Phenylketonuria is an autosomal recessive disorder that results from a mutation in the gene that codes for phenylalanine hydroxylase (PAH), the enzyme that hydroxylates phenylalanine to tyrosine. The complete hydroxylation system consists in part of PAH, a PAH stabilizer, the tetrahydrobiopterin cofactor (BH4), dihydropteridine reductase (DHPR), which is required to recycle BH4, and a BH4-synthesizing system. This system involves guanosine triphosphate (GTP) cyclohydrolase and 6-pyruvoyl tetrahydropteridine synthetase (Fig. 1.1) (14).

PAH is normally found in liver, kidney, and pancreas but not in brain or skin fibroblasts. The enzyme is an iron-containing metalloprotein dimer of identical subunits with a molecular weight of approximately 100,000. The gene coding for the enzyme has been cloned and localized to the long arm of chromosome 12 (12q22–q24). The gene is approximately 90 kilobases (kb) long with 13 exons, and codes for a mature RNA of approximately 2,400 bases. Availability of this clone has facilitated the molecular genetic analysis of patients with PKU and has confirmed that PKU is the consequence of numerous mutant alleles arising in various ethnic groups. The majority of these mutations result in deficient enzyme activity and cause hyperphenylalaninemia. Mutations occur in all 13 exons of the gene and flanking sequence. Some cause phenylketonuria; others cause non-PKU hyperphenylalaninemia, whereas still others are silent polymorphisms present on both
normal and mutant chromosomes (15,16). Some PAH alleles are more prevalent; five account for approximately 60% of European mutations and they tend to cluster in regions or are on only one of a few haplotypes. More than 495 mutations have been recorded. They are catalogued at http://www.pahdb.mcgill.ca.

Most mutations are missense mutations, although splice, nonsense, and silent mutations as well as single–base-pair frameshifts and larger deletions and insertions have been found. Most patients with PKU are compound heterozygotes rather than homozygotes in the precise meaning of the term (17). As a rule, the biochemical phenotype (i.e., the degree of phenylalanine elevation) does not correlate with the activity of PAH as predicted from the genetic mutation. In addition, no good correlation exists between PAH activity and intellectual function of untreated patients (18). Scriver (3,5) and others have pointed out that various environmental factors and modifying genes, such as the blood–brain barrier phenylalanine transporters, and variations in brain phenylalanine consumption rate may affect intellectual function in phenylketonuric patients (19,20).

The infant with classic PKU is born with only slightly elevated phenylalanine blood levels, but because of the absence of PAH activity, the amino acid derived from food proteins and postnatal catabolism accumulates in serum, CSF, and brain and is excreted in large quantities. In lieu of the normal degradative pathway, phenylalanine is converted to phenylpyruvic acid, phenylacetic acid, and phenylacetylglutamine.

The transamination of phenylalanine to phenylpyruvic acid is sometimes deficient for the first few days of life, and the age at which phenylpyruvic acid can be first detected varies from 2 to 34 days. From the first week of life, o-hydroxyphenylacetic acid also is excreted in large amounts.

In addition to the disruption of phenylalanine metabolism, tryptophan and tyrosine are handled abnormally. Normally, transport across the blood–brain barrier of a large neutral amino acid such as phenylalanine is determined by its plasma concentration and its affinity to the stereo-specific L-type amino acid carrier system (20). Supraphysiologic levels of phenylalanine competitively inhibit transport of tryptophan and tyrosine across the blood–brain barrier (21). Intestinal transport of L-tryptophan and tyrosine is also impaired in PKU, and fecal content of tryptophan and tyrosine is increased. These abnormalities are reversed by dietary correction of the plasma phenylalanine levels (22). Miyamoto and Fitzpatrick suggested that a similar interference might occur in the oxidation of tyrosine to 3-(3,4-dihydroxyphenyl) alanine (dopa), a melanin precursor, and might be responsible for the deficiency of hair and skin pigment in phenylketonuric individuals (23). The biochemical pathology of PKU is reviewed to a greater extent by Scriver and Kaufman (14).

Alterations within the brain are nonspecific and diffuse. They involve gray and white matter, and they probably progress with age. Three types of alterations exist.

Phenylketonuric infants appear healthy at birth. In untreated infants, vomiting, which at times is projectile, and irritability are frequent during the first 2 months of life. By 4 to 9 months, delayed intellectual development becomes apparent (28). In the untreated classic case, mental retardation is severe, precluding speech and toilet training. Children in this category have an IQ below 50. Seizures, common in the more severely retarded, usually start before 18 months of age and can cease spontaneously. During
infancy, they often take the form of infantile spasms, later changing into tonic-clonic attacks.

The untreated phenylketonuric child is blond and blue-eyed, with normal and often pleasant features. The skin is rough and dry, sometimes with eczema. A peculiar musty odor, attributable to phenylacetic acid, can suggest the diagnosis. Significant neurologic abnormalities are rare, although hyperactivity and autistic features are not unusual. Microcephaly may be present as well as a mild increase in muscle tone, particularly in the lower extremities. A fine, irregular tremor of the outstretched hands is seen in approximately 30% of the patients. Parkinsonian-like extrapyramidal symptoms also have been encountered (29). The plantar response is often extensor.

A variety of electroencephalographic (EEG) abnormalities has been found, but hypsarrhythmic patterns, recorded even in the absence of seizures, and single and multiple foci of spike and polyspike discharges are the most common (30).

MRI is abnormal in almost every patient, regardless of when treatment was initiated. On T2-weighted imaging, one sees increased signal in the periventricular and subcortical white matter of the posterior hemispheres. Increased signal can extend to involve the deep white matter of the posterior hemispheres and the anterior hemispheres. No signal abnormalities are seen in brainstem, cerebellum, or cortex, although cortical atrophy may be present (Fig. 1.4) (31,32). The severity of the abnormality is unrelated to the patient’s IQ but is significantly associated with the phenylalanine level at the time of imaging. In adult patients with PKU who had come off their diets, resumption of dietary treatment can improve MRI abnormalities within
a few weeks or months, an observation that strongly suggests that at least some of the MRI changes are the result of edema (32).

Heterozygous mothers tend to have elevated plasma phenylalanine levels and somewhat reduced tyrosine values during the latter part of pregnancy and after delivery. Mothers suffering from PKU have a high incidence of spontaneous abortions. It is now clear that when maternal blood phenylalanine levels are greater than 20 mg/dL (1,212 μmol/L) during pregnancy, fetal brain damage is almost inevitable, with mental retardation encountered in 92% of offspring and microcephaly in 73%. Offspring have an unusual facies: upturned nose, underdeveloped philtrum, and a thin upper lip (33). Additionally, a significant incidence of congenital heart disease and prenatal and postnatal growth retardation occurs (34). MRI in these children shows hypoplasia or partial agenesis of the corpus callosum and delayed myelination (35). These observations are best explained by a deleterious effect of elevated phenylalanine on the myelinating ability of oligodendrocytes.

Much, if not all, of the fetal damage appears to be preventable by placing phenylketonuric mothers on a phenylalanine-restricted diet before conception and maintaining blood phenylalanine levels below 6 mg/dL (360 μmol/L) throughout pregnancy (36). In the data compiled by Koch and coworkers the optimal outcome, as measured by the Wechsler Intelligence Scale for Children (WISC) score of offspring obtained at 7 years of age, was obtained when maternal blood phenylalanine levels of 120 to 360 μmol/L (2 to 6 mg/dL) were obtained by 8 to 10 weeks’ gestation and maintained at those levels throughout pregnancy. However, there was considerable scatter in IQ even when the phenylalanine levels were maintained below 5 mg/dL (37). Birth weight and head circumference in infants of mothers so managed were normal, and no evidence existed for fetal nutritional deficiency. Offspring of mothers who experience mild PKU as defined by phenylalanine levels of less than 400 μmol/L (6.6 mg/dL) appear to be normal, and mothers do not require dietary intervention (38).

The pathogenesis of mental retardation in PKU is not completely understood (39). No evidence exists that phenylpyruvic acid or any of the other phenylalanine metabolites are neurotoxic at concentrations in which they are seen in PKU (40). Probably no single factor is responsible; instead, impairment of amino acid transport across the blood–brain barrier; disruption of the brain amino acid pool; with consequent defective proteolipid protein synthesis, impaired myelination, and low levels of neurotransmitters, such as serotonin, are responsible to varying degrees (41,42).

With the worldwide practice of neonatal screening for phenylketonuria, the diagnosis is usually made during the first week of life, and it is now rare to encounter an older infant or child with undiagnosed PKU. As mentioned in the section on Molecular Genetics and Biochemical Pathology, plasma phenylalanine levels are elevated in the cord blood of phenylketonuric infants and increase rapidly within a few hours of birth. A screening program that involves spectrofluorometric or microbiologic estimation of blood phenylalanine is used commonly. Tandem mass spectrometric analysis of phenylalanine and other amino acids as well as of the acylcarnitines of the various organic acids is being used in some laboratories (43), and trials of this method for newborn screening are being instituted in several states. Whatever the method, the screening test requires a drop of whole blood placed on a filter paper. If the test is positive [a value greater than 2 to 4 mg/dL (120 to 240 μmol/L)], the diagnosis is confirmed by quantitative analysis of phenylalanine and tyrosine in blood. Although most infants whose blood phenylalanine levels are above the threshold value do not have PKU, such patients require prompt reevaluation by an appropriate laboratory to determine whether hyperphenylalaninemia is persistent and whether it is caused by PAH deficiency.

Of considerably greater concern are the false-negative test results. It is now clear that routine screening tests, when correctly performed after 12 hours of life, detect all infants with classic PKU. In a group of such infants studied by Koch and Friedman the lowest phenylalanine value recorded at 24 hours was 5.6 mg/dL (339 μmol/L); at 48 hours, 7.5 mg/dL (454 μmol/L); and at 72 hours, 8.4 mg/dL (509 μmol/L). Thus, even at 24 hours, none of the infants with classic PKU would have escaped detection (44). The Canadian experience of Hanley and coworkers is similar (45). As a rule, breast-fed PKU infants have higher phenylalanine levels than those who are formula fed. In some infants with mild hyperphenylalaninemia, the increase in blood phenylalanine is sufficiently slow to allow them to escape detection unless an assay is used, such as tandem mass spectroscopy, that can measure both phenylalanine and tyrosine, which decreases false-positive results. Some screening agencies obtain a second screening blood specimen from all infants whose first blood specimen was drawn during the first 24 hours of life (46,47).

Most infants with elevated blood phenylalanine detected by means of the newborn screening program do not have classic PKU; instead they have a transient or mild form of hyperphenylalaninemia that is usually benign. Patients with mild PKU have phenylalanine levels between 600 and 1200 μmol/L (10 to 20 mg/dL) on an unrestricted diet, and patients with mild hyperphenylalaninemia have levels below 600 μmol/L (10 mg/dl) on an unrestricted diet (48). A large proportion of these patients represent compound heterozygotes for a mutation that abolishes catalytic activity of PAH and one that reduces it (16,49,50,51). With normal protein intakes, the majority of such infants appear to have unimpaired intelligence, even when untreated (48).

Elevated blood phenylalanine levels also are observed in 25% of premature infants and occasionally in full-term newborns. In all of these patients, tyrosine levels are increased to a much greater extent than are phenylalanine levels.

Prenatal diagnosis can be performed by detection of the specific mutations in PAH identified in the family or by genetic linkage analysis (52,53). Modifier genes have not been identified.

Two treatment strategies could be employed in PKU: modification of the phenotypic expression of the defective gene and definitive treatment by correcting the gene defect. Only the first approach has been used in clinical practice.

On referral of an infant with a confirmed positive screening test result, the first step is quantitative determination of serum phenylalanine and tyrosine levels. All infants whose blood phenylalanine concentration is greater than 10 mg/dL (600 μmol/L) and whose tyrosine concentration is low or normal (1 to 4 mg/dL) should be started on a low-phenylalanine diet immediately. Infants whose blood phenylalanine concentrations remain below 10 mg/dL (600 μmol/L) on an unrestricted diet are generally considered not to require a diet (54).

The accepted therapy for classic PKU is restriction of the dietary intake of phenylalanine by placing the infant on one of several low-phenylalanine formulas. The diet should be managed by a team consisting of a nutritionist, a physician with expertise in metabolic disorders, and a person to ensure dietary compliance. To avoid symptoms of phenylalanine deficiency, regular formula is added to the diet in amounts sufficient to maintain blood levels of the amino acid between 2 and 6 mg/dL (120 and 360 μmol/L). Generally, patients tolerate this diet quite well, and within 1 to 2 weeks, the serum concentration of phenylalanine becomes normal. Serum phenylalanine determinations are essential to ensure adequate regulation of diet. These are performed twice weekly during the first 6 months of life and twice monthly thereafter.

Strict dietary control should be maintained for as long as possible, and most centers strive to keep levels below 6.0 mg/dL (360 μmol/L), even in patients with moderate and mild PKU. Samples of low-phenylalanine menus are given by Buist and colleagues (55), and the nutritional problems inherent in prolonged use of a restricted diet are discussed by Acosta and colleagues (56). Dietary lapses are common, particularly in patients 15 years or older (57). They frequently are accompanied by progressive white
matter abnormalities on MRI. Some workers have suggested supplementation of the low-phenylalanine diet with tyrosine, but no statistical evidence exists that this regimen results in a better intellectual outcome. This possibly could be due to failure to achieve adequate tyrosine levels in the brain even with tyrosine supplementation of 300 mg/kg (58). Failure to treat patients with mild hyperphenylalaninemias does not appear to produce either intellectual deficits or MRI abnormalities.

Patients with mild PKU or mild hyperphenylalaninemia have been treated successfully with tetrahydrobiopterin (BH4; 10 to 20 mg/kg per day) (59,59a). Most of these individuals have at least one missense mutation in the gene for phenylalanine hydroxylase, leading to some residual enzyme activity (60). The mechanism for this effect is unknown, as is the question of how to best select patients responsive to the cofactor. BH4 is being tested, but it is not yet approved for therapy in the United States.

Dietary therapy has for the greater part been effective in preventing mental retardation in patients with severe PKU. It has become apparent that the outcome depends on several factors. Most important is the age at which the diet was initiated. Smith and coworkers found that patients’ IQ fell approximately 4 points for each month between birth and start of treatment (61). The average phenylalanine concentration while receiving treatment also affects outcome, with optimal average phenylalanine levels in the most recent cohorts being 5.0 to 6.6 mg/dL (300 to 400 μmol/L). Additionally, hypophenylalaninemia during the first 2 years of life [i.e., the length of time that phenylalanine concentration was below 2.0 mg/dL (120 μmol/L)] also affected outcome adversely. Even patients with normal IQ scores and with the most favorable diagnostic and treatment characteristics have lower IQs than other members of their families and suffer from cognitive deficits, educational difficulties, and behavioral problems, notably hyperactivity (61,62,63). As was already noted, dietary supplementation with tyrosine, although used in several centers, does not appear to improve performance on neuropsychologic tests (64). The most likely explanations for these deficits are inadequate dietary control and unavoidable prenatal brain damage from elevated phenylalanine (see Fig. 1.2) (24).

When patients who already have developed symptoms caused by classic PKU are placed and maintained on a low-phenylalanine diet, the epilepsy comes under control and their EEGs normalize. Microcephaly, if present, can correct itself, and abnormally blond hair regains its natural color.

Considerable uncertainty exists about when, if ever, to terminate the diet (65). In the series of Waisbren and coworkers (66), one-third of youngsters whose diet was discontinued at 5 years of age had a reduction in IQ of 10 points or more during the ensuing 5 years. The blood phenylalanine level when the children were off the restrictive diet predicted the change in IQ. Of the children whose IQs dropped 20 or more points, 90% had blood phenylalanine levels of 18 mg/dL (1,090 μmol/L) or higher, and 40% of those whose IQs rose 10 points or more had a phenylalanine level of less than 18 mg/dL (1,090 μmol/L). In young adult patients with PKU, discontinuation of the diet was accompanied by progressive spasticity and worsening white matter abnormalities (67). Reinstitution of the diet results in clear clinical improvement and resolution of new MRI abnormalities. Reports such as these speak against early relaxation of the restrictions on phenylalanine intake and indicate that dietary therapy for patients with classical PKU should be life-long (68,69).

The relative inadequacies of dietary therapy underscore the need for a more definitive approach to the treatment of PKU. Allogeneic or autologous bone marrow transplants are being used for the treatment of a variety of genetic diseases (Table 1.7). The likelihood that this procedure will cure or at least stabilize a genetic disease depends on the tissue-specific expression of the normal gene product, the patient’s clinical symptoms, and the cellular transport of the normal gene product. In PKU, the defective enzyme is not normally expressed in bone marrow–derived cells, and bone marrow transplantation has no therapeutic value (see Table 1.7).

Another approach to treating the patient with PKU is the introduction of PAH gene into affected hepatic cells. Recombinant viruses containing human PAH have been introduced into mouse hepatoma cells, where they are able to continue expressing the human enzyme, lowering the phenylalanine level and normalizing coat color (70). The next step would be to find a virus that can infect human liver, maintain itself there without inducing damage to the organ, and allow the human gene to continue functioning in the new host. Ding and colleagues reviewed various approaches for nonviral gene transfer (71).

MSUD is characterized by the accumulation of three branched-chain ketoacids: α-ketoisocaproic acid, α-ketoisovaleric acid, and α-keto-β-methylvaleric acid, the derivatives of leucine, valine, and isoleucine, respectively (104,105). Their accumulation is the consequence of a defect in oxidative decarboxylation of branched-chain ketoacids (Fig. 1.5).

The branched-chain α-ketoacid dehydrogenase complex is located within the mitochondrial inner membrane matrix compartment. It is a multienzyme complex comprising six proteins: E_1α and E_1β. which form the decarboxylase; E₂; E₃; and a branched-chain–specific kinase and phosphatase. The last two enzymes regulate the activity of the complex by phosphorylating and dephosphorylating the dehydrogenase. E₁ is a thiamin pyrophosphate-dependent enzyme. The second enzyme (E₂), dihydrolipoyltransacylase, transfers the acyl group from the first enzyme to coenzyme A. The third enzyme (E₃), dihydrolipoyldehydrogenase, a flavoprotein, reoxidizes the disulfhydryl form of lipoamide. The same enzyme is common to other α-ketoacid dehydrogenases, such as pyruvate and α-ketoglutarate dehydrogenase (106). The complex removes carboxyl groups from all three branched-chain ketoacids and converts those ketoacids to their respective coenzyme A derivatives (see Fig. 1.5, step 2). Chuang and coworkers reviewed the crystal structure of the various enzyme components and the structural basis for the various MSUD mutations (103).

With six genes involved in the function of the branched-chain ketoacid dehydrogenase complex, considerable room for heterogeneity exists. Mutations in the genes for E_1α, E_1β, E₂, and E₃ have been described, with many MSUD patients being compound heterozygotes. These induce a continuum of disease severity that ranges from the severe, classic form of MSUD to mild and intermittent forms.

As a consequence of the enzymatic defect, the branched-chain ketoacids accumulate in serum and CSF and are excreted in large quantities in urine (105). Plasma levels of the respective amino acids (e.g., leucine, isoleucine, and valine) are elevated secondary to the increase in ketoacid concentrations. Alloisoleucine, which is formed by transamination of α-keto-β-methylvaleric acid, also has been found in serum (107). In some cases, the branched-chain hydroxyacids, most prominently α-hydroxyisovaleric acid (108), are excreted also. Sotolone, a derivative of α–ketobutyric acid, the decarboxylation of which is impaired by accumulation of α-keto-β-methylvaleric acid, is responsible for the characteristic odor of the patient’s urine and perspiration (109).

Structural alterations in the nervous system in untreated infants with MSUD are similar to, but more severe than, those seen in PKU. In infants dying during the acute phase of the disease, diffuse edema occurs (101). The cytoarchitecture of the cortex is generally immature, with fewer cortical layers and the persistence of ectopic foci of neuroblasts, an indication of disturbed neuronal migration. Dendritic development is abnormal, and the number of oligodendrocytes and the amount of myelin are less than would be seen in a healthy brain of comparable age (110). Marked astrocytic gliosis and generalized cystic degeneration occur (111). Little clinical or pathologic evidence exists for demyelination in patients who are treated early (112,113). On chemical examination, the concentration of myelin lipids is markedly reduced, with cerebrosides, sulfatides, and proteolipid protein almost completely absent. These abnormalities are not found in infants dying of the disease within the first days of life or in patients treated by restriction of branched-chain amino acid intake (113).

Various mutations result in five fairly distinct clinical phenotypes of MSUD. Patients can be homozygotes for the same allele or compound heterozygotes for different alleles. The classic form of MSUD accounts for approximately 75% of patients (103). Mutations in the E₁ decarboxylase component of the enzyme are associated with this phenotype (114). In the original four patients reported by Menkes and coworkers (101) as well as in subsequent cases, dystonia, opisthotonos, intermittent increase in muscle tone, and respiratory irregularities appeared within the first week of life in babies apparently healthy at birth (115). Subsequently, rapid deterioration of the nervous system occurred, and all but one died within 1 month. In some patients, cerebral edema is marked and can be fatal (116). Other patients, spastic and intellectually retarded, survived without treatment for several years. A fluctuating ophthalmoplegia correlates in intensity with serum leucine levels (117). Presentation with pseudotumor cerebri also has
been reported. Approximately 50% of patients with the classic form of MSUD develop severe hypoglycemia; this is probably the consequence of a defective gluconeogenesis from amino acids, particularly alanine (118).

MRI during the acute stage of the disease before treatment is characteristic. It demonstrates edema that is maximal in cerebellar deep white matter, the posterior portion of the brainstem, and the posterior limb of the internal capsule. Edema also is seen in the cortical U fibers, the head of the caudate, and the putamen (119,120). These findings are consistent with the location of status spongiosus noted on pathologic examination. The cause of the
acute cerebral edema and its unique localization are unknown. Diffusion MRI suggests that in the myelinated areas of the brain there is an intramyelinic cytotoxic edema, whereas in the unmyelinated areas there is vasogenic interstitial edema (121,122). Because neurologic symptoms become apparent with relatively mild increases in plasma leucine concentrations, whereas there is little apparent toxicity associated with increased levels of isoleucine or valine, damage probably results from leucine accumulation or from accumulation of its ketoacid metabolite. As is the case in PKU, chronic brain damage is probably caused by interference with amino acid transport into the brain, a deranged amino acid environment, and, consequently, failure in biosynthesis of proteolipids, myelin, and neurotransmitters (123). In treated patients, the MRI discloses symmetric bilateral periventricular high signal intensity on T2-weighted images. This picture is similar to that seen in PKU and suggests focal dysmyelination (112).

The intermittent form of MSUD results from a variety of mutations in the gene for E₂, and branched-chain dehydrogenase activity is higher than in the classic form, usually 5% to 20% of normal (124). The clinical picture is that of intermittent periods of ataxia, drowsiness, behavior disturbances, and seizures that make their first appearance between ages 6 and 9 months. Attacks are generally triggered by infections, immunizations, or other forms of stress (125).

In the intermediate form, the clinical picture is one of mild to moderate mental retardation (126). Branched-chain dehydrogenase activity ranges from 5% to 20% of normal, and the defect is usually in the gene coding for E₁ (106).

A thiamin-responsive variant represents an entity in which, in some cases at least, a mutant exists in the gene for E₂ (103). Chuang and coworkers proposed that binding of the mutated, inactive E₂ component to the wild-type E₁ component enhances wild-type E₁ activity, and that the augmented E₁ activity is responsible for the response to thiamine (114).

Mutants defective in the gene for E₃ present with hypotonia, rapid neurologic deterioration, and severe lactic acidosis. This entity is discussed with the various other organic acidemias.

The most common presentation of MSUD is that of a term infant who initially seems to be well for a few days and then deteriorates. The rate of deterioration varies, and most infants initially are believed to be septic. The neurologist called in to consult on such an infant should always consider the diagnosis of an inborn error of metabolism. Basic investigations at this point include blood or plasma pH, blood gases, glucose, electrolytes, liver function tests, ammonia, and plasma for amino acids and acyl carnitines. Urine for sugars, ketones, and organic acids is also indicated (127). In addition to MSUD, neurologic deterioration during the neonatal period is seen in various organic acidurias, urea cycle defects, fatty acid oxidation defects, and the congenital lactic acidoses. Clinically, MSUD is distinguished by the characteristic odor of the patient, a positive urine 2,4-dinitrophenylhydrazine test, and an elevation of the plasma branched-chain amino acids. The characteristic increase in plasma leucine, isoleucine, and valine is seen by the time the infant is 24 hours old, even in those infants who have not yet been given protein (128). Routine newborn screening for the condition using a bacterial inhibition assay analogous to that used for the neonatal diagnosis of PKU is performed in many states in the United States and in many other countries (129). Tandem mass spectroscopy also can be used and has the advantage of obtaining rapid quantitative measurements of all three branched-chain amino acids (43). The presence of the branched-chain ketoacid decarboxylases in cultivated amniocytes and chorionic villi allows the antenatal diagnosis of the disease as early as 10 weeks’ gestation (130).

Treatment consists in inhibiting endogenous protein catabolism, sustaining protein synthesis, preventing deficiencies of essential amino acids, and maintaining normal serum osmolarity (131). Morton and coworkers stressed that restriction of the dietary intake of the branched-chain amino acids through the use of one of several commercially available formulas is secondary in importance to inhibition of protein catabolism and enhancement of protein synthesis (131). For optimal results, infants should be placed on the diet during the first few days of life and should receive frequent measurements of serum amino acids. Prompt and vigorous treatment of even mild infections is mandatory; a number of children on this synthetic diet have died of septicemia (115,131).

Peritoneal dialysis or hemodialysis has been used to correct coma or other acute neurologic symptoms in the newly diagnosed infant (132). Another, simpler approach is to provide intravenously or by nasogastric tube an amino acid mixture devoid of leucine but containing large amounts of tyrosine, glutamine, and alanine (133). Morton and coworkers believe that the brain edema that is frequently seen in the acutely ill neonate results from hyponatremia and responds promptly to addition of oral and intravenous saline (131). Most of the children in whom long-term dietary therapy was initiated during the first 2 weeks of life and whose dietary control was meticulously maintained achieved normal or nearly normal IQs (131,134). In the experience of Hilliges and coworkers, the mean IQ of MSUD patients at 3 to 16 years of age was 74 ± 14, as compared with 101 ± 12 for early-treated patients with PKU. The length of time after birth that plasma leucine
concentrations remain elevated appears to affect the IQ, as does the amount, if any, of residual branched-chain ketoacid dehydrogenase activity (135). The thiamin-responsive child is treated with 10 to 1,000 mg of thiamin per day (103).

Argininosuccinic acid is a normal intermediary metabolite in the synthesis of urea (see Fig. 1.6). A deficiency in argininosuccinate lyase, an enzyme whose gene has been mapped to chromosome 7cn–q11.2, has been demonstrated in liver and skin fibroblast cultures (157).

The synthesis of urea is only slightly depressed, but a large proportion of labeled ammonium lactate administered to affected individuals is converted to glutamine (158). The manner in which children synthesize urea is not clear. It appears likely that in argininosuccinic aciduria, as well as in the other defects of urea cycle, substrate accumulates to a concentration at which the decreased substrate-binding capacity of the mutant enzyme is overcome by the accumulation of precursor to levels greater than the K_M for the mutated enzyme (159).

The liver architecture is abnormal, with increased fat deposition. The brain of a neonate who succumbed to the disease was edematous, with poor demarcation of gray and white matter. The cortical layers were poorly developed, and myelination was defective with vacuolated myelin sheaths and cystic degeneration of white matter (160). An older patient had atypical astrocytes similar to the Alzheimer II cells seen in Wilson disease and in severe chronic liver disease (161).

As ascertained by newborn screening, the incidence of argininosuccinic aciduria is 1 in 70,000 in Massachusetts and 1 in 91,000 in Austria (162). Three distinct clinical forms have been recognized, each resulting from a different genetic mutation (163).

The most severe entity is the neonatal form. Infants feed poorly, become lethargic, develop seizures, and generally die within 2 weeks (160,164). In a second form, progression is less rapid, but similar symptoms appear in early infancy. In the majority of patients, including those described by Allan and coworkers (156), the presenting symptoms are mental retardation, recurrent generalized convulsions, poorly pigmented, brittle hair (trichorrhexis nodosa), ataxia, and hepatomegaly (165). Some patients have been seizure free, however, and have presented with little more than learning difficulties, and others (approximately 20% of all affected children) have normal intelligence without treatment (166).

The presence of elevated blood ammonia should suggest a disorder in the urea cycle. Initial evaluation of such a child should include routine blood chemistries, plasma lactate levels, liver function tests, quantitative assay of plasma amino acids, and assay of urine for organic acids and orotic acid (153). The specific diagnosis of argininosuccinic aciduria can be made by a significant elevation of plasma citrulline and the presence of large quantities of plasma, urinary, and CSF argininosuccinic acid. In some instances, fasting blood ammonia level can be normal or only slightly elevated, but marked elevations occur after protein loading. Increased excretion of orotic acid is seen in all urea cycle defects, with the exception of CPS deficiency (167).

Hyperammonemic coma in the neonate caused by any of the urea cycle defects requires prompt intervention. In essence, treatment consists of detoxification and removal of the excess ammonia and reduction in the formation of ammonia. Quantitative amino acid chromatography should be performed on an emergency basis, and the infant should be given a high dose of intravenous glucose with insulin to suppress protein catabolism. The elevated ammonia levels can be reduced by hemodialysis if available or by peritoneal dialysis (168). Details of treatment are presented by Brusilow and Horwich (166) and Batshaw and colleagues (169). Treatment for increased intracranial pressure, which frequently accompanies neonatal hyperammonemia, is symptomatic.

The long-term management of an infant who suffers a urea cycle defect is directed toward lowering blood ammonia levels and maintaining them as close to normal as possible. This is accomplished by providing the infant with alternative pathways for waste nitrogen synthesis and excretion. Infants with argininosuccinic aciduria are placed on a protein-restricted diet (1.2 to 2.0 g/kg per
day), which is supplemented with L-arginine (0.4 to 0.7 g/kg per day), which promotes the synthesis of citrulline and argininosuccinate as waste nitrogen products, and citrate, which improves weight gain and reduces hepatomegaly (166,169). Sodium phenylbutyrate is added to divert ammonia from the urea cycle. A number of centers are now recommending the use of liver transplantation for argininosuccinic aciduria (170).

Episodes of hyperammonemia are generally triggered by intercurrent infections. Prevention of a rapid progression to death requires hospitalization and the use of intravenous therapy (166).

On this regimen, some children with argininosuccinic aciduria do well. Reduction of blood ammonia levels is accompanied by improved growth, reduction in liver size, cessation of seizures, and, in some patients, normal hair. Intellectual function is significantly impaired, however. In the experience of Batshaw and coworkers, all infants with the severe, neonatal form of the disease survived, although those who have been followed the longest have shown a significant lowering of their IQ (169). In the French long-term follow-up of 15 patients with argininosuccinic aciduria, none were doing well (171). Children with the late-onset variant fare much better, and, with therapy, achieve normal development (165). As a rule, the individual’s ultimate IQ is a function of the severity and duration of hyperammonemic coma, and children who have not recovered from coma within 5 days do poorly (172). Valproic acid cannot be used in the treatment of seizures associated with this and with the other urea cycle defects because it induces severe hyperammonemia at even low doses (173).

The gene coding for argininosuccinic acid synthetase has been cloned. It is carried on chromosome 9 (175). At least 50 different genetic mutations have been recorded for infants with neonatal citrullinemia (176).

As a result of the enzymatic defect, the concentration of citrulline in urine, serum, and CSF is markedly increased, and administration of a protein meal results in a dramatic increase of blood ammonia and urinary orotic acid. Blood and urinary urea values are normal, indicating that urea production is not completely blocked. CT and MRI studies performed on patients with the neonatal form of citrullinemia show lesions in the thalami, basal ganglia, cortex, and subcortical white matter. Diffusion-weighted MR images indicate the presence of cytotoxic edema. Follow-up studies reveal subcortical cysts, ulegyria, and atrophy (177,178).

In Western countries, the most common presentation is in the neonatal period with lethargy, hypotonia, and seizures (179). In other instances, the disease is less severe, even though recurrent bouts of vomiting, ataxia, and seizures can start in infancy. A third form presents with mental retardation. Completely asymptomatic individuals also have been encountered (174,179).

Treatment for citrullinemia is similar to treatment for argininosuccinic aciduria, except that for long-term therapy, the low-protein diet is supplemented with arginine and sodium phenylbutyrate (166,169).

Citrullinuria in the absence of citrullinemia has been seen in patients with cystinuria. In this instance, citrulline is derived from arginine, which is poorly absorbed from the intestine (180).

Late-onset citrullinemia with loss of enzymatic activity in liver, but not in kidney or fibroblasts, is seen predominantly in Japan, where it constitutes the most common form of citrullinemia (181). It presents with cyclical changes in behavior, dysarthria, and motor weakness. It is due to mutations in citrin, a mitochondrial aspartate glutamate carrier (182,183).

Another cause for intermittent hyperammonemia is ornithinemia. Clinically and biochemically this is a heterogeneous entity, At least two conditions have been delineated.

Transient hyperammonemia with consequent profound neurologic depression can be encountered in asphyxiated infants or with significant dehydration (217,218). This state should be differentiated not only from the various urea cycle defects, but also from the various organic acidemias, notably methylmalonic acidemia and propionic acidemia, which can induce hyperammonemia (219). In these conditions, the accumulation of organic acids inhibits the formation of N-acetylglutamine, the activator of mitochondrial CPS, and the activities of all five enzymes of the urea cycle are depressed. On a clinical basis, the organic acidemias can be distinguished from urea cycle defects in that infants with a urea cycle defect are asymptomatic for the first 24 hours of life and only rarely develop coma before 72 hours. Additionally, they demonstrate tachypnea rather than a respiratory distress syndrome. In contrast to the distressed neonates with hyperammonemia, infants with the various organic acidemias demonstrate ketonuria or ketonemia as well as acidosis (220).

Asymptomatic hyperammonemia is relatively common in low-birth-weight neonates. It probably is caused by shunting of blood away from the portal circulation of the liver into the systemic circulation.

Mild hyperammonemia can also be seen in associated with hypoglycemia and hyperinsulinism due to mutations in the glutamate dehydrogenase gene (221).

The increased excretion of homocystine is a manifestation of several inborn errors of methionine metabolism. The most common of these errors is marked by multiple thromboembolic episodes, ectopia lentis, and mental retardation. Although discovered as late as 1962 by Field and reported subsequently by Carson and coworkers (222), the prevalence of homocystinuria varies considerably from one country to another, ranging from 1 in 65,000 in Ireland to approximately 1 in 335,000 worldwide (223). The gene for this autosomal recessive condition has been cloned; it is localized to the long arm of chromosome 21 (21q22.3).

Several other conditions are marked by elevated plasma methionine levels. The most common of these is a transitory methioninemia, seen in infants, many of who are premature and receiving a high-protein diet (at least 7 g/kg per day). It is likely that the biochemical abnormality is caused by delayed maturation of one or more of the enzymes of methionine metabolism.

Methioninemia, with or without tyrosinemia, accompanied by hepatorenal disease (tyrosinemia I) is considered in the section on tyrosinosis and tyrosinemia (see Table 1.9).

Persistent methioninemia associated with a deficiency of hepatic methionine adenosyltransferase is a benign metabolic variant unaccompanied by neurologic symptoms or impairment of cognition (246).

The symptoms are the result of an extensive disturbance in sodium-dependent transport of neutral amino acids across the membrane of the brush border of the small intestine and the proximal renal tubular epithelium. Four main biochemical abnormalities exist: a renal aminoaciduria, increased excretion of indican, an abnormally high output of nonhydroxylated indole metabolites, and increased fecal amino acids. These deficits are discussed in detail by Milne and colleagues (265) and Scriver (266).

Pathologic changes in the brain are nonspecific and are limited to neuronal degeneration and dysmyelination (267).

The incidence of the biochemical lesion responsible for Hartnup disease is 1 in 18,000 in Massachusetts, 1 in 70,000 in Vienna, and 1 in 33,000 in New South Wales, Australia (268). Clinical manifestations of Hartnup disease are the consequence of several factors. Polygenic inheritance is the major determinant for plasma amino acid levels, and symptoms are seen only in patients with the lowest amino acid concentrations. Because protein malnutrition further lowers amino acid levels, the disease
itself, as distinguished from its biochemical defect, is seen mainly in malnourished children. Whenever dietary intake is satisfactory, neither neurologic nor dermatologic signs appear (268). In addition, no difference exists in rate of growth or IQ scores between groups with Hartnup disease and control groups. In the series of Scriver and coworkers, 90% of Hartnup patients had normal development (269). However, low academic performance and impaired growth were seen in those patients with Hartnup disease who, for genetic reasons, tended to have the lowest plasma amino acid levels (269).

When present, symptoms are intermittent and variable, and tend to improve with increasing age. A characteristic red, scaly rash appears on the exposed areas of the face, neck, and extensor surfaces of the extremities. This rash resembles the dermatitis of pellagra and, like it, is aggravated by sunlight. Cerebral symptoms can precede the rash for several years. They include intermittent personality changes, psychoses, migraine-like headaches, photophobia, and bouts of cerebellar ataxia. Changes in hair texture also have been observed. The four children of the original Hartnup family underwent progressive mental retardation, but this is not invariable. Renal and intestinal transport is impaired in 80% of patients and renal transport alone in 20% (269). The MRI is nonspecific; it demonstrates delayed myelination (270).

Hartnup disease should be considered in patients with intermittent cerebral symptoms, even without skin involvement.

Numerous metabolic disorders with a partial enzymatic defect produce intermittent cerebellar ataxia. These include MSUD, lactic acidosis, pyruvate dehydrogenase deficiency, and some of the diseases caused by defects in the urea cycle. Additionally, episodic ataxia should be considered. This condition is caused by mutations in the calcium-channel voltage-dependent, P/Q type, alpha 1A subunit gene (CACNA1A), which is highly expressed in the cerebellum. Another rare condition to be considered in the differential diagnosis of Hartnup disease is hypertryptophanemia (see Table 1.12) and a disorder in kynurenine hydroxylation (271). Chromatography of urine for amino acids and indolic substances in the presence of a normal serum pattern is diagnostic for Hartnup disease. The absence of increased proline and hydroxyproline in the urine distinguishes Hartnup from generalized aminoaciduria that can be the result of many other disorders.

The similarity of Hartnup disease to pellagra has prompted treatment with nicotinic acid (25 mg/day). Tryptophan ethylester also has been effective (272). However, the tendency for symptoms to remit spontaneously and for general improvement to occur with improved dietary intake and advancing age makes such therapy difficult to evaluate.

In 1917, Göppert demonstrated that galactosemic children excreted galactose after the ingestion of lactose (milk) and galactose (293). In 1956, Schwarz and associates found that administration of galactose to affected children gave rise to an accumulation of galactose-1-phosphate (294). This was confirmed by Kalckar and his group, who were able to demonstrate a deficiency in galactose-1-phosphate uridyltransferase (GALT), the enzyme that catalyzes the conversion of galactose-1-phosphate into galactose uridine diphosphate (UDP-galactose) (Fig. 1.8) (295). The gene for galactosemia has been mapped to the small arm of chromosome 9 (9p13). It has been cloned and sequenced, and numerous mutations have been identified (296,297). Two point mutations (Q188R and K285N) account for 69% to 80% of galactosemia in whites and result in a complete enzyme deficiency; most of the remaining mutations result in detectable amounts of enzyme activity (296,297). In African-American patients, the most common mutation (S135L) accounts for 45% of the mutant alleles. Patients homozygous for the S135L allele have residual red cell GALT activity and a milder clinical course (298). Compound heterozygotes of a classic galactosemia allele and the Los Angeles or Duarte variant are asymptomatic.

In classic galactosemia, the metabolic block is essentially complete. Lactose of human or cow’s milk is hydrolyzed to galactose and glucose, the latter being handled in a normal manner. The metabolism of galactose, however, stops after the sugar is phosphorylated to galactose-1-phosphate. This phosphate ester accumulates in erythrocytes, lens, liver, and kidney (299). Galactitol, the alcohol of galactose, also is found in the lens, brain, and urine of galactosemic individuals (300), and can be demonstrated in brain by MRS (303). Berry and coworkers postulated that galactitol induces cerebral edema that is occasionally present in neonates (301). Factors responsible for mental retardation can include a deficiency of uridine diphosphate galactose, a reduction of glycolytic intermediates, and a loss of adenosine triphosphate (ATP) (300).

Administration of galactose to affected infants results in marked hypoglycemia. This has been explained by an increased insulin release, prompted by the large amounts of circulating reducing substance, or by assuming interference by galactose-1-phosphate with normal glycogen
breakdown. The peripheral hypoglycemia is enhanced by competition between glucose and galactose at the level of the hexose transport across the blood–brain barrier. As a consequence, the brain in galactosemic patients is in a constant hypoglycemic environment.

Intolerance to galactose decreases with increasing age. In part, this intolerance might be a result of the decreasing importance of milk as a food item. In patients with classic galactosemia who have been tested repeatedly, no increase in erythrocyte transferase activity has been found (302).

The main pathologic lesions are found in the liver and brain. In the liver, several stages are recognized. Initially one sees a severe, diffuse, fatty metamorphosis. The hepatic cells are filled with large, pale, fat-containing vacuoles (303). If the disease remains untreated, the liver cell cords are transformed into pseudoglandular structures. The final stage is pseudolobular cirrhosis. Cerebral alterations are nonspecific. Edema, fibrous gliosis of white matter and marked loss of cortical neurons and Purkinje cells are the most prominent findings (304).

Infants with galactosemia appear healthy at birth, although their cord blood can already contain abnormally high concentrations of galactose-1-phosphate, and a few already have cataracts and hepatic cirrhosis, probably as a consequence of intrauterine galactosemia (305). In severe cases, symptoms develop during the first week of life. These include vomiting, diarrhea, listlessness, and failure to gain weight. Increased intracranial pressure due to cytotoxic cerebral edema can be a presenting sign (306). Infants are jaundiced. This may represent a persistence of neonatal jaundice or can appear at age 3 to 5 days (307). On a normal diet, hypoglycemia is not common. By age 2 weeks, hepatosplenomegaly and lenticular opacifications are easily detectable. The cataracts can be cortical or nuclear and can be present at birth (300). The most frequently observed opacity is a central refractile ring (308). The infants are hypotonic and often have lost the Moro reflex. Sepsis caused by Escherichia coli occurs with high frequency and is responsible for the majority of deaths during the neonatal period (309). Other secondary effects of deranged galactose metabolism include ovarian failure or atrophy and testicular atrophy (310). The MRI commonly shows multiple areas of increased signal in white matter, predominantly in the periventricular region (311).

If galactosemia goes untreated, growth failure becomes severe and the infant develops the usual signs of progressive hepatic cirrhosis. In some infants, the disease can be less severe and does not manifest until age 3 to 6 months, at which time the presenting symptoms are delayed physical and mental development. By then, cataracts can be well established and the cirrhosis far advanced.

In another group of galactosemic individuals, the diagnosis is not made until patients are several years old, often on evaluation for mental retardation. They might not have cataracts or albuminuria. Intellectual retardation is not consistent in untreated galactosemic children. When present, it is moderate; IQs range between 50 and 70. Asymptomatic homozygotes also have been detected. Most of these have some residual transferase activity.

The enzyme defect is best documented by measuring the erythrocyte GALT activity. Several methods are available and have been be used for statewide and nationwide screening. In the Austrian screening program reviewed by Item and coworkers, only 1 in 190 infants who tested positive initially actually had galactosemia (297). Some of these were galactosemia carriers or Duarte/galactosemia compound heterozygotes.

Galactosuria, usually in combination with glucosuria or fructosuria, is seen in severe hepatic disorders of the neonatal period (e.g., neonatal hepatitis, tyrosinosis, congenital atresia of the bile ducts). Families in which several members are mentally defective and have congenital cataracts without an abnormality in galactose metabolism have been described by Franceschetti and others (312).

The antenatal diagnosis of galactosemia can be made by assay of GALT activity on cultured amniocytes or chorionic villi (315).

When milk is withdrawn and lactose-free products such as Nutramigen or Prosobee are substituted, gastrointestinal symptoms are rapidly relieved and normal growth resumes. The progression of cirrhosis is arrested, and in 35% of patients, the cataracts disappear (307). Because infants have developed hypoglycemia when first placed on a galactose-free diet, it might be useful to add some glucose to the formula.

The propensity of galactosemic neonates for E. coli sepsis requires securing cultures of blood, urine, and CSF and treating against this organism until the test results are negative (309). In the larger series of Waggoner and coworkers, sepsis was suspected in 30% of neonates and confirmed in 10% (314).

Recommendations for the management of infants and children with galactosemia have been published by Walter and coworkers (315). Maintenance of the galactose-free diet and avoidance of milk and milk products is recommended until at least after puberty. In several children who returned to a milk-containing diet before puberty, cataracts flared up. Even after that age, some continue to be sensitive to milk products and prefer to avoid them. Intermittent monitoring of erythrocyte galactose-1-phosphate levels has been suggested. Even
children with well-controlled galactosemia have elevated galactose-1-phosphate levels. Endogenous formation of galactose-1-phosphate from glucose-1-phosphate by way of the epimerase reaction (see Fig. 1.8) is believed to be responsible.

The long-term outlook for galactosemic patients is not as good as was initially believed, even when the diet is carefully monitored. A number of cases of progressive cerebellar ataxia and extrapyramidal movement disorders have been reported (316,317). In the series of Waggoner and colleagues, cerebellar deficits were seen in 18% (314); in the series of Kaufman and colleagues, the incidence was 27% (317). The cause for this syndrome is unclear, and MRI studies on patients with this syndrome do not differ from those without it (317). Rogers and Segal postulated that the syndrome results from an endogenous product of galactose-1-phosphate from UDP-glucose via the epimerase reaction (318). Another possibility is that a deficiency of UDP-galactose could limit the formation of cerebral glycoproteins and galactolipids. Administration of uridine, which has been suggested to prevent these complications, has not been effective (319).

Cognitive deficits are common. In 70% of galactosemic children treated from birth, the IQ was 90 or higher; however, approximately 50% of the youngsters had significant visual and perceptual deficits, and 33% had EEG abnormalities (320). Studies confirm that most patients have cognitive deficits in one or more areas. Verbal dyspraxia occurs in some 62%. Patients are unable to program their speech musculature and also show frequent disturbances in speech rhythm. Receptive language is normal (317,321). In addition, there appears to be a progressive decline in IQ with age (314,317). Neither IQ scores nor the presence of the dyspractic speech disorder are highly related with the age at which therapy is initiated or quality of control (314). Instead, cognitive and language deficits are likely to result from the in utero formation of potentially neurotoxic galactose-1-phosphate and the continued generation of galactose from glucose via UDP-galactose-4-epimerase (see Fig. 1.8) (300,322).

Two other defects of galactose metabolism have been recognized. A deficiency of galactokinase (see Fig. 1.8) is the more common (OMIM 230200). Cataracts are present in most patients and pseudotumor cerebri has been seen occasionally. The outlook in terms of mental function is better than for patients with galactosemia, and most appear to be normal (323). A very rare disorder, generalized deficiency of epimerase (OMIM 230350), the enzyme that converts UDP-glucose to UDP-galactose (see Fig. 1.8), can result in galactosuria, failure to thrive, sensorineural deafness, dysmorphic features, and mental retardation (324).

This is a recessive disorder with an incidence of approximately 1 in 30,000. It is caused by a defect in the gene for glutaryl-CoA dehydrogenase. Biochemically, the condition is marked by the excretion of large amounts of glutaric, 3-hydroxyglutaric, and glutaconic acids. The clinical picture is protean, and the disorder is frequently undiagnosed or misdiagnosed. In approximately 20% of patients, glutaric aciduria I takes the form of a neurodegenerative condition commencing during the latter part of the first year of life and characterized by hypotonia, dystonia, choreoathetosis, and seizures (377). In most of the remaining patients, development is normal for as long as 2 years of age, and then, after what appears to be an infectious, an encephalitic, or a Reye syndrome–like illness or following routine immunization, neurologic deterioration occurs (378,379). In yet other cases, the clinical picture
resembles extrapyramidal cerebral palsy (380). Rare cases remain asymptomatic. Macrocephaly is noted in 70% of cases (381). Lack of appetite, sleeplessness, and profuse sweating also is noted, as is hypoglycemia (382,383). No molecular basis for the clinical variability exists, and the severity of the clinical phenotype seems to be closely linked to the development of encephalopathic crises rather than to residual enzyme activity or genotype (384).

Neuroimaging shows frontotemporal cortical atrophy giving what has been termed a bat wing appearance. This is often accompanied by increased signal in the basal ganglia on T2-weighted images and caudate atrophy. Bilateral subdural hematomas have also been described (Fig. 1.9). When these are accompanied by retinal hemorrhages, an erroneous diagnosis of child abuse frequently is made (390).

Neuropathology shows temporal and frontal lobe hypoplasia, degeneration of the putamen and globus pallidus, mild status spongiosus of white matter, and heterotopic neurons in cerebellum (385). It is not clear why so much of the damage is localized to the basal ganglia. Kolker and coworkers postulated that 3-hydroxyglutaric acid, which is structurally related to glutamic acid, induces excitotoxic cell damage, and that in addition 3-hydroxyglutaric acid and glutaric acid modulate glutamatergic and gamma-aminobutyric acid (GABA)-ergic neurotransmission. Secondary amplification loops could also potentiate the neurotoxic properties of these organic acids (386).

A low-protein, high-calorie diet supplemented with a formula lacking lysine and tryptophan and containing carnitine has sometimes prevented further deterioration, but in the Scandinavian series of Kyllerman and colleagues almost all patients were left with a severe dystonic-dyskinetic disorder (387). The experience of Hoffmann and coworkers was similar (381). Strauss and colleagues, who worked with the disease in the Pennsylvania Amish population, however, found that good dietary care can reduce the incidence of basal ganglia injury to 35% (388).

In this condition the defect has been localized to one of three genes involved in the mitochondrial β-oxidation of fatty acids: those coding for the α and β subunits of the electron transfer flavoprotein (ETF) and that coding for the electron transfer flavoprotein dehydrogenase (389,390). The clinical picture of GA II due to the different defects appears to be nondistinguishable; each defect can lead to a range of mild or severe cases, depending presumably on the location and nature of the intragenic lesion, that is, mutation, in each case. As a consequence of the enzyme deficiency there is increased excretion not only of glutaric acid, but also of other organic acids, including lactic, ethylmalonic, isobutyric, and isovaleric acids.

The heterogeneous clinical features of patients with MADD fall into three classes (391): a neonatal-onset form with congenital anomalies (type I), a neonatal-onset form without congenital anomalies (type II), and a late-onset form (type III).

Glutaric aciduria II can present in the neonatal period with an overwhelming and generally fatal metabolic acidosis coupled with hypoglycemia, acidemia, and a cardiomyopathy. Affected infants can have an odor of sweaty feet. As a rule, there is good correlation between the severity of the metabolic block, rather than its location and the severity of the disease, with null mutations producing the development of congenital anomalies and the presence of a minute amount of enzymatic activity allowing the development of type II disease (392).

Dysmorphic features are prominent in approximately one-half of cases (393). They include macrocephaly, a large anterior fontanel, a high forehead, a flat nasal bridge, and malformed ears. This appearance is reminiscent of Zellweger syndrome (Fig. 1.10). Neuroimaging discloses agenesis of the cerebellar vermis and hypoplastic temporal lobes (394). The neonatal-onset forms are usually fatal and are characterized by severe nonketotic hypoglycemia, metabolic acidosis, multisystem involvement, and excretion of large amounts of fatty acid- and amino acid-derived metabolites.

Standard treatment consists of a high-carbohydrate, low-fat and protein diet supplemented with carnitine. Van Hove and coworkers suggested that this condition be treated with D,L-3-hydroxybutyrate (430 to 700 mg/kg
per day). In their experience this treatment resulted in improvement of neurologic function and cardiac contractility (395).

Symptoms and age at presentation of late-onset MADD (type III) are highly variable and characterized by recurrent episodes of lethargy, vomiting, hypoglycemia, metabolic acidosis, and hepatomegaly often preceded by metabolic stress. Muscle involvement in the form of pain, weakness, and lipid storage myopathy also occurs. The organic aciduria in patients with the late-onset form of MADD is often intermittent and only evident during periods of illness or catabolic stress. Other cases present in early childhood with progressive spastic ataxia and high signal intensity on T2-weighted MRI in supratentorial white matter, a picture mimicking a leukodystrophy (398).

The neuropathologic picture is marked by diffuse gliosis of cerebrum, brainstem, and cerebellum, with foci of leukomalacia and striatal degeneration (393,397).

Some infants with glutaric aciduria II respond dramatically to riboflavin (100 mg three times a day). This riboflavin-responsive multiple acyl-CoA dehydrogenase deficiency may be identical to ethylmalonic-adipic aciduria (396,398,399).

This condition is the result of a defect of peroxisomal glutaryl CoA oxidase. It results in a persistent isolated glutaric acid excretion, but the phenotype of this disorder is unknown, and it may well represent a benign inborn error (400).

The other, rarer organic acidurias are summarized in Table 1.15.

The other disorders of fatty acid β-oxidation are much less common. They are summarized in Table 1.14. In VLCAD the defect is at the first step of the fatty acid β-oxidation cycle. Clinically, failure of long-chain fatty acid β-oxidation leads to hypoketotic hypoglycemia associated with coma, liver dysfunction, skeletal myopathy, and hypertrophic cardiomyopathy. Patients are unable to metabolize fatty acids with 12 to 18 carbons (420,421,422,423). Three clinical forms have been recognized: a severe form with early onset and a high incidence of cardiomyopathy; a milder, childhood-onset form; and an adult form with rhabdomyolysis and myoglobinuria (422,423,424). The clinical picture appears to correlate with the amount of residual enzyme activity (417). VLCAD can be effectively treated with avoidance of fasting and a diet low in long-chain fats supplemented with medium-chain triglyceride oil and carnitine. Uncooked cornstarch at night can be added to the regimen (425).