Ictal onset baseline shifts and infraslow activity.

Ictal onset baseline shifts, in surface and intracranial EEG, have been shown to localize focal epilepsies. However, whether direct current EEG amplifiers are required to detect infraslow activity is unresolved. We retrospectively analyzed intracranial EEG data from 24 patients with temporal lobe seizures, who underwent long-term invasive EEG investigation and subsequent surgery. Data were recorded using a DeltaMed/Natus (Paris, France) system with Braintronics Brainbox EEG-1164 (Almere, The Netherlands) with an input filter of 0.1 Hz. Visual comparison of infraslow activity with seizure activity in conventional frequency bands was performed using BESA software (Megis, Gräfeling, Germany). Ictal onset baseline shifts were seen in 52 of 88 partial seizures and in all 11 secondarily generalized tonic-clonic seizures. They preceded ictal activity in conventional frequencies in some cases by several seconds. Topographical distribution was concordant with seizure onsets and distant sites with suspected involvement in seizure generation. It is concluded that ictal onset baseline shifts can be detected by commonly used EEG systems with an input filter of 0.1 Hz and may contribute to identify seizure onset and areas involved in seizure generation.