Intrinsic ictal dynamics at the seizure focus: Effects of secondary generalization revealed by complexity measures

Purpose: Partial seizures (PSs) may be self-limited regional events or propagate further and secondarily generalize. The mechanisms and dynamics of secondarily generalized tonic-clonic seizures (GTCSs) are not well understood. Methods with which to assess the dynamic of those events are also limited. Methods: Seizures were analyzed from patients with intractable partial seizures undergoing monitoring with intracranial electrodes. Inclusion in this study required patients to have at least one PS and one GTCS. From >120 patients, seven patients fulfilled these criteria, three with mesial temporal (MTLE) onset seizures and four with neocortical lesional (NCLE) onset seizures. In total, 50 seizures were analyzed by using the matching pursuit (MP) method and the Gabor atom density (GAD), a measure of signal complexity derived from the MP method. Results: The GAD complexity pattern at the seizure focus for the initial ictal period is remarkably consistent in a given patient, regardless of whether secondary generalization occurs. Secondary generalization produces greater modification of seizure activity at the focus in patients with NCLE than in patients with MTLE. In seizures from four patients with NCLE, secondary generalization resulted in an average increase of 115% in complexity at the focus compared to PSs. Conclusions: GAD shows that seizure dynamics of PSs are often very stereotyped from seizure to seizure in a given patient, particularly during early ictal evolution. Secondary generalization is more likely to produce changes in the duration and dynamics at the seizure focus in NCLE patients compared with MTLE patients. These observations suggest distinct mechanisms (e.g., feedback) that are operational during secondary generalization.