Abstracts

Rationale: Intracranial ictal onset patterns in bilateral temporal lobe epilepsy (TLE) have not been comprehensively studied. A significant correlation is known regarding the presence of periodic preictal spiking and reduced CA1 cell counts (Spencer 95) in mesial temporal lobe seizures. Our study assesses whether there is a difference in the presence of ictal onset patterns including periodic preictal spiking in unitemporal and bitemporal TLE which may suggest an electrographic distinction between the two conditions.Methods: The Comprehensive Epilepsy Program electronic database at Henry Ford Hospital was searched to identify patients with intractable temporal lobe onset seizures who underwent intracranial EEG monitoring (IEM). The study group consisted of patients with independent bitemporal onset ictal patterns. A comparison group consisted of patients with unitemporal onset ictal patterns on IEM all of whom had temporal lobe resection with seizure-free outcome. Age at epilepsy onset and at surgery and IEM ictal onset patterns (Table 1) were identified. MR imaging features were also noted. Ictal onset patterns were based on independent visual frequency analyses performed by two electroencephalographers (SG, VSW) and categorized as type A (<12Hz), type B (12-40Hz) and type C (>40Hz). Preictal spiking was defined as periodic sharp waves/spikes appearing rhythmically at least 5 sec preceding ictal onset. Time for propagation to the contralateral temporal lobe was also noted in the study group. T-tests were used to assess the differences in percent of preictal spiking and frequencies of IOPs between unitemporal and bitemporal ictal onset groups.Results: Eight patients (5 F/3 M) with a mean age of 38.6 13.0 y and, at epilepsy onset, of 8.9 4.6 y, were evaluated. The four patients (1 4) in the unilateral TLE group had a mean age at epilepsy surgery of 35 16.7 y with continued seizure freedom after a mean followup of 5.25 0.5 y. Mesial temporal sclerosis was identified in 3 of the 4 patients; however, other findings were discordant necessitating IEM. The remaining four patients (5 8) in the study group had no lesional MR imaging features. Subdural electrocorticography (46 96 contacts) was performed over 8 13 d (mean 11 d). A total of 250 ictal events were analyzed (unitemporal group, 71; bitemporal group, 179). Preictal spiking in the unitemporal cases (mean 80.6 15) was much higher (p = 0.001) than in the bitemporal cases (mean 2.2 2.8). High frequency patterns were also more evident (p = 0.004) in the unitemporal group (mean 92.4 10.5) than the bitemporal group (mean 24.5 28.4). No differences were detected between the unilateral and bilateral groups regarding age at onset or at presentation.Conclusions: This study finds preictal spiking on IEM to be an indicator of unilateral mesial TLE favoring a seizure-free outcome after resection. The near absence of preictal spiking in epilepsy with bilateral independent foci may suggest a different pathophysiological evolution. Prospective studies are necessary to confirm these findings.