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Phase Reversal of Somatosensory Evoked Potentials Triggered by Gracilis Tract Stimulation: Case Report of a New Technique for Neurophysiologic Dorsal Column Mapping

Simon, Mirela V. MD*; Chiappa, Keith H. MD*; Borges, Lawrence F. MD

doi: 10.1227/NEU.0b013e31822e0a76
Case Report
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BACKGROUND AND IMPORTANCE: Reliable visual identification of the median raphae, essential for the preservation of function of the posterior dorsal columns during intramedullary spinal cord tumor resection, is not possible in many cases, because of distorted local anatomy. In such cases, intraoperative neurophysiologic mapping of the dorsal columns offers invaluable information to the surgeon, and guides the myelotomy. We hereby describe such a new technique.

CLINICAL PRESENTATION: A 41-year-old man with a C3-C4 intramedullary spinal cord tumor underwent successful myelotomy and tumor resection. Dorsal column mapping was performed by use of an 8-contact minielectrode strip placed on the dorsal spinal cord. Direct electrical stimulation was applied via 2 adjacent contacts of the strip at a time, in an attempt to stimulate in succession the left and right dorsal columns. Somatosensory evoked potentials (SSEPs) were recorded after each stimulation, via scalp electrodes. A sharp change in polarity of the recorded scalp SSEPs (phase reversal) indicated when the stimulation of the opposite dorsal column occurred. Myelotomy was performed in between the minielectrode contacts identified as being situated closest to the raphe. The posterior tibial SSEPs were continuously monitored during and after myelotomy and until the dura closure. No changes from premyelotomy SSEPs were present. Postoperatively, the patient had preservation of the posterior column function.

CONCLUSION: SSEP phase-reversal technique is a promising new method to identify the neurophysiologic midline in intramedullary tumor resection. Fast and easy to perform, its final role in neurophysiologic dorsal column mapping awaits confirmation in future applications.

*Neurophysiology Division, Department of Neurology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts

Department of Neurosurgery, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts

Correspondence: Mirela Simon, MD, WACC 739-G, 55 Fruit St, Massachusetts General Hospital, Boston, MA 02114. E-mail:

Received September 3, 2010

Accepted May 11, 2011

Copyright © by the Congress of Neurological Surgeons