Stimulus-inadequate sensations in cochlear implant patients.

From August 1984 to April 1987 the Department of Otolaryngology at the Medizinische Hochschule in Hannover implanted the NUCLEUS 22 channel cochlear implant system (CI) in their first 56 adults. Since implantation, 27 (51%) patients have described undesirable sensations from their implant. These were defined as stimulus-inadequate sensations (SIS) and were classified into three types: 1) pain-like sensations; 2) high-frequency sensations; and 3) non-stimulation. Twenty-three patients were radiographically investigated by polytomography. All of the intracochlear electrodes were visualised and a correlation between SIS and the radiological findings was established. It was found that i) SIS similar to pain are caused by electrical stimulation of the mucous membranes and periosteum of the tympanic cavity; ii) the high-frequency SIS seemed to be due to non-physiological electrical stimulation of the basal ganglion cells; and iii) the reason for non-stimulation is probably damaged electrodes and fibrosis around the electrode. Conventional tomography is the method of choice for estimating the number and configuration of intracochlear electrodes.