Prevailing View on Age-Related Hearing Loss Challenged

Scientists at Massachusetts Eye and Ear 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.

The inner ear, where most types of hearing impairment originate, cannot be biopsied, and its delicate structures can only be resolved in specimens removed at autopsy. Understanding the true cellular causes of age-related hearing loss impacts how future treatments are developed and how appropriate candidates will be identified, and can also suggest how to prevent or minimize this most common type of hearing damage, according to the authors of the study published in Journal of Neuroscience today.

"Our study upends the dogma about the major cause of age-related hearing loss," said senior author Pei-zhe Wu. "Documenting the dominant role of progressive hair cell loss in the hearing impairment of normal aging means that the millions who suffer with this condition could benefit from the hair cell regenerative therapies that are the focus of ongoing research across the world. No one is focusing on approaches to regenerate the stria."

The team examined 120 inner ears collected at autopsy. They used multivariable statistical regression to compare 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 that allowed Wu and colleagues to see the tiny bundles of sensory hairs (> 200 times thinner than a typical human hair), that helped them identify and count the small number surviving hair cells. Prior studies scored hair cells as "present," even if only one or two remained.

The data also showed that hair cell degeneration in aging humans is dramatically worse than in animal models of presbycusis. Laboratory animals are aged in sound-controlled enclosures, where they are not exposed to the constant barrage of moderate and high-intensity noises that surround us: those we choose to listen to and those we can't avoid. "The greater hair cell death in human ears suggests that the high-frequency hearing losses that define presbycusis may be avoidable, reflecting mainly accumulated damage from environmental noise exposures," said M. Charles Liberman, Ph.D.  a co-author of the study.