Joint effect of modifiable risk factors on the risk of aneurysmal subarachnoid hemorrhage: a cohort study.

BACKGROUND AND PURPOSE The joint effect of risk factors on the risk of aneurysmal SAH (aSAH) has been studied sparsely. METHODS We examined the potential synergism between cigarette smoking, hypertension, and regular alcohol consumption and the risk of aSAH in a prospective, population-based cohort of participants from the Nord-Trøndelag Health Study and the Tromsø Study in Norway. Interaction was assessed on additive and multiplicative scales. RESULTS We identified 122 cases of aSAH over 977 895 person-years of follow-up. Interaction was observed between current smoking and hypertension on the additive scale, (relative excess risk because of interaction, 6.40; 95% CI, 0.88-11.92, adjusted for sex and age). We found no significant interaction between hypertension and regular alcohol consumption or current cigarette smoking and regular alcohol consumption on the additive scale. No significant interaction was detected on the multiplicative scale. CONCLUSIONS The joint effect of current smoking and hypertension on the risk of aSAH was stronger than was the sum of the independent effects of each factor. Persons at risk of aSAH should be advised of a markedly stronger risk for aSAH with the combination of current smoking and hypertension. In addition, the finding suggests that combining smoking cessation and blood pressure lowering may have an extra risk reduction effect on preventing aSAH.