Objective: The aims of this study were to explore the prevalence of the A1555G mutation among a group of Japanese patients and to assess the pathophysiology of the hearing impairment associated with the mutation.
Study design: Genetic study and retrospective chart review.
Methods: We screened for the mitochondrial DNA A1555G mutation in 138 unrelated Japanese deaf patients, including 63 sporadic cases and 75 familial cases with different patterns of inheritance. When available, patients carrying the mutation received audiovestibular examinations, including speech audiometry, distortion-product otoacoustic emission (DPOAE) testing, electrocochleography (ECochG), and electronystagmography.
Results: One of 63 sporadic cases (1.6%) and 6 of 75 familial cases (8.0%) carried the A1555G mutation. Patients with the mutation and a familial history included two with autosomal recessive inheritance and four with maternal inheritance. In addition, two of six patients (33.3%) presenting with aminoglycoside-induced sensorineural hearing loss (SNHL) were associated with the A1555G mutation. All but one of the patients carrying the mutation showed high-frequency SNHL. Distortion-product levels of DPOAE were reduced to the noise levels, suggesting the A1555G mutation caused cochlear deafness. Cochlear microphonics in ECochG showed elevation of the detection thresholds and corresponding audiometric thresholds. The ECochG data implied that patients with high-frequency SNHL had impairment of the cochlear hair cells that was most severe toward the basal turn. The electronystagmographic findings indicated no apparent vestibular dysfunction.
Conclusions: Screening for the A1555G mutation, even in patients with idiopathic bilateral SNHL, likely would be useful for preventing further development and/or acceleration of the deafness.