Electrical stimulation of the facial nerve with a prognostic function in parotid surgery

Continuous electromyography during parotidectomies and direct stimulation of the facial nerve as an intraoperative identification technique significantly lower the rate of post-operative morbidity. To determine the usefulness of intra-operative neurophysiological parameters registered by means of electrical stimulation of the facial nerve as values capable of predicting the type of lesion and the functional prognosis. Our sample consisted of a correlative series of 20 cases of monitored parotidectomies. Post-operative facial functioning, type of lesion and its prognosis were compared with the variations in latency/amplitude of the muscle response between two stimulations of the facial nerve before and after resection, as well as in the absence or presence of muscle response to stimulation after resection. All the patients except one presented motor evoked potentials (MEP) to stimulation after resection. There was no facial damage following the operation in 55% of patients and 45% presented some kind of paresis. The 21% drop in the amplitude of the intra-operative MEP and the mean increase in latency of 13.5% correspond to axonal and demyelinating insult, respectively, with a mean recovery time of three and six months. The only case of absence of response to the post-resection stimulation presented permanent paresis. The presence of MEP following resection does not ensure that functioning of the nerve remains undamaged. Nevertheless, it can be considered a piece of data that suggests a lower degree of compromise, if it is present, and a better prognosis. The variations in latency and amplitude of the MEP tend to be intra-operative parameters that indicate the degree of compromise and functional prognosis.