EEG focal delta slowing in focal epilepsy – A didactic review

Focal slowing in scalp electroencephalography (EEG) is classically seen as an unspecific entity in terms of its association with both epilepsy and distinct aetiologies. This could be due to a lack of standardization in the assessment of slowing morphology, in distinction to precise guidelines utilized in the evaluation of pathological activities, such as interictal epileptiform discharges (IEDs). However, consistent features can be identified, which can be informative about the underlying brain pathology and risk of seizures. In this review, we discuss the different kinds of focal slowing, focusing mainly on the delta range (0.5–4 Hz), their respective pathological processes and their prognostic value in terms of likelihood to be associated with acute seizures or epilepsy. We then present new findings on source localization of focal slowing. Last, we review recent evidence of interaction between focal slow activities and epileptic activities. Our didactic approach will guide readers through important aspects of an EEG entity that might have been neglected despite past and recent observations of its relevance in epilepsy.