Abstract
Autosomal dominant optic atrophy (DOA) is a retinal neuronal degenerative disease characterized by a progressive bilateral visual loss. We report on two affected members of a family with dominantly inherited neuropathy of both optic and auditory nerves expressed by impaired visual acuity, moderate pure tone hearing loss, and marked loss of speech perception. We investigated cochlear abnormalities accompanying the hearing loss and the effects of cochlear implantation. We sequenced OPA1 gene and recorded cochlear receptor and neural potentials before cochlear implantation. Genetic analysis identified R445H mutation in OPA1 gene. Audiological studies showed preserved cochlear receptor outer hair cell activities (otoacoustic emissions) and absent or abnormally delayed auditory brainstem responses (ABRs). Trans-tympanic electrocochleography (ECochG) showed prolonged low amplitude negative potentials without auditory nerve compound action potentials. The latency of onset of the cochlear potentials was within the normal range found for inner hair cell summating receptor potentials. The duration of the negative potential was reduced to normal during rapid stimulation consistent with adaptation of neural sources generating prolonged cochlear potentials. Both subjects had cochlear implants placed with restoration of hearing thresholds, speech perception, and synchronous activity in auditory brainstem pathways. The results suggest that deafness accompanying this OPA1 mutation is due to altered function of terminal unmyelinated portions of auditory nerve. Electrical stimulation of the cochlea activated proximal myelinated portions of auditory nerve to restore hearing.
Highlights
Autosomal dominant optic atrophy (DOA) is characterized by a slowly progressive bilateral visual loss beginning in childhood
We show that the OPA1 mutation affects synchrony of neural discharges in unmyelinated dendrites of the auditory nerve while receptor potentials of hair cells were normal
SPL, 90 dB nHL) were absent unilaterally in both subjects whereas the other ear showed only low amplitude Wave V of delayed latency (7.5 ms II-2, 6.9 ms III-2, normal < 6 ms). These results are consistent with abnormal synchrony of auditory nerve in the presence of normal receptor outer hair cell activities
Summary
Autosomal dominant optic atrophy (DOA) is characterized by a slowly progressive bilateral visual loss beginning in childhood. Axons of ganglion cells within the retina are unmyelinated, of small diameter, and contain numerous mitochondria that provide energy for transmitting graded neural potentials. Some OPA1 mutations have additional clinical features including hearing loss, ataxia, and peripheral neuropathy (Amati-Bonneau et al, 2008; Chen et al, 2007; Hudson et al, 2008; Ke et al, 2006). OPA1 proteins have been localized in both inner and outer hair cells, auditory nerve terminals, and spiral ganglion cells (Chen et al, 2007) but the site(s) of abnormal function in the cochlea are not yet known. We show that the OPA1 mutation affects synchrony of neural discharges in unmyelinated dendrites of the auditory nerve while receptor potentials of hair cells were normal. The myelinated portions of auditory nerve remain capable of responding to electrical stimulation from cochlear implants to restore both hearing and neural synchrony in auditory brainstem pathways
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