Research revises view on Noise-induced SNHL mechanism

Written by earpeace™ | Phil

Category: Hearing Loss

A recently published article[1] in the Journal of the Acoustical Society of America alters what has been the classic view of sensorineural hearing loss (SNHL). Hair cells have long been considered the most valuable elements of the inner ear, for the focus of research efforts, in establishing the threshold elevations noise-induced and age related hearing loss has on their demise.

Our ears are continuously exposed to loud noises, in this modern industrialised society, which are capable of permanently damaging the inner ear, where the delicate hair cells or nerve fibres are destroyed without obvious warning signs or pain, except for temporary tinnitus.

Take a look at this short video explaining how the ear works. In summary, for the normal ear, sound waves are transmitted through the middle ear bones (collectively called the ossicles, which include the malleus, incus, and stapes), to the inner ear where they cause vibrations in the sensory epithelium called the “organ of Corti”. Here this mechanical function is turned into electrical pulse trains in the fibres of the cochlear nerve, which then carries the information to the brain for analysis of the acoustic scene.

Researchers at Eaton Peabody Laboratory, Massachusetts Eye and Ear Infirmary, in Boston, USA, now challenge this view of SNHL where hair cells are the “primary” targets and that cochlear-nerve loss is “secondary” to hair cell degeneration. Their work in mouse and guinea pig studies shows that in noise-induced hearing loss, exposures causing only reversible threshold shifts (no hair cell loss) nevertheless cause permanent loss of more than 50% of cochlear-nerve / hair-cell synapses. Notably, in age-related hearing loss, degeneration of cochlear synapses precedes both hair-cell loss and threshold elevation. According to Liberman, et al, this primary neural degeneration has remained hidden for two reasons:

• “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
• the degeneration is selective for cochlear-nerve fibers with high thresholds”

They continue to add that although not required for threshold detection in quiet (e.g. threshold audiometry, auditory brainstem response threshold), these high-threshold fibres are critical for hearing in noisy environments. The conclusion of their research suggests that

• noise exposure guidelines should be re-evaluated, since they are based on the faulty premise that threshold audiograms are the most sensitive measures of noise damage, and
• primary neural degeneration is an important contributor to the perceptual handicap in SNHL.

This suggests that every time we go to loud concerts or use power tools without ear protection, we may be losing cochlear nerve fibres and increasing our degree of hearing impairment.

[1] Liberman, M.C., Kujawa, S. G. “Hidden hearing loss: Permanent cochlear-nerve degeneration after temporary noise-induced threshold shift” J. Acoust. Soc. Am. 135, 2311 (2014) http://dx.doi.org/10.1121/1.4877618

Prevention

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