Percutaneous Threshold of Facial Nerve Stimulation Predicts Facial Canal Dehiscence.

Iatrogenic facial nerve (FN) injury is one of the most feared complications of otologic surgery. Dehiscence of the bony covering of the FN within the temporal bone increases FN vulnerability to accidental injury. High-resolution computed tomography (HRCT) of the temporal bone is used preoperatively to assess middle ear and mastoid anatomy; however, it is unreliable for detecting facial canal dehiscence. In this study, our aim was to determine if preoperative percutaneous FN stimulation could predict middle ear facial canal dehiscence. Between January 2015 and February 2017, we performed preoperative HRCT and percutaneous FN stimulation on adult patients who underwent otologic surgery at our institution. Stimulation was performed with a monopolar probe placed on the skin over the stylomastoid foramen. Electrical stimuli ranged from 0 to 40 milliamperes (mA). Recordings were made from ipsilateral facial muscles. Dependent variables included threshold to compound muscle action potential (CMAP), threshold to maximum amplitude of CMAP, and maximum amplitude of CMAP for each muscle. A retrospective chart review was performed. Seventy patients met inclusion criteria. Of the 24 with an intraoperatively confirmed dehiscence, 10 were identified preoperatively by the attending surgeon on HRCT. Averages of the lowest recorded threshold to CMAP (5.1mA v. 9.1mA), and an average of the threshold to CMAP (8.9 mA. 11.8 mA) of dehiscent versus non-dehiscent nerves were significantly different (p < .05). In conclusion, percutaneous FN stimulation is a simple and cost-effective tool that can give the surgeon important preoperative information about FN anatomy.