A new study from Massachusetts Eye and Ear (Mass Eye and Ear) investigators provides evidence that tinnitus—the persistent perception of ringing or other phantom sounds in the ears—stems from a loss of connections between the ear and brain in people with seemingly normal hearing.
The work, published in Scientific Reports, brings fresh hope for understanding and ultimately silencing these often debilitating symptoms affecting over 10% of adults.
“Beyond the nuisance of ringing in the ears, tinnitus symptoms cause real suffering for many patients, including sleep deprivation, social isolation, anxiety and depression,” explained senior study author Dr. Stéphane F. Maison, Principal Investigator at Mass Eye and Ear and Clinical Director of the Mass Eye and Ear Tinnitus Clinic.
“We won’t be able to cure tinnitus until we fully understand its origins in the brain and ear. By demonstrating auditory nerve damage underlies tinnitus even with normal audiometric thresholds, this work represents a critical step toward that goal.”
-Dr. Stéphane F. Maison
Tinnitus and Hidden Hearing Loss
The longstanding neurophysiological model suggests tinnitus perception arises from maladaptive plasticity when the brain boosts activity to compensate for weakened input from a damaged inner ear. However, some tinnitus patients have clinically normal hearing thresholds, seemingly contradicting the model.
Recent discoveries of “hidden hearing loss” help reconcile this discrepancy. In 2009, Massachusetts Eye and Ear researchers described cochlear synaptopathy, a selective degeneration of connections between inner ear sensory cells and the auditory nerve, which persists even when hair cells remain intact.
Since synaptopathy occurs prior to threshold shifts, patients with significant neural damage still test normal on routine audiometry. “We think subtle neural loss is more common than recognized, arising from noise, aging, genetics, and other causes,” said study co-author Dr. M. Charles Liberman, Director of the Eaton-Peabody Laboratories at Mass Eye and Ear.
Leveraging these insights, Maison’s team set out to determine whether tinnitus and hyperacusis might represent early symptoms of hidden neural degeneration in humans with normal hearing thresholds.
Neural Degeneration and Tinnitus
Their study recruited 294 participants aged 18-72 who reported no hearing difficulties on standard clinical exams. A subset described chronic symptoms like ringing, buzzing, or roaring sounds. By measuring electrophysiological responses from the auditory nerve and brainstem, the researchers confirmed neural deficits in the chronic tinnitus group after accounting for other factors like high frequency loss.
“Our work reconciles the neurophysiological model with the existence of tinnitus in audiometrically normal patients. It appears tinnitus may be triggered by covert loss of auditory nerve connections, likely inducing maladaptive gain increases in the central pathways.”
–-Dr. Stéphane F. Maison
Brain imaging studies provide further evidence that boosting central gain drives tinnitus perception. Patients show heightened activity in auditory processing networks, which correlates with phantom sound severity and resolves when symptoms disappear. Treatments may therefore need to address both neural damage and cortical hyperactivity for optimal efficacy.
These insights bring hope that some of the suffering caused by tinnitus could be relieved by restoring lost auditory nerve inputs. “Recent animal research shows regrowing broken connections with neurotrophin drug treatments can ‘silence’ tinnitus,” Maison said. “Identifying which patients have underlying nerve deficits could reveal who might benefit from such an approach.”
Since most clinical assessments miss hidden neural loss, the electrophysiology protocols used here could provide an important diagnostic tool. Tracking onset and progression of neuropathy in humans will help facilitate translation of emerging therapies aimed at rebuilding damaged sensory structures.
Reference:
- Vasilkov, V., Caswell-Midwinter, B., Zhao, Y. et al. Evidence of cochlear neural degeneration in normal-hearing subjects with tinnitus. Sci Rep 13, 19870 (2023). https://doi.org/10.1038/s41598-023-46741-5
