- Medium chain acyl-CoA dehydrogenase deficiency (MCAD)
- Short chain acyl-CoA dehydrogenase deficiency (SCAD)
- Long chain acyl-CoA dehydrogenase deficiency (LCAD)
- Very long chain acyl-CoA dehydrogenase deficiency (VLCAD)
Description - Fatty acid oxidation disorders are autosomal recessive disorders characterized by an enzyme defect in the fatty acid metabolic pathway.
Fatty acid transport and mitochondrial oxidation is a complex pathway that plays a major role in energy production during times of fasting and metabolic stress. Once free acids are released into the blood they are taken up by the liver and muscle cells and activated to coenzyme A esters. Then they are transported into the mitochondria and oxidized in a cyclic fashion by four sequential reactions that are each catalyzed by one of multiple enzymes.
The Acyl-CoA dehydrogenases are chain-length specific enzymes. Deficiencies or abnormalities in these result in very long chain acyl-CoA dehydrogenase (VLCAD) deficiency, long chain acyl-CoA dehydrogenase (LCAD) deficiency, medium chain acyl- CoA dehydrogenase (MCAD) deficiency, and short chain acyl-CoA dehydrogenase (SCAD) deficiency.
Clinical Features - Clinically, individuals may present with hypoglycemia, liver disease, near SIDS, encephalopathy, myopathy, cardiomyopathy, or sudden death. Symptoms may appear at any age from birth to adulthood. Any illness may lead to a fasting state that result in the depletion of glucose stores. Once this occurs, fatty acid metabolism becomes the dominant energy source. If there is an abnormality in fatty acid metabolism, life-threatening episodes of metabolic decompensation can occur. Relatively simple dietary management may avoid symptoms.
MCAD deficient children are typically normal at birth and develop episodes of hypoketotic hypoglycemia, vomiting, lethargy and seizures associated with fasting. The first episode usually occurs between 6 months and 2 years of age. The plasma acylcarnitine profile is diagnostic and a common gene mutation is found in the majority of patients. Urine organic acids in these patients typically show elevations of dicarboxylic acids, gylcine conjugates and acylcarnitines.
Deficiency of VLCAD, LCAD and SCAD is rare compared to MCAD. Individuals with VLCAD and LCAD are very similar. They can present with SIDS, hypoglycemia, hepatomegaly, myopathy, Reye syndrome, and cardiomyopathy. The plasma acylcarnitine profile reveals elevated long chain acyl-carnitine esters. Urine organic acids typically show elevations of dicarboxylic acids. Diagnosis is confirmed by enzyme assay in fibroblasts.
SCAD deficiency presents in the neonatal period with failure to thrive, hypotonia, and metabolic acidosis. Hypoglycemia is not a common feature. Hyperammonemia and lactic acidosis have been reported. The plasma profile reveals elevated short chain acyl-carnitine esters. Urine organic acids show increased excretion of short chain organic acids (ethylmalonic acid) and butyrycarnitine. Diagnosis is confirmed by enzyme assay in fibroblasts.
Treatment - MCAD deficiency treatment includes avoidance of fasting, aggressive support during infections and carnitine supplementation. VLCAD and LCAD deficiency treatment includes avoidance of fasting, aggressive support during infections, and medium chain triglyceride (MCT) oil and carnitine supplementation. SCAD deficiency treatment is not effective but the restriction of fats and supplementation of carnitine has been tried.
For treatment of all fatty acid oxidation disorders, consultation with a metabolic/genetic specialist is strongly advised.