Epilepsy is the most common serious neurological condition, affecting over 60 million people worldwide; the majority of these people live in countries where they have limited access to treatment.

Epidemiology is concerned with the natural history and distribution of disease and determinants of health in populations. Epidemiological studies can be characterized into three broad categories: descriptive, analytical, and experimental (Table 4.1). In epilepsy, epidemiological research tends to be primarily descriptive (frequencies) and analytical (risk factors), with little experimental research.

It was long believed that epilepsy was a chronic progressive condition with little chance of recovery. Such a view was succinctly expressed by 19th-century neurological luminary William Gowers, who wrote that “The spontaneous cessation of the disease is an event too rare to be reasonably anticipated in any given case.” Such a pessimistic viewpoint was routinely upheld until the early 1980s, when studies from sources other than tertiary referral centers challenged it.

Basic requirements in epidemiology include diagnostic accuracy, complete case ascertainment, and the need to control or minimize selection bias in the population under review. For instance, the fact that a negative understanding of the prognosis of epilepsy (where 70–80% of people in a cohort can be reasonably expected to achieve long remission) was maintained for so long underlies the importance of controlling for selection bias in study populations.

Many epidemiological studies from both developed and resource-poor countries have been published, but methodical differences, lack of standardized classifications, difficulties with case ascertainment, and diagnostic uncertainty have led to disparities in study findings. The International League Against Epilepsy (ILAE) has published several reports, most recently in 2011, attempting to standardize research definitions so that direct comparison of results from different studies can be made.

Case ascertainment and diagnostic accuracy are particular issues in epilepsy, as seizures can be a manifestation or symptoms of multiple etiologies, emphasizing the heterogeneous nature of the condition. Nevertheless, the majority of people with epilepsy will not have any overt physical manifestations between seizures. Ultimately, the diagnosis of epilepsy, which is normally made after two or more unprovoked seizures, is dependent on the clinical history, detailed eye-witness accounts (which can be notoriously unreliable), the chance witnessing or recording of an event, and the diagnostic skill and experience of the clinician. Given these difficulties, it is not surprising that a significant proportion of people referred to tertiary centers with chronic ^{epilepsy do not, in fact, have epilepsy (the most common alternative diagnoses being psychogenic nonepileptic attacks and syncopal episodes). Many people with epilepsy do not seek medical attention; this may be due to ignorance, lack of awareness of the symptoms, or fear of stigmatization. Indeed, in many cases absence and complex partial seizures are only recognized in retrospect following presentation with a generalized seizure. Some people with a confirmed diagnosis of epilepsy may deny the fact for fear of the social and legal implications of such a diagnosis. For these reasons, door-to-door surveys in large populations, using standardized screening tools, are considered the gold-standard methodology for epidemiological research. For logistical reasons, these are, however, easier in theory than in practice.}

Table 4.1. Different streams of epidemiological research.^a

^aEpilepsy epidemiological research is predominantly descriptive or analytical.

Incidence and prevalence

There are two main types of disease frequency that are used in epidemiological studies: the incidence and the prevalence (Table 4.2).

Incidence refers to the number of new cases in the population and is estimated using cohort or closed population studies, where a defined population (e.g., the population of a region) is followed up over a year and the number of new cases in the population is identified. Prevalence refers to the number of people with the condition in a population at one time; prevalence studies are carried out using cross-sectional studies that are much easier and less expensive to perform than incidence studies. Consequently, there are many more epilepsy prevalence studies than incidence studies from both developed and resource-poor countries. Prevalence studies are important for healthcare planning and service provision (Table 4.3).

Table 4.2. Different epidemiological terms to measure disease frequency.

Table 4.3. Reported incidence rates by region.

Incidence studies

Most incidence epilepsy studies have been retrospective and carried out in developed countries, although more recently, prospective community-based incidence studies have been performed. The incidence of epilepsy in the developed world is remarkably consistent across different countries, with a median incidence rate of around 50 (range 40–70) per 100,000 per year. In a recent systematic review of 33 studies, 9 of which were from low-or low-middle–income counties, the median annual incidence was 50.4 per 100,000. The median incidence was 45.0 per 100,000 in high-income countries and 81.7 per 100,000 in low- and low-middle–income countries. In contrast, the incidence for single unprovoked seizures is probably between 50 and 70 per 100,000 per year; however, this figure is likely to be much higher in resource-poor countries.

Incidence studies from resource-poor countries are far less common, with much greater variation in reported rates between regions and countries. In the systematic review, the median incidence rate from the seven studies from resource-poor countries was 68.7 per 100,000 (compared to 43.4 per 100,000 for developed countries). Reported incidence rates are highest in South America and Sub-Saharan Africa (where there is great variation in rates between and even within countries). In contrast, reported incidence rates in Asia (India and China) are more in line with figures from developed countries.

Most incidence studies demonstrate a slighter higher rate in males, although this difference is rarely statistically significant. In the systematic review, no significant differences were seen between males and females. In a US study, the cumulative incidence of epilepsy was 1.2% by age 24 years, 3% by age 75, and 4.4% by age 80. Later studies from Scandinavia have corroborated these findings.

Most studies have not demonstrated any significant difference in ethnicity-specific incidence rates; any differences seen are largely attributable to differences in socioeconomic circumstances. Indeed, there is a strong association between age-adjusted incidence rates and socioeconomic deprivation, which was demonstrated in a study from London, where the age-adjusted incidence rate of epilepsy was 2.3 times as high in the most socially deprived fifth of the population as in the least deprived fifth.

Incidence studies from developed countries typically show a bimodal age distribution, with a very high incidence in infancy and early childhood and a subsequent decrease in adolescence and earlier adulthood, then a steady increase after the age of 50. While most studies have shown that the overall incidence of epilepsy has not changed appreciably over the past 20–30 years, there has been a decrease in the incidence in children, with a corresponding increase in the elderly (particularly those aged above 80) to the point that the elderly are now the most common age group with newly diagnosed epilepsy.

One area where there is a dearth of studies is in the incidence of epileptic syndromes, in part due to the difficulty in utilizing such a classification in epidemiological field studies. One of the few studies published is a population study from Iceland, where localization-related epilepsy predominated (18.6 per 100,000 person-years), with essentially equal figures for symptomatic and cryptogenic epilepsies. The incidence of isolated unprovoked seizures or status epilepticus was 22.8 per 100,000 person-years. Much more work is needed to accurately determine the epidemiology of the epileptic syndromes.

Prevalence studies

Studies from developed countries typically give a prevalence of active epilepsy of between 4 and 10 per 1000, with most having a prevalence of 4–7 per 1000. Findings from prevalence studies reinforce those from incidence studies with respect to gender, ethnicity, the impact of socioeconomic factors, and age-specific variations. Some of the reported variation in prevalence rates, particularly with regard to active epilepsy, relates to the heterogeneous range of definitions employed and underlines the need for the use of standardized definitions in epidemiological research.

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