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Febrile convulsions that did not have complex features were simple. Other studies have
adopted very similar definitions, however Hesdorffer et al found that the distribution of first
febrile seizure duration was best modelled by assuming two populations and their data
suggested that ten minutes should be the upper limit for a simple febrile seizure22.

Febrile status epilepticus
Defined as a febrile convulsion lasting 30 minutes or more or a series of febrile convulsions
without full return to consciousness during that period.

Incidence, prevalence and recurrence

Overall rates
Between 2 and 4% of all children have one or more febrile convulsions by the age of five
years. Some studies find higher rates in boys than in girls but others do not. In America
Nelson and Ellenberg10 reported racial differences  the prevalence rates being 3.5% of white
and 4.2% of black children. There are geographical differences  e.g. a prevalence of 8.3%
by three years of age in Tokyo23 and an incidence rate of 6.9% at age four years in Finland24.

Age
Febrile convulsions most commonly start in the second year of life. Children are at greatest
risk between six months and three years of age19. The age of onset has been reported to vary
between two months of age and seven years nine months14.

Type of febrile convulsion
Population-based studies that include children who are not admitted to hospital have found
that the following proportions of first febrile convulsions are complex  18% in America9,
22% in Britain14 and 8.6% in Scandinavia5.

Febrile recurrences
‘Recurrence’ in this context means more than one episode of febrile convulsions, as opposed
to ‘multiple’ which means more than one convulsion during an episode of fever. Berg et al25
performed a meta-analysis and found that the overall risk of a recurrence was 34.3%. Young
age at onset (one year or less) and a family history of febrile seizures predicted increased risk.
Focal, prolonged and multiple convulsions were only associated with a small increase. Other
studies have found similar results. Most recurrences occur within three years of the first19.

Aetiology

Genetic factors
There is an expanding literature on the genetics of febrile convulsions. Population-based
studies suggest that family history is important and that febrile convulsions and epilepsy each
provide an independent contribution to the familial risk of febrile convulsions26,27. Forsgren6
concluded that multifactorial inheritance was most likely. However family studies have
shown that simple febrile seizures may be inherited as an autosomal dominant trait with high
penetrance28 and also show an occurrence rate ranging from 10% to 46% in children with a
positive family history of febrile convulsions29. It seems clear that febrile convulsions make
up an extremely heterogeneous group for which there is no single mode of inheritance.
Causative genes have not been identified in most patients with febrile convulsions; however
population-based studies have shown at least one positive association with febrile
convulsions for 14 of 41 investigated genes29. Mutations in the voltage-gated sodium channel
alpha-1, alpha-2 and beta-1 subunit genes (SCN1A, SCN2A and SCN1B) and the GABA(A)
receptor gamma-2 subunit gene (GABRG2) have been identified in families with
‘generalised epilepsy with febrile seizures plus’ (GEFS+)30. Patients with GEFS+ can have
febrile seizures followed by afebrile (often generalised) seizures31. There is evidence that the