Myopathy with MTCYB mutation mimicking Multiple Acyl-CoA Dehydrogenase Deficiency.

We describe two patients with mitochondrial DNA mutations in the gene encoding cytochrome b (m.15579A>G, p.Tyr278Cys and m.15045G>A p.Arg100Gln), which presented as a pure myopathic form (exercise intolerance), with an onset in childhood. Diagnosis was delayed, because acylcarnitine profile showed an increase in medium and long-chain acylcarnitines, suggestive of multiple acyl-CoA dehydrogenase deficiency, riboflavin transporter deficiency or FAD metabolism disorder. Implication of cytochrome b in fatty acid oxidation, and physiopathology of the mutations are discussed.