Abstracts

Rationale: Data from electrophysiological studies have suggested temporal lobe epilepsy (TLE) as a syndrome that incorporates multiple subtypes with ictal generators localized within the mesial, lateral or other neocortical structures. The dynamics among these ictal generators during ictogenesis can be classified as –1) A centrifugal origin, defined by onset at a single focus followed by progressive recruitment and synchronization among multiple structures within these subtypes (often defined as “ictal network”), or 2) multi-focal origin, where multiple discrete seizures emanate independently from different epileptogenic structures. These patterns are different on visual analysis of intracranial EEG, with the latter considered “multifocal” given its origin from discrete ictal generators. Studies by Bartolomei et al have demonstrated that a longer duration of epilepsy correlated with the number of structures involved within the ictal network. However, it is unclear what additional factors determine multifocality in TLE syndrome. Based on previous studies that demonstrated etiology (e.g.: head injury) as a determinant for multifocal epilepsy, we hypothesize that underlying etiology determines multifocality in unilateral TLE syndrome.Methods: We performed a six-year (2009-2014) retrospective analysis of consecutive Stereo-EEG evaluations performed in adults with suspected TLE. Demographics and outcomes were analyzed. Visual analysis of EEG was done to identify seizure onset zone and was classified as – a) mesial (M); b) lateral(L); c) temporo-polar(TP); d) temporal plus(T-plus)- if onset was in temporal structures and at insula, orbitofrontal or parieto-occipital regions. Ictal onset HFO’s (80-500 Hz) and ictal baseline shift-(<0.01 Hz) were used as an adjunct to identify seizure onset zone. Etiology was classified as global if the initial epileptogenic insult had global involvement of brain (like febrile seizures) or regional/focal if the insult involved focal area of brain (like intracranial hemorrhage, gunshot wound). Bilateral TLE was excluded from the study.Results: Thirty-two adults with a mean age of 38.5 (range 20-58) underwent S-EEG evaluation for suspected TLE. Eleven (35%) had multi-focal but unilateral TLE with seizure-onset zones as follows: - a) 5 had M and L onset; b) 4 had M and TP onset; c) one had L and T-plus (insula); and d) one had M and T-plus (insula). There was no statistically significant difference (unpaired T-test) in the duration of epilepsy between uni- or multi-focal forms of unilateral TLE (25 versus 31 yrs; two-tailed P= 0.24). Etiology for multi-focal TLE were- encephalitis (n=3; two had post infection status epilepticus), head injury (n=4), unknown/unidentified (n=4). Among the identified causes of uni- and multi-focal TLE, there was higher prevalence of regional/focal insult in multi-focal TLE (Fisher's exact test, p=0.0047).Conclusions: Approximately one-third of patients with unilateral TLE had multi-focal seizure-onset zones that were mapped by SEEG. Underlying etiology (like encephalitis,head injury) rather than duration of epilepsy determined multi-focality.