Localization of Somatosensory Cortex Using SSEPs

SSEP mapping to identify the primary somatosensory gyrus provides a quick, reliable means of Rolandic localization in both adult and pediatric populations [28]. SSEPs can be performed under general anesthesia or in awake patients. SSEP mapping has the advantage over stimulation mapping that seizures cannot be evoked because the cortex itself is not stimulated. When performed under general anesthesia, halogenated anesthetic agents should be avoided because they may increase the latency of the cortical SSEPs. High-dose barbiturate or propofol anesthesia may lead to burst-suppression activity, and is therefore contraindicated. Nitrous oxide combined with Pentothal or low-dose propofol provides excellent general anesthetics for these studies.

Techniques for intraoperative SSEP mapping are similar to those used for routine diagnostic studies (Fig. 10.6). A peripheral nerve is stimulated - most often the median nerve at the wrist due to the robust signal that can be recorded at the cortical surface. However, other nerves, such as the tibial nerve, can also be used. Stimulation is performed at a rate of 2-5 Hz with a 0.10.3 ms pulse duration, and the current adjusted to produce a minimal (not painful) twitch so that muscle activity can just be visualized. Stimulation can be accomplished with mechanical and thermal stimulation as well. The stimulus generates a signal that is transmitted via the spinothalamic pathways, to the medial lemniscus, then the thalamus, and finally to contralateral somatosensory cortex. Compared with scalp recordings, SSEP recordings made from the cortical surface have much higher voltages (10-100 mV). Recording typically uses a low, cut-off frequency of 1 Hz, a high-frequency filter of 3,000 Hz, and an analysis time of 100 ms. Usually, trials of 100-200 stimuli are needed to elicit well-defined responses from the somatosensory cortex. Cortical responses have a number of different components designated by their positive (P) or negative (N) polarity with respect to the reference electrode, followed by a number representing the typical latency (in ms) of the peaks. For instance, following median nerve stimulation, the contralateral somatosen-sory gyrus shows an initial N20 component followed by a P25 component.

Following craniotomy and durotomy, an array of electrodes is placed in the axial (transverse) plane on the cortical surface. An eight-contact electrode strip (1 cm center-to-center spacing) is quite adequate. The electrode contacts should extend over areas of the brain anterior and posterior to the presumed somatosensory gyrus. Alternatively, electrodes may be placed on the dura (this is especially helpful for re-operative and post-meningitis cases where the dura is adherent to the underlying cortex rather than directly on the cortical surface). If the craniotomy does not expose Rolandic cortex, an electrode strip can be slid beneath the edge of the craniotomy to reach distant cortical regions. A series of recordings are then made from the cortical surface by moving the electrode to different areas to verify localization of the somatosensory cortex.

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