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Inborn errors of metabolism comprise a diverse group of inherited genetic disorders caused by pathogenic variants that impair enzymes, transporters, or cofactors required for normal metabolic pathways. These disruptions result in toxic substrate accumulation, product deficiency, or impaired energy production, thereby affecting carbohydrate, amino acid, fatty acid, or complex-molecule metabolism. Clinical presentation varies widely, ranging from neonatal, sepsis-like metabolic crises after a symptom-free interval to later-onset, intermittent manifestations with vomiting, altered mental status, seizures, myopathy, cardiomyopathy, or progressive neurodegeneration. Early recognition remains central to preventing irreversible morbidity and mortality, particularly during catabolic stress. Initial evaluation in acute settings prioritizes rapid identification of metabolic derangements using critical blood and urine samples, with subsequent biochemical and genetic testing to confirm diagnosis and refine long-term management. Course participation enables the clinician to recognize phenotypic patterns suggestive of inborn errors of metabolism, initiate a time-sensitive diagnostic workup, and implement acute stabilization strategies, including avoiding potential toxin intake, reversing catabolism with dextrose-based support, and, when indicated, toxin scavenging for hyperammonemia. Skills gained include selecting and interpreting first-line metabolic studies, identifying disorders not captured by newborn screening, and developing individualized long-term care plans that address dietary therapy, pharmacologic options, surveillance for complications, and emergency protocols. Collaboration with an interprofessional team enhances patient outcomes through earlier diagnosis, rapid initiation of metabolic protocols, improved nutritional adequacy and adherence, reduced preventable hospitalizations, and stronger transitions from pediatric to adult care supported by a shared care plan and consistent communication. Objectives: Identify the clinical signs and symptoms indicative of an inborn error of metabolism in neonates and children, distinguishing them from common conditions like sepsis. Implement appropriate acute management steps, including ceasing protein intake and initiating anabolic support, for a patient presenting with a suspected metabolic crisis.

Identify the clinical signs and symptoms indicative of an inborn error of metabolism in neonates and children, distinguishing them from common conditions like sepsis. Implement appropriate acute management steps, including ceasing protein intake and initiating anabolic support, for a patient presenting with a suspected metabolic crisis. Differentiate between disorders of intoxication, energy metabolism, and complex molecules based on patient history, physical examination, and initial metabolic screening results. Coordinate timely care within an interprofessional team, including geneticists, dietitians, and nurses, to ensure rapid diagnosis, timely newborn screening follow-up, and strict adherence to long-term dietary management plans. Access free multiple choice questions on this topic.

Inborn errors of metabolism (IEM) constitute a diverse group of congenital disorders caused by pathogenic or likely pathogenic variants in genes coding for enzymes, transporters, or cofactors essential for metabolic pathways. These defects lead to the accumulation of toxic substrates or a deficiency of essential products, disrupting the metabolism of carbohydrates, fatty acids, proteins, or complex molecules.[1] While individual disorders are rare, collectively, IEMs are significant, with an estimated incidence of approximately 1 in 15,000 to 1 in 2 million live births.[2] Clinical presentation varies widely, ranging from acute, life-threatening metabolic crises in the neonatal period to insidious, late-onset manifestations in adulthood. Early recognition and diagnosis are critical, as timely intervention can prevent morbidity and mortality.

Inborn errors of metabolism are inherited genetic disorders, most commonly with autosomal recessive inheritance, meaning both parents are carriers of a pathogenic variant. However, other inheritance patterns exist: X-linked: Examples include ornithine transcarbamylase deficiency, Hunter syndrome (mucopolysaccharidosis type III [MPS II]), and Fabry disease. Autosomal dominant: Examples include acute intermittent porphyria and certain types of familial hypercholesterolemia. Mitochondrial inheritance: Disorders affecting mitochondrial DNA are transmitted maternally to all offspring (eg, mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes; myoclonic epilepsy with ragged red fibers).[3] The underlying pathology results from a single-gene defect that impairs the function of a specific enzyme or transport protein. Epigenetic modifications and environmental factors, such as infection or nutritional status, can significantly influence the phenotypic expression and disease severity.[4]

The combined incidence of IEMs is estimated to range from 1 in 15,000 to 1 in 2 million live births, although this varies by population and screening methods.[2] In populations with high consanguineous rates, the prevalence of autosomal recessive IEMs is significantly higher.[5] Data from expanded newborn screening programs using tandem mass spectrometry demonstrate that amino acid disorders, organic acidemias, and fatty acid oxidation disorders are among the most frequently identified conditions.[2] However, many IEMs, particularly those not included in standard newborn screening panels (eg, certain lysosomal storage disorders and mitochondrial diseases), may remain undiagnosed until clinical symptoms appear. Approximately 50% of cases present after the neonatal period, underscoring the need for clinical vigilance beyond infancy.

The pathophysiology of IEMs is best understood through the classification system proposed by Saudubray and colleagues, which categorizes disorders into 3 functional groups: Disorders of intoxication: Defects in the catabolism of small molecules (amino acids, organic acids, urea cycle) lead to the accumulation of toxic compounds proximal to the metabolic block. Patients typically present with a symptom-free interval followed by acute intoxication (vomiting, lethargy, coma) triggered by catabolic stress (fasting, illness). Disorders of energy metabolism: These involve defects in the production or use of energy (mitochondrial disorders, glycogen storage diseases, fatty acid oxidation defects, disorders of gluconeogenesis). Symptoms include hypoglycemia, lactic acidosis, hypotonia, myopathy, and cardiomyopathy. Disorders of complex molecules: These affect the synthesis or remodeling of complex cellular structures (lysosomal storage diseases, peroxisomal disorders, congenital disorders of glycosylation). These conditions usually present with progressive, permanent symptoms such as dysmorphism, organomegaly, and neurodegeneration, independent of food intake.[1][6][7] Note: For additional details, see the relevant StatPearls articles on specific IEMs.

The patient history and physical examination offer clues to the type of IEM. History Neonates: A history of a symptom-free interval followed by rapid deterioration (a sepsis-like presentation) suggests a small-molecule disorder. Key triggers include the introduction of feeding (protein or galactose), prolonged fasting, or minor viral infections. Childhood: Features such as developmental delay, learning difficulties, intellectual disability, seizures, abnormal movements, recurrent altered consciousness (eg, lethargy, obnubilation), persistent or recurrent vomiting, muscle cramps, and fatigue. Adolescence/adulthood: Features similar to childhood, ranging from mild to moderate severity, but often predominated by recurrent episodes of muscle cramps, fatigue, and inability to focus. Family history: Consanguinity, unexplained neonatal deaths (including sudden infant death syndrome), or siblings with similar symptoms. Physical Examination General: Failure to thrive, microcephaly, or dysmorphic features (suggesting complex molecule disorders like Smith-Lemli-Opitz or Zellweger syndrome) Neurologic: Hypotonia (floppy infant), hypertonia, seizures (especially intractable), abnormal movements, or altered mental status (lethargy to coma) [6][8] Ophthalmologic: Cherry-red spot (Tay-Sachs), cataracts (galactosemia), or optic atrophy Hepatic: Hepatomegaly (glycogen storage disease, lysosomal storage disease), jaundice, or liver failure [9] Odors: Unusual body fluid odors, such as maple syrup (maple syrup urine disease), sweaty feet (isovaleric acidemia), or musty (phenylketonuria)

While definitive diagnosis often requires specialized genetic or enzymatic testing, initial evaluation in the acute setting focuses on identifying the metabolic derangement. Initial Metabolic Screen (Critical Samples) Blood: Complete blood count, electrolytes (including calculation of the anion gap), arterial blood gas (to assess for metabolic acidosis or alkalosis), serum glucose, serum lactate, serum ammonia, and liver function testing Urine: Urinalysis (including ketones) and reducing substances Secondary Testing (Diagnostic) Plasma: Amino acids, acylcarnitine profile, free and total carnitine, very long-chain fatty acids, red blood cell plasmalogens, and carbohydrate-deficient transferrin Urine: Organic acids, urine amino acids, urine oligosaccharides, or urine glycosaminoglycans Cerebrospinal fluid: Lactate, pyruvate, and neurotransmitter-related markers Genetics: Gene panel (grouping genes relevant to the main clinical feature), whole exome sequencing, or whole genome sequencing are increasingly used as first-tier diagnostic tools for complex presentations Neuroimaging: Magnetic resonance imaging or magnetic resonance spectroscopy imaging may show specific patterns (eg, basal ganglia involvement in mitochondrial disease) Note: Samples should be collected during the acute crisis, if possible, as metabolites may normalize during clinical stability. However, appropriate clinical care should not be delayed to obtain the samples. The tests listed above should be interpreted cautiously, depending on the suspected condition or the dominant clinical feature.

Acute management follows the suspend, support, scavenge approach: Suspend intake: Immediately stop all oral intake of potential toxins (protein, galactose, fructose) if an intoxication disorder is suspected. Support anabolism: Reverse catabolism by providing high-calorie intravenous fluids (typically dextrose 10% with electrolytes appropriate for age) at 1.5 times the maintenance fluid requirements. Insulin may be required to manage hyperglycemia while promoting anabolism. Caution: Avoid hyperglycemia in suspected pyruvate dehydrogenase deficiency or mitochondrial disorders. Scavenge toxins: Treat hyperammonemia aggressively with nitrogen scavengers (sodium benzoate, sodium phenylacetate) or hemodialysis if ammonia levels are critical (>500-600 µmol/L) or neurological status deteriorates. Cofactors: Empiric administration of vitamin cofactors (eg, biotin, thiamine, vitamin B12, riboflavin) is often initiated.[10] Long-Term Management: Dietary: Lifelong restriction of the offending substrate (eg, low-protein diet for phenylketonuria) and use of specialized medical formulas.[10][11] Pharmacologic: Nitrogen scavengers help counter toxic byproducts.[12] Enzyme replacement therapy may be used for lysosomal storage disorders (eg, Gaucher, Pompe disease), substrate reduction therapy, and small molecule chaperones. Transplant: Liver or hematopoietic stem cell transplant is for specific disorders (eg, urea cycle defects, MPS I). Gene therapy: Recent developments in gene therapies may be used for conditions such as metachromatic leukodystrophy and sickle cell disease have made this a long-term reality for some disorders.[13][14]

The differential for IEMs is broad and overlaps with common pediatric emergencies, including: Sepsis: The most common mimic; patients with IEM are often misdiagnosed with culture-negative sepsis. Cardiac disease: Congenital heart defects, ductal-dependent lesions (eg, coarctation), or cardiomyopathy. Gastrointestinal disorders: Pyloric stenosis (vomiting with metabolic alkalosis), malrotation with volvulus, or intussusception. Neurologic disorders: Hypoxic-ischemic encephalopathy, intracranial hemorrhage, central nervous system infection (eg, meningitis, encephalitis). Toxic ingestion: Accidental poisoning.[7][15]

Prognosis is highly variable and depends on the specific disorder, variant severity, and the timing of diagnosis. Promising: For disorders such as phenylketonuria and biotinidase deficiency, early initiation of therapy after newborn screening may lead to normal development and life expectancy.[16] Guarded: For disorders like urea cycle defects or organic acidemias, patients may have cognitive impairment, chronic kidney disease, or recurrent metabolic decompensations despite treatment.[17] Poor: Severe mitochondrial disorders (eg, Leigh syndrome) or untreated neurodegenerative lysosomal storage diseases often result in early mortality.[18]

Unmanaged or severe IEMs can lead to multisystem complications: Neurologic: Intellectual disability, cerebral palsy, intractable epilepsy, cortical blindness, and sensorineural hearing loss Please see StatPearls' companion resource, "Arginase Deficiency," for further information.[19] Hepatic: Cirrhosis, hepatocellular carcinoma (eg, tyrosinemia type I), and acute liver failure Cardiac: Dilated or hypertrophic cardiomyopathy (Pompe disease, fatty acid oxidation disorders) and conduction abnormalities Please see StatPearls' companion resource, "Glycogen Storage Disease Type II," for further information.[20] Nutritional: Failure to thrive, micronutrient deficiencies due to restrictive diets, and feeding aversion requiring gastrostomy tubes

Prevention and education strategies should focus on: Genetic counseling: Cpunseling is essential for families with an affected child to understand recurrence risks (typically 25% for autosomal recessive disorders) and reproductive options (eg, carrier testing, prenatal testing, preimplantation genetic diagnosis). Newborn screening: Educating parents on the importance of newborn screening and the need for immediate follow-up if a screening test has positive results is vital. Emergency protocols: Families must be equipped with an emergency letter detailing the diagnosis and immediate management steps (eg, initiating an intravenous dextrose infusion immediately) to present to emergency clinicians during illnesses. Dietary adherence: Education on reading food labels, preparing medical formulas, and recognizing signs of metabolic decompensation (eg, vomiting, confusion) is essential.

Managing IEMs requires a coordinated, interprofessional approach to optimize outcomes: Medical geneticist/biochemical geneticist: Leads the diagnostic process and develops a specific management plan Metabolic dietitian: Crucial for creating specialized diets, monitoring growth, and preventing nutritional deficiencies Primary care clinician: Monitors general health and vaccinations, and recognizes early signs of illness that could trigger a crisis Social worker/psychologist: Supports the family with the psychosocial burden of chronic disease, facilitates insurance authorizations for medical foods, and assists with transition to adult care Emergency clinicians: Must recognize the urgency of metabolic protocols and initiate rapid resuscitation without delay. For the initial diagnosis, they must have a high index of suspicion to initiate workup of symptoms that may indicate an underlying IEM. Other: Cardiology, gastroenterology, hepatology, neurology, or nephrology are required to monitor secondary complications Effective communication between these healthcare professionals, often facilitated by a comprehensive care plan, significantly reduces hospital admissions and improves quality of life.