Interictal epileptiform discharges (IEDs) are transient graphoelements (or—more simply—waveforms) found on EEG and MEG corresponding to an increased predisposition to unprovoked seizures. Ebersole continues to cite the 1983 IFCN glossary for its definition; the updated 2017 glossary says that IED “describes transients distinguishable from background activity with a characteristic morphology typically, but neither exclusively nor invariably, found in interictal EEGs of people with epilepsy.”

By and large IEDs consist of sharp or spiky waveforms with aftergoing slow potentials that disrupt the prevailing MEEG background—although temporal intermittent rhythmic delta activity, or TIRDA, is also thought to be the equivalent of finding sharp waves or spikes. The first link above is to the validation study of the IFCN’s proposed IED-identifying criteria.

In the MESS trial follow-up data the presence of interictal discharges was associated with an increased chance of seizure recurrence. In patients with at least one seizure and uncertainty of whether to treat or not, the presence of IEDs without any other risk factors could be estimated to result in a probability of recurrent seizure of 56% by 5 y. The AAN/AES guideline on management of an unprovoked first seizure in adults, published 2015, echoed that the risk of recurrence is increased with epileptiform EEG.

IEDs don’t have to mean people have epilepsy, however. Ebersole says the “presence of epileptiform discharges does not necessarily indicate that the patient has a seizure disorder” citing Sam & So 2001. Moreover as demonstrated by, among others, Kang and Krauss 2019, not infrequently are benign MEEG transients misinterpreted as IEDs. What a biomarker it is! Not exclusively associated with epilepsy and prone to be misdiagnosed especially by yeoman EEG readers.

“It is well established that IEDs interfere with the occurrence of seizures,” writes Chvojka et al who goes on to cite literature showing that antiictogenic effects are associated at least with hippocampal spikes although other types of spikes are either proictogenic or even preictal.

What is the nature of interictal discharges after all?

Changing perspective could help. In this paper the authors derive the paradoxical anti- and pro-ictal effects of IEDs by conceiving of them as perturbative forces in a dynamical background that has two stable equilibria: interictal and ictal. In the process of critical slowing-down, as the resilience of the interictal equilibrium declines and the mean time to recover from perturbations increases, IEDs go from being stochastic knocks insufficient to force a transition to the ictal state to more-than-adequate knocks from one domain to the other. (They also hypothesize a separate effect of IEDs in more resilient states of increasing resilience).

Then there’s this work arguing that the broad field of IEDs betrays their nature as traveling waves of potential changes that propagate approximately isotropically along the grey matter. They reason that the small delays of spikes measurable across different intracranial electrodes represent the slightly different distances of the sensors from the circumscribed source. What’s more, given several electrodes’ delays they can triangulate (or properly “multilaterate”) the position of the source.

I’ll leave you with one of my favorite papers: this 2016 paper that put people with IEDs behind the (virtual) wheel in a driving simulation. Reaction times were delayed and virtual crashes happened more when there were IEDs, especially with generalized typical spike-wave discharges. If IEDs are so great after all why do they affect reaction time so much?