As hearing specialists, one of the sometimes frustrating things we encounter in our practice is that the conditions that have caused hearing loss in our patients 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. Our sense of hearing is the result of these vibrations being translated into electrical energy and sent to the brain for decryption.
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 at risk of damage. Aging, infections, certain medications or prolonged exposure to high-volume sounds (resulting in noise-induced hearing loss/NIHL) are all possible sources of damage. In humans, once these hair cells have become damaged or destroyed, they can’t be regenerated or “fixed.” Instead, hearing professionals and audiologists must use technologies such as hearing aids or cochlear implants to compensate for hearing loss that is essentially irreversible.
Things would be a lot less complicated if we humans were more like fish and chickens. That may sound like a peculiar statement, but it’s true, because – unlike humans – some fish and birds can regenerate the hair cells in their inner ears, thereby regaining their hearing once it has become lost. For reasons that are not fully understood, zebra fish and chickens have the ability to spontaneously replicate and replace damaged inner ear hair cells, and thus achieve full functional recovery from hearing loss.
Could hearing loss in humans be reversed? Flickers of hope are emerging from the groundbreaking research of the Hearing Restoration Project (HRP), but the research is preliminary and no practical benefits for humans have yet been achieved. This research, financed by the non-profit Hearing Health Foundation, is currently taking place at 14 labs in Canada and the US. What the HRP researchers are trying to do is identify the compounds that allow this replication and regeneration in animals, with the purpose of finding some way of enabling similar regeneration of hair cells in humans.
The research is painstaking and difficult, because so many distinct molecules either help with replication or prevent inner ear hair cells from replicating. By determining which of the compounds regulate this process in fish or avian cochlea, the scientists are hoping to establish which molecules promote hair cell growth. The HRP researchers are taking a divide and conquer approach to achieve their collective goal. While some labs pursue gene therapies others focus on approaches using stem cells.
As noted before, this work 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 cure their hearing loss as easily as chickens do.
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