Criteria for defining interictal epileptiform discharges in EEG: A clinical validation study

Abstract

Objective: To define and validate criteria for accurate identification of EEG interictal epileptiform discharges (IEDs) using (1) the 6 sensor space criteria proposed by the International Federation of Clinical Neurophysiology (IFCN) and (2) a novel source space method. Criteria yielding high specificity are needed because EEG over-reading is a common cause of epilepsy misdiagnosis.

Methods: Seven raters reviewed EEG sharp transients from 100 patients with and without epilepsy (diagnosed definitively by video-EEG recording of habitual events). Raters reviewed the transients, randomized, and classified them as epileptiform or nonepileptiform in 3 separate rounds: in 2, EEG was reviewed in sensor space (scoring the presence/absence of each IFCN criterion for each transient or classifying unrestricted by criteria [expert scoring]); in the other, review and classification were performed in source space.

Results: Cutoff values of 4 and 5 criteria in sensor space and analysis in source space provided high accuracy (91%, 88%, and 90%, respectively), similar to expert scoring (92%). Two methods had specificity exceeding the desired threshold of 95%: using 5 IFCN criteria as cutoff and analysis in source space (both 95.65%); the sensitivity of these methods was 81.48% and 85.19%, respectively.

Conclusions: The presence of 5 IFCN criteria in sensor space and analysis in source space are optimal for clinical implementation. By extracting these objective features, diagnostic accuracy similar to expert scorings is achieved.

Classification of evidence: This study provides Class III evidence that IFCN criteria in sensor space and analysis in source space have high specificity (>95%) and sensitivity (81%-85%) for identification of IEDs.

Figure 1. Infographic summarizing the 6 IFCN criteria for identifying IEDs

(1) Di- or tri-phasic wave with pointed peak; (2) different wave duration than the ongoing background activity; (3) asymmetry of the waveform; (4) followed by a slow after-wave; (5) the background activity is disrupted by the presence of the IEDs; and (6) voltage map with distribution of the negative and positive potentials suggesting a source in the brain corresponding to a radial, oblique, or tangential orientation of the source. For further details, see the Methods section. The voltage maps in window (6) show a tangential orientation (source in the left middle frontal gyrus) and a radial orientation (source in the left superior frontal gyrus); the irregular distribution of the potentials in the last voltage map does not imply a source in the brain (it was an artifact with sharp morphology) and does not fulfill this criterion; negative potentials are in blue, and positive potentials are in red. IED = interictal epileptiform discharge; IFCN = International Federation of Clinical Neurophysiology.

Figure 2. Examples of IED (A–C) and nonepileptiform sharp transient (D–F)

Montages: longitudinal bipolar (A and D), common average (B and E), and source space (C and F). Voltage maps are shown in G and H. The transient shown in A–C with voltage map in G (oblique distribution corresponding to a source in the right temporal pole) fulfills all IFCN criteria in sensor space, except for criterion 5, and hence, it qualifies as an IED; in source space, it also qualifies as an IED, and the propagation from the temporal pole to the basal temporal region can be observed (C). The transient shown in D–F with voltage map in H only fulfills 2 IFCN criteria in sensor space (1 and 6), and hence, it does not qualify as an IED; in source space, one can observe that the transient belongs to the background activity from the right basal temporal region (fragmented during drowsiness); the orientation of the voltage map corresponds to a source in the right basal temporal region (fulfilling criterion 6). Voltage maps are useful in distinguishing IEDs from artifacts (figure 1); however, nonepileptiform transients originating from the brain can show voltage distributions similar to IEDs. IED = interictal epileptiform discharge; IFCN = International Federation of Clinical Neurophysiology.

Figure 3. Receiver operating characteristic (ROC) curve

The numbers in the curve represent the cutoff values (number of fulfilled IFCN criteria). The stippled vertical line corresponds to a specificity of 95%; points to the left of this line have a specificity of >95%. AUC = area under the receiver operating characteristic curve; IFCN = International Federation of Clinical Neurophysiology; ROC = receiver operating characteristic.