Age-related hearing loss is a prevalent condition in the elderly population. According to the National Institute on Deafness and Other Communication Disorders (NIDCD), about 30-35% of adults aged 65 and older have hearing loss and 40-45 percent of people aged 75 and older. Unfortunately, this condition is irreversible and requires hearing aids, implants, or other devices. Recently published literature has suggested a link between hair cell loss and age-related hearing loss. A study by Wu et al. demonstrated that presbycusis is caused by damage to hair cells in the inner ear that convert sound vibrations into electrical signals which are transmitted to the brain by the auditory nerve.
The inner ear is a very sensitive structure that cannot be biopsied and can only be removed at autopsy. Having the knowledge regarding the causes of age-related hearing loss is pivotal in the development of future treatments, identification of the appropriate study participants, and understanding how to minimize hearing damage. “Our study upends the dogma about the major cause of age-related hearing loss,” said Dr. Wu. “Documenting the dominant role of progressive hair cell loss in the hearing impairment of normal aging means that the millions who suffer from 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.”
New developments to investigate the cause of age-related hearing loss
Research recently published by Wu et al. examined 120 human inner ears by quantitative microscopic analysis of the cells and fibers obtained at autopsy. Multivariable regression was used to compare the data of the cells with the subjects’ audiograms to identify the predictor associated with hearing loss. The results demonstrated that greater hair cell loss was associated with high-frequency cochlear regions compared to low-frequency cochlear regions.
Previously published data contained a small sample size of ears and did not employ quantitative data analysis. Additionally, previous studies miscalculated the loss of hair cells due to the lack of state-of-the-art microscopy techniques. Wu et al. were able to meticulously observe tiny bundles of sensory hairs and were able to carefully count the number of surviving cells.
The mechanism behind age-related hearing loss
Previous data in animals have suggested that presbycusis is caused by loss of structure in the stria vascularis – a structure responsible for maintaining the ion composition located in the inner ear. The stria vascularis serves as the powerhouse of hair cells which converts mechanical motions into electrical signals. In laboratory animals that were aging such as gerbils, it was shown that there was very little hair loss compared to humans. Although, there was severe damage to the stria which should contribute to hearing loss. Prior to the study indicated above by Wu and colleagues, most researchers assumed that the gerbil data was also applicable to the human presbycusis.
The novel results are a push in the right direction given the recent progress in the development of therapies for hair cell loss. If age-related hearing loss was due to strial damage, then hair cell regeneration therapy would not be effective. This new study by Wu et al. gives promise to elderly patients with hearing loss as they could potentially benefit from new therapies within the next decade.
Results suggest the importance of protecting ears from damage
Another important finding is the fact that hair cell loss in aging humans is worse than in animal models of presbycusis. Laboratory animals age in a certain sound-controlled environment in where they are not exposed to high-intensity noises that are likely to surround humans. “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 co-author Liberman. Therefore, if we work on protecting our ears from childhood, we can hear better by the time we reach old age.