Background: Inverse associations between IQ and stroke have been reported in a few studies, but none of them has investigated subtypes of stroke, nor have they studied both fatal and non-fatal stroke separately. Stroke is a heterogenic disease and strength of associations with IQ and putative causal pathways cannot be assumed to be identical for different subtypes of stroke.
Methods: IQ was measured for 1.1 million Swedish men, born 1951 to 1976, during military conscription. We linked data from several national registers and followed the cohort until the end of 2006 for non-fatal, and 2004 for fatal stroke. Hazard ratios (HR) with 95% confidence intervals (CI) adjusted for age, BMI, blood pressure and socioeconomic factors were estimated using Cox proportional hazards models.
Results: We found inverse associations between IQ and all stroke subtypes, fatal and non-fatal, and that the strength of the associations differed by subtype, with the strongest relative risk found for hemorrhagic stroke. In adjusted models using IQ as a continuous variable over a standard nine point scale, HR for mortality in all stroke was 0.89 (95% CI 0.85, 0.93), i.e. a 11% decrease in stroke risk per unit increase in IQ. For non-fatal stroke, the corresponding HR was 0.92 (95% CI 0.91, 0.93). The results were based on a rather young cohort and results should therefore be generalized to early stroke events rather than the general population.
Conclusions: Inverse associations were found between IQ and all stroke subtypes, fatal and non-fatal. For all types of non-fatal stroke the inverse associations with IQ remained after adjustments for childhood and adult SEP.
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