What are the far-field 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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Answer

The stationary cervical potential overlaps in time with a far-field SEP component, P14. The dipole orientation of P14 is such that it appears as a positive peak in recordings between the dorsal scalp (input 1) and a noncephalic electrode (input 2) (see image below). While the origin of P14 has been the subject of some controversy, it most likely reflects activity in the dorsal column nuclei and/or the caudal medial lemniscus within the lower medulla. When a forehead (ie, Fpz) reference is used, this far-field cervicomedullary component becomes a negativity (N14) at the C5S recording location and summates with the near-field N13 negativity picked up by that dorsal neck electrode. [7]

Normal median nerve somatosensory evoked potential Normal median nerve somatosensory evoked potentials (SEPs) recorded using the minimal (4-channel) recording montage recommended by the American EEG Society (AEEGS) guidelines. Negativity at input 1 is shown as an upward deflection. Courtesy of American Electroencephalographic Society, 1994.

The presence of overlapping negative peaks in the C5S-Fpz recording channel (a channel that was recommended in older SEP guidelines), may make attenuation of one of them difficult or impossible to recognize. Thus, as mentioned above, this recording channel has been replaced by separate C5S-Epc and Cpi-Epc recording channels in the minimal montage recommended in the most recent set of AEEGS guidelines.

For intraoperative monitoring, the cervicomedullary far-field potential may be recorded between the forehead (Fpz) and the inion, mastoid, or earlobe; this montage prevents contamination by, and confusion with, the N13 component. Depending on which electrode is designated as input 1, the cervicomedullary SEP component may be recorded as either an N14 or a P14. This component can be monitored to determine whether activity in afferent somatosensory pathways reaches the level of the cervicomedullary junction (see image below).

Cortical (left) and cervicomedullary N14 (right) s Cortical (left) and cervicomedullary N14 (right) somatosensory evoked potentials (SEPs) to stimulation of the right median nerve, recorded during the initial phases of surgery for resection of a right vestibular schwannoma. The cortical SEPs show prominent anesthetic-related changes. While the waveforms recorded in the A2-Fpz channel contain some volume-conducted cortical SEPs, the N14 far-field component (arrowhead) is unaffected by the changes in the anesthetic regimen. Courtesy of Legatt, 1995.

Anesthesia affects the cortical SEP (N20) more than it does the N14 component, because at least 2 more synapses (in thalamus and cortex) intervene. Therefore, monitoring of the cervicomedullary SEP may permit SEP monitoring of the cervical spinal cord when cortical SEPs are of poor quality because of high anesthetic levels and/or preexisting neuronal damage.

If the region of the nervous system in jeopardy is rostral to the medulla, the cervicomedullary SEP component can be monitored to determine whether changes in the cortical SEPs are due to rostral nervous system dysfunction versus peripheral nerve or technical problems. This is similar to the intraoperative use of the peripheral nerve SEP component described above. Optimally, both components should be monitored for 2 reasons: (1) the cervicomedullary SEP provides an alternative way of differentiating the possible causes of a cortical SEP change if peripheral nerve SEP recordings are suboptimal, and (2) if peripheral nerve CAPs are interpretable and remain unchanged while cortical SEPs deteriorate, examination of the cervicomedullary recordings can localize further the neural dysfunction responsible for the cortical SEP changes above or below the foramen magnum.

Another far-field component, N18, overlaps in time with the primary cortical SEP and may account for multiple negative peaks in the cortical recordings in some subjects (see image below). N18 has a wide bilateral distribution over the scalp. It is best seen in recordings with a noncephalic reference, though it also may be demonstrated with a frontal reference. While N18 has been attributed to a thalamic generator, several cases have been reported in which N18 was still present despite the presence of thalamic lesions that eradicated the primary cortical SEP. N18 most likely reflects activity in multiple subcortical structures that are activated by the somatosensory stimulus, including brainstem structures. Thus, examination of N18 cannot be used to localize the cause for cortical SEP changes (as being rostral versus caudal to the thalamus).

Normal median nerve somatosensory evoked potential Normal median nerve somatosensory evoked potentials (SEPs) recorded using the minimal (4-channel) recording montage recommended by the American EEG Society (AEEGS) guidelines. Negativity at input 1 is shown as an upward deflection. Courtesy of American Electroencephalographic Society, 1994.

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