What is near-field somatosensory evoked potentials (SEPs)?

Updated: Feb 26, 2019
  • Author: Sombat Muengtaweepongsa, MD, MSc; Chief Editor: Selim R Benbadis, MD  more...
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In some patients who are undergoing surgery for lesions that have already caused spinal cord dysfunction, both cortical and far-field SEPs may be absent in surface recordings because of a combination of anesthetic effects and temporal dispersion of the afferent volley. Electrodes positioned close to the spinal cord may pick up reproducible near-field SEPs, permitting intraoperative monitoring. Because the spinal cord volley in these patients is desynchronized and/or because the near-field electrode picks up activity in several different fiber populations, the SEPs picked up by electrodes on the dorsal surface of the spinal cord often are complex and polyphasic. This may preclude simple latency and amplitude measurements, but SEP changes caused by manipulation of the spinal cord or irrigation with cold fluids still can be recognized (see image below).

Desynchronized, polyphasic somatosensory evoked po Desynchronized, polyphasic somatosensory evoked potentials (SEPs) to posterior tibial nerve stimulation, recorded from the spinal cord during removal of an intradural extramedullary neuroma that was compressing the spinal cord in a 44-year-old woman. Cervical SEPs were highly inconsistent and not suitable for monitoring; cortical SEPs were absent. The bipolar recording electrodes were placed on the dorsal surface of the spinal cord rostral to the lesion. Note the reversible changes with manipulation of the spinal cord and with irrigation of the cord with cold fluids. Courtesy of Legatt, 1991.

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