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There have been few studies investigating the mechanism and nature of the hearing loss that occurs in the mitochondrial disorders. We studied 18 patients with the MELAS A3243G point mutation from four different kindreds. Pure tone audiometry, speech discrimination testing, acoustic reflexes, tympanometry, and brain stem auditory evoked responses were performed to localize the site of pathology in the auditory pathways. In 12 patients, we performed electrocochleography and otoacoustic emissions to assess cochlear involvement. Neuroimaging and promontory nerve stimulation were performed to exclude retrocochlear pathology. Audiological testing confirmed sensorineural hearing loss in 14 of the 18 patients studied; hearing loss was usually gradual in onset, was symmetrical, and initially affected the higher frequencies. In some patients, there were features that distinguished the hearing loss from …