What are the cortical 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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Answer

Answer

The primary cortical SEPs following lower limb stimulation are recorded as near-field positivities: P37 following posterior tibial nerve stimulation and P27 following peroneal nerve stimulation. In contrast to the N20 cortical component of the median nerve SEP, which is maximal over the lateral parietal area, the cortical SEPs to lower limb nerve stimulation often are maximal near the midline, reflecting the more medial location of the foot and leg areas of the somatosensory homunculus. However, the topography of these components varies across subjects, reflecting individual differences in the location and orientation of somatosensory cortex (see image below) and may differ between the 2 sides in the same subject.

Diagram showing 2 possible locations for the foot 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.

When the equivalent dipole of the cortical generator is oriented vertically, the maximum positivity is in the midline (see A in the image above; also see the image below). When the cortical generator is located in the mesial wall of the hemisphere, the dipole assumes a more horizontal orientation, producing a paradoxical maximum positive cortical SEP over the hemisphere ipsilateral to the stimulus (see B in the image above; also see B in the image below).

Cortical somatosensory evoked potentials (SEPs) to 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.

In the latter situation, a negative cortical SEP may be recorded over the contralateral hemisphere, and a midline electrode may pick up a much smaller and less well-defined cortical SEP. Rarely, the activated cortex is on the dorsolateral convexity, producing a scalp positivity maximum over the hemisphere contralateral to the stimulated leg. To cover all of these possibilities, the extended recording montage suggested in the AEEGS guidelines includes the following cortical recording channels:

  • CPc-Fpz

  • CPz-Fpz

  • Cpi-Fpz

When the dipole is oriented horizontally, producing an ipsilateral positivity and a contralateral negativity, a CPi-CPc recording channel may yield a larger cortical SEP than any of the recordings referred to Fpz. Thus, some laboratories incorporate a Cpi-CPc derivation in their recording montage. However, the latencies of the ipsilateral positivity and the contralateral negativity may not be identical, and the Cpi-CPc SEP may be a composite potential reflecting multiple cortical sources. Thus, a Cpi-CPc SEP should be compared only to normative data recorded with the same derivation.

Since the largest cortical SEP is located at CPz in some patients, while in others with a more horizontal dipole a small cortical SEP is recordable at this electrode but a substantial SEP can be recorded over the ipsilateral hemisphere, the optimal cortical SEP recording electrode sites may differ from patient to patient. If the monitoring equipment does not permit simultaneous recording from multiple channels (eg, CPz-FPz and CPi-FPz), the electrode derivations used for intraoperative monitoring should be customized on the basis of the results of the patient's preoperative SEP studies. However, CPz-Fpz and CPz-CPc are the most recommended cortical channels. [8] Therefore these preoperative studies must include recordings of the cortical SEPs from both midline and lateral electrodes. 

With both peroneal nerve and posterior tibial nerve stimulation, longer latency cortical SEP components follow the primary cortical P27 or P37 components. The second cortical positivity, which typically has a latency of 50-60 milliseconds after posterior tibial nerve stimulation, may be substantially larger than the primary cortical SEP (see image below). In noisy recordings with a limited montage, a low-amplitude P37 component may not be recognized, and the secondary cortical positivity may be identified erroneously as a markedly delayed cortical SEP (see Pz-Fpz waveforms in the image below). Thus, cortical SEPs to lower limb nerve stimulation should be interpreted with caution when their peak latencies appear to be delayed abnormally to this latency range.

Cortical somatosensory evoked potentials (SEPs) to 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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