How are diagnostic somatosensory evoked potentials (SEPs) interpreted?

Updated: Feb 26, 2019
  • Author: Sombat Muengtaweepongsa, MD, MSc; Chief Editor: Selim R Benbadis, MD  more...
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SEP amplitudes may vary considerably across subjects, and the interpretation of clinical diagnostic SEP studies is based primarily on component latencies. For upper limb SEPs, the AEEGS guidelines include identification of the obligate components N9, N13, P14, N18, and N20. Measurement of the N9-N20, N9-P14, and P14-N20 interpeak intervals is specified; interpeak intervals involving the N13 component are listed as options. The N9-P14 interpeak interval measures neural conduction from the brachial plexus to the lower medulla, P14-N20 from the lower medulla to primary somatosensory cortex, and N9-N20 from the brachial plexus to primary somatosensory cortex. The N13 component reflects activity within the lower cervical spinal cord. Absolute component latencies are less useful than interpeak intervals because of the effect of arm length on the latencies of N9 and subsequent components.

For posterior tibial nerve SEPs, the AEEGS guidelines mandate at minimum the identification of the SLP and the primary cortical SEP component (P37), measurement of their peak latencies, and calculation of the SLP-P37 interpeak interval. The latter approximates the conduction time between the lumbar spinal cord and primary somatosensory cortex. Some laboratories also measure the latency of the initial far-field component (P31) and calculate SLP-P31 and P31-P37 interpeak latencies, corresponding to lumbar spinal cord-to-medulla and medulla-to-primary somatosensory cortex conduction times, respectively.

For lower limb SEPs, conduction distances along both the peripheral nerve and the spinal cord portions of the afferent pathway vary considerably among subjects as a function of height. Some laboratories use height-adjusted norms for the analysis of lumbar SEP components; others measure the conduction distances and calculate conduction velocities. When no height corrections are used, interpeak latencies should be interpreted with caution in patients whose heights are at the extremes of the range of heights for which the normative data were collected.

The major criteria for abnormality are absence of obligate components and abnormal prolongation of interpeak intervals. As with any evoked-potential test involving unilateral stimulation, excessive symmetries between the measurements following left-sided stimulation and those following right-sided stimulation also may reveal abnormalities. The stationary cervical/stationary lumbar and cervicomedullary far-field components may be difficult to identify in some healthy subjects who are not sedated. In the presence of normal cortical SEPs, the inability to identify some of the more caudally generated SEP components may not be significant.

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