Topographic EEG mapping of 3/s spike-and-wave complexes during absence seizures

Topographic color mapping has recently been introduced for the study of ictal EEG manifestations of absences. We recorded 2-4 3/s spike-and-wave (sw) bursts in 12 patients with absence epilepsy, and performed a spectral analysis of the EEG under baseline conditions and during the 2 sec preceding the 3/s sw bursts. An increase in delta and theta bands was found in preseizure conditions. Moreover, the serial mapping of the sw complexes showed a different field distribution of the various components when the first sw was compared with the subsequent ones. In particular, the negative peak of the spike was mainly frontal during the first complex, with a tangentially oriented dipole, but it became better represented over the midline in the following sw, with a radially oriented dipole. We conclude that EEG changes can be detected before the onset of 3/s sw activity in absences, and that they are probably related to the strong inhibitory mechanisms acting in this type of seizure; moreover a frontal cortical mechanism seems to be prevalently active at the beginning of the seizure, which is soon captured by a cortico-subcortical process as the attack progresses.