In many cases, the febrile seizure is the first clear symptom of illness. The first febrile seizure is complex in approximately 25% of cases. About 75% of febrile convulsions are “simple.” In the National Collaborative Perinatal Project study of 55,000 infants, 1706 experienced a first febrile seizure and were followed to 7 years of age. Twenty-eight percent of the initial febrile seizures were “complex”: 4% focal, 8% prolonged greater than 15 minutes, and 16% with recurrence within 24 hours. A Todd paresis (transient focal post-ictal weakness) occurred in 0.4% (Nelson and Ellenberg 1978). In another prospective cohort study of first febrile seizures, 35% of 428 children had 1 or more features of a complex febrile seizure (Berg and Shinnar 1996). A retrospective study from Singapore reported similar findings (Lee and Ong 2004).
There is a bimodal distribution of febrile seizure duration: most are brief (82% lasting a mean of 3.8 minutes), whereas the remaining 18% constitute a seemingly separate population whose seizures last a mean of 39.8 minutes (Hesdorffer et al 2011). Prolonged febrile seizures are relatively uncommon, but concern about subsequent development of mesial temporal sclerosis and epilepsy prompted a multicenter study of the phenomenology and imaging findings in 199 affected children (median age 16 months; interquartile range 12 to 24 months). Cases were collected in several emergency departments, and they had a median status epilepticus duration of 70 minutes. Most of these febrile status epilepticus episodes were focal at onset (68%) and nearly all (98%) were convulsive seizures (Hesdorffer et al 2012). Prolonged postictal unconsciousness (greater than 30 minutes), although rare, has been associated with febrile seizures that are focal or last longer than 5 minutes (Okumura et al 2004).