Case Based Pediatrics For Medical Students and Residents
Department of Pediatrics, University of Hawaii John A. Burns School of Medicine
Chapter IV.4. Inborn Errors of Metabolism
Catherine Y.H. Wagoner, MD
June 2002

Return to Table of Contents

This is a 2 day old male infant who is referred from an outside hospital for persistent hypotonia and mild respiratory distress since birth. He also has feeding intolerance characterized by emesis following each feeding. He is the product of a full term pregnancy to a 26 year old G1P1Ab0, O+, hepatitis B negative, rubella immune, STS negative mother via NSVD with Apgar scores of 7 and 8 at 1 minute and 5 minutes, respectively. Birth was complicated by a nuchal cord x 1 and a maternal fever to 102 degrees just prior to delivery for which one dose of ampicillin was given. After delivery, the infant required some blow-by oxygen and was transferred to the newborn nursery. In the newborn nursery, the infant continued to require oxygen until 2 hours of life when he was noted to have adequate oxygen saturations in room air. A CBC was done which was reported as unremarkable. The baby nursed overnight without any difficulty, and was able to pass his first meconium at less than 24 hours old. However, his first breastfeeding was followed by emesis. A second feeding of water also resulted in emesis. Due to the persistent hypotonia, feeding intolerance, and continued mild respiratory distress despite adequate oxygenation, the infant was transferred to a tertiary care neonatal intensive care unit (NICU).

Exam: VS T 36.5, P 136, RR 44, BP 58/41, oxygen saturation 100% in room air. Wt: 3.95 kg (80%ile). He is a term-appearing male infant who is noted to be slightly tachypneic and intermittently grunting. His head, ears, eyes, nose and oropharyngeal structures are without obvious abnormalities, except for his tongue which is remarkable for lateral fasciculations. His neck is supple. His lungs are clear, but he has notable intermittent grunting. His heart is regular with no murmurs. His abdomen is flat and soft, but his liver is palpable 2 cm below RCM. His extremities are normal, with 1+ pulses. His DTRs are absent. Genitalia are normal. He is hypotonic with poor head control.

A full sepsis workup is done and he is started on empiric antibiotics. An ABG demonstrates a severe metabolic acidosis with pH 7.22 and Bicarbonate 10. Anion gap is 23. Lactic acid and ammonia levels are elevated.

A bicarbonate infusion is initiated to treat the acidosis, dropping the base deficit from -13 to -5, and then to +6. After consultation with genetics, it is felt that the infant likely has a defect in energy metabolism based on the persistent hypotonia and severe acidosis. A metabolic defect workup is done, including urine for organic acids, plasma for amino acids, and muscle biopsy for fibroblast culture and electron microscopy analysis. He is started empirically on a vitamin cocktail consisting of thiamine, niacin, riboflavin, B12, biotin, and L-carnitine for the possibility of a fatty acid oxidation or mitochondrial defect. An MRI on day 3 of life reveals severe cerebral atrophy and developmental brain anomalies including agenesis of the corpus callosum. He later decompensates requiring life support. The severe metabolic acidosis recurs and additional sodium bicarbonate infusions are required. Despite this level of support, he does not improve. A decision is made by his parents and the medical team to withdraw support, since the infant's condition is felt to be terminal.


Inborn errors of metabolism (IEM) are a diverse group of disorders. They are genetically based defects of the normal biochemical processes of the body that are required to maintain homeostasis (1). The potential for a problem is great since there are a great number of biochemical reactions that must be enzymatically carried out for normal metabolism. An enzyme defect in any pathway (e.g., Krebs cycle, urea cycle, oxidative phosphorylation, etc.) will potentially result in a condition that is incompatible with life, unless a therapeutic way to circumvent the metabolic defect can be accomplished. Unfortunately, many and probably most of the diseases in this group can lead to a debilitating and even tragic ending, as illustrated in the case above. Due to the sheer number of disorders classified as inborn errors of metabolism, literally hundreds with more being discovered each year, it would be impossible to give a comprehensive review of the subject in a single book chapter. However, there are several overriding principles and practices that are fundamental to the diagnosis and management of the patient with a suspected inborn error of metabolism.

The objectives of this chapter will be to: 1) Understand the basic genetic mechanisms which underlie inborn errors of metabolism. 2) Know some of the more commonly affected biochemical pathways that manifest as metabolic disease. 3) Learn to recognize the subtle signs and some of the constellations of symptoms that may point toward a specific metabolic disease. 4) To learn about the Hawaii State Newborn Screening Program, its goals, and the systems in place to ensure the identification of babies who may have a correctable or treatable disorder.

Some of the great discoveries of science in the 20th century were the identification of biochemical pathways in the body that facilitate the existence of life in all organisms. These biochemical processes, such as the Krebs cycle that converts glucose to energy in the form of ATP and the urea cycle that converts ingested nitrogens into a form that can be excreted, form the basis for the routine production of energy from food, excretion of waste products, and the regulation of the internal environment of the body.

There are literally hundreds of individual steps in the body's processes. Roche Pharmaceuticals Group has an interactive diagram of most of the known biochemical processes on their website (2). It is these biochemical steps that are the sites of defects that result in metabolic disease. Since each step is dependent on the product of the previous step to provide the substrate for a reaction, a "mistake" in either the substrate produced or the enzyme required for the reaction will result in a seriously magnified effect on all the products of the reactions downstream from the defective step. Ultimately, this may result in the inability to produce a necessary end product or to carry out a detoxification process.

The extent to which a metabolic defect may affect the function of the body is highly dependent on the final product of a biochemical cascade. For example, in ornithine transcarbamylase deficiency (OTC), a part of the urea cycle, carbamyl phosphate and ornithine are converted to citrulline. Citrulline is able to move outside the mitochondria, thereby transporting waste products of respiration to the cytoplasm where they can be processed further and ultimately excreted from the body. In OTC deficiency, there is accumulation of the upstream products of the reaction that leads to hyperammonemia. Over time, the high levels of ammonia will affect the brain, due to the toxicity of the waste product on the neurons. This will lead to the ataxia, seizures, cerebral atrophy, and encephalopathy observed in this disease (1-12).

It is of primary importance to understand the variable genetic mechanisms that can cause abnormal biochemical functioning. DNA is transcribed into ribonucleic acid (RNA) that is then processed into messenger RNA, which is then transcribed to a protein that may undergo further processing to become a functional enzyme, carbohydrate, signaling protein, hormone or structural element. Within this whole process of DNA to protein, there are multiple regulatory processes and feedback loops that will alter the rate of the transcription of DNA, processing of RNA, and the production of the final product. Thus, any defect in the DNA will lead to a change in the whole cascade and ultimately may affect multiple systems and biochemical processes.

One might postulate that a metabolic defect would render an individual unable to survive without modern medical interventions (e.g., transplantation, life support machines, exclusionary diets with special supplemental formulas). In fact, that is very likely true. Many infants who died in early childhood prior to the advent of sensitive diagnostic testing may have succumbed to a potentially treatable metabolic disease. It is easy to see that there is a natural selection against individuals with metabolic diseases, especially those who have a decrease in survival or basic life functioning.

It is, therefore, not surprising that many of the metabolic diseases are inherited in an autosomal recessive, X-linked recessive (males only), or sporadic (new mutation) pattern (1). Conditions that are lethal prior to reproductive age would not survive in the gene pool unless the condition was recessive (the heterozygous state could survive in the gene pool). Thus ALL lethal conditions are recessive or spontaneous new lethal mutations. In urea cycle defects, every known deficiency is autosomal recessive except OTC deficiency which is X-linked recessive (1). This also holds true for the lipidoses (disorders in lipid metabolism leading to the accumulation of lipoid material within cells). Niemann-Pick, Gaucher, Krabbe, and metachromatic leukodystrophy are all autosomal recessive, if a case is not sporadic (3). The other causes of neonatal degenerative encephalopathies, such as peroxisomal disorders and mitochondrial disorders are also autosomal recessive or X-linked recessive (4).

How might one go about determining whether an infant has a metabolic disease? The answer will always be clinical suspicion. There are a few "classic" presentations that should trigger consideration of an inborn error of metabolism as a reasonable possibility. These include metabolic acidosis, hyperammonemia, hypoglycemia and unusual odors. The respective clinical manifestations of these abnormalities are described below.

In urea cycle defects, the common toxin is ammonia (NH3), since the urea cycle is designed to excrete excess nitrogen. Therefore, the presentation of this group of defects is quite similar. Hyperammonemia causes an encephalopathic picture. Presenting signs and symptoms include vomiting, lethargy, poor activity, poor feeding, decreased mental status, and even coma. They may eventually develop spasticity, mental retardation, seizures, and ataxia. These disorders usually present in the first few days to weeks of life as the ammonia waste product accumulates quickly, leading to serum ammonia levels which are described as "sky-high" (>1000 umol/L). The afflicted newborn very rapidly decompensates (1,4). The initial clinical presenting signs and symptoms (other than hyperammonemia) more commonly signal a serious infection of the newborn. It is very difficult to differentiate the two disease processes. However, the existence of these symptoms along with low risk for a neonatal infection may raise the index of suspicion that would lead the clinician to conduct a thorough laboratory evaluation for a metabolic condition along with the sepsis workup.

A group of metabolic disorders related to the urea cycle defects is the organic acidemias. These are caused by defective processing of the amino acids resulting in accumulation of organic acid byproducts or lack of production of a necessary end product. The symptoms are very similar to the urea cycle defects; however, there are subtle laboratory differences. While urea cycle disorders result in hyperammonemia without acidosis and only occasionally hypoglycemia, the organic acidemias (as the name suggests) result in metabolic acidosis and hyperammonemia that is more on the order of 200-900 umol/L. One of the best understood diseases from this class of metabolic diseases is Maple Syrup Urine Disease (MSUD).

MSUD occurs in the immediate neonatal period, presenting with acute decompensation within the first 2 weeks of life. Symptoms include lethargy, poor feeding, vomiting, and seizures, which eventually lead to coma and cerebral edema. Laboratory evaluation yields hypoglycemia, hyperammonemia (to a lesser degree than in urea cycle defects), acidosis, and ketosis. The urine from these patients has a striking odor of maple syrup.

Phenylketonuria (PKU), one of the best understood genetic disorders and the first to be screened for in Hawaii (1966) (5), results from phenylalanine hydroxylase (PAH) deficiency. The enzyme deficiency leads to build-up of phenylalanine that is toxic in high levels to brain growth and nerve myelination. This causes mental retardation, abnormal behaviors and skin rashes. In addition, the PAH enzyme is necessary to convert phenylalanine to tyrosine which is required for melanin synthesis. With a defective enzyme, the individual is unable to produce proper levels of tyrosine which results in poor pigmentation of skin and hair (1,5).

Galactosemia is one of the commonly occurring disorders of carbohydrate metabolism. This disease occurs from a deficiency of galactose-1-phosphate uridyltransferase with the deficiency most noticeable in those organs which utilize the most energy (liver, brain, kidney and adrenal gland). Over time, there is an accumulation of galactose-1-phosphate, which manifests as vomiting, lethargy, diarrhea, cataracts, developmental delay and mental retardation, liver and kidney disease. In galactosemia, there is an increased likelihood of sepsis from gram negative organisms that may cause death in the neonatal period (1,4,5).

In an infant who has signs and symptoms consistent with a metabolic disorder, there are certain diagnostic steps that can help delineate what type of metabolic disorder could exist. Of course, the most important evaluation is always the history and physical. With metabolic disorders, one must always ask if there is a family history of early infant death or disability, developmental delay, mental retardation, or seizures. There should also be an assessment of the likelihood that the presenting illness is sepsis, which is much more common than metabolic disorders (4). Thus, some investigation for neonatal infection risk factors should be conducted (e.g., maternal fever, prolonged rupture of membranes, maternal colonization with group B strep, intrapartum maternal antibiotic treatment, fever or temperature instability in the infant, the presence of respiratory distress, etc.).

When a reasonable suspicion of a metabolic disease is established, then an appropriate workup can be undertaken. Laboratory evaluation should include a full sepsis workup (CBC, blood culture, urine culture, chest x-ray, cerebrospinal fluid studies and culture), since sepsis may not be easily distinguished from an inborn error of metabolism. In addition to the sepsis workup, metabolic screening laboratories should include a glucose level, electrolytes, an arterial blood gas, ammonia level, lactic acid level, urinary ketones, and liver function tests. Definitive diagnosis will depend on what type of metabolic disorder is suspected.

Once the screening laboratories are available, one can systematically eliminate possible diagnoses until there are only a few possibilities left. Then, a few specific diagnostic tests can be performed to hopefully, identify the type of metabolic disorder that is present.

The first useful marker is the ammonia level. Urea cycle defects have extremely elevated ammonia levels, sometimes in excess of 2000 ug/dL. Organic acidemias and benign transient hyperammonemia of the newborn (THAN) have ammonia elevations that can overlap, but are not usually as high as those found in urea cycle defects.

The next useful laboratory marker is the presence or absence of hypoglycemia. Infants with elevated ammonia levels in the presence of hypoglycemia have a reasonable likelihood of having an organic acidemia. Infants with hyperammonemia without hypoglycemia tend to have urea cycle defects. Hypoglycemia without hyperammonemia can signal a carbohydrate metabolism defect (e.g., galactosemia, defect in gluconeogenesis, or a glycogen storage disease) or a fatty acid oxidation deficiency in the older infant.

Metabolic acidosis is a key tool in the differentiation of urea cycle defects versus organic acidemias, but it is also quite useful in the evaluation of respiratory or energy transport chain defects. Persistent, severe, metabolic acidosis with absence of urine organic acids will signal primary lactic acidosis. The presence of primary lactic acidosis usually means a defect in pyruvate metabolism (leading to inability to convert lactic acid back to pyruvate to enter the Krebs cycle), gluconeogenesis disorder (leading the body to scavenge pyruvate which is converted to lactic acid with ATP production), respiratory chain defect (causing inability to produce ATP during the Krebs cycle), or a mitochondrial disorder (e.g., error in oxidative phosphorylation). If the metabolic acidosis is due to a primary lactic acidosis, a lactate/pyruvate ratio may be helpful to further narrow the differential diagnosis (4).

Although it may be possible to determine the general class of metabolic defect, it is often not possible to determine the exact enzyme which is defective or lacking. For example, since there are so many enzymes involved in oxidative phosphorylation, a defect of any one of these will result in a lethal condition.

Current diagnostic studies for inborn errors of metabolism have limitations. The primary drawback is the 3-4 day turnaround time from receipt of the sample to the results being available. A critically ill infant may not be able to survive that time period without appropriate treatment. Thus, in the first few days it is crucial to initiate empiric therapy. This should be done with the available clinical and laboratory evaluation in conjunction with a metabolic specialist guiding treatment.

The presence of a possible organic acid or urea cycle defect requires that the patient undergo protein restriction to prevent accumulation of toxic metabolites or hyperammonemia. However, this requires prevention of catabolism of body protein for conversion to energy (i.e., gluconeogenesis) to prevent additional nitrogen waste production. Thus, an infusion of proper carbohydrate calories should be initiated as soon as possible. In infants, this is usually 10% dextrose (a simple hexose) water intravenously at 80-100 ml/kg/day (1,4).

Many of the organic acidemias and urea cycle defects can be mitigated with the use of vitamins and cofactors that can bypass the defect by shunting the toxic metabolites to an alternate pathway or they can serve as transport molecules to shuttle byproducts in and out of the mitochondria. For example, in citrullinemia and argininosuccinic aciduria, infusions of arginine can result in significant drops in the ammonia level. Biotin can be used in carboxylase deficiencies while vitamin B12 can be useful in some forms of methylmalonic acidemias (1).

After 2-3 days, the body's natural processes will require amino acid input for normal functioning. Otherwise, catabolism and metabolic decompensation can occur. If a definitive diagnosis has not been established, there are commercially available protein-free formulas (e.g., Prophree by Ross) that can be used until a tailored diet can be established after the diagnosis is made (4).

Fatty acid oxidation disorders can present with hypoglycemia with lack of ketones present in the urine. Obviously, the hypoglycemia can easily be treated with a glucose infusion. The inability to process fatty acids means that these individuals will need their dietary fat restricted since they would not be able to metabolize fat. In addition, L-carnitine, a transport molecule in the liver, should be given as a supplement.

Galactosemia, which is suspected by the presence of reducing substances in the urine, is treated by elimination of galactose and lactose (glucose + galactose) from the diet. In infants, this can be accomplished by the exclusive use of soy formulas (no cow's milk or breast milk) (1,4).

Other therapy is strictly supportive: ventilatory support for those infant who are in imminent respiratory failure, bicarbonate infusions for infants with severe, unremitting acidosis, volume expansion for signs of hypoperfusion. Exchange transfusions or hemodialysis may be used in patients with high levels of ammonia. However, if empiric therapy and protein restriction are implemented early with the suspicion of a metabolic disorder, many infants may never have to undergo dialysis or exchange transfusion. Definitive therapy will depend on the metabolic defect that is identified.

The State of Hawaii has a newborn metabolic screening program which began in 1966 with PKU, but has since progressed to the inclusion of metabolic, endocrine, and hemoglobin disorders. The newest panel of disorders for which all newborns are screened includes congenital hypothyroidism, phenylketonuria, hemoglobinopathies, biotinidase deficiency, galactosemia, maple syrup urine disease, and congenital adrenal hyperplasia (5).

The Newborn Screening Program (NSP) in Hawaii has been enormously successful in capturing nearly all infants born in the State of Hawaii. Newborn Screening Programs exist in all 50 of the United States. Each state has jurisdiction over what panel of screening tests exists and the timing of the blood collection (http://www.aap.org/policy/01565t1.htm). For example, in Hawaii, the blood collection is conducted during the newborn period, prior to discharge from the hospital. Even in the Neonatal Intensive Care setting, the blood collection is done in the newborn period, but may be done earlier if the neonate is going to receive a blood transfusion.

Almost all results are reported out to the primary care provider within 2 weeks of the testing. The abnormal results are flagged for the physician and instructions for diagnostic testing, either at the Regional Laboratory (Oregon for all tests from Hawaii) or at a laboratory of the physician's choice, are included. Most abnormal results are also mailed with an instructional pamphlet for the family and physician of the affected child describing the disorder and possible diagnostic, therapeutic, and reproductive considerations.

The NSP was designed as a key preventive public health tool to identify disorders that have a treatment that when started early, can lead to reductions in mental retardation, physical disability or death.


Questions

1. True/False: Infants with an inborn metabolic defect are always symptomatic within the first two weeks of life.

2. Many of the metabolic defects can present clinically like which of the following:
. . . . . a. sepsis.
. . . . . b. formula intolerance or gastroesophageal reflux.
. . . . . c. necrotizing enterocolitis.
. . . . . d. neonatal hepatitis with liver failure.
. . . . . e. all of the above.

3. Newborn screening is designed with which of the following principles in mind:
. . . . . a. To identify all infants with the metabolic diseases that are included in the screening panel.
. . . . . b. To generate more paperwork for the physician.
. . . . . c. To screen for diseases that have no cure, but that can be alleviated through early intervention.
. . . . . d. To ensure early screening of future offspring for the family of affected infants.
. . . . . e. To screen for all possible metabolic diseases.
. . . . . f. To disseminate information regarding genetic/metabolic disease to the public and the physicians.

4. True/False: None of the metabolic diseases have a cure.

5. An infant with hyperammonemia, metabolic acidosis, and hypoglycemia most likely has what class of defect:
. . . . . a. fatty acid oxidation disorder.
. . . . . b. galactosemia.
. . . . . c. organic acidemia.
. . . . . d. urea cycle defect.
. . . . . e. lipid storage disease.


References

1. Wappner RS, Hainline BE. Section I. Inborn Errors of Metabolism. In: McMillan JA, DeAngelis CD, Feigin RD, Warshaw JB (eds.). Oski's Pediatrics: Principles and Practice, third edition. 1999, Philadelphia: Lippincott Williams & Wilkins, pp. 1823-1900.

2. Michal G (ed). Biochemical Pathways, third edition, Part 1 and 2. 1993, Germany: Roche Molecular Biochemicals.

3. Crumrine PK. Degenerative Disorders of the Central Nervous System. Pediatr Rev 2001;22(11):370-379.

4. Burton BK (Electronic Article). Inborn Errors of Metabolism in Infancy: A Guide to Diagnosis. Pediatrics 1998 Dec;102(6):e69.

5. The Northwest Regional Newborn Screening Program. Newborn Screening Practitioner's Manual, sixth edition. July 1997.

6. Cassidy SB, Allanson JE. Introduction. Management of Genetic Syndromes. 2001, USA: Wiley-Liss, pp. 1-8.

7. Jones KL. Approaches to Categorical Problems of Growth Deficiency, Mental Deficiency, Arthrogryposis, Ambiguous External Genitalia. In: Need editors Smith's Recognizable Patterns of Human Malformation, 5th edition. 1997, W.B. Saunders Company, pp. 677-694.

8. AAP Policy Statement. Molecular Genetic Testing in Pediatric Practice: A Subject Review (RE0023). Pediatrics 2000;106(4):1494-1497.

9. AAP Policy Statement: General Principles in the Care of Children and Adolescents with Genetic Disorders and Other Chronic Health Conditions (RE9717). Pediatrics, 1997 Apr; 99 (4): pp. 643-644.

10. AAP Committee on Bioethics. Ethical Issues with Genetic Testing in Pediatrics (RE9924). Pediatrics 2001;107(6):1451-1455.

11. Bowman JE. Legal and Ethical Issues in Newborn Screening. Pediatrics 1989;83(5,part 2):894-896.

12. Verma IC. Burden of Genetic Disorders in India. Indian J Pediatr 2000;67(12):893-898.


Answers to questions

1. False: Many infants with metabolic defects classified as storage disorders (lipid storage disorders) and fatty acid oxidation defects will present at many months of age.

2. e. And, there are many other disorders that can be on the list of possibilities, including child abuse (shaken baby).

3. c, d, f. The other answers are incorrect because: a. Newborn screening is not a diagnostic tool; it merely indicates need for further definitive testing. b. Obviously, physicians do not need more paperwork. e. Ideally, newborn screening could identify all metabolic disease, however, since cost and technology are prohibitive, the current principles are to screen for diseases which have a "significant" prevalence in a population and have some potential for treatment.

4. True: Unfortunately, there are no permanent cures, only lifelong supportive measures to mitigate the effects of the metabolic disease.

5. c


Return to Table of Contents

University of Hawaii Department of Pediatrics Home Page