EEG abnormality Interictal EEG abnormality 2017-03-15T13:25:46Z interictal electroencephalographic abnormality http://www.case.edu/EpSO.owl#InterIctalPattern HPO:probinson Interictal refers to a period of time between epileptic seizures. Electroencephalographic (EEG) patterns are important in the differential diagnosis of epilepsy, and the EEG is almost always abnormal during a seizure. Some persons with seizures may show EEG abnormalities between seizures, while others do not. In some cases, multiple interictal EEGs must be recorded before an abnormality is observed. In most cases the electrographic pattern of seizure onset is completely different from the activity recorded during interictal discharge.