Likewise, when Dr. Dhar’s group evaluated DPOAE loss, they found that individuals between the ages of 25 and 29 years experienced the first clinically significant DPOAE loss. Taken together, the results are consistent with an overall degradation of the cochlea. Dr. Dhar pointed out that these findings also mean that it is now possible to reliably evaluate cochlear function at higher auditory frequencies.
Explore This IssueNovember 2017
Our testing paradigms for “routine” audiometry will significantly change in the next few years to be able to detect these hearing losses that currently remain hidden. —Anil Lalwani, MD
Noise Exposure Contributes to “Hidden” Hearing Loss
Sharon G. Kujawa, PhD, associate professor of otolaryngology at Harvard Medical School in Boston, discussed “hidden” Sharon G. Kujawa, PhD, associate professor of otolaryngology at Harvard Medical School in Boston, discussed “hidden” hearing loss, saying that hair cells have traditionally been considered the primary targets in many forms of acquired sensorineural hearing loss. This is likely because hair cells have demonstrated vulnerability and undergo rapid loss that can be easily linked to insult. Moreover, their loss produces overt hearing loss that is readily apparent in the audiogram.
In contrast, she noted that loss of auditory neurons occurs with a delay and can follow a protracted time course, suggesting that the neural loss in such injury occurs as a secondary consequence of the loss of the hair cell targets.
In recent years, Dr. Kujawa and her colleagues have shed new light on these assumptions using animal models of common forms of acquired sensorineural hearing loss. Beginning with noise-induced hearing loss, Dr. Kujawa said that she and her colleagues used an exposure that would have once been considered benign, producing only temporary threshold elevations and no hair loss. When the ears of these animals were examined with powerful new immunostaining and imaging techniques, however, the investigators found that nearly 50% of the synaptic communications between inner hair cells and afferent neurons were permanently destroyed. She reminded the audience that such synaptic and neural loss can be massive without altering thresholds. Thus, the significant injury is “hidden” in the normal threshold audiogram.
The investigators then examined the role of synaptopathy in aging. They found that synapse loss begins early in life, before threshold sensitivity and hair cell declines begin, and progresses gradually with age. “As synapse number declines, neural response amplitudes fall in proportion, providing a non-invasive reflection of the underlying injury,” said Dr. Kujawa. Moreover, as ears age after noise exposure, ongoing changes with age are exaggerated.