Neurosurgery

the posterior wall of the central sulcus, receives sensory impulses from the thalamus. This region at least partially contributes to the generation of the N20 wave. Neurons within the precentral gyrus are thought to be responsible for generating the P22 deflection, as ablation of the postcentral gyrus does not eliminate this component of the SSEP. The P22 wave probably results from direct projections from the thalamus to the motor strip, but may be influenced by association fibers from area 3b.

SSEPs may also be recorded at the spinal level to monitor for insult to neurological tissues during spinal surgery, assuming that the location of peripheral stimulation is optimized to assess the level of cord at risk during a particular procedure (Fig. 1.4). SSEP monitoring is commonly used during a number of spinal procedures, including correction of scoliosis, resection of spinal AVM or tumor, therapeutic embolization of spinal AVMs, correction of spinal instability, and therapy for syringomyelia. Changes in spinal SSEP after the placement of hardware can suggest a need for changes in positioning of the hardware. Electrodes may be placed in the subarachnoid or epidural space, on the interspinous ligament, or attached to a spinous process. With the exception of subarachnoid leads, these leads may be placed percutaneously or at the site of surgical exposure. Recording evoked potentials at the spinal level has some advantages over cor-tically recorded SSEPs. Spinal evoked potentials have larger amplitudes, and repetition rates may be increased (which can reduce acquisition time). In addition, SSEPs recorded from the spinal cord are more resistant to the effects of anesthetic agents than are cortically detected SSEPs.

While median nerve stimulation has been commonly used for monitoring SSEP during cervical spine procedures, caudal portions of the cervical cord may not receive appropriate coverage with this modality. The ulnar nerve may offer more complete representation of lower cervical levels. For procedures placing the thoracic or lumbar cord at risk, SSEPs generated through the posterior tibial or common peroneal nerves can be used. Recordings taken simultaneously from both the upper and lower limb may allow for an internal control in certain procedures; specifically, evoked potentials that are lost from both the upper and lower extremity during a procedure which places the thoracic cord at risk suggest a technical error in stimu

Fig. 1.4. Left and right median nerve somatosensory evoked potentials in a patient who underwent laminectomy and exposure of an intradural, intramedullary tumor of the cervical spine. The right P14 waveform is initially diminished at baseline and then is permanently lost during the midline myelotomy. Left-sided tracings are unaffected. The patient awoke with a permanent right hemi-proprioceptive loss.

Fig. 1.4. Left and right median nerve somatosensory evoked potentials in a patient who underwent laminectomy and exposure of an intradural, intramedullary tumor of the cervical spine. The right P14 waveform is initially diminished at baseline and then is permanently lost during the midline myelotomy. Left-sided tracings are unaffected. The patient awoke with a permanent right hemi-proprioceptive loss.

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