Medium Chain AcylCoA Dehydrogenase Deficiency

Ihe optimal chain length specificity of MCAD is 8 carbons. Ihe ability to oxidize fatty acids beyond medium- chain length is impaired 20 percent or less of control rates in cultured fibroblasts. Light microscopy of liver in acute cases shows steatosis, which disappears on recovery. Ihe hepatic mitochondria show increased matrix density and intracristal widening, giving a condensed appearance. Ihere may be crystalloids in the matrix. Ihese are in contrast to the findings of Reye's syndrome, in which electron microscopy shows matrix swelling and rarefaction. MCAD is the most common mitochondrial beta-oxidation disorder, with an estimated frequency being 1 in 10,000 to 20,000. Previous unexplained sibling deaths should raise MCAD as a possible diagnosis.

Most patients present between ages 3 and 15 months, rarely after 4 years. Phenotypical heterogeneity prevails, ranging from sudden infant death syndrome, to recurrent Reye's-like syndrome, to episodic nonketotic hypoglycemic coma. A common presentation is vomiting and lethargy, followed by fasting, associated with a prior viral respiratory or gastrointestinal infection. Presentation is usually to an emergency department as an acute toxic encephalopathy or coma, with hypoketotic hypoglycemia, hyperammonemia, and abnormal liver function tests. Ihe serum carnitine value is low, and urine acylcarnitines are increased, with a specific profile. Enzyme assay for MCAD activity can be done on cultured skin fibroblasts, muscle, liver, or blood lymphocytes. Ihe differential diagnosis includes other fatty acid oxidation disorders, exogenous toxic encephalopathies, and true Reye's syndrome. Specific management involves intravenous dextrose 10 percent, avoidance of fasting with frequent short feeds, L-carnitine 100 mg/kg/day in divided doses orally, and preventive management by neonatal screening of subsequent siblings and screening with acylcarnitine profiles of present siblings. Ihe risk of death with the first episode (sudden infant death syndrome presentation) is about 20 percent. In delayed treatment there is a risk for developmental retardation, speech and language delay, behavioral problems and attention deficit disorder, muscle weakness, seizures, and failure to thrive.

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