Interictal infraslow activity in patients with epilepsy

•Interictal infraslow activity (ISA) can provide additional information about the epileptogenic process.•It can be assessed with conventional EEG systems.•ISA activity is more widely distributed in localisation-related epilepsies than might be assumed.

ObjectiveTo evaluate if interictal infraslow activity (ISA), as obtained from a conventional EEG system, can contribute information about the epileptogenic process.MethodsThe entire long-term intracranial monitoring sessions of 12 consecutive patients were evaluated on an XLTEK system for ISA. Three additional patients had long-term scalp recordings.ResultsIn intracranial as well as scalp recordings, the ISA background was consistently higher in the waking state than during sleep. From this background emerged intermittently focal changes, which could achieve in intracranial recordings millivolt amplitudes, while they remained in the microvolt range in scalp recordings. Although they were mainly contiguous between adjacent channels, this was not necessarily the case and intermittent build-up could be seen distant from the epileptogenic zone or radiographic lesion.ConclusionsInterictal ISA can be detected in routine intracranial and scalp recordings, without the need for DC amplifiers, and can provide additional information.SignificanceSince ISA is a separate element of the electromagnetic spectrum, apparently non-neuronal in origin, its assessment should be included not only in the pre-surgical evaluation of epilepsy patients but also in patients with other neurologic disorders and normal volunteers.