Many people suffer from hearing loss. However, only about a third of those patients will develop tinnitus; this amounts to about 23 million people annually in the United States.
Why do some people with hearing loss develop tinnitus while others don’t? This is a question that has puzzled doctors for many years. Now, researchers have suggested a unique theory for while some people are at risk for tinnitus but others aren’t.
Josef Rauschecker, Ph.D., D.Sc., and some of his colleagues in the Department of Neuroscience, the Division of Audiology, and the Department of Otolaryngology at Georgetown University believe that the limbic system functions as a “gatekeeper” to block the neural impulses that produce tinnitus from reaching the brain. They suggest that the limbic system—which is a network of brain structures that are involved with long-term memory, emotion and behavior—doesn’t work properly in patients who develop tinnitus. To extend the analogy, the “gate” is “broken.”
Doctors have long believed that tinnitus starts because the brain is trying to “hear” sound frequencies that have been lost because of some kind of damage to the ear. In this model, the brain “turns up” the volume of the sound from the hair cells that receive adjacent frequencies, which results in phantom noise reaching the brain.
Now Dr. Rauschecker and his working group have suggested that what determines whether the brain receives these signals or not is the interaction of the limbic system and the auditory system. This interaction helps the brain sort out and ignore many common sounds in our environment that are meaningless to us, he reasoned, so it makes sense that this “filter” also would work for phantom sound produced internally by the ear. Their theory is supported by imaging studies of patients’ brains; patients who developed tinnitus had significant differences in the interface between the auditory and limbic systems.
So what does all of this mean? Although the current state of research doesn’t tell us how and why some people develop tinnitus and other don’t, it gives doctors and scientists a place to look for a definitive answer to what causes tinnitus, and this eventually could lead to a cure for this condition. If further research bears out what Dr. Rauschecker and his colleagues believe, then they would be able to work toward developing or finding medications that are active in that part of the brain and which would provide relief from unrelenting head noise.
What Dr. Rauschecker and his team discovered also may explain another aspect of tinnitus. Overall, the incidence of depression in patients with tinnitus is more than twice the national average. What makes this statistic interesting is that patients with depression also exhibit a malfunction in the same area of the brain. This means that medications used to treat depression—particularly those medications that increase serotonin activity in these parts of the brain—also could be used to target tinnitus and accompanying depression at the same time.