Cortical Language Mapping in Epilepsy: A Critical Review

Abstract

One challenge in dominant hemisphere epilepsy surgery is to remove sufficient epileptogenic tissue to achieve seizure freedom without compromising postoperative language function. Electrical stimulation mapping (ESM) of language was developed specifically to identify essential language cortex in pharmacologically intractable epilepsy patients undergoing left hemisphere resection of epileptogenic cortex. Surprisingly, the procedure remains unstandardized, and limited data support its clinical validity. Nevertheless, ESM for language mapping has likely minimized postoperative language decline in numerous patients, and has generated a wealth of data elucidating brain-language relations. This article reviews the literature on topographical patterns of language organization inferred from ESM, and the influence of patient characteristics on these patterns, including baseline ability level, age, gender, pathology, degree of language lateralization and bilingualism. Questions regarding clinical validity and limitations of ESM are discussed. Finally, recommendations for clinical practice are presented, and theoretical questions regarding ESM and the findings it has generated are considered.

Spatial-temporal functional mapping of language at the bedside with electrocorticography

“…29,30 It has long been recognized that cortical stimulation can affect function at a distance, 7 and that ESM does not always predict postoperative language outcomes. 10,26,31 This concern is perhaps best illustrated in somatosensory and motor cortices, where the effects of lesions are more predictable.…”

“…3 The current gold standard, electrocortical stimulation mapping (ESM), is time-consuming 4,5 and often induces afterdischarges or seizures. 4,6,7 Additionally, it is difficult to rule out distant effects through diaschisis or action potentials evoked by stimulation. [7][8][9] These limitations have long motivated the investigation of passive electrocorticographic (ECoG) recordings as a tool for rapidly and safely mapping cortical function prior to resective surgery.…”

“…4,6,7 Additionally, it is difficult to rule out distant effects through diaschisis or action potentials evoked by stimulation. [7][8][9] These limitations have long motivated the investigation of passive electrocorticographic (ECoG) recordings as a tool for rapidly and safely mapping cortical function prior to resective surgery. [10][11][12] Here we introduce and test the feasibility and clinical utility of an online trial-based system for spatial-temporal functional mapping (STFM) of language cortex.…”

What do patients with epilepsy tell us about language dynamics? A review of fMRI studies

“…Approximately 35% of patients suffering from focal epilepsy have refractory epilepsy for which curative surgery may be considered (see Noachtar et al, 2003;Kahane, 2004 for a review of eligibility criteria for epilepsy surgery) if the EZ can be identified clearly and removed safely . Major functional regions must be spared to prevent postoperative cognitive deficits (see Hamberger, 2007 for a review). After clinical exploration and video-EEG recording of seizures, an assessment of hemispheric dominance with respect to language and memory is necessary before surgery can be planned (Bookheimer, 2007).…”

Prediction of neuropsychological outcome after resection of temporal and extratemporal seizure foci

“…58 Postoperative word-finding difficulties are more likely to occur with more extensive resection of lateral temporal cortex, 29 which is one area that stores semantic knowledge. 24 Other predictors of postoperative word-finding difficulties include an absence of hippocampal sclerosis or other imaging abnormalities before surgery, 11 MR tractography showing more lateralized white matter tracts in the language-dominant hemisphere, 48 fMRI temporal lobe laterality index, 51 the absence of risk factors for seizures (for example, febrile seizures in childhood), 56 cessation of language in tissue to be resected during intraoperative electrical stimulation mapping, 23 and better presurgical naming ability. 10 Semantic knowledge acquired later in life is the most vulnerable to loss after surgery.…”