What are the lumbar components of lower limb 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

Answer

An electrode placed over the lower thoracic or upper lumbar spine records a combination of the CAP in the primary afferent neuron, propagating within the cauda equina and fasciculus gracilis, and a stationary lumbar potential (SLP) that is derived from postsynaptic neurons in the gray matter of the spinal cord. Recordings with a distant reference, such as the iliac crest, emphasize the SLP, which is analogous to the stationary cervical potential (N13) recorded over the lower cervical spine following median nerve stimulation.

Bipolar recordings between a pair of rostrocaudally separated electrodes over the lower spine record the propagating CAP. However, they also contain a component derived from the SLP, representing the difference in the amplitude of the SLP between the 2 recording electrodes. The relative magnitudes of the CAP and SLP contributions vary across subjects; therefore, referential recordings show less intersubject latency variability and should be used for clinical diagnostic SEP testing.

The normal amplitude maximum of the SLP is at the T10-T12 vertebral level but may be fairly restricted (see image below).

Somatosensory evoked potentials (SEPs) recorded si Somatosensory evoked potentials (SEPs) recorded simultaneously over multiple vertebral levels to posterior tibial nerve stimulation, with an iliac crest reference. The amplitude of the stationary lumbar potential (SLP) is maximal at the T12 level. Negativity at input 1 is shown as an upward deflection. Courtesy of Legatt et al, 1986.

Thus, referential recordings from multiple electrodes over the lumbar and lower thoracic spine may be useful in demonstrating the SLP. The following channels are included in an extended montage suggested in the AEEGS guidelines:

  • T10-IC

  • T12-IC

  • L2-IC

In patients with tethered spinal cords, recordings from such an array of electrodes often demonstrate caudal displacement of the maximal amplitude of the SLP, reflecting the anatomical displacement of the lower spinal cord, or may show no identifiable SLP. The lumbar SEP components are sometimes not identifiable in unsedated healthy subjects, especially if they are obese.


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