Hidden Hearing Loss

Developing Better Tests for Discovering “Hidden” Hearing Loss

By Hari Bharadwaj, Ph.D., with Inyong Choi, Ph.D.

Conventionally, hearing loss is thought to be a consequence of damage to delicate sensory hair cells in the inner ear (cochlea). However, over the past decade animal studies have shown that nerve endings in the cochlea are considerably more vulnerable to damage than the sensory hair cells, and that such nerve damage is likely to happen before conventionally recognized forms of hearing loss occur.

Emerging Research Grants (ERG) recipients Bharadwaj and Choi, and colleagues, systematically investigated the many sources of variability that obscure cochlear nerve damage (“synaptopathy”) to provide recommendations for how best to measure such nerve damage.

Emerging Research Grants (ERG) recipients Bharadwaj and Choi, and colleagues, systematically investigated the many sources of variability that obscure cochlear nerve damage (“synaptopathy”) to provide recommendations for how best to measure such nerve damage.

Unfortunately, damage to cochlear nerve endings cannot be detected by current clinical hearing tests. Yet, this “hidden” damage can hypothetically still affect hearing in everyday noisy environments such as crowded restaurants and busy streets. Therefore, it is important to develop tests to detect such damage in humans, and there is considerable interest among hearing scientists toward this enterprise.

In our paper published in Neuroscience on March 8, 2019, we considered noninvasive tests that can potentially reveal such nerve damage and systematically investigated other extraneous sources of variability that might reduce the sensitivity and specificity of these tests. This helped us come up with recommendations for how we can best apply these tests. Funding from Hearing Health Foundation’s Emerging Research Grants contributed to experiments that helped understand and articulate the role of two key variables: how variations in the anatomy of individuals (e.g., brain shape and size) affected our noninvasive tests; and how certain cognitive factors like attention may affect hearing independently of how well the inner ear is capturing the information in sounds.

Armed with the knowledge about these variables and other factors described in the paper, we anticipate that hearing scientists will be able to design more powerful experiments to understand the effects of damage to cochlear nerve endings, and build more powerful tests to detect such damage in the clinic. This work is crucial in enabling clinical translation of the basic science that has been uncovered over the past decade.

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A 2015 Emerging Research Grants (ERG) scientist, Hari Bharadwaj, Ph.D., is an assistant professor at Purdue University in Indiana with a joint appointment in speech, language, and hearing sciences, and biomedical engineering. Inyong Choi, Ph.D., is an assistant professor in the department of communication sciences and disorders at the University of Iowa. Choi’s 2017 ERG grant was generously funded by the General Grand Chapter Royal Arch Masons International.

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Second Cause of Hidden Hearing Loss Identified

By Michigan Medicine - University of Michigan

Some people can pass a hearing test but have trouble understanding speech in a noisy environment. New research identifies a new mechanism for this condition just years after its discovery. Credit: Michigan Medicine

Some people can pass a hearing test but have trouble understanding speech in a noisy environment. New research identifies a new mechanism for this condition just years after its discovery. Credit: Michigan Medicine

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."

Materials provided by Michigan Medicine - University of Michigan. Co-author Guoqiang Wan, Ph.D., was a 2014 Emerging Research Grants recipient funded by the Wes Bradley, M.D. Memorial Grant.

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