Researchers here provide evidence for age-related deafness to be caused by the loss of viable hair cells in the inner ear, rather than other possible mechanisms. As pointed out, this is perhaps the best outcome for such a study, given the numerous approaches to hair cell regeneration or hair cell replacement that are underway in the scientific community. While it is interesting to compare this result with earlier data suggesting that hair cells survive in old individuals, but are disconnected from the brain, it nonetheless boosts the prospects for near term reversal of age-related hearing loss.
Scientists have demonstrated that age-related hearing loss, also called presbycusis, is mainly caused by damage to hair cells, the sensory cells in the inner ear that transform sound-induced vibrations into the electrical signals that are relayed to the brain by the auditory nerve. Their research challenges the prevailing view of the last 60 years that age-related hearing loss is mainly driven by damage to the stria vascularis, the cellular "battery" that powers the hair cell's mechanical-to-electrical signal conversion.
Researchers examined 120 inner ears collected at autopsy. They compared data on the survival of hair cells, nerve fibers, and the stria vascularis with the patients' audiograms to uncover the main predictor of the hearing loss in this aging population. They found that the degree and location of hair cell death predicted the severity and pattern of the hearing loss, while stria vascularis damage did not. Previous studies examined fewer ears, rarely attempted to combine data across cases and typically applied less quantitative approaches. Most importantly, prior studies greatly underestimated the loss of hair cells, because they didn't use the state-of-the art microscopy techniques.
Previous animal studies suggested that presbycusis is caused by atrophy of the stria vascularis, a highly vascularized cluster of ion-pumping cells, located in the inner ear adjacent to the hair cells. The stria serves as a "battery" that powers the hair cells as they transform sound-evoked mechanical motions into electrical signals. In aging laboratory animals, such as gerbil, there is very little loss of hair cells, compared to humans, even at the end of life. However, there is prominent damage to the stria vascularis, and damage to the stria will, indeed, cause hearing loss. Prior to this new study, most scientists have assumed that the aging gerbil data also apply to human presbycusis.
The researchers say the new findings are good news given recent progress in the development of therapies to regenerate missing hair cells. If presbycusis were due primarily to strial damage, hair cell regeneration therapy would not be effective. This new study turns the tables, suggesting, that vast numbers of hearing impaired elderly patients could likely benefit from these new therapies as they come to the clinics, hopefully within the next decade.