Chapter 6 - The epidemiology of the epilepsies
Introduction
Epilepsy is the commonest neurological condition affecting people of all ages, races, and social classes. Worldwide it is estimated that over 50 million people have epilepsy, of whom three-quarters live in resource-poor countries with little or no access to medical services and treatment (Meinardi et al., 2001, Ngugi et al., 2010).
Epidemiology is the study of health determinants, and the distribution and natural history of disease in populations. Epidemiological studies can be descriptive, analytical, or experimental. Descriptive epidemiology deals with the distribution and vital statistics (incidence and prevalence) of a condition in a population, without regard to causation, and usually involves observational studies. Analytical epidemiology tests hypotheses, for example examining risk factors for a disease, often using cohort and case–control studies. Experimental epidemiology consists of studies in which relevant risk factors are controlled. Epidemiological studies in epilepsy are mostly descriptive or analytical (Sander, 2003).
Until the 1960s, epidemiological studies in epilepsy were carried out in tertiary referral centers; this suggested that epilepsy was a chronic, progressive, incurable condition with little chance of remission (Sander, 2003).
Methodological differences, lack of standardized classification, problems with case ascertainment and diagnostic accuracy, as well as the heterogeneous nature of epilepsy, result in disparity in study findings.
Diagnostic accuracy is a particular problem in epilepsy as seizures are a symptom of diverse underlying cerebral etiologies and may not have permanent physical manifestations (Sander, 2003). The diagnosis of epilepsy is dependent on the chance recording of an event, the availability of a detailed eye-witness account (which may be misleading; Rugg-Gunn et al., 2001), and the expertise and experience of the diagnosing specialist, with opinions differing in ambiguous cases (van Donselaar et al., 1989, Stroink et al., 2004). Consequently a definitive diagnosis is often made only after an extended period of follow-up (Hauser and Kurland, 1975, Sander et al., 1990) Moreover, 20–30% of those attending tertiary referral centers with refractory epilepsy do not have epilepsy (Smith et al., 1999); the most common differential diagnoses are dissociative seizures and syncope. Neurologists are better at diagnosing epilepsy than nonspecialists (misdiagnosis rate 5.6% versus 18.9%) (Leach et al., 2005), but at least 5% of diagnoses may be wrong.
People with epilepsy may not seek medical attention, through either ignorance or lack of awareness of the symptoms. Some absence and complex partial seizures may be recognized only in retrospect following a convulsion (Hauser et al., 1990). One study found that only 20% of patients with seizures suspected the diagnosis prior to consultation (Hopkins and Scambler, 1977). A community study using a door-to-door survey found that one-quarter of people with seizures had not sought medical attention (Zielinski, 1974a).
Fear of stigmatization may also cause concealment. This may be a particular problem in certain communities; in one study, the low prevalence rate was interpreted as being the result of concealment (Rocca et al., 2001). In another study, 10% of those with a confirmed diagnosis of epilepsy denied having seizures (Beran et al., 1985).
Thus, epidemiological studies of epilepsy should be community-based, prospective (to identify risk factors), and independent of existing medical records (or supplemented by use of door-to-door surveys, questionnaires, or interview) to allow as complete as possible case ascertainment.
Section snippets
Definitions
To allow meaningful comparisons, the same definition of epilepsy and a standardized classification of seizures need to be applied across all studies. The International League Against Epilepsy (ILAE) (Commission on Epidemiology and Prognosis of the ILAE, 1993) provides guidelines for epidemiological studies in epilepsy. Epilepsy is defined as a condition characterized by recurrent (two or more) seizures that are unprovoked by any identifiable cause. Febrile seizures and neonatal seizures are
Incidence studies
Incidence is defined as the number of new cases of epilepsy or seizures in a well-defined population during a specified time period (usually 1 year) and is normally expressed as the number of cases per 100 000 people in the population per year. Incidence studies are important for identifying risk factors as well as for providing information on prognosis. Most incidence studies have been conducted in developed countries and are retrospective, although some recent studies have been population-based
Prevalence
Prevalence is defined as the number of people with a disease in a defined population as a proportion of the total population and is usually expressed as the number of cases per 1000 population. Point prevalence is the number of existing cases in the population at a particular point in time, whereas period prevalence is the prevalence over a particular interval (usually 1 year). Information about a disease's prevalence is important for service provision and planning. Prevalence studies use
Risk factors for epilepsy
The etiology of any chronic condition is best determined by use of population-based studies of newly diagnosed cases. Prevalence studies of active epilepsy will miss cases at both ends of the severity spectrum (severe cases with causes associated with increased mortality and more benign cases with early remission). Epilepsy is divided into different categories according to causation: remote symptomatic, progressive symptomatic, idiopathic (presumed genetic), and cryptogenic or unknown.
In
Prognosis
For patients with seizures, the prognosis is the risk of further seizures after a single unprovoked seizure, or the likelihood of achieving seizure freedom after a pattern of recurring seizures has been established (Sander, 1993). Various aspects of the prognosis of epilepsy can be considered: the likelihood of recurrence following a single seizure; the impact of early versus late treatment; the probability of relapse after prolonged remission, following epilepsy surgery, or following
Mortality
People diagnosed with epilepsy need to know whether this will have any impact on their life expectancy. It is accepted that people with epilepsy have a 2–3-fold increased risk of premature death compared with the general population. It has consistently been shown that people with neurological deficits or symptomatic seizures have a significantly increased risk of premature death, whereas the risk for those with idiopathic or cryptogenic epilepsy is broadly similar to that of the general
Conclusion
Further epidemiological studies should be prospective population-based incident cohort studies, focusing on outcome and possible temporal changes of the incidence of different epileptic syndromes in defined populations. Furthermore, research should focus on differences (real or perceived) between people of different ethnicity and social background. This may, in turn, lead to the identification of inherent risk factors in particular subpopulations for the subsequent development of epilepsy.
The
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