Interictal Epileptiform Activity: Diagnostic and Behavioral Implications: 2002 ACNS Presidential Address

Interictal epileptiform activity (IEA) is an electrographic term that describes the abrupt appearance of spikes or sharp waves, often associated with aftergoing slow waves that, unlike ictal epileptiform activity, are limited in duration and do not evolve in frequency and distribution over time. Although the term ’interictal’ would seem to imply the absence of a behavioral correlate, the definition of IEA does not address neuropsychological function. The implications of IEA have continued to evolve with research in: (1) the genetic and metabolic basis of electrophysiological events (Noebels, 2003; Pedley, 1997), (2) intracranial electrophysiology (Bragin et al., 2002a, b; Staba 2002), and (3) the relationship of electrophysiological events to neuropsychological function (Aarts et al., 1984; Browne et al., 1974; Shewmon and Erwin, 1988a, b, c). Accumulating information has begun to shift the older concept of IEA away from that of an electrographic event unassociated with cognitive or behavioral consequence to an electrophysiological event that is the phenotypic expression of a variety of cellular disorders (Noebels, 2003; Pedley, 1997). For the clinician, the study of IEA remains essential to the diagnosis and treatment of epilepsy. While preparing this discussion, I had the opportunity to discuss IEA with neurology residents and practicing neurologists. I also conducted informal surveys during the 2002 EEG Scholars Program for EEG fellows at the annual American Clinical Neurophysiology Society meeting and during the 2003 Clinical EEG course at the annual American Academy of Neurology meeting to get a better idea of how neurologists view IEA. Similar to the experience of Hughes (1989) and Goodin and Aminoff (1984) over a decade ago, I was surprised to find that that most neurologists do not have a good understanding of the diagnostic or potential neuropsychological implications of IEA. Perhaps, as suggested by Hughes (1989), this is because of an educational approach in which the EEG is regarded as a graphic entity with little relationship to actual biophysical cellular function. The adage “never treat a lab test” has undoubtedly been applied to EEG more often than to any other neurodiagnostic procedure, despite the fact that there are situations in which brain dysfunction is treated according to the cortical electrophysiological abnormality, i.e., the EEG. The clinical relevance of IEA depends on how the following questions are answered: