MCAD Deficiency

Synonyms: Medium-chain acyl CoA dehydrogenase (MCAD) deficiency, ACADM Deficiency.

This is an autosomal recessive inherited disorder of fatty acid metabolism, caused by a mutation of the medium-chain acyl-CoA dehydrogenase gene (ACADM) on chromosome 1. The gene has been mapped to locus 1p31; several allelic variations have been reported. The most common mutation is called G985.¹ This is because of a substitution of a guanine for an adenine nucleotide at the 985th residue.

Fatty acids are unable to be metabolised beyond the medium chain size (8-12 carbon stage), and gluconeogenesis is effectively inhibited. In response to any fasting or metabolic stress (e.g. illness) the body is unable to metabolise fat (so no ketones are produced), and continues to metabolise glucose producing hypoglycaemia.² The clinical result is severe hypoglycaemia and hypoketonuria, with accumulation of monocarboxylic fatty acids and dicarboxylic organic acids.

Incidence: 1:8000 to 1:15,000 births.¹ There is an equal gender predisposition.

• Age at presentation is quite variable.
• Most commonly presents in infants aged >3 months, when overnight feed reduce in frequency. The gap between feeds is then long enough for acute hypoglycaemia to occur, producing symptoms of preprandial irritability, drowsiness, jitteriness, sweating, coma and seizures.
• It can present as sudden death in adults. There is a 25% mortality rate in undiagnosed cases.³
• It can present with:
  • Life-threatening hypoketotic hypoglycaemic coma
  • Metabolic acidosis
  • Encephalopathy
  • Hepatomegaly and fatty infiltration of viscera.
• In later childhood it may present with episodic hypoglycaemia e.g. with sweating, collapse, confusion, or developmental delay.
• It has been reported presenting after a first episode of alcohol intoxication.⁴
• It very occasionally presents in adulthood with muscle weakness and fatigue.
• Survivors of acute episodes may have severe hypoglycaemia induced brain damage.

These may be normal in between attacks.

• Acutely - hypoglycaemia
• U&E may show high or low bicarbonate and reduced anion gap
• LFTs may show elevated enzymes, low plasma carnitine.
• Urine - Medium chain dicarboxylic aciduria and absent ketones

Skin biopsy can be performed to confirm diagnosis of primary carnitine deficiency - demonstrating reduced carnitine transport in fibroblasts. Fibroblasts may be used for fatty acid oxidation studies or enzyme assay.

• Avoidance of fasting. A maximum duration of fasting in children with MCAD deficiency of:
  • Between 6 months and 1 year of age - 8 hours
  • In the second year of life - 10 hours
  • Thereafter - 12 hours.⁵
  • There are no firm guidelines on the duration of fasting during situations of intercurrent illness, especially with fever.
• Because the fundamental biochemical defect is in fatty acid oxidation, the composition of the diet should be adjusted to provide greater calories in carbohydrates and proteins, while minimising lipids.
• Daily carnitine has been suggested by some specialists; this allows conjugation and excretion of toxic biproducts of blocked fatty acid metabolism. Supplementation may promote clearance of accumulating acylcarnitines during exercise, but there is no apparent beneficial effect on clinical and biochemical indices.^6,7
• Genetic counselling should be provided for family members.
• The heterozygous state is quite common. Testing for the gene should be offered to first-degree relatives of an affected child.

• Frequent episodes of severe hypoglycaemia carry a risk of adverse effects in the CNS.
• Hypoglycaemia and hyperammonaemia may cause cerebral oedema and prolonged coma.

• Prognosis is difficult, because of the broad clinical spectrum.
• The majority of children do well, if appropriately treated.

This is technically possible by demonstrating a marked reduction in octanoate oxidation in cultured amniotic cells obtained via amniocentesis.⁸

The Department of Health have funded a pilot study in six UK centres screening more than 700,000 babies.⁹ The remaining 7 centres will be funded from April 2009.