Several studies describing the diurnal occurrence of febrile seizures have reported greater seizure frequency early or late in the evening relative to midnight or early morning. However, no articles have reported on the diurnal occurrence of complex febrile seizure. Moreover, no studies have addressed the relationship between seizure severity and diurnal occurrence. We retrospectively evaluated complex febrile seizures in 462 children needing hospitalization, and investigated the relationship between severity and diurnal occurrence according to four categorized time periods (morning, afternoon, evening, and night). Our study showed that complex febrile seizures occurred most often in the evening, peaking around 18:00 (18:00–18:59), and least often at night (02:00–02:59). In addition, the frequency with which patients developed status epilepticus or needed anticonvulsant treatments was also lower during the night. However, the seizure duration and the proportion of the patients who needed anticonvulsant treatment were the same among the four time periods. Furthermore, we compared three subclasses (repeated episodes of convulsions, focal seizures, and prolonged seizures (≧ 15 min)), two of the complex features (focal seizures and prolonged seizures), and all complex features among the four time periods. However, they were the same among the four time periods. Taken together, our data indicate that although the severity of seizures was stable over a 24-hour period, the occurrence of seizures in our cohort of pediatric patients with complex febrile seizures requiring hospitalization was highest in the evening and lowest at night.
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