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Background: Mitochondrial DNA point mutations are especially deleterious to tissues with high energy demand, including the peripheral nervous system. Neuropathy has been associated with several mitochondrial diseases, including MELAS (mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes). Objective: To evaluate nerve conduction in a genotypically and phenotypically homogeneous group of patients with MELAS and the A3243G mutation. Design: We studied 30 patients with MELAS and the A3243G mutation using neurophysiological techniques, medical history questionnaires, laboratory tests, and a standardized neurological examination. Results: Twenty-three subjects (77%) had abnormal nerve conduction measures. Symptoms suggestive of neuropathy were present in only half of the patients, but almost all had decreased reflexes or distal sensory findings on examination. Nerve conduction abnormalities were predominantly axonal and sensory and mainly present in the legs. Patients with nerve conduction abnormalities tended to be older and were more likely male. Conclusions: Peripheral nerve impairment is common in those with MELAS and the A3243G mutation, and may be subclinical. Male sex and older age may add to the genetic disposition to develop neuropathy. [ABSTRACT FROM AUTHOR]