Cecilie Margaret Lander

Objective: Progressive clonic slowing is a common electroclinical phenomenon observed during the clonic phase of bilateral tonic–clonic seizures (BTCS) and reflects an inhibitory phenomenon. We aimed to explore the relationship between progressive clonic slowing and biomarkers implicated in the pathophysiology of sudden unexpected death in epilepsy (SUDEP), specifically postictal immobility and respiratory dysfunction. Methods: We retrospectively reviewed video‐EEG data of 110 BTCS (24 females; median age 31.5 years) from 52 patients, evaluating the presence of progressive clonic slowing and durations of seizure phases, postictal immobility, and EEG suppression. Respiratory dysfunction was assessed using pulse oximetry data (available in 76 seizures), measuring the extent and duration of oxygen desaturation during and after seizures. Results: The presence of progressive clonic slowing was associated with greater oxygen desaturation (median extent of desaturation: 18.3% vs. 7.8%; p = 0.01) and longer duration of postictal immobility (median: 48.0 s vs. 20.5 s; p < 0.001). The absence of progressive clonic slowing strongly predicted that moderate to severe (negative predictive value [NPV]: 88.9%; p = 0.002) and severe desaturation (NPV: 100%; p = 0.027) would not occur. In BTCS with desaturation, the duration of postictal immobility was longer (23.0 s vs. 49.0 s; Mann–Whitney U , p < 0.001). While the presence of PGES was associated with both a higher likelihood and extent of desaturation, the median duration of PGES did not differ between groups with and without progressive clonic slowing. No independent association between progressive clonic slowing and PGES duration was identified. Significance: Progressive clonic slowing is an electroclinical marker associated with prolonged postictal immobility and oxygen desaturation. Shared mechanisms underlying progressive clonic slowing and postictal immobility may contribute to SUDEP pathophysiology. Recognition of progressive clonic slowing as a seizure severity marker may have important implications for SUDEP risk assessment and prevention strategies. Plain Language Summary: This study found that during some convulsive seizures, the jerking movements of the body slow down gradually before the seizure ends. This pattern, called progressive clonic slowing, was linked to more severe breathing problems and longer periods of immobility afterward. Recognizing this pattern may help doctors identify seizures that carry higher risks and guide efforts to prevent sudden death in people with epilepsy.