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When a patient gets her first cochlear implant, it can transform her world from silence to sound. About a year later, she may become a candidate for a second implant in her other ear, which can enrich her newfound hearing by giving her a better understanding of speech in noisy situations, the ability to hear in stereo (from various directions) and the ability to figure out the direction sounds are coming from. But the Children's Hospital Boston's cochlear implant team can't be too careful when it comes to this second (bilateral) implant, since issues of balance can arise—issues which haven't been greatly studied, since bilateral pediatric cochlear implants are still quite new.
The vestibular, or balance, system is part of the inner ear, connected to the cochlea, the spiraled cavity of the inner ear. When a patient undergoes cochlear implant surgery in one ear, the balance system on the other side can generally accommodate for the disruption. However, when a patient has bilateral implants, the vestibular system is affected on both sides, causing potentially severe balance problems. This may occur because the side that had "made up" for any balance disturbance may now be impaired as well, due to the presence of the second cochlear implant.
Because of this risk, the implant team has been testing children before and after cochlear implant surgeries. Researcher and cochlear implant expert Margaret Kenna, MD, MPH, and her colleagues Greg Licameli, MD, and Guang Wei Zhou, ScD, assess patients' balance by measuring the function of different anatomic parts of the vestibular system, including a piece of the inner ear, called the saccule, which can sense sounds and send signals to the neck muscles; contraction of the neck muscles in response to the sound stimulus can then be measured. To evaluate how well this process works, researchers place electrodes on a patient's head and neck. The patient then moves her head and neck in certain directions while listening to a specific sound stimulus; by recording the neck muscle activity, researchers can figure out what's happening in the inner ear. Although the idea that neck muscles contract in response to sound seems strange, the response likely represents a safety reflex that helps a patient "right" her head if the vestibular system is disturbed.
Another study measures the Subjective Visual Vertical. For this test, a child sits in a chair in a dark room and, using both eyes, watches a light bar moving along a wall while the examiners measure how well her eyes can align the light bar with a fixed straight line. If the patient can't follow the light while righting themselves properly, it indicates that the balance system may be off.
"The idea is to try to isolate the problem in the balance system," says Kenna. "Then, we take the results and that becomes part of counseling the patient and family when it comes time to decide about the second implant." On a wider scale, Kenna and her colleagues also hope to study children with varying degrees of hearing loss to shed light on how different causes of hearing loss affect equilibrium. "It's a very understudied area, so it's very exciting," she says.
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