Cochlear nerve damage associated with tinnitus

More than 10% of adults worldwide will experience tinnitus during their lifetimes. For many people, this condition—a ringing, roaring, clicking, hissing or buzzing in the ears—is a temporary nuisance. But for others, it can be permanent and debilitating. Tinnitus may lead to sleep deprivation, anxiety, depression, and other symptoms that can damage quality of life.

The causes of tinnitus are not well understood, and there’s currently no cure. Most people with tinnitus have some degree of hearing loss. One theory of tinnitus suggests that, when hearing is damaged, the brain becomes hyperactive, leading to the perception of phantom sounds. But many people with tinnitus appear to have normal hearing on an audiogram test.

To look more closely at the association between tinnitus and nerve activity, a research team led by Dr. Stéphane Maison from Mass Eye and Ear recruited almost 300 people who had normal hearing results on an audiogram test.

The participants were sorted into one of three groups based on their self-reports of tinnitus. These included 201 people with no tinnitus (beyond the occasional phantom sound that resolved quickly); 64 people with intermittent tinnitus (defined as being continuous but lasting for less than 6 months at a time); and 29 people who had been experiencing chronic tinnitus for more than 6 months.

The researchers performed a range of testing on the participants. This testing covered numerous aspects of sound processing, from the sensory hair cell responses in their inner ears to the auditory reflexes of their brainstems. The study, which was funded in part by NIH, was published on November 30, 2023, in Scientific Reports.

The team found that people with chronic tinnitus were much more likely to have reduced responses to sound in their cochlear nerve compared with people in the other two groups. This nerve, which is also called the auditory nerve, carries sound information from the sensory hair cells to the brain. People with chronic tinnitus also had a weaker reflex in response to sound in the muscles of the middle ear.

Those with chronic tinnitus, however, were more likely to have increased activity in related nerves in the brainstem. This combination of findings supports the idea that reduced functioning in the cochlear nerve may lead to increased activity in the brain to cause phantom sounds.

“Our work reconciles the idea that tinnitus may be triggered by a loss of auditory nerve, including in people with normal hearing,” Maison says.

As not all people who experience cochlear nerve damage develop tinnitus, more research is needed to understand other contributors to the condition. Maison and his team plan to explore approaches to boost regeneration of the auditory nerve as a potential treatment for tinnitus.

—by Sharon Reynolds