Nov 2019 The Varsity

In conversation with neurosurgery specialist Dr. Martin Jakobs about the treatment’s effects for hearing impairment

Deep brain stimulusNew research is offering the possibility of a treatment for tinnitus, an incurable condition that causes a patient to perceive sound, even if there is no external source for it. Dr. Martin Jakobs, a physician at the Department of Neurosurgery at University Hospital Heidelberg, co-authored an editorial in September on the first-ever study involving a particular form of deep brain stimulation (DBS) to treat severe, treatment-resistant tinnitus. DBS involves implants in the body that target subconscious processes, showing how important they are in the process of perception.

Dr. Andres Lozano, a professor at U of T’s Department of Surgery, was the second co-author, and Dr. Steven Cheung, a professor at the University of California, led the study that Jakobs and Lozano analysed in their editorial.

What causes tinnitus?

A common underlying cause for tinnitus is sensory hearing loss that results from exposure to loud noises, ageing, or as an adverse reaction to medication. When the hearing loss occurs, other parts of the hearing system become hyperactive to compensate for it. This leads to a reorganisation which causes unusual synchronisation in the primary auditory cortex, a part of the brain that is involved in hearing, which then triggers the effects of tinnitus. Patients suffering from tinnitus usually opt to wear special hearing aids, which help to counteract the condition by playing tones from certain frequencies.

Deep brain stimulation as a potential alternative

The DBS system consists of three components: the lead, which is implanted into the scalp; the internal pulse generator (IPG), which is usually implanted under the skin near the collarbone; and the extension, which connects the lead to the IPG. DBS is considered to be an invasive treatment, as the operation requires creating an incision into the patient’s body.

Cheung’s study was in the first phase of a clinical trial, which evaluates the efficacy and safety of a new treatment. At the trial’s conclusion, following 24 weeks of continuous stimulation, three of the five remaining patients saw significant improvement within a metric called the “Tinnitus Functional Index score.” This score measures the “intrusiveness of tinnitus, the sense of control the patient has, cognitive interference, sleep disturbance, auditory issues, relaxation issues, quality of life, and emotional distress.” Four of the patients also saw improvement in their “Tinnitus Handicap Inventory score,” which is a separate measuring system that quantifies the effects of tinnitus.

Cheung’s team found that the treatment could preserve hearing safety, because the patients’ hearing thresholds were not significantly changed during the process. “The results of this group are very interesting and encouraging,” wrote Jakobs. “The concept of trying to block precepts from reaching awareness is fascinating and tells you how important subconscious processes are in the process of perception.”

Cheung’s research group is now trying to better understand “where the most effective target area within [a brain area named] the caudate nucleus is located,” what connections are affected by the stimulation, and eventually, where the neurological patterns originate in the brain. Once these aspects are investigated, an optimised stimulation target for each patient might be uncovered.

The limitations of deep brain stimulation

However, even though no apparent safety issue has been found in this study, DBS is not a universal solution. In treating progressive diseases such as Parkinson’s disease, more electricity is needed to achieve beneficial effects. When the electrical field is enlarged and the volume of activated tissue grows, not only are neurons in the target structure affected, but parts of neurons called axons are impacted as well. These axons, when stimulated, may cause unwanted harm. Whether this is true for hearing impairments, like tinnitus, remains to be investigated in future studies.

There are other, non-traditional methods to treat tinnitus that are incision-free, such as using magnetic resonance-guided focused ultrasound (FUS), which heats brain tissues to create a lesion. However, unlike DBS, the FUS treatment is irreversible. DBS could therefore be a promising option for patients with tinnitus which medical professionals could recommend before a patient considers FUS treatments.

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