Cochlear implants and brain stem implants

This chapter describes the development of two implantable prosthetic neurostimulators which, in the last 20 years, have revolutionised the management of severe-to-profound sensorineural deafness. We have witnessed their rapid evolution from the realms of esoteric laboratory abstraction, with many critics and little perceived clinical use, to a routine treatment which is safe, effective and, indeed, cost effective. It is one of the great triumphs of biomedical and surgical collaboration, and is without any doubt the greatest ever advance in the treatment of deafness. The anatomy and physiology of the auditory pathways have been described in a previous special issue 1 . To summarise, a sound wave entering the ear causes the tympanic membrane and ossicular chain to vibrate. The stapes, the smallest and innermost of the three ossicles moves in-and-out in the oval window and is the interface between the middle and inner ears. The mechanical movements of the middle-ear sound-conducting apparatus are transmitted to the inner-ear fluids and a wave, the travelling wave of von Bekesy, passes up the cochlea to reach a maximum at a point determined by its frequency. For the process to continue, the physical energy of the travelling wave has to be converted into electrical energy that can be propagated through the auditory nerve to the brain stem and from there to the higher auditory centres. This process of transduction occurs in the organ of Corti. Depolarisation in the inner hair cells initiates transmission through the first order neurones, the cell bodies of which are in the spiral ganglion. The synapse with the second order neurone occurs in the cochlear nucleus which is situated in the lower pons cranial to the foramen of Luschka. The more cranial nuclear projections in the brain stem and auditory cortex are complex and a detailed understanding of their anatomy is not necessary for the understanding of this essay. Many disease processes may lead to loss of hair cells in the organ of Corti. The commonest is the natural process of ageing in which there is a progressive loss of cells typically starting in the basal turn of the cochlea and advancing apically, accompanied by a hearing loss that initially affects the high frequencies and, with time, the middle and lower frequencies. These cells are incapable of spontaneous regeneration