Abstracts

Rationale: Cortical Stimulation Mapping (CSM) and the Wada procedure have long been considered the gold standard for localizing motor and language-related cortical areas and for determining the language and memory-dominant hemisphere, respectively. In recent years, however, non-invasive methods such as magnetoencephalography (MEG), functional magnetic resonance imaging (fMRI), and transcranial magnetic stimulation (TMS) have emerged as promising alternatives to the aforementioned procedures, particularly in cases where the invasive localization of eloquent cortex has proven to be challenging. To illustrate this point, we will first introduce the evidence of the compatibility of invasive and non-invasive methods and present a selection of surgical cases highlighting the utility of non-invasive procedures in specifying the extent of sensorimotor and language-specific cortex, in instances where invasive localization of these functions was deemed to be sub-optimal or impossible. Methods: Three cases where invasive pre-surgical mapping was difficult or impossible were selected for presentation: (a) Patient 1- a 21-year-old male with intractable seizures, status post right frontal topectomy and multiple subpial transections; (b) Patient 2- a 24-year-old female with intractable seizures, status post resection of a low-grade oligodendroglioma; and (c) Patient 3- a 14-year-old male with a left opercular glioma. For Patients 1 and 2, MEG mapping of the primary somatosensory cortex was conducted following tactile stimulation of the index fingers. Motor mapping was performed using single pulse TMS, applied to the primary hand motor cortices, and with fMRI using a self-paced finger tapping task. In Patient 3, hemispheric dominance for expressive language was assessed with MEG using a covert single word reading paradigm, and the extent of language cortex was further assessed in the context speech disruption during repetitive TMS, and fMRI, as the patient performed an object-naming task. Results: In Patients 1 and 2, the importance of the non-invasive procedures in successfully identifying the central sulcus and determining the topography of the motor cortex is underlined by the fact that invasive localization was not possible due to prior dural scarring and adhesion to the pia arachnoid over the motor area. In Patient 3 successful localization of language related cortex in the medial aspect of the lesion not accessible to CSM using MEG and TMS further highlights the efficacy of non-invasive methods in identifying eloquent cortex during presurgical planning. Conclusions: Based on the cases reviewed here, MEG, fMRI and TMS, either separately or in combination, can facilitate presurgical planning, especially in cases where invasive localization of eloquent cortex is not practical. It will be argued that whenever available, non-invasive functional imaging methods are a viable alternative to the invasive ones for the purposes of presurgical functional mapping and only if their combined results are ambiguous or unconvincing should the latter ones be performed. Funding: Neuroscience Institute- Le Bonheur Children's Hospital