Inborn Errors of Metabolism

Inborn errors of metabolism are a large, heterogeneous group of genetically determined disorders, predominantly inherited in an autosomal recessive pattern. Each results from deficiency of a specific enzyme or transport protein in a metabolic pathway. Affected infants are typically normal at birth, then develop acute, severe, rapidly progressive metabolic decompensation.

The group includes:

• Disorders of amino acid metabolism
• Disorders of organic acid metabolism
• Fatty acid oxidation disorders
• Urea cycle disorders
• Lysosomal storage disorders
• Disorders of carbohydrate metabolism

Infants are normal at birth because the placenta effectively performs the relevant metabolic function during fetal life. Decompensation follows once the affected pathway is called upon after birth. It is particularly pronounced once regular feeding commences or during any subsequent intercurrent illness or catabolic stress.

A high index of suspicion in any previously well neonate or infant with unexplained, acute, severe illness is the foundation of timely diagnosis and treatment.

Classification and pathophysiology

• Disorders of intoxication include the amino acid disorders (such as phenylketonuria and maple syrup urine disease), organic acidaemias (including propionic and methylmalonic acidaemia), and urea cycle disorders. A toxic metabolic intermediate accumulates proximal to the enzymatic block. This produces progressive, often severe clinical toxicity that characteristically worsens with feeding (particularly protein intake) and during catabolic states. A characteristic symptom-free interval follows birth before clinical deterioration, as the toxic intermediate accumulates.
• Disorders of energy metabolism include the fatty acid oxidation disorders and certain organic acidaemias. They reflect impaired cellular energy production. This is most apparent during states of increased metabolic demand or reduced caloric intake (including prolonged fasting or intercurrent illness with reduced oral intake). They characteristically present with hypoglycaemia and lethargy. In fatty acid oxidation disorders specifically, there is an important association with sudden, unexpected infant death if unrecognised.
• Lysosomal storage disorders reflect deficiency of a specific lysosomal enzyme required for normal breakdown of complex macromolecules. The undegraded substrate accumulates progressively within cells. The clinical course is generally more gradual and progressive than in the intoxication and energy metabolism categories. Multiple organ systems are characteristically affected over time, rather than acute, dramatic decompensation.
• Urea cycle disorders reflect impaired conversion of ammonia to urea for excretion. This produces hyperammonaemia, a particularly dangerous and rapidly neurotoxic metabolic derangement. It requires urgent, aggressive treatment, given the substantial risk of severe, irreversible neurological injury if not promptly and adequately addressed.

• Affected infants are characteristically entirely well at birth. Symptoms develop after an interval of well-being lasting typically hours to days. This reflects the time required for accumulation of the toxic metabolite, or the onset of a physiological stress such as feeding or catabolism.
• This symptom-free interval is a useful diagnostic clue. It distinguishes many inborn errors of metabolism from congenital structural anomalies or infections present from birth.
• Common, though non-specific, presenting features include poor feeding, vomiting, lethargy progressing to reduced consciousness, hypotonia, and seizures. More severe cases progress to coma and cardiovascular collapse.
• This non-specific presentation frequently closely resembles neonatal sepsis. It is a common source of initial diagnostic confusion or delay.
• Specific clues may point toward an inborn error of metabolism, rather than sepsis or another more common cause of neonatal illness:
  • An unusual odour: classically a maple syrup or burnt sugar odour in maple syrup urine disease, or a sweaty feet odour in isovaleric acidaemia, among various other recognised, specific odour associations.
  • A family history of unexplained infant death or a similarly affected sibling.
  • Parental consanguinity (increasing the likelihood of a rare, autosomal recessive condition).
  • A specific pattern of laboratory abnormality not readily explained by sepsis alone.
• Lysosomal storage disorders more typically present with a gradual, progressive clinical course. Features include progressive organomegaly (particularly hepatosplenomegaly), characteristic dysmorphic facial features (coarsening of facial features over time in several specific conditions), progressive developmental regression, and, in several specific conditions, skeletal abnormality. This pattern is distinct from the acute, intoxication-type presentation more characteristic of several of the other inborn error of metabolism categories.
• A previously well infant with unexplained, severe, or rapidly progressive illness should prompt urgent consideration of an inborn error of metabolism. This applies particularly with any of the specific clues, or a relevant family history. Urgent, targeted metabolic investigation follows.

• Expanded newborn screening uses tandem mass spectrometry analysis of the routine newborn dried blood spot sample. It has substantially expanded the range of inborn errors of metabolism identifiable before the onset of clinical symptoms. It is an increasingly important component of the diagnostic pathway for a growing number of conditions.
• Not every relevant condition is included within every screening programme. Screening does not eliminate the possibility of clinical presentation before screening results are available, or for conditions not included within the screening panel used.
• Basic first-line metabolic investigation in any acutely unwell infant with suspected inborn error of metabolism should include blood glucose, venous or arterial blood gas (assessing for metabolic acidosis and calculating the anion gap), ammonia, lactate, liver function tests, and urinalysis for ketones. The pattern of abnormality across these parameters gives initial diagnostic guidance toward the likely broad category of metabolic disorder.
• More detailed metabolic investigation includes plasma amino acid analysis, urine organic acid analysis, and acylcarnitine profile. It characterises the condition specifically and is essential for definitive diagnosis. It generally requires specialist metabolic laboratory involvement and interpretation.
• Sample timing and handling are critical for accurate interpretation. Samples for specific metabolic investigation should ideally be obtained during the acute, symptomatic episode itself. Obtain them before or during initial treatment where possible, without significantly delaying essential emergency treatment. Diagnostic abnormalities can normalise or become less apparent once treatment has been initiated and the acute metabolic derangement corrected.
• Genetic testing targets the specific gene or genes relevant to the biochemically suspected condition. It provides definitive molecular confirmation. It supports accurate genetic counselling regarding recurrence risk for future pregnancies, given the predominantly autosomal recessive inheritance of the great majority of conditions in this category.

• Immediate, emergency management of a suspected acute metabolic decompensation prioritises reversal of catabolism and removal of the accumulated toxic substrate.
• The foundational approach is to stop any dietary intake implicated in the suspected condition. Stop protein particularly where an amino acid or organic acid disorder, or urea cycle disorder, is suspected. Provide adequate caloric intake through intravenous dextrose. This suppresses the catabolic state that would otherwise further drive accumulation of the toxic metabolic intermediate.
• Specific, targeted treatment depends on the suspected or confirmed condition. It may include:
  • Cofactor supplementation. Certain enzyme deficiencies retain residual, partial enzyme activity that high-dose vitamin cofactor can enhance.
  • Nitrogen-scavenging medication for hyperammonaemia associated with urea cycle disorders.
  • Extracorporeal removal techniques, including haemodialysis or haemofiltration, for the most severe, refractory metabolic derangement. This is particularly relevant for severe hyperammonaemia, where rapidly reducing ammonia levels is critical to minimise the risk of severe, irreversible neurological injury.
• Long-term management for the substantial majority of conditions is structured, specific dietary management. For several conditions this includes protein or specific amino acid restriction. For fatty acid oxidation disorders it includes avoidance of prolonged fasting and a specific, structured emergency feeding regimen during any intercurrent illness.
• For a subset of conditions, enzyme replacement therapy (an important treatment advance for several of the lysosomal storage disorders) or other targeted pharmacological treatment is relevant.
• Full, structured "sick day" management planning is an essential component of long-term management. Give the family clear, specific, written guidance on the prompt actions required during any intercurrent illness, given the substantial risk of acute metabolic decompensation during any period of reduced oral intake or increased catabolic demand.
• Provide an accessible, emergency point of contact with the specialist metabolic team. Where relevant, provide an emergency treatment letter for presentation to any healthcare provider unfamiliar with the condition. Rapid, severe deterioration is possible if an intercurrent illness is not promptly managed according to this individualised plan.
• Full, structured, long-term multidisciplinary follow up through a specialist metabolic service incorporates ongoing developmental surveillance, nutritional management, and structured biochemical monitoring. It includes genetic counselling for the family regarding recurrence risk for future pregnancies and, where relevant, cascade testing of at-risk relatives.
• This underpins lifelong management of this broad, diverse, and individually rare but collectively significant diagnostic category. Early diagnosis (increasingly supported by expanding newborn screening programmes) and prompt, appropriate treatment prevent or minimise the potentially severe, sometimes irreversible neurological and other complications of delayed recognition and treatment.