Intraoperative Neurophysiological Monitoring for the Spinal Accessory Nerve: Case Illustrations

The spinal accessory nerve, cranial nerve XI, begins at the level of the medulla and travels through the neck to supply motor innervation to the trapezius and sternocleidomastoid (SCM) muscles. The spinal accessory nerve has the potential for injury during radical head and neck surgeries resulting in weakness of the muscles supplied. Intraoperative neurophysiologic monitoring is performed by placing dual subdermal needle electrodes into the ipsilateral upper trapezius muscle on the side of dissection. A Prass probe monopolar cathode stimulator with an adjacent anode reference needle electrode placed in nearby muscle is used by the surgeon to directly stimulate when the spinal accessory nerve is localized. Stimulus intensities of 0.5-4 milliamps (mA) are utilized. Free running EMG activity is also monitored from the trapezius muscle electrodes during the course of surgery for any indirect stimulation of the spinal accessory nerve. All waveforms and data were obtained utilizing Cadwell Cascade® intraoperative monitoring recording equipment.