Hidden Hearing Loss: Synaptopathy in noise-induced and age related cochlear damage: Berkeley Ear Club
Colloquium | April 17 | 4 p.m. | Tolman Hall, Beach Room 3105
Charles Liberman, Mass Eye & Ear Eaton Peabody Lab Boston
The classic view of sensorineural hearing loss (SNHL) is that the primary targets are hair cells, and that cochlear-nerve loss is secondary to hair cell degeneration. Our recent work in mouse and guinea pig has challenged that view. In noise-induced hearing loss, exposures causing only reversible threshold shifts (and no hair cell loss) nevertheless cause permanent loss of >50% of cochlear-nerve / hair-cell synapses. Similarly, in age-related hearing loss, degeneration of cochlear synapses precedes both hair cell loss and threshold elevation. This primary neural degeneration has remained hidden for two reasons: 1) the spiral ganglion cells, the cochlear neural elements commonly assessed in studies of SNHL, survive for years despite loss of synaptic connection with hair cells, and 2) the degeneration is selective for cochlear-nerve fibers with high thresholds. Although not required for threshold detection in quiet (e.g. threshold audiometry or auditory brainstem response threshold), these high-threshold fibers are critical for hearing in noisy environments. Our research suggests that 1) primary neural degeneration is an important contributor to the perceptual handicap in SNHL, 2) it may be key to the generation of tinnitus and hyperacusis, and 3) in cases where the hair cells survive, neurotrophin therapies can elicit neurite outgrowth from spiral ganglion neurons and re-establishment of their peripheral synapses.