Ghai Essential Pediatrics8th

and the irreversible damage or handicap can be prevented or reduced through several therapeutic approaches:

i.The deficiency of the metabolic endproduct may be made up by replacement or administration of the product. Thus, thyroxine restoresthethyroid function in familial goiterogenous cretinism; cortisone suppresses excess ACTH production and androgen synthesis in adrenogenital syndrome and adminis­ tration of Factor VIII/IX prevents bleeding in cases of hemophilia.

11.The intake of substances which cannot be meta­ bolized bythebody should be reduced, especially if their accumulation is potentially toxic, e.g. in galactosemia, galactose cannot be metabolized adequately.Aslactoseinthemilkishydrolyzedinthe body to glucose and galactose, milk in the diet of the affected infant is substituted by lactose free dietary formulaetoobviatedamageduetoexcessof galactose in tissues. The phenylketonuric infants placed on restricted phenylalanine in the diet may escape irreversible neurological damage.

iii.Certaindrugs,whichprecipitateadversesymptomsin metabolicdisorders,suchasbarbituratesinporphyria andoxidatingagentsinglucose-6-phosphatedehydro­ genase deficiency, should never be administered to

affected patients.

1v. Patientswith hemophiliaand osteogenesisimperfecta should be protected from trauma and other environ­ mental hazards to prevent excessive bleeding and fractures, respectively.

v.Surgery helps to reduce the functional or cosmetic disability in many structural defects.

vi.The excretion of certain metabolites canbe promoted by chelating agents, e.g. penicillamine promotes excretion of copper in patients with Wilson disease and desferrioxamine can be used to chelate iron in patients with thalassemia and hemochromatosis.

vii.Certain enzyme systems which may be immature or reduced at certain phases of life may be induced or stabilizedby theuse of chemicalagents.Forexample, phenobarbitone is used to induce hepatic micro­ somal enzymes like glucuronyl transferase in cases of neonatal hyperbilirubinemia or Crigler-Najjar syndrome.

viii.In some metabolic disorders, enzymatic block can be bypassed by administration of large quantities of the coenzyme, e.g. pyridoxine in homocystinuria.

ix.Enzymereplacementtherapyhasbecomefeasiblewith the availability of deficient enzymes for Gaucher disease, Hurler syndrome, Hunter syndrome, mucopolysaccharidosis type VI, Fabry disease and

Pompedisease.Thecostofthetreatmentisprohibitive.

x.Bisphosphonates, both intravenous and oral, have been useful in cases of osteogenesis imperfecta.

xi.Stem cell transplantation is recommended for many genetic disorders likethalassemiamajor, severe form

Genetic Disorders -

PREVENTION OF GENETIC DISORDERS

Carrier Screening

It is now possibleto detect the carrier state in a large num­ ber of autosomalrecessive orX-linkedrecessive disorders.

Female carriers of Ouchenne muscular dystrophy may showhighserum levelsoftheenzymecreatinine phospho­ kinase, but can be tested more precisely using molecular techniques. Female carriers of glucose-6-phosphate dehydrogenase deficiency are detected by demonstrating relativelylowlevelofenzymesintheirerythrocytes.HbA2 levels are usefulinidentifying carriers of pthalassemiatrait in high-risk communities. Molecular techniques are increasingly used for detection of individuals who are m ore likely to give birth to offspring with hereditary dis­ orders.

Newborn Screening

This is an example of secondary prevention by early diagnosis and treatment. Newborn infants are screened routinely for some endocrine disorders and inborn errors of metabolism in developed countries. This is of special value for detecting the affected cases during the newborn period, sothat thehandicapcanbepreventedor minimized by early treatment, e.g. in cases of congenital hypo­ thyroidism, congenital adrenal hyperplasia, phenylketo­ nuria, galactosemia and tyrosinemia.

Prevention of Neural Tube Defects (NTD)

Folic acid supplementation is recommended at a dose of 0.4 mg daily from one month before to three months after conception to prevent NTD. Expectant mothers at high­ risk of NTD (e.g. previous fetus with NTD) should con­ sume 4 mg of folic acid daily to prevent recurrence of neural tube defects.

Prenatal Diagnosis and Selective Termination of Affected Fetuses

This is a successfully used modality for preventing birth of affected babies and reducing the load of lethal, chronically disabling, untreatable or difficult-to-treat genetic disorders in the community. The prenatal screening or diagnostic modalities can be noninvasive or invasive. Noninvasive techniques include fetal ultrasono­ graphy and maternal serum screening.

Maternal Serum Screening

Estimation of pregnancy associated plasma protein A (PAPP-A) andfree P-humanchorionicgonadotropin(hCG) in the first trimester and serum alpha-fetoprotein, hCG, w1conjugated estriol and inhibin A in second trimester are useful biochemical markers to detect aneuploidies. If the risk of bearing a child with Down syndrome is more than 1:250, prenatal fetal karyotyping can be offered. Fetal ultrasonography helps to detect fetuses who are at high­ risk for chromosomal abnormalities. Important findings in thesecondtrimester which are markers of Down syndrome include increasednuchalfold thickness(measured over the occiput andnot the spine), short femur and humerus length and duodenal atresia. In the first trimester, nuchal trans­ lucency and nasal bone are robust markers. Ultrasound findingshelp in cow1seling, particularly if the parentshave opted for initial screening with maternal serum markers. Both maternal serum screening and fetal ultrasound are screening techniques and cannot rule out Down syndrome. The detection rate of triple test in the second trimester is about 65% with a false positive rate of 5%. First trimester screening using dual markers have high detection rates, which improves further if ultrasound markers are com­ bined. Alpha-fetoproteinand estriol are low, whereas hCG is high, in pregnancies with Down syndrome fetuses. All three markers are reduced in fetuses with trisomy 18. Elevated alpha-fetoprotein level in maternal blood is also a very sensitive marker for fetuses affected with open neural tube defects.

Invasive Prenatal Testing

This includes chorionic villus biopsy (done at 10-12 weeks of gestation or later), amniocentesis (16-20 weeks) and

cord blood sampling (after 18 weeks). Procedure related risk is lowest withamniocentesis(- 0.5%), whilechorionic villus biopsy carries a risk of fetal loss in about 2%. These samples can be used for chromosomalstudies, DNA based tests or enzyme assays. Amniotic fluid is the preferred sample for chromosomal studies and chorionic villus tissue for DNA based tests. Single gene disorders with a known gene can be diagnosed prenatally. Some common examples are thalassemia, sickle cell anemia, hemophilia, Duchenne muscular dystrophy and cystic fibrosis.

Genetic Counseling

Genetic counseling is a communication process, which deals with problems associated with the occurrence and recurrence of a genetic disorder in a family. Counseling should be undertaken by a physician with proper under­ standingof thegeneticmechanisms.Someimportantindi­ cations for genetic counseling are as follows: (i) known or suspectedhereditarydiseasein apatientorfamily; (ii) birth defects inpreviouschildren; (iii) unexplainedmentalretar­ dation, dysmorphism, multiple malformations in a child; (iv) consanguinity; (v) exposure to a teratogen during pregnancy; and (vi) identification of malformation(s) by ultrasonography during pregnancy.

Suggested Reading

Cassidy SB, Allanson JE Management of genetic syndromes, 3rd edition, Wiley Blackwell, USA,2010

Harper PS. Practical Genetic Cow1seling, 5th edn. Wright Publishers, Bristol, 2004

Reardon W. The Bedside Dysmorphologist. Oxford University Press, 2008

Rimori DL, Cooner JM, Pyeritz RE, Korf BR. Principles and Practice ofMedical Genetics, 5th edn., Churchill Livingstone, Philadelphia, 2006

Inborn errors of metabolism (IEM) are conditions caused by genetic defects related to synthesis, metabolism, transport or storage of biochemical compounds. The metabolic error usually results in the deficiency of one or more enzymes required for the formation or transport of proteins. The worldwide incidence ofIEMs is 3-4/1000 live births; most are inherited in an autosomal recessive manner.

SUSPECTING AN INBORN ERROR OF METABOLISM

IEMs may present in the newborn period, in early or late childhood, or in adults. The diagnosis is often delayed, and requires a high index of suspicion, since symptoms are nonspecific, leading to evaluation for other pediatric illnesses like sepsis and hypoxic ischemic encephalopathy.

Features of Metabolic Disorders

•Sudden and rapid illness in a previously normal baby precipitated by fever, vomiting or fasting

•Nonspecific, unexplainedfeatures such as poor feeding, lethargy, vomiting, hypotonia, failure to thrive, respi­ ratory abnormalities, hiccups, apnea, bradycardia and hypothermia, with normal sepsis screen

•Rapidly progressive encephalopathy of unknown etiology

•Persistent or recurrent hypoglycemia, intractable metabolic acidosis, unexplained leukopenia or throm­ bocytopenia

•Hyperammonemia

•E. coli sepsis

•Organomegaly

•Peculiar odor (musty in phenylketonuria; cabbage like in tyrosinemia; maple syrup like in maple syrup urine disease; like sweaty feet in isovaleric acidemia, or glutaric acidemia type II; like cat urine: in 3-methyl- crotonyl CoA carboxylase or multiple carboxylase deficiency)

•Family history of unexplained neonatal deaths or progressive neurological disease, HELLP (hemolysis,

Neerja Gupta, Madhulika Kabra

elevated liver enzymes, low platelet counts) syndrome in mother

• Parental consanguinity

Classification

Based on the pathophysiology, IEMs can be classified as follows:

Intoxication group includes disorders of intermediary metabolism, with accumulation of toxic compounds resulting in acute or progressive symptoms. Amino­ acidopathies (e.g. phenylketonuria andmaple syrup urine disease), organic aciduria, urea cycle defects, disorders of carbohydrate and copper metabolism and porphyrias belong to this category. Symptoms are often precipitated by catabolic state (fever, infections, immunization, dehydration or fasting).

Defects in energy metabolism include conditions associated with deficient energy production or utilization within liver, muscle, heart andbrain, e.g. mitochondrial disorders, disorders of glycolysis, glycogen metabolism and gluco­ neogenesis and hyperinsulinism. Failure to thrive, hypo­ glycemia, hepatomegaly, hypotonia, cardiomyopathy, myopathy, high lactate, neurological symptoms, circulatory collapse or sudden death may be seen.

Disorders of complex molecules include lysosomal storage diseases, peroxisomal disorders, a,-antitrypsin deficiency and congenital disorders of glycosylation. Symptoms are usually progressive and permanent and do not have precipitating factors.

Metabolic disorders can have either acute or chronic presentation (Table 23.1).

Acute Presentation

Neonates with metabolic disorders appear normal at birth since the small intermediary metabolites are eliminated by the placenta during fetal life. Disorders of glucose, protein and fat breakdown usually present early, although

Table 23.1: Classification of inborn errors of metabolism

premature neonates with transient hyperammonemia of newborn (THAN) and term babies with glutaric acidemia type II or pyruvate carboxylase deficiency may present on the first day of life. In general, an early onset of clinical symptoms is associated with severe disease. The onset of illness is delayed in the intermittent or milder forms.

An important clue to diagnosis is unexpected deteriorationafter normal initial period in afull term baby. Neonates with organic acidurias,ureacycledisordersandsomearninoacidurias may present with lethargy, poor feeding, persistent vomiting, seizures, tachypnea, floppiness and body or urine odor. Common conditions such as sepsis, hypoxic ischemic encephalopathy and hypoglycemia should be excluded.

Older children show acute unexplained, recurrent episodes of altered sensorium, vomiting, lethargy pro­ gressing to coma, stroke or stroke like episodes, ataxia, psychiatric features, exercise intolerance, abdominal pain, quadriparesis or arrhythmias. The symptom free period may be prolonged, often longer than a 1 yr and patients are normalin between the episodes. Intercurrent illnesses, high protein intake, exercise, fasting and drug intake (enzyme inducers) mayprecipitate symptoms. Encephalo­ pathy occurs with little warning in previously healthy individuals; progresses rapidly, may be recurrent and show fluctuatingconsciousnessand is not associatedwith focal neurological deficits.

Physical examination may show altered sensorium, apnea or hyperpnea and hypotonia. Facial dysmorphism, structural anomalies of brain, cataract, retinopathy, deaf­ ness, hypertrophic or dilated cardiomyopathy, hepato­ megaly, multicystic dysplastic kidneys, myopathy and peculiar urine odor suggest specific diagnoses.

Laboratory Investigations

Biochemical tests may be normal when the child is asymptomatic.The initial screeninginvestigationsinclude total and differential counts and blood levels of sugar, electrolytes, bicarbonate,calcium,transarninases, ammonia, lactate and pyruvate. During neonatal period, ammonia levels are <200 µg/dl; beyond neonatal age, levels

<80 µg/dl are considered normal. In urea cycle disorders, blood ammonia levels exceed 1000 µg/dl and cause respiratory alkalosis with compensatory metabolic acidosis. In organic acidurias, ammonia levels are <500 µg/dland in fattyacidoxidation defects <250 µg/dl.Urine metabolicscreenincludes pH,ketones, reducing substances and ferric chloride, dinitrophenylhydrazine (DNPH) and nitroprusside tests.

Specializedtestssuchasquantitativeurinaryandplasma amino acids analysis by high performance liquid chroma­ tography (HPLC), plasma carnitine and acylcarnitine by tandem mass spectrometry (TMS) and urinary organic acids by gas chromatography and mass spectrometry (GCMS) are helpful in reaching a conclusive diagnosis. Cerebrospinalfluid,chestX-ray, echocardiography, ultra­ sound abdomen, computed tomography (CT) head, magnetic resonance imaging of brain and electroence­ phalogram (EEG)are useful in specific cases.

Acutely presenting IEMs are classified into five major categories (Table 23.2). Figure 23.1 describes the initial approach in such patients.

Biochemical Autopsy

In a severely ill or dying child, where an IEM is suspected, parents should be advised about the need for a bio­ chemical autopsyforconfirmation of diagnosis. Following informed written consent, the following samples should be obtained postmortem to facilitate diagnosis.

Blood: 5-10 ml each in heparin (for plasma) and EDTA (leukocytes); store at -20°C

Urine: Store at -20°C

Cerebrospinal fluid: Store at -20°C

Skin biopsy (including dermis: Store at 37°C in culture medium or saline with glucose).

Liver, muscle, kidney, heart biopsy: Tissue frozen Clinical photograph and infantogram

Management

Treatment isofteninstitutedempirically; promptmanage­ ment may be lifesaving. Dietary or parenteral intake of potentially toxic compounds (such as protein, fat, galac­ tose, fructose) is eliminated; adequate calories are pro­ vided using 0.2% saline in 10% dextrose intravenously. lntralipids (2-3 g/kg/day) may be infused if fatty acid oxidation defect is not suspected. Metabolic acidosis (pH <7.30, bicarbonate <15 mEq/1, anion gap >16 mEq/1) shouldbe corrected. Bloodlevelsofsugar,pH and electro­ lytes should be monitored.

The excretion of toxic metabolites is enhanced by hemo­ dialysis or using alternative pathways for nitrogen excre­ tion. Immediate measures to decrease plasma levels of ammonia are necessary as the risk for irreversible cerebral damage is related to its concentration. IV phenylacetate andsodiumbenzoatewithL-arginine (Table 23.3) are used as detoxifying agents. Dialysis is initiated if plasma ammonia levels exceed 500-600 µg/dl, or if levels do not

GCMS Gas chromatography and mass spectrometry; TMS Tandem mass spectrometry; + present; N normal; i

Poor feeding, persistentSuspectedvomiting,metabolicseizures,disorderfloppiness, encephalopathy

Plasma +ammonia

increased

No acidosisl

Aminoacidopathies Nonketotic hyperglycinemia Galactosemia Peroxisomal disorders

fall within 2 hr after initiation of IV treatment. Hemo­ dialysis is preferred to peritoneal dialysis and exchange transfusion.

Carnitine eliminates organic acids as carnitine esters. Carnitine may be used in life-threatening situations associated with its deficiency, at a dose of 25-50 mg/kg IV given over 2-3 minutes, followed by 25-100 mg/kg/ day orally. L-carnitine should not be administered with sodium benzoate. Intractable seizures without metabolic acidosis or hyperammonemia are treated with pyridoxine 100-200 mg IV.

Ifclinicalimprovement isobservedand afinaldiagnosis is not established, some amino acid intake should be

provided after 2-3 days of complete protein restriction. Essential amino acids or total protein is provided orally or IV at an initial dose of 0.5 g/kg/day and gradually increasedto1.0-1.5g/kg/day,untildiagnostic evaluation is complete and plans are made for longterm therapy. Appropriate amino acidformula (free of precursor amino acids) or protein free infant formula with breast milk is gradually introduced with careful clinical and laboratory monitoring. Expressed human milk is preferred as it can be measured and total protein intake quantified.

Empiric cofactor or coenzyme therapy may be adminis­ tered (Table 23.4) to maximize residual enzyme activity awaiting final diagnosis. Longterm strict adherence to

2hr

* The dose of arginine hydrochloride can be decreased to 200 mg/kg for carbamoyl phosphate synthetase or ornithine transcarbamylase deficiency; IV intravenous

dietary and pharmacologic regimen is recommended. Prompt recognition and avoidance of physiologic stresses (fever, infection, trauma, surgery, fasting) and changes in diet that may precipitate symptoms is important in pre­ venting metabolic decompensation.

Chronic and Progressive Presentation

This group of metabolic disorders is characterized by variable but insidious onset from birth to adulthood. Unexplained developmental delay with or without seizures, organomegaly, coarse facies, cataract, dislocated lens, chronic skin lesions, abnormal hair, abnormal urine color on standing andfailure to thrive are important clues. These forms are divided into subgroups depending upon the involvement of specific system. The approach to a patient with chronic encephalopathy is shown in Fig. 23.2.

Neurologicfindingsare developmental delayor progressive psychomotor retardation, seizures, ataxia, spasticity, variable hearing and visual impairment, and extra-

pyramidal symptoms. Psychomotor or developmental delay is the chief manifestation and tends to be global and progressive. History of regression of milestones may be present. Severe irritability, impulsivity, aggressiveness and hyperactivity and behavioral patterns such as auto­ matism, stereotypes, compulsive chewing of thumbs and fingers, self-mutilation and nocturnal restlessness arecom­ mon. Complex partial or myoclonic seizures occur early in course of the disease and are often resistant to therapy. Signs include change in tone and pyramidal or extra­ pyramidal deficit. Differentiating between involvement of either gray matter or white matter is helpful in narrowingthedifferentialdiagnosis (Table. 23.5). Movement disorders are intermittent or progressive, in form of ataxia, dystonia, choreoathetosis and Parkinsonism. Underlying conditions include organic acidurias, late-onset neuronal

Gray matter diseas

Pyridoxine dependency Biotinidase deficiency Neuronal ceroid lipofuscinosis

GM2 gangliosidosis (early onset) Mitochondrial, e.g. Leigh disease, MELAS

White matter disease

Motor difficulties

Tone abnormalities

Central only: Canavan disease, Alexander disease, GM2 and GM1 gangliosidosis (late),

adrenoleukodystrophy, aminoacidopathies, organic aciduria

Central and peripheral: Metachromatic leukodystrophy, Krabbe disease, peroxisomal disorders

Sialidosis II

Zellweger syndrome

Table 23.5: Differentiating features of gray matter and white matter disorders

ceroid lipofuscinosis, lysosomal storage disorders and urea cycle disorders.

Muscular disorderspresentingwithmyopathy are usually due to defects in energy metabolism. Myopathy can be progressive (glycogen storage disease, GSD types II and III), exercise intolerance with cramps and myoglobinuria (GSD V, VI), or as part of multisystem disease (mito­ chondrial myopathies).

Hepatic presentations include the presence of unconju­ gated or conjugated jaundice, hypoglycemia and hepato­ megaly with or without hepatocellular dysfunction. Coexisting deranged lipid profile is seen in GSD type I and hepatosplenomegaly a feature of GSD III and lyso­ somal storage disorders. Hepatocellular dysfunction is seen in galactosemia, GSD IV and III, Niemann-Pick type B, ai-antitrypsin deficiency. Disordersleading to cirrhosis include tyrosinemia, galactosemia, hereditary fructose intolerance and Wilson disease.

Cardiac manifestations may occur in fatty acid oxidation defects, mitochondrial disorders, GSD type II, methylma­ lonic acidemia, Fabry disease, Kearns-Sayre syndrome,

Inborn Errors of Metabolism --

familial hypercholesterolemia, mucopolysaccharidoses and GMl gangliosidosis.

Dysmorphic features are presentinpatientswithZellweger syndrome,glutaricaciduriatype 2andstoragesyndromes.

Renal manifestations are seen in patients with cystinosis, galactosemia, hereditary fructose intolerance and tyro­ sinemia (renal tubular acidosis); progressive renal failure is common in patients with cystinosis. Enlarged kidneys are seen in patients with GSD type I.

OcularfindingsoftenprovideacluetotheunderlyingIEM. The presenceofcataract(s)suggestsgalactosemia,peroxi­ somal disorders, Lowe syndrome and Wilson disease. Cornealabnormalitiesareseeninmucopolysaccharidoses, Wilson disease and Fabry disease. Patients with homo­ cystinuria show lens dislocation. Cherry-red spots are foundin various lysosomal storage diseases, suchas Tay­ Sachs disease, GMl gangliosidosis and Niemann-Pick disease.

Skin may show an eczematous rash associated with alopecia in biotinidase deficiency. Angiokeratoma are characteristic of Fabry disease, but can be seen in fucosi­ dosis and -mannosidosis.

Evaluation

Investigations should include complete hemogram, liver and renal function tests and serumelectrolytes. Pancyto­ penia may be seen in patients with methylmalonic acidemia and propionic acidemia. The peripheral smear may show vacuolated lymphocytes in neuronal ceroid lipofuscinosis,fucosidosisandsialidosis;acanthocytosisin abetalipoproteinemia and Hallervorden-Spatz disease (pantothenate kinase associated neurodegeneration). Adrenal insufficiency is frequent in patients with adrenoleukodystrophy. Metabolic acidosis and evidence ofproximalrenaltubulardysfunctionispresentinpatients with Lowe syndrome, cystinosis, Wilson disease and galactosemia. Investigationsthatenablespecificdiagnosis include neurological imaging and electrophysiological studies and skeletal survey. Specific enzyme assays and estimation of plasma levelsof lactate, ammonia, verylong chainfattyacidsandaminoacidsareusefulincertaincases.

Management

Amultidisciplinary team ofmetabolicspecialists, pediatric neurologists, clinical geneticist, cardiologist, orthopedic surgeon and physiotherapist is required to maximize the supportivecare inthese patients. Other treatmentoptions include cofactor and megavitamin therapy, special diets, enzyme replacement therapy and organ transplantation. Most IEMs are inherited in autosomal recessive manner and risk of recurrence in subsequent pregnancy is 25%. Few disorders are X-linked, autosomal dominant and mitochondrial in inheritance. Prenatal diagnosis is possi­ ble by enzyme assays or mutation testing in fetal DNA in chorionic villus biopsy metabolites in amniotic fluid and using fetal DNA (Chapter 22).

SPECIFIC DISORDERS

Aminoacidopathies

These disorders do not have a common phenotype but have unique features depending upon the site of defect.

Phenylketonuria

Phenylketonuria (PKU) is a disorder of phenylalanine metabolism and occurs due todeficiencyof phenylalanine hydroxylase (PAH).

Clinicalfeatures. Affected individuals have profound and irreversible intellectual disability, microcephaly, epilepsy and behavioral problems. These patients often have a musty body odor and skin conditions such as eczema caused by excretion of excessive phenylalanine and its metabolites. Decreased skin, hair and eye pigmentation may also be present due to associated inhibition of tyrosinase and reduced melanin synthesis (Fig. 23.3).

Diagnosis. Modalities include (i) newborn screening: PKU can be detected in virtually 100% of cases by various methods of newborn screening such as Guthrie card bacterial inhibition assay (BIA), fluorometric analysis and tandem mass spectrometry. In classic PKU, plasma phenylalanine level is >1000 µmol/1 with <1% residual PAH activity, (ii) molecular genetic testing of the PAH gene.

Treatment. A low-protein diet and use of phenylalanine­ free medical formula as soon as possible afterbirth to achieve plasma concentrations 120-360 µmol/1 (2-6 mg/ dl) is recommended. A significant proportion of patients with PKU may benefit from adjuvant therapy with single daily dose of 5-10 mg/kg tetrahydrobiopterin.

Maple Syrup Urine Disease (MSUD)

MSUD is due todecreasedactivityofbranchedchainalpha ketoacid dehydrogenase (BCKAD) complex, a mito-

Fig. 23.3: Blond hair in a child with phenylketonuria

chondrial enzyme involved in degradation of branched chain amino acids (leucine, isoleucine and valine). There are five different phenotypes based on clinical findings and response to thiamine. This enzyme has four subunits: El(Xf Ei p, E2 and E3.

Clinicalfeatures. Thefirst sign of classic MSUD in untreated neonates is maple syrup odor in cerumen at 12-24 hr afterbirth. By48-72hr,poorfeeding, ketonuria, irritability and drowsiness develops followed by unexplained progressive coma. Thecharacteristic urine smell develops on day 5-7 of life. In advanced stage, intermittent apnea, bradycardia, hypothermia, generalized hypertonia, opisthotonusand involuntary movements such asfencing and bicycling may appear. Individuals with acute intermittent late onset forms of MSUDcanhave recurrent episodes of severe metabolic decompensation and encephalopathy during any catabolic stress. Chronic progressiveformscanpresentwithvariable manifestations such as developmental delay or progressive psychomotor retardation, seizures, failure to thrive, sleep disturbances, hyperactivity, mood swings and movement disorders.

Diagnosis. (i) Urine 2,4-dinitrophenylhydrazine (DNPH) test to detect ketonuria by adding DNPH to urine which produces a yellow-white precipitate due to presence of branched chain ketoacids, (ii) elevated plasma levels of leucine, isoleucine, valine (5 to 10-fold greater than normal)andalloisoleucine, (iii)enzymaticand/or genetic testing are useful for confirming the diagnosis.

Treatment. Treatment during acute stage shouldfollow the abovementioned principles along with rapid removal of branched chain amino acids from body tissues and fluids using either peritoneal or hemodialysis. Cerebral edema is a common complication should be treated promptly in an intensive care setting with mannitol, hypertonic saline and diuretics. During recovery high calorie, branched chain amino acid free formula is initiated early with regularplasma amino acid monitoring. Somepatientswith milder forms may respond to thiamine. Orthotopic liver transplantation is effective therapy for classic MSUD.

Hepatorenal Tyrosinemia Type I

The condition is caused by deficiency of enzyme fumaryl­ acetoacetate hydrolase (FAH), encoded by FAH gene. Enzyme is mainly expressed in liver and kidney.

Clinical features. It is a disorder of tyrosine metabolism, classicallypresentsassevereliverdiseasein young infants. Severe forms present during infancy with vomiting, diarrhea, bleeding diathesis, hepatomegaly, jaundice, hypoglycemia, ascites and coagulopathy. Children older than sixmonthsof age maycome to medical attention with variable degree of renal dysfunction, hypophosphatemic rickets and aminoaciduria. Untreated children may have repeated, often unrecognized, neurologic crises lasting 1-7 days that can include change in mental status, abdominal

pain,peripheralneuropathy,autonomicdysfunctionand/ or respiratory failure requiring mechanical ventilation. Death in the untreated child usually occurs before age ten years, typically from liver failure, neurologic crisis or hepatocellular carcinoma.

Diagnosis The following tests are useful: (i) Deranged liver function tests; prolonged prothrombin and partial thromboplastin times, (ii) generalized aminoaciduria; radiological evidence of rickets, (iii) markedly elevated serum concentration of alpha-fetoprotein(average160,000 ng/ml) (normal: <1000 ng/ml for infants 1-3 months; <12 ng/ml for age 3 months to 18 yr), (iv) increased succinylacetone concentration in the blood and excretion in the urine. Elevated plasma concentration of tyrosine, methionine and phenylalanine, (v) enzyme assay, and (vi) molecular genetic studies for FAH gene.

Treatment Nitisinone or 2-(2-nitro-4-fluoromethyl­ benzoyl)- 1,3-cyclohexanedione(NTBC) treatment should begin as soon as the diagnosis of tyrosinemia type I is confirmed. It blocks tyrosine degradation at an early step topreventtheproductionof downstreammetabolitessuch as fumarylacetoacetate and succinylacetone. It is given at doses of1mg/kg/day. Dietary restriction ofphenylalanine and tyrosine is required to prevent tyrosine crystals from forming in the cornea. In Western countries, prior to the availability of nitisinone, the only definitive therapy for tyrosinemia type I was liver transplantation, which now isreservedfor thosechildren who have severe liver failure at presentation and fail to respond to nitisinone therapy or have documented evidence of malignant changes in hepatic tissue. Nitsinone is not readily available in India and is expensive.

Classic Homocystinuria

This occurs due to cystathionine -synthase deficiency leading to accumulation of homocysteine, which has deleterious effects on the central nervous system, vessels, skin, joints and skeleton. Two clinical variants exist: B6 (pyridoxine)-responsive homocystinuria and B6-non­ responsive homocystinuria. B6-responsivehomocystinuria is typically, but not always, milder than the nonresponsive variant and has a better outcome than the nonresponsive ones.

Clinical features. Patients typically present with ocular, skeletal, CNS and vascular manifestations usually after 3--4 yr of age. Developmental delay, seizures, psychiatric problems and extrapyramidal signs such as dystonia, downward lens dislocation and/or severe myopia, marfanoid habitus, osteoporosis with or without thromboembolic complications are the usual presenting features. They can also have hypopigmentation and pancreatitis. Ectopia lentis occurs by age eight years. Thromboembolism is a major cause of early death and morbidity.

Inborn Errors of Metabolism -

Diagnosis. Quantitative plasma amino acid analysis showing increased levels of methionine, homocysteine with no cystathionineconfirmsthe diagnosis. Plasmatotal homocysteine levels are important for monitoring the treatment(normallevels<15µmol/1;homocystinuria >200 µmol/1). Confirmation can be done by CBS enzyme acti­ vity or molecular testingfor CBS gene. Urinenitroprusside test is a good screening test.

Treatment. Treatment is directed towards lowering the plasma homocysteine levels asclose tonormalas possible. About half of all patients respond to vitamin B6 therapy (200-1000 mg/day). In patients with folate and vitamin B12 deficiency, folic acid (5 mg/day) and hydroxy­ cobalamin(1mg intramuscularlyper month)isalsogiven. Patients nonresponsive to pyridoxine require lifelong methionine restricted diet with frequent biochemical monitoring. Oral betaine at 150 mg/kg/day (in two divided doses) is effective in lowering homocysteine levels. Vitamin C supplementation (1 g/day) ameliorates endothelial dysfunction.

Alkaptonuria

This was the first inborn error of metabolism described by Garrod in 1902 and is caused by defect of the enzyme homogentisate 1,2-dioxygenase (homogentisic acid oxidase). The most prominent symptoms are related to connective tissues and joints. These manifestations are rarely noticed before the age of 20 to 30 yr.

Clinical features. The disorder comes to attention due to change in color of urine to brownish black or staining of diapers. Pigment deposits irritate the articular cartilage, resulting in degeneration and osteoarthritis like changes. Intervertebral disks are degenerated,spaces are narrowed and calcification occurs. Ochronotic arthritis commonly involves shoulders and hips. Pigment deposits in the kidney manifest as renal stones. A grayish discoloration of sclera and the ear and nose cartilage (ochronosis) usually occurs after 30 yr. The pigment in ochronosis is a polymer of homogentisic acid.

Diagnosis. The urine becomes dark on standing, especially if the pH of urine is alkaline. Excessive urine homo­ gentisate resultsin positive reducing substances. Organic acid analysis by GCMS can identify and quantify homo­ gentisic acid.

Treatment. No specific therapy is known. Administration of vitamin C prevents deposits of the ochronotic material in cartilage but has no effect on the basic metabolic defect. Nitisinone inhibits the enzyme that produces homo­ gentisic acid and may prove useful.

Urea Cycle Defects

The urea cycle is the main pathway for the removal of highly toxic ammonia, derived from the catabolism of

amino acids, in the form of urea. It is basically composed of six enzymes as demonstrated in Fig. 23.4. Defects of any of these enzymes are characterized by hyperam­ monemia and deranged amino acid metabolism.

Clinical Features

Presentation is highly variable. In the classical forms, neonates present within first few days of life with poor feeding, recurrent vomiting, tachypnea, hypothermia, irritability, seizures and lethargy progressing to coma. Partial deficiencies of these enzymes represent milder forms and have symptoms that are often subtle and may notoccur for months or years. These are usually diagnosed by hyperammonemic episodes manifesting as poor appetite, vomiting, lethargy and behavioral problems and are often triggered by stress or illness. These patients are intolerant to and dislike protein food. Arginase deficiency has more specific symptoms such as spastic diplegia, dystonia and ataxia.

Glutamic acid + Acetyl CoA

N-acetyl glutamatesynthasel

N-acetyl glutamate1

Carbamoyl phosphate!

synthase (1) Ammonia

traOrnithinescarbamoylase (2)

Citrulline

Aspartate

Arginosuccinate

Ng

Diagnosis

The diagnosis of a urea cycle disorder is based on clinical suspicion and biochemical screening. Presence of hyperammonemia (plasma ammonia >80 µg/dl after neonatal period) associated with normal anion gap and normal glucose level suggests a urea cycle defect. Plasma amino acid analysis and urinary erotic acid can distinguish the specific defects (Fig. 23.5). A definitive diagnosis of a urea cycle defect depends on either DNA analysis or measurement of enzyme activity.

Management

Treatment is based on the principles highlighted in the section on management of acute presentation and includes rapid removal of ammonia and inhibition of its

Fig. 23.4: Pathways for ammonia disposal and ornithine metabolism. Deficiency of enzymes results in the following: (1) CPS deficiency,

(2) OTC deficiency, (3) citrullinemia, (4) arginosuccinic aciduria and

(5) argininemia

production along with treatment of any intercurrent illness and correction of dehydration or electrolyte imba­ lance. Maintenance therapy includes nutritional manage­ ment with restriction of protein, pharmacological therapy with sodium benzoate/phenylacetate (250-500 mg/kg/ day), essential amino acids (0.25 g/kg/day) and arginine (200-600 mg/kg/day).