A six year old boy with progressive myopathy and motor neuropathy associated with complex I deficiency is reported from the Institutes of Neurology, Pediatrics and Cell Biology, St. Radboud University Hospital, Nijmegen, Netherlands. There was a three month history of weakness in the legs, sudden falling, inability to run and difficulty in climbing stairs. Within two months he developed weakness in his hands, a wobbling gait and a positive Gowers’ sign. Muscle tone was impaired and proximal muscles were wasted. Biochemical studies on muscle tissues showed a defect of NADH dehydrogenase (complex I). Dramatic improvement followed oral treatment with L-carnitine 2 g daily and riboflavin 9 mg daily. Seven months later the complex I activity was normal, Gowers’ sign was negative, deep tendon reflexes had returned and muscle weakness was limited mainly to the peroneal muscles. Nine further cases of a pure myopathy associated with complex I deficiency are reviewed. 
COMMENT. The present case report of a mitochondrial myopathy differed from nine other reports. There was no history of excessive fatiguability or exercise intolerance, the serum lactate concentration was normal, and the muscle biopsy did not show ragged red fibers. The dramatic improvement with riboflavin and carnitine and the return to normal of complex I activity were unusual findings. The authors stress the clinical, biochemical and morphological heterogeneity in complex I deficiency. Biochemical studies of mitochondrial metabolism in the muscle are advisable in children with progressive weakness and exercise intolerance when a precise diagnosis is unclear.