The risk of unprovoked seizures after encephalitis and meningitis.

A population-based cohort of 714 survivors of encephalitis or meningitis between 1935 and 1981 was followed in order to evaluate the risks of unprovoked seizures after CNS infections. The 20-year risk of developing unprovoked seizures was 6.8%, and the ratio of observed to expected cases of unprovoked seizures was 6.9. The increased incidence of unprovoked seizures was highest during the first 5 years after the CNS infection but remained elevated over the next 15 years of follow-up. The type of CNS infection and the presence or absence of seizures during the acute phase of the CNS infection greatly influenced the risks of subsequent unprovoked seizures. The 20-year risk of developing unprovoked seizures was 22% for patients with viral encephalitis and early seizures, 10% for patients with viral encephalitis without early seizures, 13% for patients with bacterial meningitis and early seizures, and 2.4% for patients with bacterial meningitis without early seizures. The 20-year risk of 2.1% for patients with aseptic meningitis was not increased over the general population incidence of unprovoked seizures.