Tonic–clonic seizures captured during ambulatory video‐ EEG are frequently unreported

OBJECTIVE: Tonic-clonic seizures (TCSs) are widely regarded as clinically obvious, yet seizure counts used for treatment decisions and risk counseling often rely on patient or caregiver diaries. We sought to quantify the frequency of unreported TCSs during prolonged ambulatory video-EEG (vEEG) monitoring and examined associations with electrographic-onset subtype and patient characteristics. METHODS: We conducted a retrospective cohort study of routinely collected ambulatory vEEG from a single national Australian service (January 2018-June 2024). Studies were eligible if the patient had epilepsy and at least one objectively captured TCSs. Reporting status was derived from diary entries and post-study questioning, and classified as reported vs unreported. The primary descriptive outcome was patient-level reporting status across captured TCSs: all captured TCSs reported, some captured TCSs reported, or no captured TCSs reported. Event-level reporting status was used for seizure-level descriptive summaries and patient-clustered analyses. Event-level associations were examined using patient-clustered generalized estimating equations, and patient-level subgroup comparisons used nonparametric and categorical tests. RESULTS: Among 130 patients with objectively captured TCSs, 69 of 130 (53.1%) reported all captured TCSs, 41 of 130 (31.5%) reported some but not all captured TCSs, and 20 of 130 (15.4%) reported no TCSs during monitoring. Overall, 61 of 130 patients (46.9%) had at least one unreported TCS. At the event level, 340 of 754 captured TCSs (45.1%) were unreported and identified only on review. Nineteen of 130 patients (14.6%; 95% confidence interval [CI] 9.0-21.9) had no documented prior TCS history in available service records and reported no TCSs during monitoring. Unreported event proportions were similar across focal-onset and generalized-onset TCSs, whereas sleep was associated with higher odds of underreporting. SIGNIFICANCE: In this selected ambulatory vEEG cohort, nearly half of patients with captured TCSs had at least one unreported TCS, including 15.4% who reported none during monitoring. These findings indicate that diary-based histories may underestimate convulsive seizure burden in some monitored patients, with implications for safety decisions, therapeutic escalation, and sudden unexpected death in epilepsy (SUDEP) counseling.