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underlying energy state of the brain. With the advent of implantable defibrillators, a clinical model of global ischemia in humans has become available, and EEG changes occurring in this state have been fairly well described. The EEG changes within 8-16 sec of ischemia. EEG changes include slowing, loss of high-frequency activity, increased amplitude of low-frequency activity, and overall decreases in amplitude. However, other patterns do occur, including loss of 5 activity or increase in 0 activity. The same general patterns seem to occur with focal ischemia, but with localized extent and more variable timing. However, the EEG is not always a sensitive monitor, and clinical neurological changes can result from ischemia without EEG changing at the time. In certain situations, monitoring for changes in evoked potentials (EP) may be better at detecting ischemia. EP repetitively test specific neural pathways and can detect ischemia somewhere along that pathway. The degree of insult detectable by EP monitoring may not produce changes in the unstimulated EEG.
SSEP
Electrodes placed on the scalp are recorded in response to stimuli applied to peripheral nerves. More relevant to surgery of the cranial base is the use of SSEP recording when stimuli are applied to the median nerve. A median nerve SSEP is particularly helpful during surgery of the upper cervical spine and around the foramen magum, since this area is not covered by the brain-stem auditory evoked potential It also correctly predicts the onset of probable brain-stem ischemia in a patient undergoing posterior fossa surgery. While minor changes in BAEP or SSEP were seen in the majority of these patients, they noted major changes in only 2.5%. Persistent changes usually predict postoperative brain dysfunction. The SSEP parallels regional blood flow, changing when blood flow falls to a near-critical level and disappearing when blood flow exceeds a critical level. The SSEP is less affected by many anesthetic agents than the EEG, and it indicates cerebral function rather than blood flow only. The latency between the cervical evoked-potential peak and the first negative thalamocortical peak recorded over the scalp is called central conduction time CCT. The addition of hypotension to a temporary vessel occlusion exaggerates the effect of ischemia, and an SSEP will reflect such a change.
Median-nerve SSEP appears to be a safe and effective method of intraoperative monitoring of the functional status of several portions of the nervous system. Monitoring both hemispheres can differentiate the effects of local-vessel clipping from systemic hypotension or effects of medication. (MSPs) are used to aid in determining the functional integrity of the somatosensory cortex, are useful both in preventing or reducing surgical morbidity. Quantitative analysis of the EP signal may improve sensitivity. Thus, EP monitoring is most useful in situations in which a clearly defined neural pathway is at risk, such as during acoustic neuroma resection or surgery involving risk to the spinal cord (e.g., aortic re (Continued on page 110)
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