Background: Little is known about the relative importance of genes and early environment in the etiology of febrile convulsions.
Methods: We performed a follow-up study using data from two nationwide registers in Denmark, 1980-1998. The study population comprised 10,224 younger siblings of children who had had febrile convulsions, and 21,218 younger siblings of children who had never been hospitalized with febrile convulsions.
Results: The study provides three main findings. First, if a previous child had had a febrile convulsion, the risk was lower for the next child if either parent changed partners. Compared with full-siblings, the hazard ratio (HR) of febrile convulsions was 0.6 for paternal half-siblings and 0.7 for maternal half-siblings. In contrast, if there was no history of febrile convulsions in the previous child, a change in partner was associated with a slight increase in risk (1.2 among paternal half-siblings and 1.3 among maternal half-siblings). Secondly, the risk of febrile convulsion was strongly associated with the number of hospitalizations for febrile convulsions experienced by the older siblings, with a doubling of risk among those whose older sibling had had three or more hospitalizations. Thirdly, the risk of febrile convulsions increased with decreasing gestational age, birth weight, and birth weight ratio regardless of family history.
Conclusions: Our data suggest that the etiology of febrile convulsions depends on a genetic susceptibility that can be transmitted through both parents, and corroborates the hypothesis that multiple febrile convulsions may constitute a separate etiological entity.