Summary: Researchers reveal an unexpected cause for hidden hearing loss.
Source: University of Michigan.
Patients who complain they can’t hear their friends at a noisy restaurant, but pass a hearing test in their doctor’s office, may be describing hidden hearing loss.
Now, less than six years since its initial description, scientists have made great strides in understanding what hidden hearing loss is and what causes it. In research published in Nature Communications, University of Michigan researchers report a new unexpected cause for this auditory neuropathy, a step toward the eventual work to identify treatments.
“If people can have hidden hearing loss for different reasons, having the ability to make the right diagnosis of the pathogenesis will be critical,” says author Gabriel Corfas, Ph.D., director of the Kresge Hearing Research Institute at Michigan Medicine’s Department of Otolaryngology – Head and Neck Surgery.
Corfas published the research with co-author Guoqiang Wan, now with Nanjing University in China. They discovered using mice that disruption in the Schwann cells that make myelin, which insulates the neuronal axons in the ear, leads to hidden hearing loss. This means hidden hearing loss could be behind auditory deficits seen in acute demyelinating disorders such as Guillain-Barré syndrome, which can be caused by Zika virus.
Corfas and Wan used genetic tools to induce loss of myelin in the auditory nerve of mice, modeling Guillain-Barré. Although the myelin regenerated in a few weeks, the mice developed a permanent hidden hearing loss. Even after the myelin regenerated, damage to a nerve structure called the heminode remained.
Synapse loss versus myelin disruption
When the ear is exposed to loud noises over time, synapses connecting hair cells with the neurons in the inner ear are lost. This loss of synapses has previously been shown as a mechanism leading to hidden hearing loss.
In an audiologist’s quiet testing room, only a few synapses are needed to pick up sounds. But in a noisy environment, the ear must activate specific synapses. If they aren’t all there, it’s difficult for people to make sense of the noise or words around them. That is hidden hearing loss, Corfas says.
“Exposure to noise is increasing in our society, and children are exposing themselves to high levels of noise very early in life,” Corfas says. “It’s clear that being exposed to high levels of sound might contribute to increases in hidden hearing loss.”
The newly identified cause — deficiency in Schwann cells — could occur in individuals who have already had noise exposure-driven hidden hearing loss as well. “Both forms of hidden hearing loss, noise exposure and loss of myelin, can occur in the same individual for an additive effect,” Corfas says.
Previously, Corfas’ group succeeded in regenerating synapses in mice with hidden hearing loss, providing a path to explore for potential treatment.
While continuing this work, Corfas started to investigate other cells in the ear, which led to uncovering the new mechanism.
There are no current treatments for hidden hearing loss. But as understanding of the condition improves, the goal is for the research to lead to the development of drugs to treat it.
“Our findings should influence the way hidden hearing loss is diagnosed and drive the future of clinical trials searching for a treatment,” Corfas says. “The first step is to know whether a person’s hidden hearing loss is due to synapse loss or myelin/heminode damage.”
Funding: NIH/National Institute on Deafness and Other Communication Disorders, Hearing Health Foundation funded the study.
Source: Haley Otman – University of Michigan
Image Source: NeuroscienceNews.com image is credited to Michigan Medicine.
Original Research: Full open access research for “Transient auditory nerve demyelination as a new mechanism for hidden hearing loss” by Guoqiang Wan & Gabriel Corfas in Nature Communications. Published online February 17 2017 doi:10.1038/ncomms14487
Transient auditory nerve demyelination as a new mechanism for hidden hearing loss
Hidden hearing loss (HHL) is a recently described auditory neuropathy believed to contribute to speech discrimination and intelligibility deficits in people with normal audiological tests. Animals and humans with HHL have normal auditory thresholds but defective cochlear neurotransmission, that is, reduced suprathreshold amplitude of the sound-evoked auditory nerve compound action potential. Currently, the only cellular mechanism known for HHL is loss of inner hair cell synapses (synaptopathy). Here we report that transient loss of cochlear Schwann cells results in permanent auditory deficits characteristic of HHL. This auditory neuropathy is not associated with synaptic loss, but rather with disruption of the first heminodes at the auditory nerve peripheral terminal. Thus, this study identifies a new mechanism for HHL, highlights the long-term consequences of transient Schwann cell loss on hearing and might provide insights into the causes of the auditory deficits reported in patients that recover from acute demyelinating diseases such as Guillain–Barré syndrome.
“Transient auditory nerve demyelination as a new mechanism for hidden hearing loss” by Guoqiang Wan & Gabriel Corfas in Nature Communications. Published online February 17 2017 doi:10.1038/ncomms14487