Risk of Epilepsy in Patients with Multiple Sclerosis: A Population‐Based Study in Iceland

Summary: Purpose: Several clinical series reported an association between multiple sclerosis (MS) and epilepsy. We conducted a total population study in Iceland to determine the risk for developing epilepsy in patients with MS compared with that expected in the general population. Methods: Medical records of the 188 incidence cases of clinically definite MS first diagnosed in Iceland during the 25-year study period (1965–1989) were reviewed. The cases were followed up through 1994 or until death to identify those developing seizures or epilepsy. The expected number of cases with epilepsy in the MS-incidence cohort were calculated based on the age-specific incidence for epilepsy in Iceland and the age-specific person years of follow-up in the MS cohort. Results: During the 2,771 person years of observation after diagnosis of clinically definite MS, three MS patients developed epilepsy. One additional case developed epilepsy after onset of MS symptoms but before diagnosis of MS. The cumulative incidence of epilepsy by 10 years after diagnosis of MS was 1.9%. Given the age-specific person years of follow-up after diagnosis of MS, only one case of epilepsy would have been expected; standardized incidence ratio (SIR), 3.0 (95% confidence interval (CI), 0.6–8.8). Conclusions: MS is a risk factor for developing epilepsy. Patients with MS have a threefold increase in risk for developing epilepsy when compared with that expected in the general population. The reason for this increased risk is unclear and needs further investigation.