What are the far-field 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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When referred to a frontal scalp reference (eg, Fpz), electrodes over the cervical spine record a biphasic waveform that was labeled originally as a cervical potential but is believed now to reflect predominantly far-field potentials generated in subcortical elements of the lemniscal somatosensory pathways. If the inputs are connected so that the cervical lead is input 1 (ie, C5S-Fpz), the waveform consists of a negativity followed by a positivity. [7]

The montage given in the AEEGS guidelines specifies this channel as an Fpz-C5S recording, with Fpz as input 1. Such a recording contains a positivity followed by a negativity, which are designated P31 and N34, respectively, following posterior tibial nerve stimulation (see image below); the peak latencies are approximately 10 milliseconds shorter, following peroneal nerve stimulation. P31 is analogous to the P14 component of the median nerve SEP and, like it, most likely is generated in the dorsal column nuclei and/or the caudal medial lemniscus within the lower medulla. The negativity that follows is most likely analogous to the N18 component of the median nerve SEP and thus is generated by activity in multiple brainstem and/or thalamic structures that are activated by the somatosensory stimulus.

Normal posterior tibial nerve somatosensory evoked 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.

A near-field origin within the cervical spinal cord had been proposed for some smaller and earlier peaks that can be picked up by the recording electrodes placed on the posterior part of the neck. Like the lumbar SEP components, the far-field SEP components elicited by lower limb nerve stimulation may be difficult to identify in recordings from unsedated subjects because of noise, particularly electromyogram (EMG) artifact from paraspinal musculature. Under surgical anesthesia, and especially with the use of neuromuscular blocking agents, these SEP components are usually clearly identifiable and reproducible.

The cortically generated SEPs elicited by stimulation of lower limb nerves are far more sensitive to the effects of anesthesia than the far-field components (see image below). During operations in which the cortically generated SEPs are markedly attenuated or completely suppressed by anesthesia or in which they show a degree of anesthetic-related variability such that changes related to surgical manipulations might not be recognized, the far-field SEPs may be used to monitor the dorsal column pathways of the spinal cord.

Serial somatosensory evoked potentials (SEPs) reco 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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