Abstract
The Somatosensory Evoked Potential Phase Reversal (SEP-PR) technique is used to identify the Central Sulcus (CS) during brain surgery. A 64-year old man was operated on for a left post-rolandic tumor. The patient had a right arm plaster cast preventing median nerve stimulation at the wrist. We report the results of phase reversal SEPs following index finger stimulation. Electrical stimulation of the right II° finger was delivered by 12 mm needle electrodes (0.2 ms duration, frequency rate 4.7 Hz, threshold intensity 20 mA). Transcranial SEPs were recorded from C3′/Cz′ electrodes. After opening of the dura, a 1 × 8 strip electrode was placed on CS. We obtained good phase reversal across electrode number 4 (motor) and electrode number 5 (sensory). Direct cortical stimulation was delivered by electrode number 4 (train of five stimuli, ISI 4 ms, threshold intensity ranging from 5 to 20 mA at 1 Hz rate) and muscle motor evoked potentials were recorded on biceps brachialis and abductor brevis pollicis muscles. This case report illustrates one of several unusual drawbacks that technicians may face in the operating room. Flexibility on a multimodal intraoperative neurophysiological approach is important to overcome these drawbacks. Finger stimulation can be a reliable alternative to perform SEP-PR whenever direct stimulation of the median nerve at wrist is not feasible.
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