Epileptic DC (slow) shifts: overview

Abstract

Clinical EEG provides us with diagnostic information of epileptogenicity by epileptiform discharges, i.e., spikes, sharp waves, which reflect the paroxysmal depolarization shifts (PDS) in the epileptic neurons. Currently advanced technology has enabled us to record wide-band EEG: direct current (DC) shifts and high frequency oscillation (HFO). The both conditions could widen the neurophysiological definition of epileptgenicity.
Ictal DC shifts was recorded by using a DC amplifier in 1960s with technical difficulty, but recently is by applying very small low frequency filter (0.016 Hz) of an AC amplifier which has the large input impedance more than 200 Mohm without difficulty in patients with invasive electrodes (Ikeda et al., 1996, 1999, 2008). It could reflect the massive, synchronized paroxysmal depolarization of the neurons in the epileptogenic area, and thus also represents associated depolarization of the glia. It could be regarded as the surrogate markers of the core epileptogenicity in human epilepsy regardless of the etiology.
Once HFO is thought to highly reflect epileptogenicity in human epilepsy, we have investigated both ictal DC shifts and HFO simultaneously in patients with intractable partial epilepsy by means of subdural electrodes (Imamura et al., 2011; Kanazawa et al., 2015). We observed that 1) both occurred together as early as electrodecremental pattern occurred or earlier than conventional ECoG changes, 2) both occurred in the same electrodes, 3) ictal DC shifts were more often observed than HFO, and 4) ictal DC shifts could occur earlier than HFO. It could suggest more active role of glia in not only generating DC shifts but also presumably in ictogenesis.