In 2011, ILAE published new guidelines for epidemiological research on epilepsy (39), because substantially differing methodologies had made comparisons of studies difficult (6, 7). Epidemiological studies commonly define epilepsy as a minimum of two unprovoked epileptic seizures during a period of more than 24 hours (39, 51). This remained ILAE’s definition until 2014, when it was expanded to also include a single unprovoked seizure if it is part of an epilepsy syndrome or if the risk of recurring seizures is estimated to exceed 60 % (50).
Studies that include single unprovoked and/or acute symptomatic seizures will naturally result in a higher prevalence of epilepsy than studies that have deemed such seizures as not qualifying for the diagnosis.
According to the ILAE definition, active epilepsy is a condition involving ongoing treatment with antiepileptic medication and/or at least one epileptic seizure over a specified period of time, commonly the last 2 – 5 years. It is recommended that epidemiological studies restrict their reporting to active epilepsy (39, 51). However, there is still room for some discrepancy – the shorter the period that has elapsed since the last seizure, the more patients will be excluded from the group with active epilepsy. For those who have been seizure-free for more than ten years and have gone without antiepileptic medication for more than five years, the ILAE has introduced the concept of «resolved epilepsy» (50). This means that their epilepsy is in remission, not that the disease has been cured. This concept has not yet been applied in epidemiological studies.
Another cause of substantially different findings is the method used for identification of patients. No method is totally watertight, and recommending a single specific procedure is therefore difficult. Studies that use a population-based questionnaire may have limitations caused by low response rates and selection bias. Moreover, such studies may for practical reasons be restricted to smaller populations or delimited age groups, as are door-to-door surveys.
Studies based on registered diagnostic codes may provide for larger study populations, although there is a risk of overestimating the prevalence of the disease. In addition, it is difficult to restrict such studies to active epilepsy. A study from Denmark showed that approximately 20 % of all patients who were registered with an epilepsy code did not fulfil ILAE’s definition of epilepsy (52). In our recent study from Buskerud county we made similar findings (13). A retrospective review of patient records to validate the diagnosis would naturally increase the specificity, but this is conditional on the correctness of the information in the records and that the registrations have been made by competent personnel.
Age variation in the study population is another problem. Only a handful of studies from the Nordic countries include all age groups. Studies that are limited to a specific age group can only with difficulty be extrapolated to the entire population, since it has previously been proven that there is an elevated prevalence of epilepsy in the oldest section of the population (12). This trend is also evident in the material we have collected for this article. In studies that have investigated the prevalence of active epilepsy in children and adolescents (0 – 19 years), the prevalence rate varies from 3.2 to 4.1 per 1 000 (14) – (20), while the prevalence of active epilepsy among adults is higher (5.5 – 8.2/1 000) (22) – (25). In the Nordic countries, no isolated studies of prevalence among the elderly have been made, nor has this been investigated in the adolescent group.
As regards incidence, there are too few studies in our material and their results are too divergent to permit any conclusions regarding the age distribution. It is known, however, that the incidence curve for epilepsy is double-humped, with a peak early in life and a new increase in the oldest age groups (53).
Unfortunately, some stigma remains attached to the diagnosis of epilepsy (54). A wish to keep this diagnosis secret on the part of some people in certain communities or age groups may cause underreporting. This source of error may perhaps be greater in older studies, and perhaps also in studies undertaken in rural areas.