Summary Purpose Extraoperative electrical stimulation mapping (ESM) to identify functional cortex is performed prior to neurosurgical resection at epilepsy surgery programs worldwide. However, the procedure remains unstandardized, with no established clinical guidelines. We sought to determine the current range in ESM practice parameters across established epilepsy surgery centers. Methods We developed and distributed a 31 question survey to 220 epilepsy centers worldwide regarding current practice parameters of ESM. Questions addressed preoperative assessment, technical stimulation parameters, language testing protocols, criteria for identification of positive or negative functional sites, management of mapping complications, and postoperative functional outcome. Key findings Survey responses were obtained from 56 centers. These revealed marked practice variability in virtually all aspects of the ESM procedure. Importantly, these aspects included critical procedure components such as electrical stimulation settings, the types of language functions tested, the operational definition of a language error, size of surgical resection margin, cortical locations mapped for language, testing in the presence of after discharges, and medical management of mapping complications. Forty-one percent of centers reported at least one persistent adverse language outcome despite preserving all eloquent sites defined by their stimulation mapping procedure. Significance The striking variations in practice across centers are likely to influence mapping results, which directly affect the boundaries of cortical resection and, consequently, might worsen either seizure or functional outcomes. Clearly, adverse functional outcomes occur despite mapping procedures that were perceived to be adequate. Investigation of critical technical and procedural aspects of stimulation mapping is warranted, with the ultimate goal of establishing empirically based practice guidelines to improve the safety and efficacy of ESM and resective epilepsy surgery.
Naming is mediated by perisylvian cortex in the left (language-dominant) hemisphere, and thus, left anterior temporal lobe resection for pharmacologically intractable temporal lobe epilepsy (TLE) carries risk for post-operative naming decline. Interestingly, this risk is lower in patients with hippocampal sclerosis (HS) relative to those without HS (non-HS). Although the hippocampus has traditionally been considered a critical structure for memory, without contribution to naming, this pattern might implicate direct hippocampal naming involvement. On the other hand, critical naming sites have been found in anterior, lateral temporal (i.e. extra-hippocampal) neocortex, the region typically removed with 'standard' TLE resection. We, therefore, speculated that the relative preservation of naming in post-operative HS patients might reflect cortical reorganization of language to areas outside this region. Using pre-resection electrical stimulation mapping, we compared the topography of auditory and visual naming sites in 12 patients with HS and 12 patients without structural brain pathology. Consistent with previous work, non-HS patients exhibited post-operative naming decline, whereas HS patients did not. As hypothesized, HS patients had proportionally fewer overall naming sites in anterior temporal cortex, the region typically removed with standard anterior temporal resection, whereas non-HS patients exhibited a more even distribution of naming sites in anterior and posterior temporal regions (P = 0.03). Although both groups exhibited the previously reported pattern of auditory naming sites anterior to visual naming sites, auditory naming sites had a significantly more posterior distribution in HS patients (P = 0.02). Additionally, non-HS patients exhibited a greater proportion of visual naming sites above the superior temporal sulcus, whereas visual naming sites in HS patients were scattered across superior and inferior temporal cortex. Results suggest that preserved naming ability in HS patients following anterior temporal resection might be attributable, at least in part, to intrahemispheric reorganization of language in response to the likely, early development of sclerosis in the medial temporal region. Furthermore, their more posterior distribution of naming sites is consistent with the more anterior propagation of EEG discharges in TLE. These results hold theoretical implications regarding the role of the dominant hippocampus in determining the cortical representation of semantic and lexical information, and raise questions regarding the specific roles of medial and lateral temporal cortex in targeted word retrieval. The different patterns of naming areas identified in patients with and without HS may also carry clinical implications, potentially improving efficiency during the time-constrained process of stimulation mapping.
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