Nov. 30 (UPI) -- People with persistent ringing in their ears, or tinnitus, are suffering from auditory nerve loss that goes undetected in standard hearing tests, a new U.S. study has found.
The research by Mass Eye and Ear scientists at Eaton-Peabody Laboratories in Boston establishes the link between cochlear synaptopathy, commonly referred to as "hidden hearing loss," and tinnitus, bringing a cure a step closer, according to the study published Thursday in the journal Scientific Reports.
The research team sought to determine if hidden damage could be associated with the tinnitus symptoms experienced by test subjects whose hearing rated normal on hearing tests.
By measuring the response of their auditory nerve and brainstem, the researchers found that chronic tinnitus was not only associated with a loss of auditory nerve, but participants also showed hyperactivity in the brainstem.
"Beyond the nuisance of having persistent ringing or other sounds in the ears, tinnitus symptoms are debilitating in many patients, causing sleep deprivation, social isolation, anxiety and depression, adversely affecting work performance, and reducing significantly their quality of life," said senior author Stephane F. Maison, a principal investigator at Mass Eye and Ear, a member of Mass General Brigham and clinical director of the Mass Eye and Ear Tinnitus Clinic.
"We won't be able to cure tinnitus until we fully understand the mechanisms underlying its genesis. This work is a first step toward our ultimate goal of silencing tinnitus."
Scientists have for many years believed the buzzing, humming, ringing or even roaring symptoms of tinnitus are caused by maladaptive plasticity of the brain in which the brain tries to compensate for "hidden" hearing loss by jacking up its activity, producing "phantom sound" that isn't really there.
However, hearing tests of some people with tinnitus come back normal.
The Mass Eye and Ear investigators' latest work builds on landmark research they conducted in the 2000s that sparked a revival of this hypothesis in 2009 by showing that significant auditory nerve loss was not incompatible with a normal score in conventional hearing tests.
"Our work reconciles the idea that tinnitus may be triggered by a loss of auditory nerve, including in people with normal hearing," Maison said.
The study was of 294 people aged 18 to 72, of whom 29 chronic tinnitus sufferers formed the test group with a control group of 265 people without tinnitus.
The tinnitus group, the vast majority of whom were male, reported a history of concussion and symptoms of anxiety and or depression more often than controls. All but one of the "chronic" test group heard noise in both ears.
The chronic tinnitus sufferers reported finding it more difficult to hear in noisy environments than the control group despite both the amount and amplitude of noise exposure reported by both groups being par.
This is in line with other studies, but the authors said its reliability was limited due to subjectivity and dependence on participants accurately recalling the number and repetitiveness of exposure episodes.
The scientists' next goal is to leverage recent work with a class of drugs called neurotrophins as a possible way to regenerate auditory nerves.
"The idea that, one day, researchers might be able to bring back the missing sound to the brain and, perhaps, reduce its hyperactivity in conjunction with retraining, definitely brings the hope of a cure closer to reality," Maison added.
The study was funded with the help of grants from the National Institutes of Health's National Institute on Deafness and Other Communication Disorders and the Lauer Tinnitus Research Center at the Mass Eye and Ear.