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factors related to its nature, stimulation and recording techniques, influence of other agents and conditions, and, finally, interpretation and significance. A VEP is a late evoked potential, with the principal peak occurring 100 to 200 ms after the stimulus. Thus, it tends to be relatively unstable, varying in amplitude in response to moderate changes in temperature, electrolytes, drugs, or level of consciousness. The best stimuli are pattern reversal stimuli, which are impractical for intraoperative use. Light flashes, the only practical intraoperative stimuli, may produce evoked potentials that are not affected by pathology or manipulation of the optic pathways. Furthermore, VEP monitoring is often performed in patients who have pre-existing visual impairment. Thus, the chance for successful intraoperative VEP is poor when preoperative disturbance of VEP is present.
Intraoperative VEP monitoring has been reported in patients with juxtasellar tumors or aneurysms. Several authors have felt that an improved intraoperative VEP heralds post-operative improvement in vision. The VEP can also help identify the optic nerve and chiasm when they are embedded in tumors. Though sometimes minor, traction or manipulation of the visual pathway often causes transient changes in VEP. VEP can perhaps give warning of chiasmal herniation into the sella turcica after removal of a large pituitary tumor. Further improvements in the technical aspects of the VEP are needed before it can be considered reliable and practical for wider clinical use. Recording potential directly from the optic nerve and the optic tract has recently been reported, but without clear advantage over the scalp recording of a VEP.
Visual evoked potentials (VEPs) are used to aid in determining the functional integrity of the visual system, primarily in the region of the optic nerves, chiasm, and optic radiations. The recorded activity is generated either at the retina (electroretinogram) or at the cortex.
Stimulation of the visual system using a bright flash is not recommended for diagnostic purposes due to inter-subject variability, except in selected situations; however, in the operating room this is a very helpful and effective technique. Four waves are typically seen in the visual evoked potential: P60, which is thought to be generated in subcortical structures; and N70, P100, and N120, which are all thought to be generated in the primary visual cortex.
For stimulation of the visual system, we use a fiber optics system, which is positioned directly under the eye, but not on the globe, and securely tape in place. This fiber optics stimulator is designed to be mounted on the flash stimulator driven by a Grass photic stimulator (P22), and this stimulator is then set at maximum intensity. Recording electrodes are placed at 0 and 02, both referenced to Cz.
The stimulus rate used for the visual data is 1.3 Hz and the observation window is 200 msec. A bandwidth of 1-100 Hz and a gain of 20,000 are used. Usually we record only one channel of (Continued on page 129)
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