Critical neuromonitoring at spinal and brainstem levels by somatosensory evoked potentials.

Electronic averaging makes it possible to analyze somatosensory evoked potentials (SEP) recorded noninvasively from the body surface in man. With noncephalic reference recording, the SEP discloses a series of components that are volume-conducted from distinct open-field generators with a geometry adequate to produce external potential gradients over the head. Farfields are brief positive dips with widespread distribution that present stationary onset and peak latencies all over. They reflect the propagated afferent volley in axons bundles, thus in brachial plexus (P9), dorsal column (P11), and medial lemniscus (P14). Somehow unexpectedly, SEP traces also disclose a widespread prolonged farfield N18 of negative polarity that reflects neural generators in the brainstem below thalamus. Nearfields can be positive or negative, and they reflect neural generators located less than about 50 mm from the electrode. They are influenced to a greater extent by the position of the recording electrodes. For example, neck electrodes can follow the upward propagation of the dorsal column volley (N11), whereas scalp electrodes can map out the distinct contralateral parietal (N20, P27) or frontal (P22, N30) cortical generators. Electrodes around the neck also disclose the posterior N13 and anterior P13 responses that reflect the two sides of the same dorsal horn generator with a horizontal axis. Bit-mapped topographic color imaging of potential fields provides detailed data on time and spatial features of the different SEP neural generators. SEP neuromonitoring can use these results to titrate input to spinal cord (nerve potentials or P9 farfield), spinal generators (N11 nearfield or N13-P13 nearfield in posterior-to-anterior neck montages), brainstem generators (P14 farfield and N18 response), or cortical generators (parietal N20-P27 or frontal P22-N30). The central somatosensory conduction time can be titrated from the spinal entry and cortical arrival times measured in neck and scalp recordings.