Physiological aspects regarding bilateral fitting of BAHAs

Abstract

Normally, when we hear a sound, the sound is transmitted through the outer and middle ear to the inner ear where the sound energy is converted to nerve impulses. These impulses are transmitted further to the cochlear nucleus and superior olivary complex where information from both ears is mixed, compared and analysed. It is here the binaural process occur which allows us to determine the direction of the sound source as well as improve hearing in noise when the useful sound source and noise source is spatially separated. Hence, a patient with a bilateral hearing loss and fitted with airconduction (AC) hearing aids generally benefits from binaural fitting compared with monaural fitting. When a bone-anchored hearing aid (BAHA) is used, the sound is transmitted via bone conduction (BC) to the inner ear. The difference between the stimulation level of the contralateral and ipsilateral cochlea, often referred to as the transcranial attenuation, is normally in the range of 0–20 dB for frequencies up to 10 kHz. Hence, the side-specific stimulation of the inner ears is less with BC than with AC, which affects the binaural effect of the hearing. Due to the overhearing between the cochleae with BC, it is questionable whether binaural hearing is possible using BC stimulation and, consequently, BAHAs. Proceedings of the 4th International Symposium on Electronic Implants in Otology 83