New Research Traces the Onset of Tinnitus to the Cellular Level

Tinnitus is popularly known as a ‘ringing in the ears’, but the term actually refers to any perception of noise in the absence of a true external sound. This can take many forms, including buzzing, ticking and whistling as well as ringing. Approximately 1 in 3 people experience tinnitus at least once in their lifetime, and the worldwide prevalence is approximately 10 percent. Tinnitus can range from an occasional annoyance to a debilitating condition resulting in fatigue, irritability and depression. Up to one million sufferers state that tinnitus significantly disrupts their lives.

Objective versus subjective tinnitus

Tinnitus can be divided into two subtypes, objective and subjective. Objective tinnitus can be heard by an outside observer using special listening devices, and usually has an anatomic basis. One example is pulsatile tinnitus, which refers to the rushing rhythmic sound corresponding to the patient’s pulse. This can be caused by abnormal positioning of a large artery next to the middle ear. Objective tinnitus can often be cured or effectively treated medically.

In contrast, subjective tinnitus, while considerably more common, is less well understood. Individuals with subjective tinnitus hear noises that cannot be detected by an outside observer. The nature of these noises can vary widely; they may be constant or infrequent, involve one or both ears, and stay the same over time or change character. Subjective tinnitus is much more difficult to effectively treat.

What causes subjective tinnitus?

In approximately 40 percent of cases of subjective tinnitus, no definite cause can be identified. The most common identifiable cause is exposure to excessively loud noise. This typically involves repeated exposure to noises such as aircraft, loud music and heavy construction equipment. These individuals also typically suffer from noise-induced hearing loss.

Another common cause is ototoxic (damaging to the ear) drugs. These include many commonly prescribed drugs including antibiotics such as erythromycin and antidepressants such as Prozac. Over the counter analgesics including aspirin and ibuprofen can also be ototoxic, particularly in large doses.

Current treatment options for subjective tinnitus

There is currently no cure for tinnitus, and no effective drug treatment. Treatment instead focuses on lessening the severity of the symptoms. Masking devices, worn like a hearing aid, provide a constant low-level noise. This is similar to the noise produced by commercially available ‘white noise’ machines, mimicking ocean surf or rainfall. This appears to work by distracting the brain from the tinnitus. If tinnitus is accompanied by hearing loss, conventional hearing aids, which work by amplifying sound, can sometimes result in at least partial relief from tinnitus as well. Tinnitus retraining therapy combines masking devices with directed cognitive therapy, to gradually teach patients to ‘ignore’ tinnitus. This can take up to one to two years to be effective.

Understanding tinnitus at the cellular level may guide future therapy

Researchers at the University of Leicester have been studying cells in a brain structure called the dorsal cochlear nucleus. These cells transmit chemical messages between nerve cells in the ear and processing centers in the brain responsible for making sense out of the sounds we hear. After repeated exposure to loud noises, cells in the dorsal cochlear nucleus become over-excited and transmit messages in an uncontrolled fashion. This leads to the irregular buzzing and ringing that characterizes tinnitus.

The researchers have traced this irregular cellular activity to a dysfunction of potassium channels. After exposure to loud noises, the potassium channels stop working effectively. This traps the cells in an excited state, in which they continue to send signals to the processing centers of the brain. This forces the brain to ‘hear’ a sound when no sound actually exists.

While scientists had already recognized the importance of the cochlear nucleus, this is the first research to specifically define the mechanism of tinnitus at the cellular level. This will allow researchers to develop therapies that target the potassium channels in the affected cells. The goal is to return to cells to a resting state, stopping the continuous signaling that results in the constant buzz of tinnitus. These drugs could also be used for prevention, when given to patients exposed to loud noises or ototoxic drugs. While still in its early stages, this breakthrough could one day lead to relief for the millions suffering from tinnitus.

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Dr. Margaret G. McKernan, MD

Margaret McKernan, MD, PhD is a practicing diagnostic radiologist and medical writer. She has an MD, PhD degree in Neuroscience from the University of Texas Medical Branch, and completed her training in Radiology at Wake Forest University Baptist Medical Center. She also completed additional fellowship training in Abdominal Imaging at Massachusetts General Hospital.

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