Testing Strategy for Inborn Errors of Metabolism in the Neonate

Early detection and management of inborn errors of metabolism (IEMs) can improve the affected infant’s prognosis. Initial screening tests can provide a general overview of the infant’s metabolic status and suggest potential IEMs. Among the clinical findings seen in many IEMs are encephalopathy, hypoglycemia, jaundice and liver disease, cardiac arrhythmias, cardiomyopathy, hypotonia, dysmorphic features, and nonimmune hydrops. Confirmatory testing (enzyme analysis or molecular DNA testing) are required to make the diagnosis. Clinicians should be aware of specific requirements for such testing to obtain the desired results. Introduction Many IEMs present in the newborn period, and early detection and management can improve an infant’s prognosis substantially. The advent of extended newborn screening has made presymptomatic diagnosis of a number of these disorders possible. However, not all IEMs are identified on an extended newborn screen, and in some instances, an infant who has an IEM may become ill before the results of newborn screening become available. It is, therefore, important for the neonatologist to consider an inborn error of metabolism in the differential diagnosis for an acutely sick neonate and initiate appropriate testing. Sudden deterioration after a period of apparent normalcy is highly suggestive of a metabolic disorder. A neonate who has an IEM may present with symptoms that frequently are attributed to more common neonatal conditions, such as sepsis or gastrointestinal pathology, which may lead to a delayed or missed diagnosis. Signs such as an unusual odor (as in maple syrup urine disease), posturing (as in glutaric acidemia type 1), or apnea (as in nonketotic hyperglycinemia) can indicate an IEM. Parental consanguinity or a family history of neonatal death also should alert the physician to the possibility of a genetic disorder. In this review, we discuss the laboratory testing strategy that may be considered in the initial evaluation of a possible IEM. Discussion of the treatment of metabolic disorders is beyond the scope of this review. Initial Testing Strategy All of the tests listed in Table 1 should be obtained for neonates suspected of having IEMs. Understandably, an acutely ill neonate in metabolic crisis requires immediate management, but it is crucial to set aside samples (at least 5 mL plasma and 5 mL urine) before attempting to correct metabolic abnormalities. This is important for certain diseases where abnormal metabolites that are critical in providing a diagnosis are detected only in times of acute decompensation. The initial test results can provide a general overview of the *Division of Genetics and Metabolism, Department of Pediatrics, University of Florida, Gainesville, Fla. Article genetics NeoReviews Vol.9 No.7 July 2008 e291 at Childrens Mercy Hosp on March 14, 2013 http://neoreviews.aappublications.org/ Downloaded from metabolic status of the neonate and suggest IEMs that might be considered in the differential diagnosis (Table 2). Some IEMs have characteristic presentations, but initial test results are normal. These presentations and appropriate testing are discussed in the relevant sections of this article. Encephalopathy An infant who has encephalopathy can present with seizures, lethargy, or poor feeding that may be associated with metabolic abnormalities such as hyperammonemia and metabolic acidosis. Occasionally, an infant who has encephalopathy and an IEM may present with no other supporting laboratory abnormalities. Encephalopathy associated with hyperammonemia can be a feature of several inborn errors, which can be differentiated based on initial test results (Fig. 1). Elevated ammonia concentration within 24 hours of birth with normal plasma amino acids values suggests the diagnosis of transient hyperammonemia of newborn, a Table 1. Initial Tests for Evaluation of Possible Inborn Error of Metabolism General Laboratory Tests ● Blood gas ● Serum electrolytes ● Blood urea nitrogen and creatinine ● Blood glucose ● Liver function tests ● Creatine kinase ● Ammonia ● Serum uric acid ● Serum lactate ● Urine ketones ● Urinalysis Metabolic Screening Tests ● Plasma amino acids ● Plasma acylcarnitine profile ● Urine organic acids Table 2. Key Laboratory Findings for Neonates Who Have Inborn Errors of Metabolism Tests Key Finding IEM Consideration Complete blood count with differential count Neutropenia Hemolytic anemia OA, GSD I G6PD deficiency, pyruvate kinase deficiency Blood gas Metabolic acidosis Respiratory alkalosis OA, Mito, PDH, PC, FAO UCD Blood glucose 2 FAO, GSD I, Galac, OA, HFI, Tyr 1 Blood urea nitrogen 2 UCD Serum lactate 1 Mito, PDH, PC, FAO, GSD I Blood ammonia 1 UCD, OA, FAO, PDH, PC Serum uric acid 1 2 GSD I Molybdenum cofactor deficiency Creatine kinase 1 FAO Ketones 2 (with hypoglycemia) 1 FAO OA Plasma acylcarnitine profile FAO, OA Plasma (free and total) carnitine FAO, OA Plasma amino acids UCD, OA Urine organic acids OA, FAO, Mito, PDH, PC Urine reducing substances Galac, HFI, Tyr 1 Very long-chain fatty acids Peroxisomal disorders Urine mucopolysaccharides Lysosomal storage disorders Urine oligosaccharides Lysosomal storage disorders 7-dehydrocholesterol Smith-Lemli-Opitz syndrome Serum transferrin glycoforms CDG CDG congenital disorder of glycosylation, FAO fatty acid oxidation defect, Galac galactosemia, GSD I glycogen storage disease type I, G6PD glucose6-phosphate dehydrogenase, HFI hereditary fructose intolerance, IEM inborn error of metabolism, Mito mitochondrial energy metabolism defects, OA organic aciduria, PC pyruvate carboxylase deficiency, PDH pyruvate dehydrogenase deficiency, Tyr 1 tyrosinemia type 1, UCD urea cycle defect genetics inborn errors of metabolism e292 NeoReviews Vol.9 No.7 July 2008 at Childrens Mercy Hosp on March 14, 2013 http://neoreviews.aappublications.org/ Downloaded from poorly understood condition that is believed to have a nongenetic cause. Encephalopathy associated with metabolic acidosis due to inborn errors of metabolism usually is characterized by an increased anion gap. It is important to determine the source of this increase. Lactic acidosis is encountered commonly and if mild, usually is due to cardiovascular compromise. Unexplained elevation of lactic acid requires the consideration of a metabolic disorder (Fig. 2). In addition, an acylcarnitine profile can aid in the diagnosis of fatty acid oxidation disorders and organic acidemias. Some IEMs can present with an encephalopathic picture without hyperammonemia or metabolic acidosis. Maple syrup urine disease (MSUD) is caused by an inability to break down branched-chain amino acids such as leucine, isoleucine, and valine, and a plasma amino acid profile is diagnostic. Ketoacidosis is observed in MSUD but may be absent at initial presentation. Nonketotic hyperglycinemia is caused by an inability to breakdown glycine in the liver and is characterized by a severe encephalopathy with no other associated abnormalities. Plasma amino acids may show an elevated glycine, and supporting laboratory results may be noncontributory. In this clinical scenario, it is essential to obtain cerebrospinal fluid to measure glycine concentrations, which can be diagnostic. Pyridoxine-dependent seizures present as severe, intractable seizures within the first few hours of birth. This disorder is marked by a dramatic response to vitamin B6 administration. Severe seizures and encephalopathy usually developing in the first postnatal week also can suggest molybdenum cofactor deficiency. This is associated with combined deficiencies of the two enzymes that depend on molybdenum: xanthine oxidase and sulfite oxidase. In xanthine oxidase deficiency, uric acid is markedly decreased (not seen in isolated sulfite oxidase deficiency). The presence of sulfite in a fresh urine sample using a special dipstick test may be suggestive of this condition. However, elevated S-sulfocysteine concentrations in urine are more definitive. Hypoglycemia Hypoglycemia is a relatively common neonatal concern. Unexplained hypoglycemia may be caused by IEMs due to defects in carbohydrate metabolism and fatty acid oxidation. Occasionally, protein metabolism defects can have associated hypoglycemia, but other metabolic disturbances predominate. The tests listed in Table 1 should be ordered, with the addition of urine for reducing substances (Fig. 3). If nonglucose reducing substances are present in the urine, galactosemia, hereditary fructose intolerance, or tyrosinemia should be considered. Abnormally low ketones in the presence of hypoglycemia may be indicative of a fatty acid oxidation disorder. Fatty acid oxidation disorders are important because of their relatively high prevalence and usually are identified based on acylcarnitine profile abnormalities. Affected infants have an impaired capacity to use stored fats in periods of extended fasting. The presentation may be similar to Reye syndrome, with metabolic acidosis, hyperammonemia, and elevated liver enzymes, particularly transaminases. Hepatic glycogen storage diseases (GSDs), especially type I, cause hypoglycemia during periods of fasting. The diagnosis of GSD type I may be missed in the neonatal period because the hypoglycemia resolves when regular feeding is established. The liver is mildly enlarged in the first 2 postnatal days but can be markedly enlarged by the end of the first week. Abnormal laboratory study results that indicate the diagnosis of GSD type I include lactic acidosis, hyperuricemia, and hypoglycemia. Metabolic acidosis Hyperammonemia Early onset and normal amino acids Tra ns ien t hy pe ram mo ne mi a of the ne wb orn U rea cy cle de fec t Abnormal plasma amino acids Lactic acidosis Abnormal urine organic acids Py ruv ate me tab . d efe cts Mi toc ho nd ria l e ne rgy me tab . d efe cts Or ga nic ac ide mi as No acidosis Figure 1. Algorithm for evaluation of hyperammonemia in a neonate who has a suspected inborn error of metabolism. metab. metabolism. genetics inborn errors of metabolism NeoReviews Vol.9 No.7 July 2008 e293 at Childrens Mercy Hosp on March 14, 2013 http://neoreviews.aappublications.org/ Downloaded from Jaundice and Liver Disease Unconjugated hyperbilirubinemia may be seen in inborn errors of erythrocyte and bilirubin metabolism. Glucose6-phosphate dehydrogenase (G6PD) deficiency and pyruvate kinase deficiency can present in the newborn period with unconjugated hyperbilirubinemia due to hemolysis. Nonspherocytic hemolytic anemia, as noted on peripheral smear, helps in diagnosis, which can be confirmed by enzymatic analysis. Disorders of bilirubin metabolism, such as Gilbert, Crigler Najjar, DubinJohnson, and Lucey Driscoll syndromes, present with unconjugated hyperbilirubinemia and normal values for other liver function tests. Most of these disorders are benign. However, hyperbilirubinemia in type I Crigler Najjar syndrome may be severe enough to cause kernicterus. Most newborn screening programs include evaluation for classic galactosemia. It presents in the neonatal period with hyperbilirubinemia that is unconjugated in the early stages and predominantly conjugated in the later stages. If the neonate continues to be fed lactose-containing formula or human milk, the jaundice worsens, hepatomegaly appears, transaminases become elevated, and coagulopathy and hypoalbuminemia can occur. When classic galactosemia is suspected, urine and blood samples should be reserved for confirmatory testing, and the patient should be switched to a nongalactose-containing soy formula. The collected urine should be tested simultaneously with Benedict solution and a glucose oxidase method (urine dipsticks). A negative glucose oxidase test result and positive Benedict reagent reaction suggests the presence of a nonglucose reducing substance, and confirmatory testing can be undertaken. Aminoacidopathies such as tyrosinemia type 1 and citrullinemia type 2 may present with severe liver disease and an abnormal plasma amino acid profile. Excretion of succinylacetone in the urine is diagnostic for tyrosinemia type 1. Fatty acid oxidation disorders also may present with hepatocellular dysfunction and are diagnosed based on an abnormal acylcarnitine profile. Traditionally, hereditary fructose intolerance does not present in the neonatal period. However, affected infants who are exposed to fructose-containing feedings (as in sucrose-containing soy formulas) may develop acute hepatocellular failure, lactic acidosis, hyperuricemia, hyperchloremia, hypophosphatemia, and metabolic acidosis. Alpha-1-antitrypsin deficiency presents with jaundice and cholestasis that may resolve spontaneously by 2 months of age only to present later with cirrhosis. The disorder is identified by low plasma concentrations of alpha-1-antitrypsin and is confirmed by protein phenotyping and mutation analysis. The perinatal form of GSD type IV presents with fetal hydrops and severe hypotonia. It often is rapidly fatal and is diagnosed by a liver biopsy. Niemann-Pick disease type C is a progressive neurodegenerative disease that can involve self-resolving jaundice and hepatomegaly in the neonatal period. Diagnosis often requires specialized studies on fibroblast cultures. Neonatal hemochromatosis can cause rapidly progressive liver failure and requires a liver biopsy for diagnosis. Gl yc og en sto rag e d ise as e I He red ita ry fru cto se int ole ran ce ↑ Lactate Metabolic acidosis