Transcranial electrical stimulation through screw electrodes for intraoperative monitoring of motor evoked potentials

Technical note

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✓ The feasibility of high-frequency transcranial electrical stimulation (TES) through screw electrodes placed in the skull was investigated for use in intraoperative monitoring of the motor pathways in patients who are in a state of general anesthesia during cerebral and spinal operations.

Motor evoked potentials (MEPs) were elicited by TES with a train of five square-wave pulses (duration 400 µsec, intensity ≤ 200 mA, frequency 500 Hz) delivered through metal screw electrodes placed in the outer table of the skull over the primary motor cortex in 42 patients. Myogenic MEPs to anodal stimulation were recorded from the abductor pollicis brevis (APB) and tibialis anterior (TA) muscles. The mean threshold stimulation intensity was 48 ± 17 mA for the APB muscles, and 112 ± 35 mA for the TA muscles. The electrodes were firmly fixed at the site and were not dislodged by surgical manipulation throughout the operation. No adverse reactions attributable to the TES were observed.

Passing current through the screw electrodes stimulates the motor cortex more effectively than conventional methods of TES. The method is safe and inexpensive, and it is convenient for intraoperative monitoring of motor pathways.

Article Information

Address reprint requests to: Katsushige Watanabe, M.D., Ph.D., Department of Anatomy and Neurobiology, College of Medicine, The University of Tennessee Memphis, 855 Monroe Avenue, Memphis, Tennessee 38163. email: katsuwatanabe-nsu@umin.ac.jp.

© AANS, except where prohibited by US copyright law.

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Figures

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    A: Photograph of a screw electrode. B: Photograph of a patient's head (supine position) showing placement of screw electrodes in the outer table of the skull over C3 and C4. The electrodes are connected to the cables from the stimulation device and are secured with transparent dressings to avoid invasion of the electroconductive fluid. C: Schematic representation of screw placement. Stimulus current was delivered between the two screw electrodes (arrows).

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    Conceptual representations of the current pathways for stimulation. A: Much of the stimulus current spreads laterally through the scalp from the scalp electrodes because of the high resistivity of the skull. B: The stimulus current passes through the skull more effectively from the screw electrodes.

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