Factors that predict recurrence of a seizure during a subsequent fever episode were studied in 230 children included in a randomized placebo controlled trial of ibuprofen prophylaxis treatment at the Sophia and Juliana Children's Hospitals, Rotterdam, the Netherlands. There were 67 febrile seizure (FS) recurrences in a total of 509 fever episodes; 35 (52%) occurred within 2 hours of fever onset and 32 (48%) after more than 2 hours of fever. The median temperature with recurrences early in the fever episode was lower (39.3°C) than that registered during delayed FS recurrences (40.0°C). The risk of FS recurrence increases with temperature at fever of onset and during the febrile episode and decreases with age. A child aged 1 year with a 30% recurrence risk has a 5 to 10% risk at 4 years, .
COMMENT. Recurrent febrile seizures occur within 2 hours of onset of a fever episode in 50% of cases. Seizures recurring after 2 hours of fever are associated with higher temperature levels than those within the first 2 hours of fever. The risk of FS recurrence with subsequent fever decreases with age and increases with temperature at fever onset and during fever episodes.
A threshold convulsive temperature, suggested by this report, was first demonstrated in laboratory and clinical studies in Jan 1959, exactly forty years ago [2, 3]. The rapidity of rise of body temperature, previously proposed as an essential mechanism for febrile seizures, was non-contributory. Factors found to modify the threshold convulsive temperature include age, brain maturation, electrolyte imbalance, and various medications. A postictal elevation in seizure threshold requires a relatively higher body temperature to induce a second seizure.