Understanding Hidden Hearing Loss
People who battle with hidden hearing loss often struggle to hear conversations in noisy environments but pass a hearing exam perfectly. As frustrating as it sounds, this type of hearing loss cannot be measured by the common hearing test and is common for a lot of patients.
Last week, we discussed hearing loss caused by damage to the hair cells, or cilia, in the inner ear. Hidden hearing loss is when there is damage to the nerve cells that connect the cochlea to the brain, making it harder for those affected to hear when there is competing background noise. Damage to the nerve cells makes hearing loss harder to diagnose because the auditory nerves are not activated during a hearing test. Scientists are working to better understand hidden hearing loss and what exactly causes it in order to ultimately find a way to treat the condition.
Causes of Hidden Hearing Loss
The most common cause of hidden hearing loss is exposure to high level of noise, especially in children and young adults. In this type of hidden hearing loss, the synapses connecting the hair cells and neurons in the inner ear are lost. In a noisy environment, all of those synapses are required to make sense of speech or noise. However, when taking a hearing test, not all of those synapses are required, thus making it easier to pass a hearing test, but not be able to understand what people are saying in a loud environment.
Gabriel Corfas, Ph.D., director of the Kresge Hearing Research Institute at Michigan Medicine’s Department of Otolaryngology – Head and Neck Surgery along with Guoqiang Wan, Nanjing University in China recently published a report that offers a different cause of hidden hearing loss: deficiency in Schwann cells. The Schwann cells are responsible for making myelin which insulates the neuronal axons in the ear. Therefore, a Schwann cell deficiency causes a loss of myelin, resulting in hearing loss. By inducing a loss of myelin in the auditory nerve of mice, Corfas and his team were able to determine that even after some myelin regeneration, the mice suffered permanent damage. More specifically, it caused permanent damage to the nerve structure or the heminode.
A patient can suffer from both noise-induced hearing loss as well as loss of myelin, producing an additive effect. While there is no treatment for hidden hearing loss, a better understanding of the condition lets researchers gain a better understanding of how to treat it. For example, in another study, Corfas, along with Charles Liberman had been researching a way to regenerate synapses with a growth factor called neurotrophin 3, which can reverse some of the signs of hidden hearing loss. The more we learn about hearing loss and hidden hearing loss, the closer we get to effective treatments.