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Summary: Purpose: Two earlier population-based studies provide conflicting information on the association between low socioeconomic status (SES) and risk for epilepsy. Seizure etiologies (e.g., head injury, stroke) associated with low SES were not addressed in prior analyses. We assess the relation between SES indices and incident epilepsy separately for children and adults and in subgroups defined by seizure etiology.
Methods: In this population-based case–control study, a surveillance system identified incident unprovoked seizure or first diagnosis of epilepsy throughout Iceland (n = 418). Controls were selected from the population registry as the next two same-sex births alive, residing in Iceland at the time of the index seizure, and without a history of unprovoked seizure on the date of the case's incident seizure (n = 835). The odds ratio measured the association between SES and epilepsy.
Results: An association was found between epilepsy and SES among adults, but not among children. Among adults, low education was associated with an increased risk for epilepsy [odds ratio (OR), 2.29; 95% confidence interval (CI), 1.21–4.34), and home ownership was protective (OR, 0.63; 95% CI, 0.43–0.92). When analyses were repeated by seizure etiology, this association remained only in the group with epilepsy of unknown cause, even after adjusting for alcohol consumption.
Conclusions: Low SES, indexed by low education or lack of home ownership, is a risk factor for epilepsy in adults, but not in children, suggesting a cumulative effect of SES on risk for epilepsy. This association is not explained by established risk factors for epilepsy (e.g., head injury, stroke). We find no evidence of a downward social drift among cases whose parents had epilepsy.
Socioeconomic Status (SES) describes the distribution of income, education, occupation, and social class. Low SES is associated with increased prevalence of epilepsy (1,2). Such cross-sectional studies do not clarify whether low SES is a risk factor for epilepsy or a consequence of the disorder. Two previous community-based studies of SES and risk for epilepsy are conflicting (3,4). An incident case–control study found no difference in SES between cases and controls (3). A prospective study, using a composite measure of SES, concluded that low SES is a risk factor for the development of epilepsy (4).
Indices of low SES are associated with many established risk factors for epilepsy, including cerebrovascular disease (5,6), head trauma (7), congenital malformations (8), central nervous system infection (meningitis, encephalitis) (9), alcohol intake or abuse (10), brain neoplasms (11), and Alzheimer's disease (12). Previously observed associations between epilepsy and low SES could be limited to cases with these symptomatic etiologies. We address the question of whether an association exists between SES and a first diagnosis of unprovoked seizure or epilepsy, and if so, whether this association exists for all etiologic subtypes of unprovoked seizure (13) and is consistent for adults and children.
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We found no association between SES and incident unprovoked seizure or epilepsy among children. Among adults, low SES indexed by low education was associated with an increased risk for epilepsy, and high SES indexed by home ownership was protective for the development of epilepsy. The risk for epilepsy increased with low education, and the risk for epilepsy decreased with home ownership for all adult cases and controls, and for the subgroup with idiopathic/cryptogenic etiology. None of the SES indices were differentially associated with epilepsy in those with remote symptomatic or with progressive symptomatic etiology; however, numbers were small in these subgroups. Downward social drift did not seem to explain the results, because the SES of cases whose parents had epilepsy did not differ from that of cases whose parents did not; however, information on the SES of parents of adult participants was unavailable.
One prior prospective study found an association between low SES and epilepsy (4). This study compared the incidence of epilepsy by categories of Carstairs score, a composite based on overcrowding, social class of the head of the household, car ownership, and unemployment. The incidence of epilepsy in the lowest fifth of the Carstairs scores was 2.3-fold greater than that in the highest fifth (4). Our results are consistent with these findings, but suggest that the effect of low SES may be limited to unprovoked seizures of unknown etiology.
Major strengths of our study are its individual ascertainment of SES and population-based design. This design avoids the potential effects of selection bias. A weakness of the study is that the association between SES and epilepsy is investigated in an egalitarian society, potentially leading to an underestimate of the effect of low SES on the risk for epilepsy.
Iceland is a society in which universal health care and state assistance programs remove much of the association between SES and access to health care. Nonetheless, associations between SES and health status exist worldwide (14) and are found in Scandinavian countries, which are similar to Iceland (14,16,18). In light of universal access to healthcare in Iceland, the association between SES and epilepsy in adults, but not in children, is notable. This absence of an association in children may be real or due to the narrow range of SES in Iceland, masking an effect that truly exists. It is difficult to determine which explanation is more likely. However, our control group appears to be representative of the Icelandic population, because the distribution of family income in our control children is very similar to that reported in a study of a representative sample of 3,007 Icelandic school children, aged 2 to 17 years (19). Other studies of Icelandic children report associations between low SES and ill health (19), high SES and increased leisure time physical activity (22), and between SES and headache (23), suggesting that the absence of an effect of SES on epilepsy in children may be real. Alternatively, SES may influence the risk for epilepsy through risk factors and diseases that are more common in adults (e.g., hypertension, stroke, Alzheimer's disease) than in children. This is consistent with the notion of a cumulative impact of SES over the life course.
The impact of SES on disease in Icelandic adults is not limited to epilepsy. Despite universal access to healthcare and state assistance, the literature reports three studies of the association between SES and disease in Icelandic adults. Low educational level has been reported to increase the risk for coronary heart disease risk factors in 18,919 adults living in Reykjavik (24). Among a random sample of 1,023 adults in Reykjavik, total and partial edentulousness was more frequent in the lower-SES group, defined by employment (25). Female industrial workers in Iceland have an almost twofold increased mortality compared with the general population (standardized mortality ratio, 1.79; 95% CI, 1.45–2.19) (26).
It is notable that some measures of low SES were associated with an increased risk for epilepsy in people with seizures of unknown etiology in our study. The lack of an association between low SES and epilepsy for remote symptomatic and for progressive symptomatic etiologies is surprising, because many of the causes of these seizures are themselves associated with low SES in other populations (5–12). However, small subgroup size for these etiologies may have contributed to an inability to detect an effect.