Interictal Epileptiform Abnormalities

Interictal Epileptiform Abnormalities

Written by Trent Tollefson, MD and Sarah Durica, MD

Evaluation for the presence of interictal epileptiform abnormalities is one of the most common indications for routine EEG. The most common epileptiform abnormalities are interictal epileptiform discharges. Epileptiform discharges are typically comprised of either a sharp wave (70-200 milliseconds) or a spike (<70 milliseconds) followed by an after-going slow wave. Epileptiform discharges are classified as either focal (Figure 1) or generalized (Figure 2). When generalized epileptiform discharges can be reliably produced by intermittent photic stimulation, this is termed a photoparoxysmal response (Figure 3). The presence of epileptiform discharges alone is not diagnostic for epilepsy, but they generally indicate that the patient is at increased risk for seizures.

Criteria defining an epileptiform discharge have been proposed by the International Federation of Clinical Neurophysiology (IFCN).

These include:

1. Di-or tri-phasic wave with sharp or spiky morphology
2. Different wave duration than the ongoing background activity
3.  Asymmetry of the sharp component of the waveform
4. The sharp component is followed by an after-going slow wave
5. The background activity surrounding the discharge is interrupted by the presence of the discharge
6. Spatial distribution suggestive of a cerebral source of the waveform

If four or more of these criteria are met, a waveform can be considered an epileptiform discharge.

 

Figure 1: Example of focal (left temporal) epileptiform discharge. Image credit: Trent Tollefson

 

Figure 2: Example of generalized epileptiform discharges. Image Credit: Trent Tollefson

 

Figure 3: Example of a photoparoxysmal response. Note the yellow bar at the top of the image indicating when the strobe light is on. Image Credit: Cemal Karakas

 

Other examples of interictal epileptiform abnormalities include temporal intermittent rhythmic delta activity (TIRDA) and paroxysmal fast activity (PFA). TIRDA is typically seen in patients who have temporal lobe epilepsy and is considered an equivalent to a focal epileptiform discharge (Figure 4). PFA is characterized by low frequency beta-range activity that stands out from the background and is often diffuse with a frontal predominance (Figure 5). Generalized PFA can be seen in epileptic encephalopathies, such as Lennox-Gastaut syndrome, but can also be associated with genetic generalized epilepsies. Less commonly, PFA can occur more focally over a region of the EEG. In order to be considered an interictal abnormality, PFA must not be accompanied by a clinical correlate. When a clinical correlate is present, this is indicative of an ictal pattern.

 

Figure 4: Example of left temporal intermittent rhythmic delta activity. Image Credit: Trent Tollefson

 

Figure 5: Example of generalized paroxysmal fast activity (red box). Image Credit: Zulfi Haneef