Abstracts

Rationale: Almost two-thirds of patients with Tuberous Sclerosis Complex (TSC) associated epilepsy suffer from drug resistant seizures. Despite multifocality of the tubers, epilepsy surgery is a highly effective option in selected cases, leading to seizure freedom in 50-60%. However, the optimal invasive EEG and operative approach remains unclear. This is in part due to the unresolved issue of tuber and surrounding cortex epileptogenicity in TSC. Methods: We retrospectively analysed 18 patients with TSC who underwent presurgical Stereo-EEG (SEEG) (seven adults). All tubers were radiologically classified as type A (FLAIR hyperintense), B (FLAIR hyperintense, T1 hypointense) or C (type B with FLAIR hypointense centre). A 20-minute interictal recording was analysed for continuous interictal epileptiform discharges (IED) in all patients. 110 seizures (mean 6/patient) were analyzed with the Epileptogenicity Index (EI). In 13 patients with adequate tuber sampling, five anatomical regions of interest (ROI) were defined: dominant tuber (tuber with highest median EI), perituber cortex (PT) , secondary tuber (tuber with second highest median EI), nearby cortex (NC, normal appearing cortex in same lobe as dominant tuber) and distant cortex (DC, in other lobes). Epileptogenicity of ROI were examined by comparing the highest EI recorded from any bipolar channel within the anatomical region. Dominant tuber centre and rim EI values were also compared. Epileptogenic zone (EZ) organisation was separated into focal tuber (median EI across all seizures > 0.3 in dominant tuber only) and complex (all other patterns). Results: The most epileptogenic structure was the dominant tuber, with higher EI than perituber cortex, secondary tuber, nearby cortex and distant cortex (p < 0.001). Dominant tuber centre EI was greater than rim (p < 0.001). A focal tuber EZ organization was identified in seven patients (figure 1). This was characterized by continuous (IED) in dominant tuber (100%), radiological type C dominant tuber (86%), dominant-tuber-stimulation induced seizures (71%) and Engel IA outcome in 80%. In contrast, six patients had a complex EZ organisation (figure 2), characterized by nearby cortex as the most epileptogenic region, an absent centre-rim gradient and 40% Engel IA outcome. At tuber level, the combination of type C, continuous IED and stimulation-induced-seizures offered 98% specificity for a focal tuber EZ organization. Conclusions: The presence of distinct EZ organizations in TSC has significant implications for EZ hypothesis generation, invasive EEG approach and resection strategy. Tubers with focal EZ organization have a striking similarity to type II focal cortical dysplasia (FCDII) and further research should identify accurate non-invasive biomarkers of FCDII-like tubers. Funding: No funding