In prior studies of febrile seizures (FSs), prolonged FSs were defined, absent empirical evidence, as lasting 10 or 15 minutes or more. We assessed the distribution of FS duration in a cohort with first FSs, and the association between FS duration and baseline characteristics of the children.


We calculated the observed cumulative probability, S(t), that a FS would last at least t minutes, S(t) = exp(−t/τ). Data were also fit using a model obtained as the sum of 2 exponential distributions (S[t] = αexp[−t1] + [1 − α]exp[−t2]). After assessing the best fit, the cutoff defining long FS was determined. Logistic regression was used to examine associations between long FSs and baseline characteristics, behavior, and development.


In 158 children with a first FS, median duration was 4.0 minutes. Duration of FS was best fit by a 2-component mixture exponential model. Using this model, we identified 1 population that accounts for 82.3% of FSs and has a mean duration of 3.8 minutes (short FS) and a second population that accounts for 17.7% of FSs and has a mean duration of 39.8 minutes (long FS). Long FSs were significantly associated with developmental delay (p = 0.010) and delays and younger age at first FS (p = 0.048).


Like the distribution of afebrile seizure duration in children, the distribution of first FS duration is best modeled by assuming 2 populations. Developmental delay and younger age are associated with prolonged FSs. Our data lend further support to defining 10 minutes as the upper limit for a simple FS. ANN NEUROL 2011