Socioeconomic variation in incidence of epilepsy: prospective community based study in south east England

Introduction

Epilepsy is associated with a wide range of markers of social and economic disadvantage, including poor academic achievement, unemployment, underemployment, and low income.1^–4 Because of this association it is often assumed that people who are socially and economically deprived are more likely to develop epilepsy. This hypothesis is supported to some extent by the observation that the incidence of epilepsy is higher in developing countries than in developed countries.5

A few epidemiological studies have confirmed an association between the prevalence of epilepsy and markers of social disadvantage.6 Prevalence studies, however, cannot establish the direction of causality, and the employment problems and social disadvantage experienced by people with epilepsy may cause downward social “drift.”7 The association between socioeconomic factors and incident epilepsy is poorly understood8 and to date has not been examined within the general community with methods that prospectively ascertain cases.

The NHS and the World Health Organization aim to reduce inequalities in health. 9 10 This can be achieved by concentrating resources on conditions that affect socially and economically deprived people. Understanding of the role that deprivation has in epilepsy gives insight into its aetiology and management. We determined the incidence of epilepsy in an unselected community based population and its variation with socioeconomic deprivation.

Methods

Over an 18 or 24 month period we prospectively ascertained all incident cases of epilepsy in an unselected community population served by eight general practices in London (86 989 person years) and 12 practices outside London (282 294 person years). For these practices we were sole providers of secondary care for seizure disorders.

We advertised the linkage scheme to all general practices within the region. Practices that were willing to cooperate were selected if their patient details were stored on a computerised database. The follow up time differed because of researcher funding.

The practices outside London were all in south east England. Epilepsy was defined as the occurrence of one or more unprovoked seizure. We excluded provoked seizures, acute symptomatic seizures, and febrile convulsions.

We used a range of methods to identify cases of epilepsy, including a fast track clinic and active surveillance. An audit that involved a systematic search of all individual primary care records was performed at the end of the study period. This was 24 months (1 June 1995 to 31 May 1997) in 12 practices and 18 months (1 January 1995 to 30 June 1997) in the eight remaining practices. The methods by which cases were ascertained have been fully described previously.11

For each patient we collected clinical and demographic data, including postcode. Data were anonymised before analysis. The postcodes were used to assign to each individual a Carstairs score of social deprivation for the enumeration district in which he or she lived. An enumeration district on average contains 140 households and is the smallest area for which census data are available. The Carstairs score is an index of deprivation based on four variables available from the 1991 census: overcrowding, social class of head of household, car ownership, and unemployment.12 The distribution of the scores was banded into fifths, with the lowest fifth denoting the least deprived and the highest fifth the most deprived.

Results

We identified 268 new cases of seizures during 369 283 person years of observation (see table A on bmj.com). We excluded 78 cases of provoked or acute seizures and febrile convulsions. The 190 remaining cases were included in the analysis, giving a crude incidence rate of 51.5 (95% confidence interval 44.4 to 59.3) per 100 000 per year.

We found a strong relation between incidence of epilepsy and age. The incidence was highest between 0 and 4 years and lowest between 45 and 64 years (table 1). Males and females had similar incidence of epilepsy. We observed a steep rise in incidence with socioeconomic deprivation (tables 1 and 2, figure). The incidence, adjusted for age and sex, in the most deprived fifth of the study population was 2.33 (1.46 to 3.72) times that in the least deprived fifth (P=0.001 for trend). There were similar socioeconomic gradients in the 0-14, 15-64, and 65 age groups (figure).

Age and sex adjusted rate ratios by fifth of Carstairs deprivation score

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Populations served by London practices, however, were more deprived on average than those outside London. When we considered mean Carstairs scores of the nine most deprived practice populations, eight were based in London and the overlap in deprivation scores of individuals in London and non-London practices was small (see table A on bmj.com). When we made an additional adjustment for area, the association between epilepsy incidence and deprivation fifth was weakened and was not significant at the 5% level (table 2).

Discussion

This prospective study based on an unselected population represents the first to examine socioeconomic status as a risk factor for the development of epilepsy. The overall incidence rates obtained are comparable with those from previous epidemiological studies of incidence in the United Kingdom.5

Our main observation was the relation between the incidence of epilepsy and Carstairs deprivation score. However, interpretation of this apparent association is complicated by the fact that the main contrasts in deprivation were those between the populations in London and outside London. Indeed, in the multivariable analyses, when we made additional adjustment for area (London versus outside London) the strong gradient with deprivation was somewhat weakened—though the broad pattern remained—and the association was no longer significant (P=0.12 for trend). The question then is whether the observed deprivation gradient represents a “cause and effect” association or whether it is a spurious (confounded) association generated by a London versus outside London difference in some factor that affects incidence or case ascertainment, or both.