THE EFFECT OF GENERAL-ANESTHESIA ON INTRAOPERATIVE ELECTRICALLY-EVOKED STAPEDIUS REFLEX MEASUREMENT

Evaluation of the intraoperative electrically evoked stapedius reflexes during cochlear implant surgery has two purposes: (1) The functioning of the device and the peripheral auditory pathways can be proven immediately and (2) The data of reflex threshold can be used for the prediction of later threshold (T)- and maximum comfortable (C) levels. This is especially useful for the later fitting of the speech processor in very young children. When trying to use the stapedius reflex data for the prediction of T- and C-levels it must be considered that during surgery the drugs used for general anaesthesia influence the values of the stapedius reflex. Depending on the drug, the stapedius reflex threshold can be increased or the reflex can even be totally blocked. This is not only due to relaxants but also to anaesthetics. To investigate this influene we first determined the acoustically evoked stapedius reflex threshold in normal hearing adults under general anaesthesia with 7 different anaesthetics without additional relaxation. It was found that especially with Dormicum(R) and Brevimytala the stapedius threshold increased only very lightly or did not change at all. In a second investigation the electrically evoked stapedius reflex thresholds were obtained in adults who underwent a cochlear implant surgery using the same anaesthetics for anaesthesia. The intraoperatively evaluated reflex data were compared with postoperatively obtained data. It was found that both reflex thresholds were similar if evaluated in Dormicum(R) or Brevimytal(R) anaesthesia. This suggests that only with the use of the right anaesthetics the data of the electrically evoked stapedius reflex obtained intraoperatively can be used for predicting T- and C-levels.