Alteration of the threshold stimulus for intraoperative brain mapping via use of antiepileptic medications

Intraoperative seizures during awake craniotomy with cortical and subcortical mapping are a common occurrence. Patients are routinely treated preoperatively with anti-convulsive medications to reduce seizure occurrence. Historically these drugs have not been believed to significantly affect awake craniotomy procedures. We report a patient undergoing intraoperative mapping with differential response and seizure occurrence based upon antiepileptic drug usage. A 43 year old female presented with history of seizures, right sided hemiparesis, electrical sensations, and difficulty with language function. She was determined to have a mass lesion involving the left frontal and temporal lobes and subsequently elected to undergo resection by awake craniotomy with intraoperative mapping. A first attempt at lesion resection was performed after a missed dose of anti-convulsant medication (levetiracetam) and was subsequently aborted because of repeated seizure activity. The threshold for seizure generation (1.75 mA) was observed to be significantly lower than expected. Therapy was begun with both levetiracetam and phenytoin prior to a second attempted resection one week later. Thresholds for cortical motor response in the second operation were significantly higher than expected (> 9.0 mA), and no intraoperative seizure activity was observed. To our knowledge this is the first quantitative example of antiepileptic drugs affecting the current required for intraoperative mapping. This case highlights the potential for higher current requirements in patients preoperatively treated with high doses of antiepileptic drugs, as well as the importance of confirming adequate dosage of antiepileptic drugs in patients at an increased risk of seizure generation.