BACKGROUND AND PURPOSE:The prognostic significance of interictal epileptiform discharges (IED) and periodic patterns (PP) after ischemic stroke has not been assessed. We sought to test whether IED and PP, detected on standard Electroencephalography (EEG) performed during the acute phase of ischemic stroke are associated with a worse functional outcome. METHODS:One-hundred-fifty-seven patients 18 years or older with a diagnosis of acute ischemic stroke presenting within 72 h from stroke onset were prospectively enrolled and followed. Patients with a pre-stroke history of seizures or epilepsy, previous debilitating neurological disease or conditions that precluded the performance of EEG were excluded. Interpretation was performed by a blinded board certified neurophysiologist. IED and PP (grouped as epileptiform activity [EA]) were defined according to proposed guidelines. Univariable and multivariable analyses were used to identify predictors of outcome (modified Rankin Scale dichotomized ≤2 vs. ≥3) at 3 months. RESULTS:In the univariable analysis, admission NIHSS (OR 1.20, 95% CI 1.12-1.28, p = 0.001), age (OR 1.03, 95% CI 1.01-1.05, p = 0.02), and presence of EA (OR 2.94, 95% CI 1.51-5.88, p = 0.001) were significantly associated with the outcome at 3 months. In the multivariable analysis, only admission NIHSS (OR 1.19, 95% CI 1.11-1.28, p < 0.001) and the presence of EA (OR 2.27, 95% CI 1.04-5.00, p = 0.04) were independently associated with the prognosis. SIGNIFICANCE:The importance of EEG in the prognosis of acute ischemic stroke warrants additional research, examining the role of medication therapy on the outcome and the occurrence of seizures for those patients with specific EEG patterns.