Our auditory system is comprised of several components; the outer and middle ear, the inner ear or cochlea, and the nerve fibers that run to our brain. When hearing loss occurs, at least one portion of the system fails to work properly. For the condition called presbycusis, or age-related hearing loss, the prevailing opinion was that the cause was due to damage of the stria vascularis. The stria vascularis is a barrier of cells lining the inner ear. It has been called the cellular battery because it converts the movement of the inner ear’s hair cells to electrical signals sent to the brain. These electrical signals are what our brain is able to understand as sound.
New research from Massachusetts Eye and Ear Infirmary has challenged that current view after examining the fine structures of the inner ear, a practice where specimens can only be retrieved at autopsy. The goals of their work were to understand the cellular causes age-related of hearing loss and to prevent or reduce the most common type of hearing damage.
The researchers were able to examine 120 inner ears from older adults at autopsy and used a procedure called multivariate statistical regression to compare the data on the survival of the hair cells, nerve fibers, and stria vascularis. This was compared to the patient’s hearing test results to determine the main predictor of the hearing loss in this aging population. The results indicated that the degree and location of the hair cell damage predicted the severity and pattern of the hearing loss – while the stria vascularis did not. In addition, their research used state-of-the-art microscopy techniques that allowed them to see the tiny bundles of hair cells and precisely count the surviving hair cells.
The research team indicated that these results are good news considering the significant progress on the development of therapies to regenerate hair cells. If the hearing loss was due to damage of the stria vascularis, these regeneration therapies would not be as effective; however, this new data shows that large numbers of elderly patients with age-related hearing loss could benefit from these therapies. It should be noted that these therapies may be many years in the future, but hopefully within the next decade.
The second important result is that this type of hair call damage can be reduced or prevented by reducing our exposure to loud environmental sounds. We are almost constantly exposed to loud sounds such as factory noise and even headsets or earphones with high volume settings. This type of hair cell damage that results in age-related hearing loss (presbycusis) can be avoided by reducing the effects of environmental sound exposure.
One of the lead authors noted that “if we were more careful about protecting our ears during exposure to noisy activities, we could all hear better into old age”.
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