Interictal epileptiform discharges

The slide shows the International Federation of Clinical Neurophysiology criteria for epileptiform discharge, based on identification of ≥4 of the 6 criteria:¹

1.sharp or spiky morphology1 (20–70 milliseconds for spike waves; 70–200 milliseconds for sharp waves²)

2.different wave duration than background activity¹

3.waveform asymmetry¹

4.after-going slow wave¹

5.disruption of background activity: flattening or low-voltage alpha or beta frequency activity after
(most frequently) or before sharp transient¹

6.distribution suggestive of cerebral source/physiologic field.¹

Note that Criteria 4 and 5 are independent of each other.¹

The paroxysmal depolarizing shift (PDS) is a large-amplitude depolarization associated with action potential bursting, it represents the cellular counterpart of the interictal spike that occurs between focal seizures on an EEG.³

References:

1.Nascimento FA, Beniczky S. Teaching the 6 criteria of the International Federation of Clinical Neurophysiology for defining interictal epileptiform discharges on EEG using a visual graphic. Neurol Educ 2023; 2 (2): e200073.

2.EEG Abnormal Waveforms. StatPearls [Internet]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK557655/. Accessed October 2025.

3.Prince DA, Avoli M. The paroxysmal depolarizing shift: the first cellular marker of focal epileptogenesis. In: Noebels JL, Avoli M, Rogawski MA, et al. (eds). Jasper’s Basic Mechanisms of the Epilepsies. 5th edition. Oxford University Press, 2024.