A reappraisal of secondary bilateral synchrony

ABSTRACT

A reappraisal was made with respect to a classical observation of the mode of instrumental phase reversals on inter-ictal EEG of seemingly bilateral synchronous spike-wave discharges in patients with either idiopathic generalized epilepsies (IGE) or symptomatic localization-related frontal lobe epilepsies (FLE). It was pointed out in the original observation by Tukel and Jasper that one phase reversal at midline or near the midline on the side of the parasagittal epileptogenic lesion designated as secondary bilateral synchrony (SBS) was found in patients with frontal lobe epilepsy (FLE), whereas a double phase reversal was found over the homologous frontal electrodes (F3 and F4) designated as primary bilateral synchrony (PBS) in patients with IGE. Twenty-three patients (IGE 15, and FLE 8) revealing bursts of seemingly bisynchronous spike-wave discharges in interictal EEGs were retrospectively studied. Discharge patterns were defined as stable phase reversal pattern if the site of phase-reversal was consistent, and as unstable pattern if the site of phase-reversal was not consistent but shifting in the same patient. Stable one phase-reversal pattern was found more frequently in FLE (50%) than in IGE patients (26.7%), and stable double phase-reversal pattern more frequently in the IGE (33.3%) than in the FLE group (12.5%). Notably, unstable pattern was found almost equally in both IGE and FLE patients (40% and 37.5%, respectively). Recognition of SBS or PBS in accordance with original observation was found not to clearly differentiate FLE from IGE in patients showing seemingly bisynchronous spike-wave complexes. The variability of instrumental phase-reversals can be accounted for by the fact that the localization of maxima of negative spike of the spike-and-wave complexes varies considerably.