18 Electrical stimulation of the cochlear nerve with unipolar and bipolar electrode systems has been devised as a cochlear prosthesis in which sounds are transformed into electrical stimuli through a processor. Clinical studies of multiple-electrode prostheses have shown that they provide significant help to postlingually deaf patients in understanding speech, especially when used in combination with lip-reading. About one half of such patients can understand some running speech without help from lip-reading (Clark et al 1987, Brown et al 1987). It has been estimated that there are about 3300 postlingually deaf adult subjects who may be considered for cochlear implant (Thornton 1986). Subjective auditory threshold estimation of electrical cochlear stimulation and recording of evoked potentials in a profoundly deaf patient have been suggested as a method of pre-operative assessment of the viability of ganglion cells and neural fibres. The function of the implanted prosthesis can be objectively assessed by stimulating through the implanted electrode and evaluating the response. Evoked potentials similar to ABR and MLR can be recorded in man using extracochlear and intracochlear electrodes. These are known, respectively, as electrically evoked ABR (EABR) and electrically evoked MLR (EMLR). Several methods of electrical stimulation have been described. Studies in experimental animals indicate that intracochlear stimulation from an electrode in the scala tympani produces better responses than does stimulation at the round window or the promontory when a large population of ganglion cells survive. In ears with almost no ganglion cells, the responses from the round window and the scala tympani are similar (Simmons et al 1987). A relatively small impedance can be achieved by using a platinum balled tip electrode placed into the round window through a small tympanotomy, and a second electrode placed on the promontory or the mastoid. The monopolar electrode used for cochlear implantation can successfully serve for pre-operative evaluation of auditory function with electrically evoked auditory potentials. Evoked potentials have been recorded by stimulating through the electrodes of an implanted extracochlear or intracochlear prosthesis (Fig. 18.1). Testing can be carried out in the operating room under anaesthesia or at a later date in the out-patient clinic. With general anaesthesia, the myogenic artefacts can be avoided on the EABR and EMLR recordings. Also, with extracochlear stimulation, higher stimulation levels can be attained without causing discomfort or pain. Fig. 18.1 ABR in response to electrical stimulation via an implant. With electrical stimulation that affects the nerve directly, the latencies are shorter than those obtained from acoustic stimulation (Upper trace: Initial result. Lower trace: two years postoperative). (After Miyamoto & Brown 1987.)
Electrically evoked potentials in cochlear stimulation
CLINICAL PROBLEM
Tests for patient selection
METHODS OF ELECTRICALLY EVOKED POTENTIALS
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