Research on both noise-induced and age-related hearing loss traditionally focuses on how the loss of hair cells affects hearing. However, recent efforts by scientists at the Harvard Medical School have determined that nerve fibers in the inner ear are even more susceptible to damage from loud noise than hair cells. These new findings have implications for both our understanding of hearing loss and for public health policies.
We know that sound waves transmit through bones in the ear, causing vibrations translated into electrical pulses in the fibers of the cochlear nerve. This information travels to the brain, where it is then processed. However, until recently, researchers did not fully appreciate that cochlear nerve loss occurs without affecting a person’s ability to detect tone in quiet. Since tone detection in quiet is a baseline component of hearing tests, most auditory specialists using the threshold audiogram test fail to recognize some inner ear nerve damage.
This new understanding of hearing loss should change public health policies. Current federal noise exposure guidelines posit that transient threshold elevation levels are not harmful to the ear. However, the research by the Harvard team clearly demonstrates in three non-human mammalian ears that transient noise-exposure damages cochlear nerves. This suggests that even single exposures to loud noises, such as at concerts, leads to cochlear nerve fiber loss and an increased risk for future hearing problems.
With these new insights comes hope for successful therapy. The death of neural cell bodies is a slow process; researchers believe there may be opportunities for the chemical regeneration of nerve terminals, leading to the re-establishment of synaptic connections necessary for hearing. This work is in its infancy and more research is required to understand possible treatments.
Source: Science Daily