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 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 Thus, as mentioned above, this recording channel has been replaced by separate C5S-Epc and Cpi-Epc recording channels in the minimal montage recommended in the most recent set of AEEGS guidelines.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) 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 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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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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Normal posterior tibial nerve somatosensory evoked potentials (SEPs) recorded using the minimal (4-channel) recording montage recommended by the American EEG Society (AAEGS) guidelines. Note that the second channel from the bottom is specified as Fpz-C5S, so that the far-field potentials appear as a P31 followed by an N34. Negativity at input 1 is shown as an upward deflection. Courtesy of American Electroencephalographic Society, 1994.
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Somatosensory evoked potentials (SEPs) recorded during resection of a posterior fossa tumor (intradural extension of a clear-cell tumor of the right middle ear) in a 46-year-old woman. The peripheral nerve compound action potentials (CAPs) to left median nerve stimulation, recorded at the elbow, and the simultaneously recorded cortical SEPs both displayed marked amplitude attenuation. The stimulating electrodes at the left wrist were replaced; the peripheral nerve and cortical SEPs both returned to their baseline values and remained there through the end of the operation. Courtesy of Legatt, 1995.
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Somatosensory evoked potentials (SEPs) to stimulation of the left median nerve, recorded from a ring of electrodes placed around the neck at the level of SC5 posteriorly and the superior border of the thyroid cartilage anteriorly. Courtesy of Emerson et al, 1984.
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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.
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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.
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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.
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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.
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Serial somatosensory evoked potentials (SEPs) recorded during spinal instrumentation and fusion surgery in a 13-year-old girl with scoliosis. Note the attenuation of the cortical SEPs resulting from administration of an intravenous bolus dose of 50 mg of fentanyl given at 1:53 pm. The far-field SEPs were relatively unaffected. In addition to the far-field components, the C2S-Fpz waveforms (labeled "SC2-Fpz") contain a volume-conducted contribution from the cortical SEPs; the contribution also was attenuated by the fentanyl. Nitrous oxide (60%) and isoflurane (0.6-0.8%) were being administered throughout these recordings. Positivity at input 1 is shown as an upward deflection in this picture.
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Diagram showing 2 possible locations for the foot area of the somatosensory homunculus (shaded area of cerebral cortex), which generates the P37 cortical component of the posterior tibial somatosensory evoked potential (SEP). The arrows represent the equivalent dipoles of the cortical SEP generators; the arrowhead marks the positive side of the dipole field. A: The maximum P37 amplitude is in the midline. B: The maximum P37 amplitude is over the hemisphere ipsilateral to the stimulus, and the negativity can be recorded over the contralateral hemisphere.
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Cortical somatosensory evoked potentials (SEPs) to stimulation of the left posterior tibial nerve in 2 different healthy subjects, showing the variability of scalp topography. The SEPs were recorded from the coronal chain of electrodes; negativity at the active electrode as compared to the Fpz reference is shown as an upward deflection. A: The cortical positivity (labeled "P38") is maximal in the midline at the vertex. B: The cortical positivity is maximal over the hemisphere ipsilateral to the stimulus and is inverted to a negativity over the contralateral hemisphere. Courtesy of Emerson, 1988.
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Cortical somatosensory evoked potentials (SEPs) to left posterior tibial nerve stimulation, showing a secondary cortical positivity (open arrow) that is much larger than the P37 primary cortical SEP component (solid arrow). If only a single Pz-Fpz channel were used to record the cortical SEP, the secondary component might be identified erroneously as a markedly delayed cortical SEP. Negativity at input 1 is shown as an upward deflection.
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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.
