Among the sometimes bothersome things about being a hearing specialist is that many of the situations we encounter that have caused our patients to lose their hearing can’t be reversed. For example, one of the extremely common reasons for hearing loss is damage to the miniature, sensitive hair cells that line the inner ear and vibrate in response to sound. These vibrations are translated by the brain into what we call hearing.
The sensitivity of these tiny hair cells enables them to vibrate in such a manner, and thus enables us to hear, but their very sensitivity makes them very fragile, and prone to damage. Infections, certain medications, aging or prolonged exposure to loud sounds (resulting in noise-induced hearing loss/NIHL) are all potential sources of damage. In humans, once these hair cells are damaged or destroyed, they can’t be regenerated or “fixed.” Since we can’t reverse the damage, hearing specialists and audiologists look to technology instead. We make up for hearing loss due to inner ear hair cell damage with hearing aids and cochlear implants.
This wouldn’t be true if humans were more like chickens and fish. Unlike humans, some fish species and birds have the ability to regenerate their damaged inner ear hair cells and regain their lost hearing. Strange, but true. Chickens and zebra fish are just 2 examples of species that have the ability to spontaneously replicate and replace their damaged inner ear hair cells, thus allowing them to fully recover from hearing loss.
Could hearing loss in humans be reversed? Glimmers of hope are emerging from the innovative research of the Hearing Restoration Project (HRP), but the research is preliminary and no useful benefits for humans have yet been achieved. This research, funded by the non-profit Hearing Health Foundation, is presently being conducted in 14 labs in Canada and the US. What the HRP researchers are trying to do is isolate the compounds that allow this replication and regeneration in animals, with the purpose of finding some way of enabling similar regeneration of inner ear hair cells in humans.
The research is painstaking and challenging, because so many different molecules either contribute to replication or hinder hair cells from replicating. By determining which of the molecules regulate this process in avian or fish cochlea, the scientists are hoping to establish which compounds promote hair cell growth. The HRP researchers are taking a divide and conquer approach to attain their joint goal. While some labs work on gene therapies others work on approaches using stem cells.
As mentioned before, this research is still in its preliminary stages, but we join with others in wishing that it will bear fruit, and that someday we’ll be able to help humans reverse their hearing loss as easily as chickens do.