Education and dementing disorders : The role of schooling in dementia and cognitive impairment

Sammanfattning: This doctoral thesis aimed to investigate the complex relationship between education, dementias, and cognitive impairment. Two different databases were used: the Faenza and the AIDS Projects. The Faenza Project is a longitudinal study on ageing and dementia, targeting 7,930 inhabitants of Faenza (including the village of Granarolo), Italy, aged 61 years and older in 1991. The study population derives from an area which has been one of the wealthiest in Italy since the beginning of the 1950s, but with a high percentage of noneducated subjects. Diagnostic and Statistical Manual of Mental Disorders, 3rd edition revised diagnostic criteria were used for the clinical diagnosis of dementia, while a person was classified as affected by cognitive impairment, no dementia if he/she scored two or more standard deviations lower than the mean score of the corrected Mini-Mental State Examination calculated among the nondemented people. The AIDS Project is a longitudinal study on HIV-1-related cognitive impairment. The study sample included 282 consecutive subjects examined at the Division of Infectious Diseases, University of Bologna, Italy. HIV cognitive impairment was defined as poor performance on at least two of the seven neuropsychological tests included in the neuropsychological battery. Poor performance in a test was considered as a score of two or more standard deviations lower than the mean of the seronegative group in the corresponding risk behavior strata (injecting drug users, hemophiliacs, and other risk behaviors). Data were analyzed with logistic regression models. The major findings from the five research papers included in this thesis are summarized below. Study I. In the subpopulation of Granarolo, the relationship between Alzheimer's disease (AD) and other dementias with education was examined. Having no education was associated with dementia independent of all other putative risk factors (OR 4.7; 95% CI=2.3-9.6). This association was stronger among younger old persons, and decreased with increasing age. Similar findings were found for AD and Vascular dementia, separately. Study II. We examined the relationship between HIV-1-related cognitive impairment and education, controlling also for risk behaviors and clinical status. Low education was a strong risk factor for cognitive impairment in HIV-1-seropositive persons: adjusted OR in subjects with less than six years of education was 18.9 (95% Cl=3.7-97.6), and 1.3 (95% Cl=0.5-3.2) in subjects with five to eight years of education, when compared to subjects with nine+ years of schooling. Study III. The effect of education on both cognitive impairment and dementia was investigated in the Faenza cohort. Very low education was a major determinant of both dementia and cognitive impairment. No education was associated with an increased risk of both CIND (OR 16.7; 95% CI=11.2-25.0) and dementia (OR 10.9; 95% G=7.0-16.7) with a dose-response relationship. Study IV. We found that stroke does not completely explain the association between low education and dementia or cognitive impairment. The effect of stroke on dementia was stronger among the higher educated subjects (four+ years of schooling, RR 6.5; 95% CI=4.98.2) as well as among the younger old. These findings support the hypothesis that having a stroke nullifies the beneficial effect of high education and younger age against dementia and cognitive impairment. Study V. Low education and low occupation-based socioeconomic status (SES) were both independent risk factors for dementia and cognitive impairment. Subjects with low education and low SES had the highest risk of dementia (OR 5.1, 95% CI=3.5-7.3) and cognitive impairment (OR 5.2; 95% CI=3.6-7.6). These findings suggest that low occupation-based SES is not a mediator for the association between low education and dementia or cognitive impairment. In summary, mental activity stimulated by education during childhood, could be a possible mechanism explaining how high education protects against cognitive decline and dementia. The cognitive reserve hypothesis could provide the biological plausibility for this theory. The additive dementia risk observed in subjects with both low education and low SES implies that exposure acting at adult life might increase the risk due to education related exposure in childhood. However, the protective effect of high SES during adult life might balance the risk given by a low education.