Mitochondrial cytopathy (MERRF) without ragged-red fibers

We report a case of a mitochondrial cytopathy (MERRF) with a late diagnosis because of the absence of Ragged-Red Fibers (RRF) in the first muscular bi opsy performed in childhood. Eight years after the onset of the disease the familial history and the clinical manifestations were very suggestive of a mitochondrial cytopathy and a second biopsy was performed confirming the d iagnosis of MERRF. The authors discuss several hypotheses to explain the ne gativity of the first muscular biopsy. The absence of ragged-red fibers doe s not exclude the diagnosis of a mitochondrial cytopathy and other investig ations are justified such as the biochemical study of the respiratory chain and the molecular analysis of mitochondrial DNA.