Objective:

To evaluate the frequency, risk factors and prognostic significance of electroencephalographic (EEG) epileptiform activity (EA) in hospitalized adults.

Background:

Epileptiform activity, encompassing lateralized and generalized periodic discharges (LPDs, GPDs) and rhythmic delta activity (LRDA, GRDA), is frequently identified in acute neurology and critical care patients undergoing continuous EEG (cEEG) monitoring. The full diagnostic and prognostic significance of these patterns is yet to be determined.

Design/Methods:

We conducted a retrospective cohort study of adults who underwent cEEG monitoring at three centers (2009–2024). Patients with ≥2-hour EEG recordings were included. Clinical data, comorbidities, and diagnoses were extracted from electronic health records using structured ICD codes and natural language processing. Cox proportional hazards models assessed associations between EA and outcomes—90-day and 1-year mortality, and new-onset epilepsy—adjusting for age, sex, Charlson Comorbidity Index, primary diagnosis, and prior epilepsy.

Results:

Among 29,809 patients (median age 65 years, 54% male), EA or seizures were observed in 25.2%. GPDs (10.5%) and LPDs (6.8%) were the most frequent EA subtypes. EA or seizures were independently associated with increased mortality at 90 days (adjusted hazard ratio [aHR] 1.53, 95% CI 1.44–1.62) and at 1 years (aHR 1.53, 95% CI 1.44–1.62). EA alone showed stronger associations (aHR 1.64 and 1.64, respectively). GPDs conferred the highest mortality risk (90-day aHR 1.99; 1-year aHR 1.92), whereas LRDA was not associated with excess mortality. New-onset epilepsy occurred in 7.3% within 90 days and 14.8% at 5 years, with highest risk among patients with LRDA and structural lesions.

Conclusions:

Epileptiform activity on EEG is a common finding in hospitalized patients and is associated with increased short- and long-term mortality and potential risk of epilepsy. These findings support the prognostic significance of EA beyond seizures and highlight the need for further studies to guide therapeutic strategies.