What are the cortical components of upper 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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The primary cortical SEP component following median nerve stimulation, N20 (see image above), is recorded as a near-field potential over the parietal area contralateral to the stimulated median nerve. Since an electrode also is within the scalp distribution of the far-field N18 component, a recording with a noncephalic reference contains an admixture of N18 and N20. The recommended montage places the reference over the mirror-image position ipsilateral to the stimulus (ie, a CPc-CPi recording linkage), which tends to cancel the bilaterally distributed N18. An Fz reference also has been used to record N20 but may yield a composite waveform consisting of N20 and the inversion of the frontally generated P22 component. A CPc-Fz or CPc-Fpz linkage may be useful for intraoperative monitoring because it yields a cortical SEP waveform that is larger in amplitude than the CPc-CPi linkage used for diagnostic recordings.

While a thalamic or subcortical origin for N20 has been suggested, most authors believe that N20 predominantly reflects activity of neurons in the hand area of the primary somatosensory cortex; multiple generators with overlapping voltage topographies may contribute to this component. N20 predominantly originates in primary somatosensory cortex in the posterior bank of the central sulcus (see first image below) and thus displays a polarity inversion across the central sulcus in epidural cortical surface recordings (see second image below) and some scalp recordings. This polarity inversion may be used to identify the central sulcus during surgery.

Diagram showing generation of the N20 component of Diagram showing generation of the N20 component of the median nerve somatosensory evoked potentials (SEP) in the primary somatosensory cortex located in the posterior bank of the central sulcus producing a horizontal dipole with a postcentral N20 and precentral P20.
Somatosensory evoked potentials (SEPs) to median n Somatosensory evoked potentials (SEPs) to median nerve stimulation recorded from cortical surface electrodes prior to resection of a right parietal arteriovenous malformation (AVM) in a 35-year-old man. Note the inversion of the N20/P20 component (arrowheads) across the central sulcus; the amplitude is largest over the postcentral gyrus, where the component is negative in polarity. The longer latency surface-positive component has a different distribution. The arrows indicate 2 large veins draining the AVM. Courtesy of Legatt, 1991.

An accumulation of air within the subarachnoid space can block the volume conduction of the cortically generated SEP components to the overlying recording electrodes; this is most likely when the patient is in the sitting position and cerebrospinal fluid (CSF) has been drained. The peripheral nerve and far-field cervicomedullary SEPs are generally not affected. Therefore, the SEP changes produced by pneumocephalus can mimic those caused by intraoperative compromise of the somatosensory pathways rostral to the cervicomedullary junction.

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