Inborn Errors of Metabolism in Neonates
Inborn errors of metabolism should be suspected in those with suspicious family or obstetric history. The commonest presentation in neonates is acute encephalopathy. Arterial blood gases with anion gap measurement, ammonia level, blood glucose, urine for ketone bodies and urine smell as well as negative sepsis screening results are helpful screening tests.
Keyword : Metabolism; Neonate
Inborn errors of metabolism are rare disorders that mimic much commoner conditions like sepsis especially during the neonatal period. Hence, a working knowledge about inborn errors of metabolism is essential for those working with neonates otherwise the true diagnosis could be missed resulting in serious and undesirable consequences.
Practical biochemistry for cell energy production1-2 (Fig. 1)
Glucose is the final common step leading to the process of energy generation from all carbohydrate. All exogenous carbohydrates like fructose, galactose, lactose, sucrose, starch, and endogenous carbohydrate, ie glycogen, are converted to glucose for generation of energy. Glucose would be converted to pyruvate in glycolysis. Pyruvate, a 3-carbon ketoacid, is converted to acetyl CoA which is fed into the Krebs cycle for generation of the reduced form of nicotinamide adenine dinucleotide (NADH). NADH is carried through oxidative phosphorylation for generation of adenosine triphosphate (ATP). ATP is the currency of energy at the cellular and molecular level.
Fat is composed of a 3-carbon backbone (glycerol) coupled to long chain fatty acids with varying degrees of saturation. Majority of energy is stored in long chain fatty acids. In a process of beta-oxidation, successive 2-carbon units are removed from the fatty acids. The 2-carbon unit is acetyl CoA which is fed into Krebs cycle for NADH and ATP production.
Protein / amino acids
Amino acid is comprised of two components, the amino group and an organic acid backbone. The amino group is metabolized in the form of ammonia, which is converted to urea through the urea cycle in the liver. Different organic acids are metabolized somewhat differently. However, acetyl CoA, pyruvate and ketones are the common end products.
Hong Kong Data (Table I)
Selected areas of inborn errors of metabolism were reported from Hong Kong.3-5 The author conducted an informal survey of various public hospitals to assess the number of cases of inborn errors of metabolism. In one hospital "A", the data was retrieved for patients with discharge diagnoses compatible with those listed in Table I from 1987 to 1997. For other hospitals, individual hospital contact person was requested to survey his/her colleagues for number of patients with diagnoses listed in Table I. As all these diagnoses are unusual, it is unlikely that doctors responsible would forget. This informal survey would reflect the cumulative incidence over the past years. The true incidence must be higher as a number of cases would have been missed, and a number of cases would be treated in private hospitals. Assuming these cases were recruited over ten years, the average annual incidence of treated error of metabolism as defined here would be around 9 cases per year in public hospitals. Given an average annual birth rate of 78,000 and assuming all affected babies born in private hospital were referred to public hospitals, the incidence of inborn errors of metabolism as listed in Table 1 would be at least 1.1 per 10,000 live birth. In Hong Kong, the commonest inborn errors of metabolism are glycogen storage diseases followed closely by organic acidurias. Urea cycle disorder would come third. Amongst organic acidurias, multiple carboxylase and ketothiolase deficiencies occurred more frequently than others.5 In China, regional screening programs for PKU have documented an incidence of 1/16,500 in Beijing, 1/17,000 in Shanghai.6-7 A pilot screening project for PKU was done in Hong Kong and no PKU was found.3 In Guangzhou, the incidence was estimated to be 5 per 100,000.8
A history of consanguinity increases the risk of genetic diseases from 2 to 4 %. As the placenta serves as an effective hemodialyser, affected infants are usually normal at birth and remain well for the first few days. Previous stillbirths or unexplained neonatal deaths would further increase the suspicion. Most of these disorders are due to protein intolerance and the affected infant often improves when feeding is withheld only to relapse with resumption of feeding. Affected neonates often present with poor feeding, depressed conscious level and convulsion. Besides the characteristic odour of isovaleric acidaemia, glutaric aciduria type II, maple syrup urine disease and cataracts in galactosaemia, specific signs are often lacking. The single most important indicator of an inborn error is altered neurologic status disproportionate to systemic symptoms and signs of the presumed etiologies, e.g. sepsis, asphyxia.9
Types of Clinical Presentations
(1) acute encephalopathy
Acute Encephalopathy (Table II, Fig. 2)
Acute encephalopathy is the commonest clinical manifestation of inborn errors of metabolism. It is usually due to diffusible small molecules and is often associated with hepatomegaly. Most of these disorders are due to defects of catabolic pathways including pathways of amino acid or organic acid degradation, defects of fatty acid oxidation, defects of ureagenesis, disorders of pyruvate metabolism and defects of the mitochondrial electron transport chain. These infants are usually well during the first few days after an uneventful pregnancy and a normal delivery. Unexpected deterioration occurs with poor feeding, vomiting, poor weight gain, increasing lethargy progressing and seizures to coma. Differential diagnoses include infection, cerebral malformation, birth trauma, intoxication.
Suggested Initial Metabolic Screening
This includes blood pH, blood gases and glucose, plasma electrolytes, and renal and liver function tests. Urine is checked for reducing substance and ketones. For those with metabolic acidosis, the anion gap should be determined. Anion gap greater than 25mmol/l makes organic acidaemia a likely cause.
Plasma ammonia level should be checked and it is raised in organic acidopathies and urea cycle defects as well as in liver disease and sepsis (Fig. 3). Lactic acidosis is an important labaratory finding, but it is difficult to interpret. It is important to remember that plasma lactate and pyruvate would be raised by two to three fold in the presence of venous obstruction by crying, torniquet or restraint. The most important difference is to differentiate primary from secondary lactic acidosis causes by hypoxia, cardiac disease, infection or convulsions. Persistently raised level of lactate and pyruvate alert one to suspect primary pyruvate metabolism defects. Hypoglycemia is seen in disorders of carbohydrate or fat metabolism. Heavy ketonuria is unusual in the neonatal period and its detection should be immediately followed by organic acid analysis.
The laboratory should be provided with full details of diet intake and drugs administration when metabolic screen is requested. Where possible a blood sample must be taken before any blood transfusion. Amino acids concentrations are estimated in blood and urine by high performance liquid chromatography. One-dimensional thin layer chromatography differentiates at least five reducing sugar residues simultaneously in urine: lactose, galactose, glucose, fructose and xylose. Gas chromatography-mass fragmentography is used for organic acid analyses. Urine orotic acid level should be checked if plasma ammonia concentration is high.
General Management for Suspected Severe Inborn Errors of Metabolism
Fluid replacement and correction of acid-base and electrolytes imbalance is the important first step. Often large doses of bicarbonate is necessary and steady infusion is usually preferred. For those with resistant acidosis, a range of treatment options are available. This includes haemodialysis, peritoneal dialysis, high-flow hemofiltration at 300ml/hr to 600ml/hr ultrafiltrate removal or continuous venous-venous hemofiltration with dialysis (CVVHD) or exchange transfusion. Assisted ventilation is often required to prevent hypoxia or hypercapnia. Adequate glucose, using 15-20% glucose with insulin (1 IU/3g glucose, or 0.05unit/kg/hour1-2 days)10 is employed to promote anabolism after first 24 hours.
A combination of vitamins in pharmacological doses are often given intravenously to sick infants while awaiting results because several inborn errors of metabolism have vitamin responsive forms. A typical megavitamin cocktail is listed in Table III.
Role of Carnitine in Inborn Errors of Metabolism11
Carnitine is an amino-acid derivative. It has several important functions including: (1) the transfer of long chain fatty acids across the inner mitochondrial membrane for oxidation, (2) esterification of potentially toxic acyl-CoA metabolites which impair the citric acid cycle, (3) gluconeogensis, (4) urea cycle and fatty acid oxidation in acute clinical crisis, (5) faciliation of branched chain alpha-ketoacid (metabolites of branched chain amino acids) oxidation.
75% of our daily carnitine requirement is derived from diet. Breast milk, diary products and red meat are rich sources of carnitine. The rest of the daily carnitine requirement is derived from endogenous production in liver and kidneys with conversion from lysine or methionine.
Carnitine is excreted in kidneys with active absorption across proximal tubules. The absorption threshold is 40 μmol/L.
Clinical conditions that benefit from carnitine supplement
Specific Causes of Acute Encepathlopathy
1. Amino acid diseases
Maple syrup urine disease is a classical example of inborn errors of metabolise. The defects lie in the branched chain 2-keto acid dehydrogenase complex. This results in a failure to catabolise the branched chain amino acids - leucine, isoleucine and valine. The incidence is reported at 1 in 180,000 world-wide. Four cases were reported from 1983-1994 in Hong Kong. One more case was diagnosed in Hospital "A" during this period, giving a minimum incidence of 1 in 175,800 for Hong Kong Chinese.
There are five clinical types with classical form being most severe and the only form that presents during neonatal period. Clinical features include lethargy, poor sucking, alternate hypotonia and hypertonia, dystonic posture, seizure and cerebral edema. The urine smells like maple syrup or burnt sugar. Urine would be positive for 2, 4 dinitrophenylhydrazine (DNPH) test and raised level of branched amino acids especially leucine is found in blood, urine and CSF.
In the acute phase, peritoneal dialysis is instituted to remove toxic metabolites. Long term management involves dietary adjustment with branched chain amino acids free formula plus a small prescribed amount of leucine, isoleucine and valine as deficiency of these three amino acids may lead to severe dermatitis resembling acrodermatitis enteropathica. Liver transplant provides definitive cure.
2. Organic acidopathies
Accumulation of organic acids occur in many disorders in the metabolism of amino acids, carbohydrate and lipids. 15 cases of organic acidurias were reported in one series from one centre in Hong Kong from 1989 to 1995.5
Propionic acidaemia is a prime example. The defect lies in biotin-dependent propionyl-CoA carboxylase which converts propionyl-CoA (an intermediate metabolite of isoleucine, valine, threonine, methionine) to methylmalonyl-CoA. Clinical features include lethargy, poor feeding. Ammonia level is raised with marked ketoacidosis. Raised glycine level is also found. Hence, this disorder is also known as ketotic hyperglycinaemia. Acute management include peritoneal dialysis or hemodialysis, trial of biotin and protein restriction. Carnitine decreases the toxic effect of acyl coenzyme A ester within the cell by conjugating with these compounds to from acylcarnitine esters, which are excreted through the kidney.
In isovaleric acidaemia, glycine conjugates with isovaleryl groups for excretion. Hence, glycine supplementation leads to metabolic correction.10 Forced diuresis with intravenous volume loads of 200 ml/kg/day of 10% dextrose with adequate sodium and calcium supplement with or without diuretic administration is the treatment of choice in acute metabolic deterioration of methylmalonic acidaemia because the clearance of methylmalonic acid by peritoneal dialyisis is less than the renal clearance.12
For propionic acidaemia, synthetic amino acid devoid of isoleucine, valine, threonine and methionine is advised. Liver transplant is an alternative treatment.
3. Fatty acid metabolic defects
Long chain fatty acid couples with carnitine to enter the mitochondria whereas medium chain and short chain fatty acids enter the mitochondria readily. Long-chain, medium-chain and short chain acyl-CoA dehyrogenases are the first enzymes in the beta-oxidation of fatty acids for production of ketone bodies which serve as fuel for the brain and muscle after prolonged fasting. MCAD is the commonest defect in this group with incidence of 1/10,000-15,000. These disorders usually present after three months of age when longer interval of feeding is introduced.
Clinical features include fasting non-ketotic hypoglycemia, coma, hepatomegaly, sudden infant death syndrome, cardiac hypertrophy, muscle hypotonia and recurrent Reye-like syndrome. Investigation reveals hyperuricaemia, hyperammonaemia, decreased total plasma carnitine, increased esterified carnitine and dicarboxylic aciduria.
Treatment involves avoidance of prolonged fasting and carnitine administration may be beneficial by augmenting urinary excretion of toxic acyl-CoA intermediates.
4. Urea cycle disorders12-13
Ammonia is formed from the catabolism of amino acids, cytosine, adenosine and amines and by gut bacteria. Ammonia is highly toxic to the central nervous system although the mechanism is unclear. As ammonia is highly toxic, it is rapidly converted to urea in the liver by a series of reactions termed the urea cycle. Ammonia enters the urea cycle in the liver mitochondria and ends with the production of urea in the cytoplasm.
Clinical features include encephalopathy, liver failure, respiratory alkalosis, pulmonary haemorrhage and intracranial haemorrhage. The plasma urea may be low but the most important investigation is the plasma ammonia concentration. Measurement of plasma amino acids, primarily citrulline level, and urine orotic acid level is helpful to determine the defective enzymes involved. Plasma citrulline level is in excess of 1000uM/L in arginosuccinic acid synthetase deficiency. Its level is between 100 and 399 uM/L in argininosuccinase deficiency. Citrulline is found in absent to trace amount in both ornithine transcarbamylase deficiency and carbamyl phosphate synthease deficiency. However, urinary orotic acid concentration is high in ornithine transcarbamylase deficiency but low in carbamyl phosphate synthetase deficiency. Specific enzyme assay is possible in erythrocytes, leucocytes, skin fibroblasts or liver biopsy.
Severe hyperammonaemia is a medical emergency. All dietary protein must be withdrawn and intravenous glucose started. Peritoneal dialysis is an effective way to bring down ammonia level. It is the oft-used method despite its inferior ammonia clearance compared with haemodialysis which is technically more difficult. Ten per cent arginine hydrochloride (6ml/kg/day), sodium benzoate (250mg/kg/day) and sodium phenylacetate (250 mg/kg/day) given intravenously are used to promote ammonia excretion by alternative pathways.12 These drugs serve as the first line treatment for urea cycle disorders. Long term management includes protein restriction, arginine supplementation for argininosuccinate synthase deficiency and argininosuccinate lyase deficiency. Sodium benzoate and sodium phenylacetate or butyrate are employed to provide an alternative pathways of nitrogen excretion in some of the urea cycle disorders.
5. Transient hyperammonemia of newborn
The mechanism is not known. It usually occurs in the first 24 hours of life with respiratory distress in premature babies. Treatment is similar to urea cycle disorders.
6. Primary lactic acidaemia (Fig. 4)
The essential feature is raised lactate level with or without acidosis in the absence of cardiac, pulmonary failure nor sepsis, i.e. tissue hypoxaemia. The incidence of primary lactic acidosis is not known and it was estimated that 250 new cases were recognised in the US per year.14 Causes include disorders of oxidative phosphorylation, pyruvate oxidation and Krebs cycle enzymes as well as defects of gluconeogensis.
7. Nonketotic hyperglycinemia
It is a severe autosomal recessive neurological disorder. The defects in one of four proteins (P,H,T,L) of the glycine cleavage system. Glycine is an inhibitory neurotransmitter in spinal cord but is an excitatory one in cerebral cortex. It also activates N-methyl-D-aspartate receptors (NMDA) thereby augmenting excitatory synaptic transmission. Preventing this effect on NMDA receptors might be of therapeutic value17 as activation of NMDA receptors causes an excessive calcium influx into neurones and may finally lead to cytolysis.18
Clinical features include rapidly progressive hypotonia, obtundation, seizure, apnoea and characteristic hiccup within the first days of life. The types of seizures can Inborn Errors of Metabolism in Neonates vary from abnormal eye movement and myclonic seizures to generalized tonic-clonic convulsion. Elevation of cerebral spinal fluid (CSF) glycine to plasma glycine ratio is the diagnostic test as plasma glycine may be normal.
Hypomagnesaemia should be corrected if it is found. Strychnine, a potent antagonist of glycine at the inhibitory glycine receptors given at dosage of 0.2 to 1mg/kg/day divided in six doses, may be helpful in mild cases to decrease muscular hypotonia. Ketamine intravenous infusion at 0.1mg/kg/hour for four weeks followed by oral ketamine 1mg/kg/day divided in 6 doses is found to be helpful in one case report to decrease convulsion frequency. 17
Neurological outcome is poor but early use of ketamine may possibly be helpful to improve the outcome.
8. Pyridoxine-Dependent Epilepsy
The defect probably lies in glutamic acid decarboxylase. Clinical features include serious seizures with no other identifiable disorders. Clinical and EEG response to pyridoxine should be sought. Trial of pyridoxine 75mg bid for 3 weeks should be attempted even if there is no immediate response.
Neonatal jaundice glucose-6-phosphate dehydrogenase (G6PD) deficiency results in decreased production of NADPH which is important to protect red cell membrane against oxidative stress and subsequent hemolysis. G6PD deficiency affects 400 million people worldwide.19 G6PD deficiency is a X-linked recessive condition. 4.42 % of Hong Kong Chinese male and 0.4% of Chinese female are affected. Heterozygous female is also at increased risk of haemolysis.20
They are prone to hemolysis by oxident drugs, infection e.g. umbilical sepsis. Even for those without demonstrable hemolysis, moderately higher bilirubin level still tends to develop. G6PD level may also be low in leucocyte, platelet, liver, kidneys and adrenals in affected individuals. Usually no organ dysfunction is seen because of the presence of alternate pathways in nucleated cells for the generation of NADPH.
Salt-losing congenital adrenal hyperplasia presents in the neonatal period with dehydration, shock and hyper-pigmentation. Females would present with ambiguous gentalia resulting from excessive androgen production. The commonest defect is 21-hydroxylase deficiency. Plasma 17 hydroxy-progesterone level is elevated.
The lysosome is a single membrane bound organelle in all cells, containing numerous enzymes. These enzymes function to degrade macromolecules for recycling. A few cases have been reported to present during the neonatal period as hydrops fetalis. Otherwise, they usually present later in life, eg mucopolysaccharidoses, sphingolipidosis. There is no effective treatment available for most of these disorders. There have, however, been some successes for a few disorder, for example, enzyme replacement therapy for Gaucher disease and bone marrow transplantation for Maroteaux-Lamy syndrome.
Peroxisome is a single membrane bound organelle found in all cells except mature red blood cells. It has both anabolic and catabolic functions. Anabolic functions include synthesis of plasmalogen (ether phospholipid found in cell membrane especially myelin), cholesterol and bile acid. Catabolic functions include oxidation of a number of amino acids and organic acids. Three kinds of defects have been reported.
It is usually found in mother with poorly controlled phenylketouria. High phenylanine levels act as teratogen and affected babies would present with microcephaly, growth retardation, congenital heart defect especially ventricular septal defect.
Diffuse Liver Disease
Causes include galactosaemia, glycogen storage disorders and tyrosinemia type I.
This is due to galactose - phosphate 1-uridyl transferase deficiency. Clinical features include E. coil sepsis in neonate, cataract, hemolysis, hypoglycaemia and prolonged jaundice. Urine for reducing substance is often negative. Urine chromatography for galactose should be performed in suspected case. Erythrocytes galactose-6-phosphate uridyl transferase activity can be measured by a commercially available assay kit.
Dietary restriction of lactose and galactose is advised. Subtle defects of intellectual function are often found despite good dietary compliance.
Glycogen storage disorders
Common defects include glucose-6-phosphotase in type I and debranching enzyme in type III. Features include hepatomegaly, hypoglycemia, hyperuricaemia, hyperlipidaemia and lactic acidosis. Hypoglycaemia can be prevented by frequent feeds during the day and continuous nasogastric feeding at night, in infancy and early childhood. Raw cornstarch (2g/kg every six hours) has been shown to be effective in preventing hypoglycaemia in older children with glycogen storage disease type I as well as decreasing the hyperlipidaemia, hyperuricaemia and lactic acidaemia.21
This involves defect in fumaryl acetoacetase, which is the last enzyme in the tyrosine catabolic pathway. Acute form presents with severe hepatocellular dysfunction, hepatomegaly and acute neurologic crises that began at mean age of 1 year. Chronic form presents with progressive liver failure, hepatocellular carcinoma and porphyria-like syndrome. Investigations show generalised amino aciduria, elevated liver transminases and markedly raised serum alpha-fetoprotein level. Raised succinylacetone level in urine is diagnostic. The enzyme deficiency can be demonstrated in lymphocytes and cultured skin fibroblasts.
NTBC (nitro-trifluoromethylbenzoyl-cyclcthexane), a pesticide derivative, brings down serum succinylacetone level and improve clinical symptoms. The oral dose is 0.1 to 0.6mg/kg/day. Liver transplant is recommended as a long term measure to decrease chance of hepatocellular carcinoma.
Management when Death seems Inevitable
Post-mortem examination must be carried out as soon as possible after death so that biochemical studies can be made on tissues that have not undergone enzymatic self digestion. If post-mortem examination is declined, limited biopsy study including liver, muscle and skin should be obtained. Serum should be frozen together with as much urine as possible.
Samples for biochemical studies should be snap-frozen (in dry ice or liquid nitrogen) and stored at -70°C until assay.10
As a group, inborn errors of metabolism is not uncommon. They would usually masquerade as sepsis during neonatal period. It is therefore important for paediatricians to have a working knowledge about this group of disorders in order to diagnose them early. This would often mean a relatively normal life versus a severely handicapped one or even death for those that are diagnosed late.
The author would like to thank Prof. CY Yeung for his guidance during the preparation of the manuscript, Ms. Clara Hung for expert secretarial assistance, Dr.KY Chan, Dr.SY Chan, Dr.J Hui, Dr.CW Law, Dr.T Leung, Dr.KT So, Dr.P Tse for providing personal data of their respective hospitals.
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