The cause of a febrile convulsion, is, as the name implies, a fever or high temperature. Any of the common childhood infections such as chickenpox, tonsillitis, upper respiratory, ear, bowel or urinary infections may cause a high temperature and therefore cause a febrile convulsion. It is unclear whether it is how quickly the temperature rises, or how high it eventually gets which determines whether a convulsion will occur. Lots of children between 6 months and 5 years of age have febrile illnesses but obviously the majority will not have a convulsion. One of the reasons why some children do, and others do not have convulsions with fever, is because of inherited factors which are important in determining whether febrile convulsions will occur. Almost one third of children will be found to have a family history of febrile convulsions in their parents or siblings (brothers and sisters). When one parent has a history of febrile convulsions, the risk to a child of developing a febrile convulsion is almost 20 per cent; if both parents have a history, then the risk is increased to 50 per cent. The brothers and sisters of a child who has had a febrile convulsion have a three times increased risk of having a febrile convulsion themselves; this risk is even higher in identical twins.
Most children who have febrile convulsions do not need any tests. Usually the cause of the infection and of the fever is obvious from the examination carried out by the doctor—for example, a sore throat (tonsillitis), red ear (otitis media), rash (for example, chickenpox), or cold and cough. Rarely, however, and particularly in children under 18 months of age, a convulsion may be the first sign of meningitis or encephalitis (group (c) above). If there is any doubt as to whether a child has meningitis (particularly in children aged 6-18 months), then a lumbar puncture must be done, and other tests may well be required. Children with simple febrile convulsions do not need to have an EEG or brain scan. However, children with complex febrile convulsions (group (b)) may well need them in order to explore what is the underlying cause of their asymmetrical or prolonged convulsion, or earlier slow development.