Abstracts

RATIONALE: Secondary bilateral synchrony (SBS) was originally described by Jasper (1951) as [dsquote]bilaterally synchronous discharges, which can be shown to arise from a unilateral cortical focus....[dsquote] SBS would likely have significant impact upon localization decisions based upon scalp electroencephalography, yet its incidence and other clinical features have been poorly defined. Therefore, we investigated the occurrence of SBS in a population of patients with intractable, localization-related epilepsy.
METHODS: Consecutive long-term, video-EEG evaluations were analyzed from May 1997 through March 2002. Patients were included if they demonstrated at least one generalized, non-lateralizing or localizing epileptiform discharge during interictal recording and had at least one typical epileptic, localization-related seizure captured with video-EEG. Patients with primary or symptomatic generalized epilepsy were excluded.
RESULTS: A total of 1,079 patients were evaluated. Among the entire patient population 448 (41%) were classified as having a localization-related epileptic syndrome. These patients were further classified into frontal epilepsy (12%), temporal epilepsy (41%), parieto-occipital epilepsy (3%) and insufficient data to localize (44%). Sixteen patients met inclusion criteria for SBS, including 13 females and 3 males. This represented 2.9% of all patients with epilepsy and 3.6% with focal epilepsy. The mean age was 29.5 years (range: 18-56). A final assessment of localization was possible in 15/16 patients and were as follows: Frontal /fronto-parietal = 9/15 (60%), mesial/neocortical temporal = 3/15 (20%), parieto-occipital = 1/15 = 7%, temporo-occipital = 1/15 (7%) and left hemisphere = 1/15 (7%). Ictal localization was obtained in 10/16 (63%): frontal/fronto-parietal = 4 (40%), temporal = 4 (40%), temporo-parietal = 1 (10%) and left hemisphere = 1 (10%). Interictally, 3/16 (19%) had exclusive generalized spike-slow wave discharges, 6/16 (38%) had localized frontal discharges, 4/16 (25%) had unilateral or bilaterally independent temporal discharges, 2/16 (12 %) had bilaterally independent fronto-temporal discharges and 1/16 (6%) had independent multifocal spike discharges. MRI results were available in 13/16 and showed extra-temporal encephalomalacia in 4/13 (31%) (frontal in 3), mesial temporal sclerosis in 4/13 (31%) and were normal in 5/13 (38%).
CONCLUSIONS: SBS, as defined as generalized epileptiform discharges in patients with exclusive focal epilepsy, is a relatively infrequent interictal finding, occurring in approximately 4% of all intractable, localization-related patients undergoing video-EEG monitoring. While less likely, the possibility of co-existing and unrecognized generalized epilepsy or an inherited generalized EEG trait cannot be absolutely excluded. The higher proportion of patients with likely frontal lobe epilepsy and SBS (60%), in contrast with the overall incidence of frontal epilepsy in this population (12%), suggests an associated increased risk for extra-temporal epilepsy.