April 2019 Korea Biomedical Review
Researchers at Asan Medical Centre have reported that the side effects of facial nerve stimulation after cochlear implant vary depending on the presence or absence of a malformation in the inner ear. Patients with severe hearing loss receive a cochlear implant that can electrically stimulate the auditory nerve to hear the sound. However, if the implant also stimulates the facial nerve near the auditory nerve, it can cause adverse effects such as pain when hearing loud noises.
The team, led by Professor Park Hong-ju of the Department of Otorhinolaryngology at the hospital, analyzed 1,151 patients who underwent cochlear implant surgery whether or not they had adverse effects after the operation. According to the result, the incidence of facial nerve stimulation in the absence of anomalies in the inner ear was very low, with 0.9 percent. When the patient had a malformation in their inner ear, however, the incidence of facial nerve stimulation increased to more than seven times to 6.4 percent. Cochlear implants require the insertion of electrodes into the cochlea. The cochlea, located in the inner ear, has a connection not only to the auditory nerve but also to the facial nerve. “In the case of cochlear anomalies, it is interpreted that the facial nerve location is distributed differently from the normal people, which can stimulate both the auditory and peripheral nerves and cause facial nerve stimulation adverse effects,” the hospital said in a press release.
If the facial nerve stimulation occurs after surgery, it is necessary to selectively block the electrode that stimulates the facial nerve and cause the sound stimulation not fully to reach the auditory nerve system, it added. The hospital recommended that those patients go through auditory rehabilitation training so that they can minimise or prevent such side effects. It also said that facial nerve stimulation patterns are different depending on the presence or absence of cochlear abnormalities, and facial nerve stimulation occurs at different frequencies depending on the type of inner ear deformity.
Professor Park’s team suggested a customised surgical guideline, which minimises side effects by adjusting the electrode type and length of the cochlear implant in such situations. “By taking a computed tomography (CT) image of the ear before cochlear implantation, physicians can predict the possible occurrence of facial nerve impairment and its appearance,” Park said. “In recent years, not only newborn babies born with hearing loss but also adults suffering from hearing impairment have become heavily dependent on cochlear implants.” The mechanism of the occurrence of facial nerve impulses and the proposed operational guidelines to avoid them will be helpful for many patients who need to undergo cochlear implant surgery, the professor added.