Abstracts

Cortical Somatosensory evoked potential (SSEP) mapping has been used increasingly for various purposes including central sulcus (CS) localization and verification of cortical stimulation. Median nerve SSEP have been used to identify the CS at the level of the hand sensory-motor area and several studies have characterized the waveforms and findings of this SSEP modality. As opposed to median nerve cortical SSEP, posterior tibial nerve cortical SSEP have been less studied and controversies exist as for some of its cortical generators. Specifically, the peaks seen at 35-40 ms were thought to be ends of a single dipole, but recent studies have suggested that they might be generated by different sources overlying different anatomic substrates. The purpose of this study was to try to characterize the cortical generators of the posterior tibial nerve SSEP, and it[rsquo]s accuracy in localizing the CS.
This was a retrospective chart review. Charts from 16 patients with intractable epilepsy that underwent invasive monitoring with subdural grids (SDG). Posterior SSEP were performed for CS localization. Recordings were made from SDG located in the mesial and lateral convexity when available. Waveforms were then analyzed for latency, amplitude, and the location of the electrode where they were recorded verified using MRI corregistration.
Cortical SSEP were recorded from 16 patients, obtaining a total of 53 separate waveforms. Each waveform was recorded from a separate electrode. Of these 47 were recorded on the fronto-parietal mesial grid, and 6 from the lateral fronto-parietal cortex.Waveforms with an initial positive polarity in the mesial grid were seen anterior to the CS in 10 cases (average latency 37.4 ms, range 29-52 ms), and posterior to the CS in 12 cases (average latency: 37.8 ms, range 28-41). Waveforms with an initial negative polarity in the mesial grid were seen anterior to the CS in 16 cases (average latency: 38.5 ms, range: 27-55) and posterior to the CS in 9 cases (average latency: 45 ms, range 25-65). One waveform with an initial positive polarity was seen in the lateral grid anterior to the CS (latency 45 ms) and one was seen posterior to the CS (latency: 42 ms). Two waves with an initial negative polarity were seen in the lateral grid anterior to the CS (average latency: 38.5 ms) and two posterior to the central sulcus (average latency: 33.5 ms).In only 4 patients a clear phase reversal was seen in the mesial fronto-parietal grid. In all of them (100%) the maximum positivity was seen in the grid posterior to the central sulcus, and the maximum negativity anterior to the central sulcus.
Clear phase reversal associated with posterior tibial nerve stimulation do not occur commonly (25%). Phase reversals when seen, best localize the central sulcus. Isolated responses were recorded in proximity to the central sulcus in all patients with mesial frontoparietal placed grids but were seen both in the SI and MI cortices.