near the same site, or with higher currents, can elicit local or generalized seizure activity. Therefore, it is important to make sure that the patient has adequate serum anticonvulsant levels pre-operatively and that a short-acting intravenous anticonvulsant is readily available in the event that seizures are elicited.
A constant current, biphasic, square wave, 60 Hz, bipolar stimulator (Ojemann Stimulator, Radionics Sales Corp.; 5 mm between electrodes) set at 2-10 mA is used to elicit movement and/or sensation in the awake patient. Higher current settings may be necessary in younger children, in patients under general anesthesia, or when stimulating through the dura. It is best to start at lower current settings and gradually increase the current until sensation or movement is elicited, as this will help to avoid eliciting seizure activity.
Fig. 10.6. a Bipolar montage from somatosensory evoked potential (SSEP) recording following stimulation of the right (contralateral) median nerve. The N20, denoting the primary somatosensory cortex is seen in channel "3". The P22, indicating motor cortex, is seen in channel "4". b Trans-dural SSEP electrode arrangement with an eight-contact electrode placed on the dura prior to durotomy. c SSEP recording directly from the cortical surface.
Using this technique, the entire sensory and motor homunculi can be mapped. The technique can also be used to identify descending subcortical motor fibers when resections extend below the cortical surface, such as during supplementary motor area resections and insular resections. When performing subcortical motor mapping, the current needed to elicit movement is the same, or lower than, the current needed at the cortical surface. When the resection is very close to functional cortex, it is helpful to periodically repeat the stimulation mapping procedure to verify that cortical and subcortical functional regions are not damaged.
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