What are the spinal cord pathways mediating the somatosensory evoked potentials (SEPs)?

Updated: Feb 26, 2019
  • Author: Sombat Muengtaweepongsa, MD, MSc; Chief Editor: Selim R Benbadis, MD  more...
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Answer

The large-fiber, rapidly conducting afferent somatosensory pathways that sustain the primary cortical SEPs to stimulation of mixed sensorimotor limb nerves travel predominantly in the dorsal columns (fasciculus gracilis and fasciculus cuneatus) within the spinal cord.

In experimental animals, transection of the dorsal column pathways almost completely obliterates the earliest cortical SEPs to stimulation of nerves whose axons enter the spinal cord caudal to the transection, while ventrolateral funiculus lesions usually have only minor effects on these SEPs. Thus, significant damage to descending motor systems can occur without causing changes in the SEPs used for intraoperative monitoring. Such false-negative cases are fortunately rare, but they have occurred. In contrast to the cortical SEPs, near-field SEPs recorded over the spinal cord (see image below) may contain components reflecting large-fiber afferent activity in both the dorsal columns and the spinocerebellar tracts.

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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