The morphological changes in the cochlea following electrical stimulation--experimental study of the cochlear implant

Abstract

The morphological changes of the guinea pig and chinchilla cochlea following extracochlear electrical stimulation were investigated. Three types of electrical pulsatile stimulation with various intensities were given through the silver ball electrodes placed on the round window membrane.Positive and negative square wave pulse stimulation caused severe damage to the organ of Corti and spiral ganglion cell. Morphological changes were found in the ears stimulated for one hour at charge densities over 212μC/cm2/phase of positive and 318μC/cm2/phase of negative square wave pulse. Even at the lower stimulation level, long term direct current stimulation caused severe damage to the cochlea. In the most severely damaged area, most outer hair cells were missing and the pillar cells were degenerated. Some damage to the inner hair cells and loss of the spiral ganglion cells were also seen. All these changes were limited to the basal turn.When symmetrical, biphasic, charge balanced pulses were used, some damage to the first row outer hair cell cilia was seen but the spiral ganglion cells showed no remarkable morphological change. This change of the stereocilia was limited to a small region of the basal turn and was much milder than those due to intracochlear electrical stimulation reported by other investigators. The electrical stimulation was also given to the cochlea damaged by streptomycin sulfate (SM) injection. The electrical stimulation did not modify the extent of SM effect on the cochlear nerve.These results indicate that electrical stimulation without direct component does not adversely affect the cochlea if operated well within the electrochemically safe limit of electrode. Thus, from the morphological point of view, an extracochlear electrode should be given the top priority when the single channel electrode system is utilized.