Inborn Errors of Metabolism

Etiology

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]

Epidemiology

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.

Pathophysiology

The pathophysiology of IEMs is best understood through the classification system proposed by Saudubray and colleagues, which categorizes disorders into 3 functional groups:

1. 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).
2. 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.
3. 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.

History and Physical

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)

Evaluation

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.

Treatment / Management

Acute management follows the suspend, support, scavenge approach:

1. Suspend intake: Immediately stop all oral intake of potential toxins (protein, galactose, fructose) if an intoxication disorder is suspected.
2. 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.
3. 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.
4. 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]

Differential Diagnosis

 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

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]

Complications

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