Although cortical mapping is an important tool, potential pitfalls must be recognized so as to use mapping safely and effectively . Below are some of the major difficulties encountered. These have been separated into: (1) inability to identify functional cortex, and (2) injury to functional cortex once it has been identified.
Inability to Identify Functional Cortex In young patients, stimulation motor mapping is often not possible. SSEPs must be used to localize Rolandic cortex. Under general anesthesia, SSEPs and motor cortex localization may prove difficult. Nitrous/narcotic anesthesia is best for mapping. Inability to identify functional cortex does not prove that one is not in functional cortex. It may be indicative that there was a problem with mapping, not that resection is necessarily safe. During localization of speech cortex, ECoG must be used to determine the after-discharge threshold. This assures that there are no local seizures elicited by stimulation.
There are often two or more essential speech areas within both the temporal and frontal lobes. Therefore, the entire region to be resected should be mapped (i.e. mapping should not be stopped simply because two speech areas have been identified). White matter underlying functional cortex can be injured. For Rolandic cortex, this can be avoided with subcortical mapping. For speech cortex, the patient should continue naming during resection of abutting cortex or white matter. Vascular injury in the neighborhood of functional cortex must be avoided. Lesional distortion of cortex superficially does not indicate that underlying white matter has (or has not) been displaced. Ascending or descending fibers may not travel perpendicular to the gyral crown.
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