Interictal epileptiform discharges

Interictal epileptiform discharges (IEDs) are waveforms seen more commonly in people with epilepsy. There is not an absolute association between IEDs and epilepsy and they can be found in the general population. Identifying IEDs though can be used to support a diagnosis of epilepsy and they may provide localising information.

IEDs include spikes and sharps. Spikes are between 20-70ms in duration. Sharps are between 70-200ms in duration.

The International Federation of Clinical Neurophysiology define six criteria used to define an IED. If four or more of these features are present, the waveform in question can be considered an IED.

The six criteria are: 1) Sharp or spiky morphology 2) Duration different from background activity 3) Asymmetric 4) After going slow wave 5) Disruption of background activity 6) Physiological field

Kural et al. investigated what combination of these features is required for a 95% specificity diagnosing epilepsy (Kural et al. Clinical Neurophysiology 2020).

There are two cardinal components of the spike-wave complex - the spike and the wave!

A positive transient of variable duration may preceed the spike.

Spikes and sharps are usually surface negative. Positive sharps and spikes are more common in depth recordings. A positive waveform on a scalp recording generally occurs in the presence of a deficit in the superficial cortex. With dipole formation (especially in benign rolandic epilepsy), a positive spike discharge may be found in a moderate distance from the negative spike focus.

Normal variants

The following may be confused with epileptiform sharps/spikes: - Wicket waves - POSTs - Vertex sharps

POSTs are generally positive, as the name suggests!