Clinical Validation of Criteria for Identification of Epileptiform Electroencephalography Discharges in Sensor Space and Source Space

Epilepsy affects 50 million people of all ages. It is the most common neurologic disorder across the lifespan, and has been found to be associated with an increased risk of mortality compared to the general population.

EEG is an important tool in the diagnostic work-up of patients with suspected epilepsy. The presence of epileptiform EEG discharges (EDs) confirm the diagnosis and provides important information that helps in classifying epilepsy.

EDs are visually identified by experts with training in reading EEG. Although EDs were defined in the previous edition of the IFCN glossary of terms, that definition was conceptual, and the inter-rater agreement for identifying EDs was only moderate. Recently, the IFCN suggested the following operational definition for EDs: Transients distinguishable from background activity with a characteristic morphology; EDs have to fulfill at least 4 of the following 6 criteria:

1. Di- or tri-phasic waves with sharp or spiky morphology (i.e. pointed peak).
2. Different wave-duration than the ongoing background activity: either shorter or longer.
3. Asymmetry of the waveform: a sharply rising ascending phase and a more slowly decaying descending phase, or vice versa.
4. The transient is followed by an associated slow after-wave.
5. The background activity surrounding epileptiform discharges is disrupted by the presence of the epileptiform discharges.
6. Distribution of the negative and positive potentials on the scalp suggests a source of the signal in the brain, corresponding to a radial, oblique or tangential orientation of the source. This is best assessed by inspecting voltage maps constructed using common-average reference.

However, these criteria are largely based on expert opinion. Data from clinical validation studies are lacking, thus it is not known what the sensitivity and specificity of these methods are. The threshold of four fulfilled criteria was arbitrary, and it is not sure whether it yields the optimal accuracy (sensitivity and specificity). Furthermore, since each criterion is based on visual evaluation, it is not known whether all experts would reach the same conclusion.