Positive interictal epileptiform discharges in adults: A case series of a rare phenomenon

•Positive interictal epileptiform discharges (IEDs) were recorded in 0.033% of EEGs, making this a rare phenomenon.•Positive IEDs were highly associated with epilepsy.•Brain surgery was the most common etiology for positive IEDs, and probable cortical malformation in the remaining patients. Positive interictal epileptiform discharges (IEDs) are rarely recorded from surface EEG, due to the orientation of the cortex and its neurons. Their frequency and significance in adults is unknown, and has only been studied as a phenomenon of the neonatal period and childhood. We aimed to evaluate the frequency and characteristics of positive epileptiform discharges in a large cohort of patients. We retrospectively reviewed 24,178 reports from 18,060 patients of non-invasively recorded EEGs for various indications. Positive IEDs were recorded in six patients (eight EEGs – 0.033%), all of which had epileptic seizures. Brain surgery was the most common reason for recording positive and not negative IEDs. Cortical malformation was the most probable etiology in the remaining patients. Positive IEDs seem to be of lower frequency in adults than in children and are highly associated with epilepsy. They appear more often, but not exclusively, in adult patients with skull defects. This is the first adult series reported. Positive IEDs must be identified for the correct diagnosis and clearly differentiated from normal variants.