Abstracts

An assumption underlying electrical mapping of language cortex is that the excitatory interference produced by stimulation is local. Hyperexcitable cortical networks near epileptic foci may invalidate this assumption, and lead to false positive identification of language sites. We report a series of patients in whom this phenomenon was encountered, and identify features that allowed the recognition of such sites.
We reviewed language mapping studies performed in the context of two-stage epilepsy surgery over the past 3 years at our center. We identified 9 patients in whom extraoperative electrical mapping identified positive language at one or more sites which were subsequently excluded for language function, and resected without impairment. All these patients had left hemispheric seizure foci, and had undergone left craniotomies with placement of subdural grid and strips for intracranial localization of seizures foci prior to resective surgery. Language mapping using electrical stimulation was performed in all of these patients after seizure localization and resumption of antiepileptic medications. In all but one of these patients specific sites were retested due to the positive language phenomena within the area of planned resection.
The patients ranged in age from 8 to 44 years. Seizure foci were in the left temporal lobe in 7 patients, left frontal in 1 and left temporo-parietal in 1. The IAP results were available for 8 patients, and demonstrated left hemisphere language dominance in 6, exclusive right language in 1, and bilateral language in 1 patient. In 7 patients language impairment was produced at one or more sites near to, or overlapping the ictal focus. In three of these patients, auras, automatisms, or early ictal behavior were noted in the absence of afterdischarges in adjacent subdural contacts. Of the remaining 2 patients, both had antero-mesial temporal seizure foci, and showed language impairment at anterior sites. Of the 9 patients, extraoperative retesting after achieving high anticonvulsant levels was able to exclude language at these sites in 3. Retesting of the sites during awake surgery (with higher anticonvulsant levels), continuous language assessment during surgical manipulation of the area, and one instance focal cooling, allowed exclusion of language function in 4 of these patients. In 2 instances resection was performed without further testing, and resulted in no impairment.
We conclude that electrical stimulation near epileptic foci can yield false positive localization of language sites. It becomes important to recognize such sites when the surgical plan for resection is significantly modified by the finding. Strategies such as retesting the sites electrically after achieving high anticonvulsant levels, or intraoperative language assessment during surgical manipulation, or focal cooling may provide ways to exclude language representation in these areas.