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Subsequent afebrile seizures

Incidence. In hospital-based series rates of subsequent afebrile seizures and/or epilepsy
(defined as ‘recurrent’ afebrile seizures) have varied from 7% to 40%19. In the population-
based American NCPP the rate of epilepsy after febrile convulsions was 2% by seven years
of age9 and in the British CHES14 it was 2.5% by ten years. There is evidence that up to 85%
of afebrile seizures occur within four years of febrile convulsions19 but it seems that
determination of the true incidence of afebrile seizures requires long follow up. Annegers et
al2 found that the risk of ‘unprovoked seizures’ after febrile convulsions steadily increased
with age  2% at five years, 4.5% at ten years, 5.5% at 15 years and 7% by age 25. The UK
National General Practice Study of Epilepsy followed up children with febrile seizures for a
mean of 21.6 years and found that 6% developed epilepsy over the whole follow-up period.
The risk seemed to decrease with time52.

Predisposing factors for later afebrile seizures

Family history of epilepsy. The information from population-based studies is conflicting. The
NCPP10 found that a history of seizures without fever in a parent or prior-born sibling was
associated with a threefold increase in the rate of subsequent epilepsy after febrile
convulsions. However Annegers et al3 found only a weak association.

Age of onset of febrile convulsions. In the population-based NCPP9 there was an increased
rate of epilepsy by seven years of age in children whose febrile convulsions began in the first
year and especially in the first six months. However there was a tendency for abnormal
children to have convulsions early which might explain the increased risk of epilepsy in this
group. Annegers et al3 found that most of the increased rates associated with age were due to
confounding by complex features of the febrile convulsions.

Abnormal neurological or developmental status. In the NCPP9 children who had neurological
or developmental abnormality before the first febrile convulsion were three times more likely
to be epileptic by the age of seven years than those who were previously normal.

Characteristics of the febrile convulsions. Afebrile seizures occur with increased frequency
after convulsions that are ‘complicated’ or ‘complex’. In the American cohort study, the
NCPP9, the rate of spontaneous epilepsy, not preceded by febrile convulsions, was 5/1000;
after ‘pure’ febrile convulsions epilepsy developed in 15/1000 while after complex febrile
convulsions epilepsy developed in 41/1000. The outcome also varied according to the type
of complex febrile convulsion  when the first convulsion had prolonged, multiple or focal
features epilepsy developed in 31, 42 and 71/1000, respectively. The British CHES14 found
very similar results, as did Annegers et al3  who found that the risk of what they called
‘unprovoked seizures’ ranged from 2.4% among those who had simple febrile convulsions
to 68% for those with a single complex feature, 1722% with two complex features and
49% with all three complex features.

Recurrent episodes of febrile convulsions. There are reports that an increase in the number of
febrile recurrences is associated with an increased risk of later epilepsy19. However neither
the NCPP9 nor the Rochester Study3 found much evidence for this.

Type of afebrile seizure after febrile convulsions
As discussed above, some studies suggest that febrile convulsions can cause temporal lobe
damage and lead to afebrile complex partial seizures. Annegers et al3 did find that children
with febrile convulsions had a higher risk of later partial rather than generalised afebrile
(‘unprovoked’) seizures. The prognostic factors for partial and generalised seizures were