Elsevier

Epilepsy Research

Volume 85, Issue 1, July 2009, Pages 31-45
Epilepsy Research

Review
The descriptive epidemiology of epilepsy—A review

https://doi.org/10.1016/j.eplepsyres.2009.03.003Get rights and content

Summary

Epilepsy is a chronic disease experienced by millions and a cause of substantial morbidity and mortality. This review summarizes prevalence and incidence studies of epilepsy that provided a clear definition of epilepsy and could be age-adjusted: requirements if comparisons across studies are to be made. Although few exceptions, age-adjusted prevalence estimates from record-based studies (2.7–17.6 per 1000), are lower than those from door-to-door surveys (2.2–41.0 per 1000). Age-adjusted incidence ranged from 16 to 51 per 100,000, with one exception in Chile, where incidence was 111 per 100,000. Variation in reported prevalence and incidence may be related to factors such as access to health care, regional environmental exposures, or socioeconomic status. A higher proportion of epilepsy characterized by generalized seizures was reported in most prevalence studies. Epilepsy characterized by partial seizures accounted for 20–66% of incident epilepsies. Virtually all prevalence and incidence studies report a preponderance of seizures of unknown cause. Additional prevalence studies are needed in regions where data does not exist, and additional incidence studies in all regions. Interpretation of differences in prevalence and incidence will require understanding of the role of cultural, social and economic factors influencing epilepsy and its care.

Introduction

Epilepsy is one of the oldest conditions known to mankind (WHO, 2001a) and still the most common neurological condition affecting individuals of all ages. At any given time, it is estimated that 50 million individuals worldwide have a diagnosis of epilepsy (WHO, 2001b). However, the heavy burden of this disease is not evenly distributed, and according to available data, there are disparities in reported prevalence and incidence across the world. Many of the reported differences can be attributed to variations in study methodology (e.g., case definition, ascertainment) and population structure (e.g., age). Increased prevalence and incidence may be related to factors such as low socioeconomic status, limited access to health care and environmental exposures such as neurocysticercosis. Prevalence or incidence may be underestimated in areas where the condition is greatly stigmatized and cultural beliefs about the causes of epilepsy or negative attitudes toward those with epilepsy lead to the concealing of symptoms of epilepsy, or its diagnosis.

In this review, all primary population-based studies of prevalence and incidence of epilepsy published in English, and accessible through Medline since 1965 are examined. The PubMed database was searched using three keyword combinations: “epilepsy epidemiology” (n = 5314), “epilepsy incidence” (n = 6534) and “epilepsy prevalence” (n = 5749). Additional references were found using reference lists from selected studies (n = 57). Studies were then limited to total population studies that provided a clear definition of epilepsy and could be age-adjusted (n = 68; 48 prevalence and 20 incidence).

Epilepsy is defined as a condition characterized by recurrent (two or more) epileptic seizures, unprovoked by any immediate identified cause (Hauser and Kurland, 1975; ILAE, 1993). Multiple seizures occurring in a 24-h period or an episode of status epilepticus (SE) are considered a single event. Individuals who have had only febrile seizures or only neonatal seizures (seizures in the first 30 days of life), and people with acute symptomatic seizures (seizures associated with acute systemic illness, intoxication, substance abuse or withdrawal, or acute neurological insults), and individuals with a single unprovoked seizure, are excluded from this category.

In some studies, “epilepsy” is defined as the above plus those with a single unprovoked seizure, any afebrile seizure, or febrile seizures. For some studies using computerized record search alone, there is uncertainty as to the inclusion or exclusion of any of the above features. This review has focused on studies of epilepsy defined as recurrent unprovoked seizures. For completeness, studies using other definitions are also reviewed.

Prevalent epilepsy is defined as a diagnosis of epilepsy (recurrent unprovoked seizures) at some point prior to the prevalence period or date. An active prevalence case is one that continues to experience the burden of epilepsy based either on recency of seizure (generally in the year prior to the prevalence date or within 5 years of the prevalence date, depending on the study) and/or recency of anti-seizure medication use. Point prevalence reflects the number of cases of active epilepsy on prevalence day, divided by the total population under study on that prevalence day. In this review, prevalence is expressed as active cases per 1000 persons. Active prevalence differs from life-time prevalence, which includes those having a history of epilepsy regardless of recency of seizures or of use of anti-seizure medication.

Incidence of epilepsy is defined as the number of new cases of epilepsy over a specified time period. In this review, the incidence is the number of new cases per year divided by the average susceptible population under study during a specified time period and is expressed as new cases per 100,000 persons per year.

Since the incidence and prevalence of epilepsy vary with age, overall population incidence and prevalence cannot be compared unless the age structures of the populations are identical. Thus, we have only included studies that provide age-adjusted or age-specific estimates. For studies which provided age-specific estimates, we performed age-adjustment using the direct method to the 2000 US standard population (Health Statistics, 2006).

The two major categories of seizure type classified by the International League Against Epilepsy (ILAE) in 1981 are epilepsy characterized by partial seizures and epilepsy characterized by generalized seizures. Epilepsy characterized by partial (focal) seizures is that in which seizures begin in a local area of the brain. This seizure type is further subdivided into simple partial (no alteration in consciousness) and complex partial seizures (alteration of consciousness). Epilepsy characterized by generalized seizures may also be categorized as partial with secondary generalization if a clinical description of an antecedent symptom (aura), or a clear electroencephalographic signature of focality is indicated. Epilepsy characterized by generalized onset seizures conceptually involves the entire brain simultaneously. Individual generalized seizure types include absence, myoclonic, tonic–clonic, atonic, tonic, and clonic symptoms. In many studies, it is not clear that “generalized seizure” is synonymous with generalized onset seizure, leading to some ambiguity in classification by seizure type. Since some seizure types co-exist, it is assumed that the classifications represent the predominant seizure type. This is seldom clear, however, and in some cases, seizure type may remain unclassifiable.

Broad etiologic categories included idiopathic, symptomatic, and cryptogenic (ILAE, 1989). Idiopathic epilepsies are assumed to have a genetic basis and generally have onset during childhood. Symptomatic epilepsies typically follow an identified brain insult. For cryptogenic epilepsies, the cause of the epilepsy is unknown but many presume that a cause could be identified with sufficient investigation.

Section snippets

Methods of case ascertainment

The spectrum of methods of case ascertainment in population-based studies of epilepsy reviewed included community surveillance, community survey, key informants, record-based review and administrative database search. Most studies used combinations of these methodologies in order to capture and properly classify cases.

Surveillance methodology typically involved systematic identification of potential cases of epilepsy at multiple points of contact. Generally, a broad case definition for case

Prevalence in studies of epilepsy (recurrent unprovoked seizures)

Forty-eight population-based studies including individuals of all ages, and for which prevalence could be age-adjusted, were identified for this review. Of these studies, 29 used door-to-door survey methodology (Table 1), 14 studies used record-based methodology (Table 2), and 5 used self-report or computerized database methodology (Table 3).

Incidence in studies of epilepsy, all unprovoked seizures and all afebrile seizures

While prevalence studies are useful in estimating the burden of disease, and making economic and other public health predictions, incidence studies provide a better understanding of the etiology and natural history of epilepsy. Due to the difficulty and expense of identifying an incidence cohort, incidence studies are conducted less often than prevalence studies.

Discussion

Findings from studies are most comparable when the definition of epilepsy is the same and population age structure differences are minimized through age-adjustment. When comparing age-adjusted estimates of studies using the same definition of epilepsy in this review, overall prevalence and incidence of epilepsy tended to be lower in developed regions (United States and Europe) in comparison to developing regions (Latin America and Africa), with Asia reporting the lowest frequency of epilepsy.

Acknowledgement

Supported in part by T32 NS 007153.

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