Clinicopathological features of MERRF.

Clinical features of MERRF are varied in terms of patterns and severity of symptoms at the early stage. The age of onset differs even in the same family. Occasionally, ragged-red fibers are not found upon the muscle biopsy. Pathological features in some patients are very mild in comparison with their clinical symptoms; genetic analysis is needed for diagnosis. Unique clinical features in members of three families with MERRF are reported.