The Ictal Signature of Thalamus and Basal Ganglia in Focal Epilepsy: A SEEG Study

OBJECTIVETo determine the involvement of subcortical regions in human epilepsy by analyzing direct recordings from these regions during epileptic seizures using stereoelectroencephalography (SEEG). METHODSwe studied the SEEG recordings of a large series of patients (74 patients, 157 seizures) with an electrode sampling the thalamus and in some cases also the basal ganglia (caudate nucleus −22 patients- and putamen—4 patients). We applied visual analysis and signal quantification methods (Epileptogenicity Index, EI) to their ictal recordings and compared electrophysiologic with clinical data. RESULTSWe found that in 86% of patients thalamus was involved during seizures (visual analysis) and that 20% showed high values of epileptogenicity (EI >0.3). Basal ganglia may also disclose high values of epileptogenicity (9% in caudate nucleus) but to a lesser degree than thalamus (p < 0.01). We observed different seizure onset patterns including low voltage high frequency activities. We found high values of thalamic epileptogenicity in different epilepsy localisations, including opercular and motor epilepsies. We found no difference between epilepsy aetiologies (cryptogenic vs malformation of cortical development, p = 0.77). Thalamic epileptogenicity was correlated with the extension of epileptogenic networks (p = 0.03, ρ 0.32). We found a significant effect (p < 0.05) of thalamic epileptogenicity regarding the post-surgical outcome (higher thalamic EI corresponding to higher probability of surgical failure). CONCLUSIONSThalamic involvement during seizures is common in different seizure types. The degree of thalamic epileptogenicity is a possible marker of the epileptogenic network extension and of post-surgical prognosis.