Investigating the Etiology of Hemifacial Spasm (HFS): The Role of Desflurane

"Intra Operative Monitoring" (IOM) is a standard and routine medical practice for patients having MVD surgery. IOM is done to make sure that the hearing and face nerves are not accidentally bumped or damaged during surgery. After anesthetic induction, the neurophysiologist places electrodes for the nerves, on the scalp and muscles. Stimulation and monitoring of muscle twitches are all common, safe and painless medical procedures.

In this study, the investigators would like to measure the nerve and muscle function on the normal side and compare it to nerve and muscle function on the abnormal side during different levels of the anesthetic desflurane. In order to monitor the normal side of the face one additional set of electrodes will be placed in the facial muscles. The investigators will compare responses to three different levels of anesthetic. The technique used will be motor evoked potentials. The effect of the anesthesia on the lateral spread response will also be examined. This response is evident only on the symptomatic side. These observations will take approximately 10 to 15 minutes and will not delay or lengthen the surgery.