Ahead of Print: Phase Reversal of Somatosensory Evoked Potentials Triggered by Gracilis Tract Stimul

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

CLINICAL PRESENTATION: A 41 year old male with C3-C4 intramedullary spinal cord tumor underwent successful myelotomy and tumor resection. Dorsal column mapping was performed using an 8 contact microelectrode strip placed on the dorsal spinal cord. Direct electrical stimulation was applied via two 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 pre-myelotomy SSEPs were present. Postoperatively, the patient had preservation of the posterior column function.

CONCLUSION: SSEPs 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.



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