Neurosurgery

of cross-clamping the ICA. Nearly all of the major EEG changes that occur upon cross-clamping will reverse with placement of a shunt.

It has been suggested that EEG monitoring has a high false- positive rate in predicting stroke during CEA, thus unnecessarily subjecting many patients to the risks of shunt placement. However, the sum of the data on EEG monitoring in the setting of CEA indicates that it can identify the subset of patients at risk of clamp-induced ischemic insult. Redekop and Ferguson described a cohort of 293 patients who underwent routine CEA without shunting. Eight percent of these individuals demonstrated major EEG changes following clamping of the ICA; of this subset, 18% had immediate postoperative deficits, compared with only 1% of the individuals who did not have clamp-related EEG changes [3]. Another large retrospective analysis demonstrated similar success with the use of intraoperative EEG during CEA; stroke occurred in only 0.3% of patients who had been monitored with EEG during their procedure (and who had shunts placed upon the appearance of significant EEG changes), compared with a stroke incidence of 2.3% in the non-monitored group [4].

It has been pointed out that the predictive value of EEG monitoring, as measured by the actual number of strokes associated with major alterations of EEG patterns, is relatively low. There are a number of factors that are relevant to this issue. The threshold between tolerable ischemia and irreversible infarction does not clearly correlate with changes in the EEG patterns. Time is also a significant variable. A patient may well tolerate relative ischemia for the short time in which the ICA is cross-clamped during endarterectomy; however, if such ischemia were to persist for a greater length of time, permanent injury could result.

Obviously, the utility of standard methods of EEG, which require the placement of a grid of scalp electrodes, is limited by specific requirements of the operative approach, so these methods are of very little practical utility for a large proportion of intracranial cases. In addition, EEG monitoring loses efficacy in cases performed under deep hypothermic circulatory arrest (e.g. complex intracranial aneurysm); in fact, EEG activity ceases at brain temperatures of 19-26°C. Conversely, the disappearance of EEG activity has been used as a method to assess the adequacy of cooling in cases where deep hypothermic circulatory arrest is required. It has been proposed that a total of 3 minutes of electrocerebral silence (ECS) is an adequate endpoint for the assessment of therapeutic hypothermia.

Electrocorticography

Electrocorticography (ECoG) has been used as a tool to identify loci of epileptiform activity or to delineate regions of eloquent cortex. As with EEG, ECoG records electrical potentials that are generated by the changing oscillatory activity of cortical neuronal groups. Unlike EEG, however, ECoG uses depth electrodes or surface electrode "grids" that are placed in direct contact with the cortical tissue, allowing for much finer spatial resolution of cortical electrical activity. Synchronous neuronal activity must be within approximately 6 cm2 of the cortical surface in order to be detectable by scalp electrodes, while ECoG is able to detect epileptiform discharges outside of this radius. As with standard EEG, the interpretation of intraoperative ECoG is complicated by the effects of anesthetic agents.

The traditional use of intraoperative ECoG has been dedicated to the identification and demarcation of the limits of resectable epilepto-genic foci, primarily based on the detection of interictal epileptiform activity. There has been no agreement, however, on which interictal discharges are predictive of continued risk of epileptiform activity. A study evaluating the implications of residual epileptogenic discharges following tumor resection suggested that surgical irritation of the cortex could induce such activity; furthermore, such discharges were not predictive of post-operative clinical seizures [5]. In addition, ECoG may not be helpful in determining whether such discharges are independent of, or propagated from, another site. The most widely accepted use of ECoG in epilepsy surgery has been in cases of extratemporal partial seizures, where it has been used routinely to set the boundaries of tissue resection. The use of ECoG in temporal lobe procedures has been more dependent on individual institutional philosophy, as some centers employ standard resection strategies or depend on pre-operative delineation of the epilepto-genic focus. The use of post-excisional ECoG

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