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More than 300 CPT II cases have been described with the myopathic form being the most common (myopathic form: 86%, severe infantile form: 8%, neonatal form: 6% of cases).

The myopathic form is the least severe and is characterized by recurrent attacks of rhabdomyolysis, muscle pain and weakness triggered by prolonged physical exercise, fasting, viral illness or extremes in temperature. The severe infantile form is characterized by a severe fasting intolerance leading to metabolic disorders such as hypoketotic hypoglycemia and hepatic encephalopathy. The lethal neonatal form includes symptoms of the infantile disease as well as dysmorphic features (e.g. cystic dysplastic kidneys).

More than 60 mutations in the CPT2 gene, resulting in general in amino acid substitutions or small deletions, cause the CPT II deficiency.

The diagnosis is made by an initial tandem mass spectrometry of serum/plasma acylcarnitines followed by mutation analysis and measurements of CPT2 enzyme activity in fresh circulating lymphocytes, muscle or fibroblasts.

The differential diagnosis for the myopathic form should include McArdle disease, Duchenne muscular dystrophy, and cytochrome c oxidase deficiency (see these terms) among others, and carnitine-acylcarnitine translocase deficiency (CACT) and very-long-chain acyl-CoA dehydrogenase deficiency (see these terms) for the infantile and neonatal forms

Prenatal diagnosis is available based on a combination of enzymatic and molecular testing.

Transmission is autosomal recessive.

Treatment is based on avoidance of prolonged fasting (>12 hr) and a low-fat and high-carbohydrate diet.

The myopathic form of CPT II has a good prognosis. The severe infantile form may lead to sudden death during infancy due, in general, to paroxysmal cardiac arrhythmias. The neonatal form is almost always lethal during the first months of life.