-100 mg/24 hr either as the ethyl ester or in the form of a triglyceride in which one of the fatty acids has been replaced by docosahexaenoic acid. The levels of this substance are greatly reduced in patients with disorders of peroxisome biogenesis because the last step of its synthesis takes place in the peroxisome. This therapy normalizes the plasma and erythrocyte levels of this substance, which has important physiologic functions in retina and brain. There are anecdotal reports of clinical improvement. The oral administration of cholic acid and chenodeoxycholic acid in a dosage of 100-250 mg/24 hr, with the aim of reducing the levels of presumably toxic bile acid intermediates, may be effective as well.

Genetic Counseling

All the peroxisomal disorders, except hyperoxaluria type 1, can be diagnosed prenatally in the 1st or 2nd trimester. The tests are similar to those described for postnatal diagnosis (see Table 80-6) and use chorionic villus sampling or amniocytes. More than 300 pregnancies have been monitored, and more than 60 affected fetuses have been identified without diagnostic error. Because of the 25% recurrence risk, couples with an affected child must be advised about the availability of prenatal diagnosis. Heterozygotes can be identified in X-ALD and in those disorders in which the molecular defect has been identified (see Table 80-3).

Etiology

The key biochemical abnormality is the tissue accumulation of unbranched saturated VLCFA, with a carbon chain length of 24 or more. Excess hexacosanoic acid (C26 : 0) is the most striking and characteristic feature. This accumulation of fatty acids is caused by genetically deficient peroxisomal degradation of fatty acid. The key biochemical defect involves the impaired function of peroxisomal lignoceroyl CoA ligase, the enzyme that catalyzes the formation of the CoA derivative of VLCFA. The gene that is defective (ABCD1) codes for a peroxisomal membrane (ALDP). More than 400 distinct mutations have been identified, and most families have a mutation that is “private” (unique to that kindred) and are updated on the website, www.x-ald.nl. The gene has been mapped to chromosome Xq28. The mechanism by which the ALDP defect leads to VLCFA accumulation and the pathology of X-ALD is unknown.

Epidemiology

The minimum incidence of X-ALD in males is 1/21,000, and the combined incidence of X-ALD males and heterozygous females in the general population is estimated to be 1/17,000. All races are affected. The various phenotypes often occur in members of the same kindred.

Pathology

Characteristic lamellar cytoplasmic inclusions can be demonstrated with the electron microscope in adrenocortical cells, testicular Leydig cells, and nervous system macrophages. These inclusions probably consist of cholesterol esterified with VLCFA. They are most prominent in cells of the zona fasciculata of the adrenal cortex, which at first are distended with lipid and later atrophy.

The nervous system can display 2 types of lesions. In the severe childhood cerebral form and in the rapidly progressive adult forms, demyelination is associated with an inflammatory response manifested by the accumulation of perivascular lymphocytes that is most intense in the parieto-occipital region. In the slowly progressive adult form, adrenomyeloneuropathy (AMN), the main finding is a distal axonopathy that affects the long tracts in the spinal cord. The inflammatory response is mild or absent.

Pathogenesis

The adrenal dysfunction is probably a direct consequence of the accumulation of VLCFA. The cells in the zona fasciculata are distended with abnormal lipids. Cholesterol esterified with VLCFA is relatively resistant to adrenocorticotropic hormone (ACTH)-stimulated cholesterol ester hydrolases, and this limits the capacity to convert cholesterol to active steroids. In addition, C26 : 0 excess increases the viscosity of the plasma membrane and this may interfere with receptor and other cellular functions.

There is no correlation between the neurologic phenotype and the nature of the mutation or the severity of the biochemical defect as assessed by plasma levels of VLCFA or between the degree of adrenal involvement and nervous system involvement. The severity of the illness and rate of progression correlate with the intensity of the inflammatory response. The inflammatory response may be cytokine mediated and may involve an autoimmune response triggered in an unknown way by the excess of VLCFA. A CD1 lipid antigen has been implicated. Mitochondrial damage and oxidative stress also contribute. Approximately half of the patients do not experience the inflammatory response. A modifier gene that sets the “thermostat” for the inflammatory response is postulated.

Clinical Manifestations

There are 5 relatively distinct phenotypes, 3 of which are present in childhood with symptoms and signs. In all the phenotypes, development is usually normal in the 1st 3-4 yr of life.

In the childhood cerebral form of ALD, symptoms are first noted most commonly between the ages of 4 and 8 yr (21 mo is the earliest onset reported). The most common initial manifestations are hyperactivity, which is often mistaken for an attention deficit disorder, and worsening school performance in a child who had previously been a good student. Auditory discrimination is often impaired, although tone perception is preserved. This may be evidenced by difficulty in using the telephone and greatly impaired performance on intelligence tests in items that are presented verbally. Spatial orientation is often impaired. Other initial symptoms are disturbances of vision, ataxia, poor handwriting, seizures, and strabismus. Visual disturbances are often due to involvement of the cerebral cortex, which leads to variable and seemingly inconsistent visual capacity. Seizures occur in nearly all patients and may represent the 1st manifestation of the disease. Some patients present with increased intracranial pressure or with unilateral mass lesions. Impaired cortisol response to ACTH stimulation is present in 85% of patients, and mild hyperpigmentation is noted. In most patients with this phenotype, adrenal dysfunction is recognized only after the condition is diagnosed because of the cerebral symptoms. Cerebral childhood ALD tends to progress rapidly with increasing spasticity and paralysis, visual and hearing loss, and loss of ability to speak or swallow. The mean interval between the 1st neurologic symptom and an apparently vegetative state is 1.9 yr. Patients may continue in this apparently vegetative state for 10 yr or more.

Adolescent ALD designates patients who experience neurologic symptoms between the ages of 10 and 21 yr. The manifestations resemble those of childhood cerebral ALD except that progression is slower. About 10% of patients present acutely with status epilepticus, adrenal crisis, acute encephalopathy, or coma.

Adrenomyeloneuropathy first manifests in late adolescence or adulthood as a progressive paraparesis caused by long tract degeneration in the spinal cord. Approximately half of the patients also have involvement of the cerebral white matter.

The “Addison only” phenotype is an important and underdiagnosed condition. Of male patients with Addison disease, 25% may have the biochemical defect of ALD. Many of these patients have intact neurologic systems, whereas others have subtle neurologic signs. Many acquire adrenomyeloneuropathy in adulthood.

The term “asymptomatic ALD” is applied to persons who have the biochemical defect of ALD but are free of neurologic or endocrine disturbances. Nearly all persons with the gene defect eventually become neurologically symptomatic. A few have remained asymptomatic even in the 6th or 7th decade.

Approximately 50% of female heterozygotes acquire a syndrome that resembles adrenomyeloneuropathy but is milder and of later onset. Adrenal insufficiency is rare.

Laboratory and Radiographic Findings

The most specific and important laboratory finding is the demonstration of abnormally high levels of VLCFA in plasma, red blood cells, or cultured skin fibroblasts. The test should be performed in a laboratory that has experience with this specialized procedure. Positive results are obtained in all male patients with X-ALD and in about 85% of female carriers of X-ALD. Mutation analysis is the most reliable method for the identification of carriers.

CT And MRI

Patients with childhood cerebral or adolescent ALD show cerebral white matter lesions that are characteristic with respect to location and attenuation patterns on MRI. In 80% of patients, the lesions are symmetric and involve the periventricular white matter in the posterior parietal and occipital lobes. About 50% show location of a garland of accumulated contrast material adjacent and anterior to the posterior hypodense lesions (Fig. 80-5A). This zone corresponds to the zones of intense perivascular lymphocytic infiltration where the blood-brain barrier breaks down. In 12% of patients, the initial lesions are frontal. Unilateral lesions that produce a mass effect suggestive of a brain tumor may occur. MRI provides a clearer delineation of normal and abnormal white matter than does CT and may demonstrate abnormalities missed by CT (Fig. 80-5B).

Figure 80-5 A, Contrast enhanced CT abnormalities in adrenoleukodystrophy (ALD) with typical parieto-occipital location, showing symmetric bilateral hypodense inactive zones (Ho). The enhancing active periphery zone of hypodensity is demarcated by arrows. Compare the anterior zone of hypodensity (arrowheads) with that on the MRI in B. CC, corpus callosum. B, MRI of the same pattern and area shown by CT. MRI T2 weighted image shows a high-intensity signal of the abnormally bright parieto-occipital white matter. Subcortical involvement is better identified on MRI. Separation of active zones may be better appreciated by CT, because both inactive and active zones are seen at high-signal areas on MRI. It is assumed, however, that such major distinctions afforded by CT will also be demonstrable when IV enhancement (paramagnetic enhancement) becomes readily available. Note the hypodense involvement of CT (arrowheads and arrows in A) compared with the well-resolved lesions on MRI in B.

(From Kumar AJ, Rosenbaum WE, Naidu S, et al: Adrenoleukodystrophy: corresponding MR imaging with CT, Radiology 165:497–504, 1987.)

Diagnosis and Differential Diagnosis

The earliest manifestations of childhood cerebral ALD are difficult to distinguish from the more common attention-deficit disorders or learning disabilities. Rapid progression, signs of dementia, or difficulty in auditory discrimination suggest ALD. Even in early stages, CT or MRI may show strikingly abnormal changes. Other leukodystrophies (Chapters 592 and 605.10) or multiple sclerosis (Chapter 593.1) may mimic these radiographic findings. Definitive diagnosis depends on demonstration of VLCFA excess, which occurs only in X-ALD and the other peroxisomal disorders. The latter may be distinguished from X-ALD by their clinical presentation during the neonatal period.

Cerebral forms of ALD may present as increased intracranial pressure and unilateral mass lesions. These have been misdiagnosed as gliomas, even after brain biopsy, and several patients have received radiotherapy before the correct diagnosis was made. Measurement of VLCFA in plasma or brain biopsy specimens is the most reliable differentiating test.

Adolescent or adult cerebral ALD can be confused with psychiatric disorders, dementing disorders, or epilepsy. The 1st clue to the diagnosis of ALD may be the demonstration of white matter lesions by CT or MRI; assays of VLCFA are confirmatory.

ALD cannot be distinguished clinically from other forms of Addison disease; it is recommended that assays of VLCFA levels be performed in all male patients with Addison disease. ALD patients do not usually have antibodies to adrenal tissue in their plasma.

Complications

An avoidable complication is the occurrence of adrenal insufficiency. The most difficult neurologic problems are those related to bed rest, contracture, coma, and swallowing disturbances. Other complications involve behavioral disturbances and injuries associated with defects of spatial orientation, impaired vision and hearing, and seizures.

Treatment

Corticosteroid replacement for adrenal insufficiency or adrenocortical hypofunction is effective (Chapter 569). It may be lifesaving and increase general strength and well-being, but it does not alter the course of the neurologic disability.

Genetic Counseling and Prevention

Genetic counseling and primary and secondary prevention of X-ALD are of crucial importance. Extended family screening should be offered to all at-risk relatives of symptomatic patients; one program led to the identification of more than 250 asymptomatic affected males and 1,200 women heterozygous for X-ALD. The plasma assay permits reliable identification of affected males in whom plasma VLCFA levels are increased already on the day of birth. Identification of asymptomatic males permits institution of steroid replacement therapy when appropriate and prevents the occurrence of adrenal crisis, which may be fatal. Monitoring of brain MRI also permits identification of patients who are candidates for BMT at a stage when this procedure has the greatest chance of success. Plasma VLCFA assay is recommended in all male patients with Addison disease. X-ALD has been shown to be the cause of adrenal insufficiency in >25% of boys with Addison disease of unknown cause. Identification of women heterozygous for X-ALD is more difficult than that of affected males. Plasma VLCFA levels are normal in 15-20% of heterozygous women, and failure to note this has led to serious errors in genetic counseling. If VLCFA levels are normal both in plasma and cultured skin fibroblasts, the risk of false-negative results is reduced but not eliminated. DNA analysis permits accurate identification of carriers, provided that the mutation has been defined in a family member, and is the procedure recommended for the identification of heterozygous women. Mutation analysis is available on a service basis.

Prenatal diagnosis of affected male fetuses can be achieved by measurement of VLCFA levels in cultured amniocytes or chorionic villus cells and by mutation analysis. Whenever a new patient with X-ALD is identified, a detailed pedigree should be constructed and efforts should be made to identify all at-risk female carriers and affected males. These investigations should be accompanied by careful and sympathetic attention to social, emotional, and ethical issues during counseling.

Transport of Exogenous (Dietary) Lipids

All dietary fat with the exception of medium-chain triglycerides is efficiently carried into the circulation by way of lymphatic drainage from the intestinal mucosa. Triglyceride (TG) and CE combine with apoA and apoB-48 in the intestinal mucosa to form chylomicrons, which are carried into the peripheral circulation via the lymphatic system. HDL particles contribute apoC-II to the chylomicrons, required for the activation of lipoprotein lipase (LPL) within the capillary endothelium of adipose, heart, and skeletal muscle tissue. Free fatty acids are oxidized, resterified for storage as triglycerides, or released into the circulation bound to albumin for transport to the liver. After hydrolysis of the TG core from the chylomicron, apoC particles are recirculated back to HDL. The subsequent contribution of apoE from HDL to the remnant chylomicron facilitates binding of the particle to hepatic LDL receptors (LDL-R). Within the hepatocyte, the chylomicron remnant may be incorporated into membranes, re-secreted as lipoprotein back into the circulation, or secreted as bile acids. Normally, all dietary fat is disposed of within 8 hr after the last meal, an exception being individuals with a disorder of chylomicron metabolism. Postprandial hyperlipidemia is a risk factor for atherosclerosis. Abnormal transport of chylomicrons and their remnants may result in their absorption into the blood vessel wall as foam cells, caused by the ingestion of CE by macrophages, the earliest stage in the development of fatty streaks.

Transport of Endogenous Lipids from the Liver

The formation and secretion of VLDL from the liver and its catabolism to IDL and LDL particles describe the endogenous lipoprotein pathway. Fatty acids used in the hepatic formation of VLDL are derived primarily by uptake from the circulation. VLDL appears to be transported from the liver as rapidly as it is synthesized, and it consists of triglycerides, cholesteryl esters, phospholipids, and apoB-100. Nascent particles of VLDL secreted into the circulation combine with apoliproteins Cs and E. The size of the VLDL particle is determined by the amount of triglyceride present, progressively shrinking in size as TG is hydrolyzed by the action of LPL, yielding free fatty acids for utilization or storage in muscle and adipose tissue. Hydrolysis of about 80% of the TG present in VLDL particles produces IDL particles containing an equal amount of cholesterol and TG. The remaining remnant IDL is converted to LDL for delivery to peripheral tissues or to the liver. ApoE is attached to the remnant IDL particle to allow binding to the cell and subsequent incorporation into the lysosome. Individuals with deficiency of either apoE2 or hepatic triglyceride lipase (HTGL) accumulate IDL in the plasma.

LDL particles account for approximately 70% of the plasma cholesterol in normal individuals. LDL receptors are present on the surfaces of nearly all cells. Most LDL is taken up by the liver, and the rest is transported to peripheral tissues such as the adrenal glands and gonads for steroid synthesis. Dyslipidemia is greatly influenced by LDL-R activity. The efficiency with which VLDL is converted into LDL is also important in lipid homeostasis.

HDL and Reverse Cholesterol Transport

As hepatic secretion of lipid particles into the bile is the only mechanism by which cholesterol can be removed from the body, transport of excess cholesterol from the peripheral cells is a vitally important function of HDL. HDL is heavily laden with apoA-I containing lipoproteins, which is nonatherogenic in contrast to B lipoproteins. Cholesterol-poor nascent HDL particles secreted by the liver and small intestine are esterified to more mature HDL-2 particles by the action of the enzyme lecithin-cholesterol acyltransferase (LCAT), which facilitates movement of chylomicrons and VLDL into the HDL core. HDL-2 may transfer cholesteryl esters back to apoB lipoproteins mediated by cholesteryl ester transfer protein (CETP), or the cholesterol-rich particle may be removed from the plasma by endocytosis, completing reverse cholesterol transport. Low HDL may be genetic (deficiency of apoA-I) or secondary to increased plasma TG.

Hypercholesterolemia (Table 80-8)

Homozygous FH

FH homozygotes inherit 2 abnormal LDL receptor genes, resulting in markedly elevated plasma cholesterol levels ranging between 500 and 1,200 mg/dL. Triglyceride levels are normal to mildly elevated, and HDL levels may be slightly decreased. The condition occurs in 1/1,000,000 persons. Receptor negative patients have <2% normal LDL receptor activity, whereas those who are receptor defective may have as much 25% normal activity and a better prognosis.

The prognosis is poor regardless of the specific LDL receptor aberration. Severe atherosclerosis involving the aortic root and coronary arteries is present by early- to mid-childhood. These children usually present with xanthomas, which may cause thickening of the Achilles tendon or extensor tendons of the hands, or cutaneous lesions on the hands, elbows, knees, or buttocks (Figs. 80-10, 80-11, and 80-12). Corneal arcus may be present. Family history is informative because premature heart disease is strongly prevalent among relatives of both parents. The diagnosis may be confirmed by measuring LDL receptor activity in cultured skin fibroblasts. Phenotypic expression of the disease may also be assessed by measuring receptor activity on the surface of lymphocytes by using cell sorting techniques.

Figure 80-10 Achilles tendon xanthoma (heterozygous familial hypercholesterolemia).

(From Durrington P: Dyslipidaemia, Lancet 362:717–731, 2003.)

Figure 80-11 Striate palmar xanthomata.

(From Durrington P: Dyslipidaemia, Lancet 362:717–731, 2003.)

Figure 80-12 Eruptive xanthomata on extensor surface of forearm.

(From Durrington P: Dyslipidaemia, Lancet 362:717–731, 2003.)

Untreated homozygous patients rarely survive to adulthood. Symptoms of coronary insufficiency may occur; sudden death is common. LDL apheresis to selectively remove LDL particles from the circulation is recommended for many children and has been shown to slow the progression of atherosclerosis. Liver transplantation has also been successful in decreasing LDL cholesterol levels, but complications related to immunosuppression are common. HMG CoA reductase inhibitors are often effective depending on the specific class of LDL receptor defect present. Combination therapy with ezetimibe, selectively blocking cholesterol adsorption in the gut, usually results in further modest decline in LDL levels; it has largely replaced the use of bile acid sequestrants.

Hypercholesterolemia with Hypertriglyceridemia

Hypertriglyceridemias

The familial disorders of triglyceride-rich lipoproteins include both common and rare variants of the Frederickson classification system. These include chylomicronemia (type I), familial hypertriglyceridemia (type IV), and the more severe combined hypertriglyceridemia and chylomicronemia (type V). Hepatic lipase (HL) deficiency also results in a similar combined hyperlipidemia.

Familial Chylomicronemia (Type I Hyperlipidemia)

This rare single gene defect, like familial hypercholesterolemia, is due to mutations affecting clearance of apoB-containing lipoproteins. Deficiency or absence of lipoprotein lipase (LPL) or its cofactor apoC-II, which facilitates lipolysis by LPL, causes severe elevation of triglyceride rich plasma chylomicrons. HDL cholesterol levels are decreased. As clearance of these particles is markedly delayed, the plasma is noted to have a turbid appearance even after prolonged fasting (Fig. 80-13). Chylomicronemia caused by LPL deficiency is associated with modest elevation in triglycerides, whereas this is not the case when the cause is deficient or absent apoC-II. Both are autosomal recessive conditions with a frequency of approximately 1/1,000,000. The disease usually presents during childhood with acute pancreatitis. Eruptive xanthomas on the arms, knees, and buttocks may be present, and there may be hepatosplenomegaly. The diagnosis is established by assaying triglyceride lipolytic activity. Treatment of chylomicronemia is by vigorous dietary fat restriction supplemented by fat-soluble vitamins. Medium-chain triglycerides that are adsorbed into the portal venous system may augment total fat intake, and administration of fish oils may also be beneficial.

Figure 80-13 Milky plasma from patient with acute abdominal pain.

(From Durrington P: Dyslipidaemia, Lancet 362:717–731, 2003.)

Familial Hypertriglyceridemia (FHTG, Type IV Hyperlipidemia)

FHTG is an autosomal dominant disorder of unknown etiology, which occurs in about 1/500 individuals. It is characterized by elevation of plasma triglycerides >90th percentile (250-1,000 mg/dL range), often accompanied by slight elevation in plasma cholesterol and low HDL. FHTG does not usually manifest until adulthood, though it is expressed in about 20% of affected children. In contrast to FCHL, FHTG is not thought to be highly atherogenic. It is most likely caused by defective breakdown of VLDL, or less often by overproduction of this class of lipoproteins.

The diagnosis should include the presence of at least one first-degree relative with hypertriglyceridemia. FHTG should be distinguished from FCHL and FDBL, as the latter require more vigorous treatment to prevent coronary or peripheral vascular disease. The differentiation is usually possible on clinical grounds, in that lower LDL cholesterol levels accompany FHTG, but measurement of normal apoB levels in FHTG may be helpful in ambiguous situations.

A more severe hypertriglyceridemia characterized by increased levels of chylomicrons as well as VLDL particles (Frederickson type V) may occasionally be encountered. Triglyceride levels are often >1,000 mg/dL. The disease is rarely seen in children. In contrast to chylomicronemia (Frederickson type I), LPL or apoC-II deficiency is not present. These patients often develop eruptive xanthomas in adulthood, whereas type IV hypertriglyceridemia individuals do not. Acute pancreatitis may be the presenting illness. As with other hypertriglyceridemias, excessive alcohol consumption and estrogen therapy can exacerbate the disease.

Secondary causes of transient hypertriglyceridemia should be ruled out before making a diagnosis of FHTG. A diet high in simple sugars and carbohydrates, or excessive alcohol consumption as well as estrogen therapy may exacerbate hypertriglyceridemia. Adolescents and adults should be questioned about excessive consumption of soda and other sweetened drinks, as it is common to encounter people who drink supersized drinks or multiple 12 oz cans of sweetened drinks daily. Cessation of this practice often results in dramatic fall in triglyceride levels as well as weight among those who are obese. HDL cholesterol levels will tend to rise as BMI stabilizes.

Pediatric diseases associated with hyperlipidemia include hypothyroidism, nephrotic syndrome, biliary atresia, glycogen storage disease, Niemann-Pick disease, Tay-Sachs disease, systemic lupus erythematosus, hepatitis, and anorexia nervosa (Table 80-10). Certain medications exacerbate hyperlipidemia, including isotretinoin (Accutane), thiazide diuretics, oral contraceptives, steroids, β-blockers, immunosuppressants, and protease inhibitors used in the treatment of HIV.

Treatment of hypertriglyceridemia in children rarely requires medication unless levels >1,000 mg/dL persist after dietary restriction of fats, sugars, and carbohydrates, accompanied by increased physical activity. In such cases, the aim is to prevent episodes of pancreatitis. The common use of fibrates (fenofibric acid) and niacin in adults with hypertriglyceridemia is not recommended in children. HMG CoA reductase inhibitors are reasonably effective in lowering triglyceride levels, and there is considerably more experience documenting the safety and efficacy of this class of lipid lowering medications in children.

Disorders of HDL Metabolism

Conditions Associated with Low Cholesterol

Disorders of apoB-containing lipoproteins and intracellular cholesterol metabolism are associated with low plasma cholesterol.

Disorders of Intracellular Cholesterol Metabolism

Lipoprotein Patterns in Children and Adolescents

Table 80-13, derived primarily from the Lipid Research Clinics Population Studies, shows the distribution of lipoprotein levels in American youth at various ages. Total plasma cholesterol rises rapidly from a mean of 68 mg/dL at birth to a level approximately twice that by the end of the neonatal period. A very gradual rise in total cholesterol level occurs until puberty, at which time the mean level reaches 160 mg/dL. Total cholesterol falls transiently during puberty, in males due to a small decrease in HDL cholesterol, and in females secondary to a slight fall in LDL cholesterol. Blood cholesterol levels track reasonably well as individuals age. High blood cholesterol tends to aggregate in families, a reflection of genetic and environmental influences.

Acceptable total cholesterol among children and adolescents is <170 mg/dL; borderline is 170-199 mg/dL; and high >200 mg/dL. Acceptable LDL cholesterol is <110 mg/dL; borderline 110-129 mg/dL; and high >130 mg/dL. HDL cholesterol should be >40 mg/dL.

Blood Cholesterol Screening

Guidelines for cholesterol measurement in children were updated by the American Academy of Pediatricians in 2008. A targeted approach to cholesterol screening for children remains in effect:

Whereas Lipid Research Clinics data referenced in the original NCEP guidelines predicted that selective blood cholesterol screening of children would apply to a quarter of American youth, more recent population-based studies such as the National Health and Nutrition Examination Surveys (NHANES), predict that nearly half of children fulfill criteria for screening. The increase is likely due to the worrisome rise in youth obesity.

Being overweight confers special risk of CVD because of the strong association with the insulin resistance syndrome (metabolic syndrome). Though there is no single definition of metabolic syndrome defined for youth, it is likely that half of all severely obese children are insulin resistant. Among a large cohort of 5th grade school children who had comprehensive screening of CVD risk factors conducted by the Coronary Artery Risk Detection in Appalachian Communities (CARDIAC) Project, 49% of those with the hyperpigmented rash, acanthosis nigricans, had 3 or more risk factors for the metabolic syndrome, including insulin resistance, hypertension, and abnormal lipid levels (triglycerides >150 mg/dL; HDL-C <40 mg/dL) (Chapter 44).

Reliance on family history of premature heart disease, or known parental hypercholesterolemia greater than 240 mg/dL, is considered by some to be too insensitive and difficult to apply. Indeed, over half of children with hypercholesterolemia will be missed by the targeted approach to screening. It was predicted that those with severe genetic predisposition to premature heart disease, such as familial hypercholesterolemia (FH), would be identified by application of selective criteria for screening. However, data from the CARDIAC Project found that universal screening of youth identified just as many children with severe dyslipidemia who did not fulfill criteria as those who did.

It is possible that, in the future, guidelines regarding blood cholesterol screening in childhood may be liberalized further to include all children because of newer information and changing circumstances. It is well documented that many parents are unaware of their own cholesterol levels, making the criteria problematic. The evidence that cholesterol screening of children may cause psychological harm to the child is less than compelling, as is the concern that universal screening might lead to overuse of cholesterol lowering medication by practitioners. Finally, the epidemic of childhood obesity, approaching 50% in some disadvantaged high-risk populations, supports broader screening to identify those with the metabolic syndrome. A fasting lipid profile is recommended in order to detect hypertriglyceridemia and/or low HDL cholesterol often found in this pre-diabetic condition. The AAP guidelines recommend that blood cholesterol testing occur sometime after age 2 yr but no later than age 10 yr.

Risk Assessment and Treatment of Hyperlipidemia

The National Cholesterol Education Program (NCEP) recommends a population-based approach toward healthy lifestyle applicable to all children, and an individualized approach directed at those children at high risk (Fig. 80-14). The important focus on maintenance of a healthy lifestyle rather than aggressive weight reduction is recommended by the AAP.

Figure 80-14 Flow chart of classification, education, and follow-up of children based on low-density lipoprotein (LDL) cholesterol levels. CVD, cardiovascular disease; HDL, high-density lipoprotein.

(From Williams CL, Hagman LL, Daniels SR, et al: Cardiovascular health in childhood, Circulation 106:143–160, 2002.)

The AHA Step I diet, with updated dietary recommendations based on new U.S. Department of Agriculture guidelines, is still applicable to most children older than 2 yr:

Persistence of an elevated LDL cholesterol >130 mg/dL indicates the need for more comprehensive evaluation and lifestyle modification. History and physical examination and additional laboratory tests aimed at ruling out secondary causes of hyperlipidemia (see Table 80-10) should be performed. Other family members should have blood cholesterol screening. If the LDL cholesterol level does not achieve the minimal goal of <130 mg/dl the AHA Step II diet should be recommended. This diet allows the same average fat consumption of no more than 30% of total calories, but restricts saturated fats to <7-8% of total calories and cholesterol intake to <200 mg per day. Follow-up lab tests, measurement of height and weight for the calculation of body mass index (BMI), and dietary history should be scheduled at 3-6 mo intervals.

The 2004 revision of the NCEP Adult Treatment Panel III raised the minimal acceptable level of HDL cholesterol from 35 mg/dL to 40 mg/dL. If low HDL cholesterol is present, counseling directed toward weight management, tobacco avoidance, and daily physical activity should be provided.

No restriction of fat or cholesterol is recommended for infants less than 2 yr of age because of rapid growth and development, especially involving the central nervous system. Overfeeding should be discouraged as increasingly infants and toddlers are exceeding weight for height standards published by the U.S. Centers for Disease Control and Prevention. The myth that “a bigger baby is a healthier baby” persists.

The safety of a heart healthy diet among children 3-19 yr has been established by the National Health and Nutrition Examination Survey (NHANES III). Despite a decrease in the average level of fat intake from the second survey there was no evidence of poor growth or compromised nutritional status. The prospective, well-controlled Dietary Intervention Study in Children (DISC) compared children consuming a low-fat Step I diet with subjects consuming the “usual” diet containing 33-34% of calories as fat, and 13% as saturated fat. No differences between groups with regard to height, weight, micronutrients, or psychological well-being were observed. Children on the low-fat diet had lower LDL cholesterol levels.

The Committee on Nutrition of the American Academy of Pediatrics suggests that as children older than 2 yr consume fewer calories from fat, they should eat more grain product, fruits, vegetables, low-fat milk products, beans, lean meats, poultry, fish, and other protein-rich foods. Low-carbohydrate, high-fat diets have become popular over the last decade as a means to achieve weight reduction. Unlimited fat intake is strongly discouraged, as is unrestricted sugar and carbohydrate consumption. Carbohydrates should comprise about 55% of calories, achieved by consuming complex carbohydrates such as pasta, some vegetables, potatoes, legumes, and whole grain cereals and bread.

Protein should provide about 15-20% of calories, and should contain all of the essential fatty acids. The exclusion of meat or fish from the diet necessitates a healthy mixture of plant proteins in order to achieve appropriate nutrient balance. Thus foods high in fiber, such as fruits, vegetables, and whole grains are recommended because of their excellent nutrient content as components of a low saturated fat eating pattern. Children should eat 5 or more fruits and vegetables daily. Canned and frozen vegetables and soups should be selected for low sodium content.

If followed, these dietary recommendations provide adequate calories for optimal growth and development without promoting obesity. Compliance on the part of children and their caregivers is challenging in today’s society. Children learn eating habits from their parents. Successful adoption of a healthier diet is far more likely to occur if meals and snacks in the home are applicable to the entire family rather than an individual child. A regular time for meals together as a family is desirable. Grandparents and other nonparental caregivers sometimes need to be reminded not to indulge the child who is on a restricted diet. The rise in obesity is prompting some school districts to restrict sweetened drink availability, and offer more nutritious cafeteria selections.

The rise in sedentary activity among our youth is contributing to the increase in obesity nationwide, in turn increasing the prevalence of other risk factors such as dyslipidemia and hypertension. The National Association for Sport and Physical Education (NASPE) recommends that children should accumulate at least 60 min of age-appropriate physical activity on most days of the week. Extended periods (2 hr or more) of daytime inactivity are discouraged, as is more than 2 hr of television and other forms of screen time.

GM₁ Gangliosidosis

GM₁ gangliosidosis most frequently presents in early infancy, but has been described in patients with a juvenile and onset subtypes. Inherited as an autosomal recessive trait, each subtype results from a different gene mutation that leads to the deficient activity of β-galactosidase, a lysosomal enzyme encoded by a gene on chromosome 3 (3p21.33). Although the disorder is characterized by the pathologic accumulation of GM₁ gangliosides in the lysosomes of both neural and visceral cells, GM₁ ganglioside accumulation is most marked in the brain. In addition, keratan sulfate, a mucopolysaccharide, accumulates in liver and is excreted in the urine of patients with GM₁ gangliosidosis. The β-galactosidase gene has been isolated and sequenced; mutations causing the disease subtypes have been identified.

The clinical manifestations of the infantile form of GM₁ gangliosidosis may be evident in the newborn as hepatosplenomegaly, edema, and skin eruptions (angiokeratoma). It most frequently presents in the first 6 mo of life with developmental delay followed by progressive psychomotor retardation and the onset of tonic-clonic seizures. A typical facies is characterized by low-set ears, frontal bossing, a depressed nasal bridge, and an abnormally long philtrum. Up to 50% of patients have a macular cherry red spot. Hepatosplenomegaly and skeletal abnormalities similar to those of the mucopolysaccharidoses, including anterior beaking of the vertebrae, enlargement of the sella turcica, and thickening of the calvarium, are present. By the end of the first year of life, most patients are blind and deaf, with severe neurologic impairment characterized by decerebrate rigidity. Death usually occurs by 3-4 yr of age. The juvenile-onset form of GM₁ gangliosidosis is clinically distinct, with a variable age at onset. Affected patients present primarily with neurologic symptoms including ataxia, dysarthria, mental retardation, and spasticity. Deterioration is slow; patients may survive through the 4th decade of life. These patients lack the visceral involvement, facial abnormalities, and skeletal features seen in type 1 disease. Adult-onset patients have been described who present with gait and speech abnormalities, dystonia and mild skeletal abnormalities. There is no specific treatment for either form of GM₁ gangliosidosis.

The diagnosis of GM₁ gangliosidosis should be suspected in infants with typical clinical features and is confirmed by the demonstration of the deficiency of β-galactosidase activity in peripheral leukocytes. Other disorders that share some of the features of the GM₁ gangliosidoses include Hurler disease (mucopolysaccharidosis type I), I-cell disease, and Niemann-Pick disease (NPD) type A, which can each be distinguished by the demonstration of their specific enzymatic deficiencies. Carriers of the disorder are detected by the measurement of the enzymatic activity in peripheral leukocytes or by identifying the specific gene mutations; prenatal diagnosis is accomplished by determination of the enzymatic activity in cultured amniocytes or chorionic villi. Currently only supportive therapy is available for patients with GM₁ gangliosidosis.

The GM₂ Gangliosidoses

The GM₂ gangliosidoses include Tay-Sachs disease and Sandhoff disease; each results from the deficiency of β-hexosaminidase activity and the lysosomal accumulation of GM₂ gangliosides, particularly in the central nervous system. Both disorders have been classified into infantile-, juvenile-, and adult-onset forms based on the age at onset and clinical features. β-Hexosaminidase occurs as 2 isozymes: β-hexosaminidase A, which is composed of 1 α and 1 β subunit, and β-hexosaminidase B, which has 2 β subunits. β-Hexosaminidase A deficiency results from mutations in the α subunit and causes Tay-Sachs disease, whereas mutations in the β-subunit gene result in the deficiency of both β-hexosaminidases A and B and cause Sandhoff disease. Both are autosomal recessive traits, with Tay-Sachs disease having a predilection for the Ashkenazi Jewish population, where the carrier frequency is about 1/25.

More than 50 mutations have been identified; most are associated with the infantile forms of disease. Three mutations account for >98% of mutant alleles among Ashkenazi Jewish carriers of Tay-Sachs disease, including one allele associated with the adult-onset form. Mutations that cause the subacute or chronic forms result in enzyme proteins with residual enzymatic activities, the levels of which correlate with the severity of the disease.

Patients with the infantile form of Tay-Sachs disease have clinical manifestations in infancy including loss of motor skills, increased startle reaction, and macular pallor and retinal cherry red spots (see Table 80-16). Affected infants usually develop normally until 4-5 mo of age when decreased eye contact and an exaggerated startle response to noise (hyperacusis) are noted. Macrocephaly, not associated with hydrocephalus, may develop. In the 2nd yr of life, seizures develop which may be refractory to anticonvulsant therapy. Neurodegeneration is relentless, with death occurring by the age of 4 or 5 yr. The juvenile and later-onset forms initially present with ataxia and dysarthria and may not be associated with a macular cherry red spot.

The clinical manifestations of Sandhoff disease are similar to those for Tay-Sachs disease. Infants with Sandhoff disease have hepatosplenomegaly, cardiac involvement, and mild bony abnormalities. The juvenile form of this disorder presents as ataxia, dysarthria, and mental deterioration, but without visceral enlargement or a macular cherry red spot. There is no treatment available for Tay-Sachs disease or Sandhoff disease, although experimental approaches are being evaluated.

The diagnosis of infantile Tay-Sachs disease and Sandhoff disease is usually suspected in an infant with neurologic features and a cherry red spot. Definitive diagnosis is made by determination of β-hexosaminidase A and B activities in peripheral leukocytes. The two disorders are distinguished by the enzymatic assay, because in Tay-Sachs disease only the β-hexosaminidase A isozyme is deficient, whereas in Sandhoff disease both the β-hexosaminidase A and B isozymes are deficient. Future at-risk pregnancies for both disorders can be prenatally diagnosed by determining the enzyme levels in fetal cells obtained by amniocentesis or chorionic villus sampling. Identification of carriers in families is also possible by β-hexosaminidases A and B determination. Indeed, for Tay-Sachs disease, carrier screening of all couples in which at least one member is of Ashkenazi Jewish descent is recommended before the initiation of pregnancy to identify couples at risk. These studies can be conducted by the determination of the level of β-hexosaminidase A activity in peripheral leukocytes or plasma. Molecular studies to identify the exact molecular defect in enzymatically identified carriers should also be performed to permit more specific identification of carriers in the family and to allow prenatal diagnosis in at-risk couples by both enzymatic and genotype determinations. The incidence of Tay-Sachs disease has been markedly reduced since the introduction of carrier screening programs in the Ashkenazi Jewish population. Newborn screening may be possible by measuring specific glycosphingolipid markers, or the relevant enzymatic activities in dried blood spots.

Gaucher Disease

This disease is a multisystemic lipidosis characterized by hematologic abnormalities, organomegaly, and skeletal involvement, the latter usually manifesting as bone pain and pathologic fractures (see Table 80-16). It is the most common lysosomal storage disease and the most prevalent genetic defect among Ashkenazi Jews. There are 3 clinical subtypes delineated by the absence or presence and progression of neurologic manifestations: type 1 or the adult, non-neuronopathic form; type 2, the infantile or acute neuronopathic form; and type 3, the juvenile or subacute neuronopathic form. All are autosomal recessive traits. Type 1, which accounts for 99% of cases, has a striking predilection for Ashkenazi Jews, with an incidence of about 1/1,000 and a carrier frequency of about 1/18.

Gaucher disease results from the deficient activity of the lysosomal hydrolase, acid β-glucosidase, which is encoded by a gene located on chromosome 1q21-q31. The enzymatic defect results in the accumulation of undegraded glycolipid substrates, particularly glucosylceramide, in cells of the reticuloendothelial system. This progressive deposition results in infiltration of the bone marrow, progressive hepatosplenomegaly, and skeletal complications. Four mutations—N370S, L444P, 84insG, and IVS2—account for about 95% of mutant alleles among Ashkenazi Jewish patients, permitting screening for this disorder in this population. Genotype-phenotype correlations have been noted, providing the molecular basis for the clinical heterogeneity seen in Gaucher disease type 1. Patients who are homozygous for the N370S mutation tend to have later onset, with a more indolent course than patients with one copy of N370S and another common allele.

Clinical manifestations of type 1 Gaucher disease have a variable age at onset, from early childhood to late adulthood, with most symptomatic patients presenting by adolescence. At presentation, patients may have bruising from thrombocytopenia, chronic fatigue secondary to anemia, hepatomegaly with or without elevated liver function test results, splenomegaly, and bone pain. Occasional patients have pulmonary involvement at the time of presentation. Patients presenting in the first decade frequently are not Jewish and have growth retardation and a more malignant course. Other patients may be discovered fortuitously during evaluation for other conditions or as part of routine examinations; these patients may have a milder or even a benign course. In symptomatic patients, splenomegaly is progressive and can become massive. Most patients develop radiologic evidence of skeletal involvement, including an Erlenmeyer flask deformity of the distal femur. Clinically apparent bony involvement, which occurs in most patients, can present as bone pain, a pseudo-osteomyelitis pattern or pathologic fractures. Lytic lesions can develop in the long bones, including the femur, ribs, and pelvis; osteosclerosis may be evident at an early age. Bone crises with severe pain and swelling can occur. Bleeding secondary to thrombocytopenia may manifest as epistaxis or bruising and is frequently overlooked until other symptoms become apparent. With the exception of the severely growth-retarded child, who may experience developmental delay secondary to the effects of chronic disease, development and intelligence are normal.

The pathologic hallmark of Gaucher disease is the Gaucher cell in the reticuloendothelial system, particularly in the bone marrow (Fig. 80-18). These cells, which are 20-100 µm in diameter, have a characteristic wrinkled paper appearance resulting from the presence of intracytoplasmic substrate inclusions. The cytoplasm of the Gaucher cell reacts strongly positive with the periodic acid–Schiff stain. The presence of this cell in bone marrow and tissue specimens is highly suggestive of Gaucher disease, although it also may be found in patients with granulocytic leukemia and myeloma.

Figure 80-18 Cells from the spleen of a patient with Gaucher disease. A characteristic spleen cell is shown engorged with glucocerebroside.

Gaucher disease type 2 is much less common and does not have an ethnic predilection. It is characterized by a rapid neurodegenerative course with extensive visceral involvement and death within the first years of life. It presents in infancy with increased tone, strabismus, and organomegaly. Failure to thrive and stridor caused by laryngospasm are typical. After a several-year period of psychomotor regression, death occurs secondary to respiratory compromise. Gaucher disease type 3 presents as clinical manifestations that are intermediate to those seen in types 1 and 2, with presentation in childhood and death by age 10-15 yr. It has a predilection for the Swedish Norrbottnian population, among which the incidence is about 1/50,000. Neurologic involvement is present. Type 3 disease is further classified as types 3a and 3b based on the extent of neurologic involvement and whether there is progressive myotonia and dementia (type 3a) or isolated supranuclear gaze palsy (type 3b).

Gaucher disease should be considered in the differential diagnosis of patients with unexplained organomegaly, who bruise easily, have bone pain, or have a combination of these conditions. Bone marrow examination usually reveals the presence of Gaucher cells. All suspected diagnoses should be confirmed by determination of the acid β-glucosidase activity in isolated leukocytes or cultured fibroblasts. In Ashkenazi Jewish individuals the identification of carriers can be achieved best by molecular testing for the common mutations. Testing should be offered to all family members, keeping in mind that heterogeneity, even among members of the same kindred, can be so great that nonsymptomatic affected individuals may be diagnosed. Prenatal diagnosis is available by determination of enzyme activity and/or the specific family mutations in chorionic villi or cultured amniotic fluid cells.

Treatment of patients with Gaucher disease type 1 includes enzyme replacement therapy, with recombinant acid β-glucosidase (imiglucerase). Most extraskeletal symptoms (organomegaly, hematologic indices) are reversed by enzyme (60 IU/kg) administered by intravenous infusion every other week. Monthly maintenance enzyme replacement improves bone structure, decreases bone pain, and induces compensatory growth in affected children. A small number of patients have undergone bone marrow transplantation, which is curative but results in significant morbidity and mortality from the procedure, making the selection of appropriate candidates limited. Although enzyme replacement does not alter the neurologic progression of patients with Gaucher disease types 2 and 3, it has been used in selected patients as a palliative measure, particularly in type 3 patients with severe visceral involvement. Alternative treatments, including the use of agents designed to decrease the synthesis of glucosylceramide by chemical inhibition of glucosylceramide synthase, are available for patients who cannot be treated by enzyme replacement.

Neimann-Pick Disease (NPD)

The original description of NPD was what is now known as type A NPD, a fatal disorder of infancy characterized by failure to thrive, hepatosplenomegaly, and a rapidly progressive neurodegenerative course that leads to death by 2-3 yr of age. Type B disease is a non-neuronopathic form observed in children and adults. Type C disease is a neuronopathic form that results from defective cholesterol transport. All subtypes are inherited as autosomal recessive traits and display variable clinical features (see Table 80-16).

NPD types A and B result from the deficient activity of acid sphingomyelinase, a lysosomal enzyme encoded by a gene on chromosome 11 (11p15.1-p15.4). The enzymatic defect results in the pathologic accumulation of sphingomyelin, a ceramide phospholipid, and other lipids in the monocyte-macrophage system the primary pathologic site. The progressive deposition of sphingomyelin in the central nervous system results in the neurodegenerative course seen in type A, and in non-neural tissue in the systemic disease manifestations of type B, including progressive lung disease in some patients. A variety of mutations in the acid sphingomyelinase gene that cause types A and B NPD have been identified.

The clinical manifestations and course of type A NPD is uniform and is characterized by a normal appearance at birth. Hepatosplenomegaly, moderate lymphadenopathy, and psychomotor retardation are evident by 6 mo of age, followed by neurodevelopmental regression and death by 3 yr. With advancing age, the loss of motor function and the deterioration of intellectual capabilities are progressively debilitating; and in later stages, spasticity and rigidity are evident. Affected infants lose contact with their environment. In contrast to the stereotyped type A phenotype, the clinical presentation and course of patients with type B disease are more variable. Most are diagnosed in infancy or childhood when enlargement of the liver or spleen, or both, is detected during a routine physical examination. At diagnosis, type B NPD patients usually have evidence of mild pulmonary involvement, usually detected as a diffuse reticular or finely nodular infiltration on the chest radiograph. Pulmonary symptoms usually present in adults. In most patients, hepatosplenomegaly is particularly prominent in childhood, but with increasing linear growth, the abdominal protuberance decreases and becomes less conspicuous. In mildly affected patients, the splenomegaly may not be noted until adulthood, and there may be minimal disease manifestations.

In some type B patients, decreased pulmonary diffusion caused by alveolar infiltration becomes evident in late childhood or early adulthood and progresses with age. Severely affected individuals may experience significant pulmonary compromise by 15-20 yr of age. Such patients have low PO₂ values and dyspnea on exertion. Life-threatening bronchopneumonias may occur, and cor pulmonale has been described. Severely affected patients may have liver involvement leading to life-threatening cirrhosis, portal hypertension, and ascites. Clinically significant pancytopenia due to secondary hypersplenism may require partial or complete splenectomy; this should be avoided if possible because splenectomy frequently causes progression of pulmonary disease, which can be life-threatening. In general, type B patients do not have neurologic involvement and have a normal IQ. Some patients with type B disease have cherry red maculae or haloes and subtle neurologic symptoms (peripheral neuropathy).

Type C NPD patients often present with prolonged neonatal jaundice, appear normal for 1-2 yr, and then experience a slowly progressive and variable neurodegenerative course. Their hepatosplenomegaly is less severe than that of patients with types A or B NPD, and they may survive into adulthood. The underlying biochemical defect in type C patients is an abnormality in cholesterol transport, leading to the accumulation of sphingomyelin and cholesterol in their lysosomes and a secondary partial reduction in acid sphingomyelinase activity (Chapter 80.3).

In type B NPD patients, splenomegaly is usually the first manifestation detected. The splenic enlargement is noted in early childhood; in very mild disease, the enlargement may be subtle and detection may be delayed until adolescence or adulthood. The presence of the characteristic NPD cells in bone marrow aspirates supports the diagnosis of type B NPD. Patients with type C NPD, however, also have extensive infiltration of NPD cells in the bone marrow and, thus, all suspected cases should be evaluated enzymatically to confirm the clinical diagnosis by measuring the acid sphingomyelinase activity level in peripheral leukocytes, cultured fibroblasts, or lymphoblasts, or a combination of these cells. Patients with types A and B NPD have markedly decreased levels (1-10%), whereas patients with type C NPD have normal or somewhat decreased acid sphingomyelinase activities. The enzymatic identification of NPD carriers is problematic. In families in which the specific molecular lesion has been identified, however, family members can be accurately tested for heterozygote status by DNA analysis. Prenatal diagnosis of types A and B NPD can be made reliably by the measurement of acid sphingomyelinase activity in cultured amniocytes or chorionic villi; molecular analysis of fetal cells can provide the specific diagnosis or serve as a confirmatory test. The clinical diagnosis of type C NPD can be supported by the demonstration of filipin stain positivity in cultured fibroblasts and/or by identifying a specific mutation in the NPC gene.

Currently there is no specific treatment for NPD. Orthotopic liver transplantation in an infant with type A disease and amniotic cell transplantation in several type B NPD patients have been attempted with little or no success. Bone marrow transplantation in a small number of type B NPD patients has been shown to be successful in reducing the spleen and liver volumes, the sphingomyelin content of the liver, the number of Niemann-Pick cells in the marrow, and radiologically detected infiltration of the lungs. In one patient, liver biopsies taken up to 33 mo post transplantation showed only a moderate reduction in stored sphingomyelin. To date, lung transplantation has not been performed in any severely compromised patient with type B disease, although two patients who underwent whole lung lavages with variable results have been reported. A phase I trial of enzyme replacement therapy for type B NPD has been completed and further clinical studies to evaluate effectiveness of this approach are planned. Clinical trials of miglustat (Acetelion, Basel, Switzerland) have been performed and the drug has been approved in Europe for the treatment of type C disease. Treatment of type A disease by bone marrow transplantation has not been successful presumably due to the severe neurologic involvement.

Fabry Disease

This condition is an X-linked inborn error of glycosphingolipid metabolism characterized by angiokeratomas (telangiectatic skin lesions), hypohidrosis, corneal and lenticular opacities, acroparesthesias, and vascular disease of the kidney, heart, and/or brain (see Table 80-16). The classic phenotype is due to the deficient activity of the enzyme α-galactosidase A and has an estimated prevalence of approximately 1/50,000 males. Later-onset affected males with residual α-galactosidase A activity may present with cardiac and/or renal disease including hypertrophic cardiomyopathy and renal failure and is more prevalent than the classic phenotype. Heterozygous females for the classic phenotype can be asymptomatic or as severely affected as the males, the variability due to random X-inactivation. The disease results from mutations in the α-galactosidase A gene located on the long arm of the X chromosome (Xq22). The enzymatic defect leads to the systemic accumulation of neutral glycosphingolipids, primarily globotriaosylceramide, particularly in the plasma and lysosomes of vascular endothelial and smooth muscle cells. The progressive vascular glycosphingolipid deposition in classically affected males results in ischemia and infarction, leading to the major disease manifestations. The cDNA and genomic sequences encoding α-galactosidase A have identified more than 500 different mutations in the α-galactosidase A gene that are responsible for this lysosomal storage disease, including amino acid substitutions, gene rearrangements, and mRNA splicing defects.

Affected males with the classic phenotype have the skin lesions, acroparesthesias, hypohidrosis, and ocular changes, whereas males with the later onset phenotypes lack these findings and present with cardiac and/or renal disease in adulthood. The classic angiokeratomas usually occur in childhood and may lead to early diagnosis (Fig. 80-19). They increase in size and number with age and range from barely visible to several mm in diameter. The lesions are punctate, dark red to blue-black, and flat or slightly raised. They do not blanch with pressure, and the larger ones may show slight hyperkeratosis. Characteristically, the lesions are most dense between the umbilicus and knees, in the “bathing trunk area,” but may occur anywhere, including the oral mucosa. The hips, thighs, buttocks, umbilicus, lower abdomen, scrotum, and glans penis are common sites, and there is a tendency toward symmetry. Variants without skin lesions have been described. Sweating is usually decreased or absent. Corneal opacities and characteristic lenticular lesions, observed under slit-lamp examination, are present in affected males as well as in about 90% of heterozygotes. Conjunctival and retinal vascular tortuosity is common and results from the systemic vascular involvement.

Figure 80-19 Typical angiokeratomas. Angiokeratomas are quite large and easily recognizable, but if only a few lesions exist or they are restricted only to the genitalia or umbilical regions, they can be easily missed.

(From Zarate VA, Hopkin RJ: Fabray’s disease, Lancet 372:1427, 2008.)

Pain is the most debilitating symptom in childhood and adolescence. Fabry crises, lasting from minutes to several days, consist of agonizing, burning pain in the hands, feet, and proximal extremities and are usually associated with exercise, fatigue, fever, or a combination of these factors. These painful acroparesthesias usually become less frequent in the 3rd and 4th decades of life, although in some men, they may become more frequent and severe. Attacks of abdominal or flank pain may simulate appendicitis or renal colic.

The major morbid symptoms result from the progressive involvement of the vascular system. Early in the course of the disease, casts, red cells, and lipid inclusions with characteristic birefringent “Maltese crosses” appear in the urinary sediment. Proteinuria, isosthenuria, and gradual deterioration of renal function and development of azotemia occur in the 2nd through 4th decades. Cardiovascular findings may include hypertension, left ventricular hypertrophy, anginal chest pain, myocardial ischemia or infarction, and heart failure. Mitral insufficiency is the most common valvular lesion. Abnormal electrocardiographic and echocardiographic findings are common. Cerebrovascular manifestations result from multifocal small vessel involvement. Other features may include chronic bronchitis and dyspnea, lymphedema of the legs without hypoproteinemia, episodic diarrhea, osteoporosis, retarded growth, and delayed puberty. Death most often results from uremia or vascular disease of the heart or brain. Before hemodialysis or renal transplantation, the mean age at death for affected men was 40 yr. Patients with the later onset phenotype with residual α-galactosidase A activity have cardiac and/or renal disease. The cardiac manifestations include hypertrophy of the left ventricular wall and interventricular septum, and electrocardiographic abnormalities consistent with cardiomyopathy. Others have had hypertrophic cardiomyopathy or myocardial infarction, or both.

The diagnosis in classically affected males is most readily made from the history of painful acroparesthesias, hypohidrosis, the presence of characteristic skin lesions, and the observation of the characteristic corneal opacities and lenticular lesions. The disorder is often misdiagnosed as rheumatic fever, erythromelalgia, or neurosis. The skin lesions must be differentiated from the benign angiokeratomas of the scrotum (Fordyce disease) or from angiokeratoma circumscriptum. Angiokeratomas identical to those of Fabry disease have been reported in fucosidosis, aspartylglycosaminuria, late-onset GM₁ gangliosidosis, galactosialidosis, α-N-acetylgalactosaminidase deficiency, and sialidosis. Later onset patients have been identified among patients on hemodialysis and among patients with hypertrophic cardiomyopathy or who have suffered cryptogenic strokes. Later-onset patients lack the early classic manifestations such as the angiokeratomas, acroparesthesias, hypohidrosis, and corneal opacities. The diagnosis of classic and later-onset patients is confirmed biochemically by the demonstration of markedly decreased α-galactosidase A activity in plasma, isolated leukocytes, or cultured fibroblasts or lymphoblasts.

Heterozygous females may have corneal opacities, isolated skin lesions, and intermediate activities of α-galactosidase A in plasma or cells. Rare female heterozygotes may have manifestations as severe as those in affected males. Asymptomatic at-risk females in families affected by Fabry disease, however, should be optimally diagnosed by the direct analysis of their family’s specific mutation. Prenatal detection of affected males can be accomplished by the demonstration of deficient α-galactosidase A activity or the family’s specific gene mutation in chorionic villi obtained in the 1st trimester or in cultured amniocytes obtained by amniocentesis in the 2nd trimester of pregnancy. Fabry disease can be detected by newborn screening and pilot studies have been conducted in Italy and Taiwan.

Treatment for Fabry disease may include the use of phenytoin and/or carbamazepine to decrease the frequency and severity of the chronic acroparesthesias and the periodic crises of excruciating pain. Renal transplantation and long-term hemodialysis are lifesaving procedures for patients with renal failure.

Recombinant α-galactosidase (Fabrazyme, Genzyme Corporation, Cambridge, Mass; Replagal, Shire, UK) is a safe and effective enzyme replacement therapy of choice for Fabry disease at a dose of 1 mg/kg every other week. It has been shown to clear microvascular endothelial deposits of globotriaosylceramide from the kidneys, heart, and skin in patients with Fabry disease with stabilization of renal disease, regression of hypertrophic cardiomyopathy, reduction of pain, and improvement in quality of life.

Fucosidosis

This is a rare autosomal recessive disorder caused by the deficient activity of α-fucosidase and the accumulation of fucose-containing glycosphingolipids, glycoproteins, and oligosaccharides in the lysosomes of the liver, brain, and other organs (see Table 80-16). The α-fucosidase gene is on chromosome 1 (1p24), and specific mutations are known. Although the disorder is panethnic, most affected patients are from Italy and the USA. There is wide variability in the clinical phenotype, with the most severely affected patients presenting in the first year of life with developmental delay and somatic features similar to those of the mucopolysaccharidoses. These features include frontal bossing, hepatosplenomegaly, coarse facial features, and macroglossia. The central nervous system storage results in a relentless neurodegenerative course, with death in childhood. Patients with milder disease have angiokeratomas and longer survival. No specific therapy exists for the disorder, which can be diagnosed by the demonstration of deficient α-fucosidase activity in peripheral leukocytes or cultured fibroblasts. Carrier identification studies and prenatal diagnosis are possible by determination of the enzymatic activity or the specific family mutations.

Schindler Disease

This is an autosomal recessive neurodegenerative disorder that results from the deficient activity of α-N-acetylgalactosaminidase and the accumulation of sialylated and asialoglycopeptides and oligosaccharides (see Table 80-16). The gene for the enzyme is located on chromosome 22 (22q11). The disease is clinically heterogeneous, and two major phenotypes have been identified. Type I disease is an infantile-onset neuroaxonal dystrophy. Affected infants have normal development for the 1st 9-15 mo of life followed by a rapid neurodegenerative course that results in severe psychomotor retardation, cortical blindness, and frequent myoclonic seizures. Type II disease is characterized by a variable age at onset, mild retardation, and angiokeratomas. There is no specific therapy for either form of the disorder. The diagnosis is by demonstration of the enzymatic deficiency in leukocytes or cultured skin fibroblasts or specific gene mutations.

Metachromatic Leukodystrophy (MLD)

This is an autosomal recessive white matter disease caused by a deficiency of arylsulfatase A (ASA), which is required for the hydrolysis of sulfated glycosphingolipids. Another form of MLD is caused by a deficiency of a sphingolipid activator protein (SAP1), which is required for the formation of the substrate-enzyme complex. The deficiency of this enzymatic activity results in the white matter storage of sulfated glycosphingolipids, which leads to demyelination and a neurodegenerative course. The ASA gene is on chromosome 22 (22q13.31qter); specific mutations are known to fall into two groups that correlate with disease severity.

The clinical manifestations of the late infantile form of MLD, which is most common, usually presents between 12 and 18 mo of age as irritability, inability to walk, and hyperextension of the knee, causing genu recurvatum. The clinical progression of the disease relates to the pathological involvement of both central and peripheral nervous system, giving a mixture of upper and lower motor neuron and cognitive and psychiatric signs. Deep tendon reflexes are diminished or absent. Gradual muscle wasting, weakness, and hypotonia become evident and lead to a debilitated state. As the disease progresses, nystagmus, myoclonic seizures, optic atrophy, and quadriparesis appear, with death in the 1st decade of life (see Table 80-16). The juvenile form of the disorder has a more indolent course with onset that may occur as late as 20 yr of age. This form of the disease presents with gait disturbances, mental deterioration, urinary incontinence, and emotional difficulties. The adult form, which presents after the 2nd decade, is similar to the juvenile form in its clinical manifestations, although emotional difficulties and psychosis are more prominent features. Dementia, seizures, diminished reflexes, and optic atrophy also occur in both the juvenile and adult forms. The pathologic hallmark of MLD is the deposition of metachromatic bodies, which stain strongly positive with periodic acid–Schiff and Alcian blue, in the white matter of the brain. Neuronal inclusions may be seen in the midbrain, pons, medulla, retina, and spinal cord; demyelination occurs in the peripheral nervous system. Bone marrow transplantation has resulted in normal enzymatic levels in peripheral blood, but no clear evidence for clinical efficacy in terms of the neurologic course; supportive care remains the primary intervention.

The diagnosis of MLD should be suspected in patients with the clinical features of leukodystrophy. Decreased nerve conduction velocities, increased cerebrospinal fluid protein, metachromatic deposits in sampled segments of sural nerve, and metachromatic granules in urinary sediment are all suggestive of MLD. Confirmation of the diagnosis is based on the demonstration of the reduced activity of ASA in leukocytes or cultured skin fibroblasts. Sphingolipid activator protein deficiency is diagnosed by measuring the concentration of SAP1 in cultured fibroblasts using a specific antibody to the protein. Carrier detection and prenatal diagnosis is available for all forms of the disorder.

Multiple Sulfatase Deficiency

This is an autosomal recessive disorder that results from the enzymatic deficiency of at least 9 sulfatases including arylsulfatases A, B, and C, and iduronate-2-sulfatase. The specific defect has been shown to be an enzyme in the C-α-formylglycine generating system (whose gene is located at 3p26), which introduces a common post-translational modification in all of the affected sulfatases and explains the occurrence of these multiple enzyme defects. Due to the deficiency of these enzymes, sulfatides, mucopolysaccharides, steroid sulfates, and gangliosides accumulate in the cerebral cortex and visceral tissues, resulting in a clinical phenotype with features of leukodystrophy as well as those of the mucopolysaccharidoses. Severe ichthyosis may also occur. Carrier testing and prenatal diagnosis by measurement of the enzymatic activities can be performed. There is no specific treatment for multiple sulfatase deficiency other than supportive care.

Krabbe Disease

This condition, also called globoid cell leukodystrophy, is an autosomal recessive fatal disorder of infancy. It results from the deficiency of the enzymatic activity of galactocerebrosidase (GALC) and the white matter accumulation of galactosylceramide, which is normally found almost exclusively in the myelin sheath. Both peripheral and central myelin are affected, resulting in spasticity and cognitive impairment coupled with deceptively normal or even absent deep tendon reflexes. The galactocerebrosidase gene is on chromosome 14 (14q31), and specific disease-causing mutations are known. The infantile form of Krabbe disease is rapidly progressive and patients present in early infancy with irritability, seizures, and hypertonia (see Table 80-16). Optic atrophy is evident in the 1st yr of life, and mental development is severely impaired. As the disease progresses, optic atrophy and severe developmental delay become apparent; affected children exhibit opisthotonos and die before 3 yr of age. A 2nd, late infantile form of Krabbe disease also exists and patients present after the age of 2 yr. Affected individuals have a disease course similar to that of the early infantile form.

The diagnosis of Krabbe disease relies on the demonstration of the specific enzymatic deficiency in white blood cells or cultured skin fibroblasts. Causative gene mutations have been identified. Carrier identification and prenatal diagnosis are available. The development of methods to measure GALC activity on dried blood spots has led to the inclusion of Krabbe disease in the newborn screening programs of some states. Treatment of infants with Krabbe disease with umbilical cord blood cell transplantation has been reported in prenatally identified asymptomatic newborns and symptomatic infants. The long term outcome of umbilical cord blood cell transplantation is being evaluated; transplanted infants develop neurologic manifestations at a slower rate but succumb to a neurologic demise.

Farber Disease

This is a rare autosomal recessive disorder that results from the deficiency of the lysosomal enzyme acid ceramidase and the accumulation of ceramide in various tissues, especially the joints. Symptoms can begin as early as the 1st year of life with painful joint swelling and nodule formation (Fig. 80-20), which is sometimes diagnosed as rheumatoid arthritis. As the disease progresses, nodule or granulomatous formation on the vocal cords can lead to hoarseness and breathing difficulties; failure to thrive is common. In some patients, moderate central nervous system dysfunction is present (see Table 80-16). Patients may die of recurrent pneumonias in their teens; there is currently no specific therapy. The diagnosis of this disorder should be suspected in patients who have nodule formation over the joints but no other findings of rheumatoid arthritis. In such patients, ceramidase activity should be determined in cultured skin fibroblasts or peripheral leukocytes. Various disease causing mutations have been identified in the acid ceramidase gene. Carrier detection and prenatal diagnosis are available.

Figure 80-20 Forearm of an 18 mo old girl with Farber disease. Note the painful joint swelling and the nodule formation. The infant was suspected of having rheumatoid arthritis.

Wolman Disease and Cholesterol Ester Storage Disease (CESD)

These are autosomal recessive lysosomal storage diseases that result from the deficiency of acid lipase and the accumulation of cholesterol esters and triglycerides in histiocytic foam cells of most visceral organs. The gene for lysosomal acid lipase is on chromosome 10 (10q24-q25). Wolman disease is the more severe clinical phenotype and is a fatal disorder of infancy. Clinical features become apparent in the 1st wk of life and include failure to thrive, relentless vomiting, abdominal distention, steatorrhea, and hepatosplenomegaly (see Table 80-16). There usually is hyperlipidemia. Hepatic dysfunction and fibrosis may occur. Calcification of the adrenal glands is pathognomonic for the disorder. Death usually occurs within 6 mo.

Cholesterol ester storage disease is a less severe disorder that may not be diagnosed until adulthood. Hepatomegaly can be the only detectable abnormality, but affected individuals are at significant risk for premature atherosclerosis. Adrenal calcification is not a feature.

Diagnosis and carrier identification are based on measuring acid lipase activity in peripheral leukocytes or cultured skin fibroblasts. Disease causing mutations have been identified in the acid ceramide gene. Prenatal diagnosis depends on measuring decreased enzyme levels or identifying specific mutations in cultured chorionic villi or amniocytes. There is no specific therapy available for either disorder, although pharmacologic agents to suppress cholesterol synthesis, in combination with cholestyramine and diet modification, have been used in patients with cholesterol ester storage disease (see Chapter 80.3).

I-Cell Disease

This disorder shares many of the clinical manifestations of Hurler syndrome (Chapter 82), although there is no mucopolysacchariduria and the presentation is earlier (see Table 80-16). Some patients have clinical features evident at birth, including coarse facial features, craniofacial abnormalities, restricted joint movement, and hypotonia. Nonimmune hydrops may be present in the fetus. The remainder of patients present in the first year with severe psychomotor retardation, coarse facial features, and skeletal manifestations that include kyphoscoliosis and a lumbar gibbus. Patients may also have congenital dislocation of the hips, inguinal hernias, and gingival hypertrophy. Progressive, severe psychomotor retardation leads to death in early childhood. No treatment is available.

Pseudo-Hurler Polydystrophy

Pseudo-Hurler polydystrophy is a less severe disorder than I-cell disease, with later onset and survival to adulthood reported. Affected children may present around the age of 4 or 5 yr with joint stiffness and short stature. Progressive destruction of the hip joints and moderate dysostosis multiplex are evident. Radiographic evidence of low iliac wings, flattening of the proximal femoral epiphyses with valgus deformity of the femoral head, and hypoplasia of the anterior third of the lumbar vertebrae are characteristic findings. Ophthalmic findings include corneal clouding, retinopathy, and astigmatism; visual complaints are uncommon (see Table 80-16). Some patients have learning disabilities or mental retardation. Treatment, which should include orthopedic care, is symptomatic.