Researchers here propose a novel form of damage and dysfunction contributing to the development of age-related hearing loss. Much of the focus of past years of research has been placed upon loss of connectivity between sensory hair cells of the inner ear and the brain, or outright loss of hair cells themselves. But there are other components of the overall problem of loss of function, as noted here, and the question of what to do about it doesn’t always have as straightforward a path towards practical therapies as exists for loss of cells.
Age-related hearing loss is common among older adults and can result from several problems in the inner ear. The disorder is usually classified into three types: (a) Neural: due to damage to auditory nerve fibres, (b) Sensory: caused by the loss of the sensory cells that detect sound, and (b) Metabolic: involving degeneration of the cells in the cochlea’s wall that help maintain the ear’s internal environment.
In the metabolic type, often considered the most common one, the positive electrical potential normally found close to the sensory cells is much reduced – and without this positive potential, sensory cells cannot function normally. However, the possibility that degeneration in the cochlea’s wall could affect the hearing organ in other ways has not been considered.
We used a physiologically based animal model to investigate what happens when cells in the cochlea’s wall stop working. Using advanced imaging techniques, we discovered that calcium levels near the sensory cells dropped. This is an important observation because calcium is a key regulator of sensory cell function. We also found that the tectorial membrane – which helps transmit sound-evoked vibration to the sensory cells – often detached from the sensory cells. This detachment made it nearly impossible for sound to reach the sensory cells. To confirm that these findings are relevant for the human disorder, we examined samples from people with metabolic age-related hearing loss. We saw the same tectorial membrane detachment, and the extent of detachment predicted the severity of hearing loss.
