Table 2 Pooled relative risks of ischaemic stroke stratified by migraine type, oral contraceptive use, and age

No of studies Relative risk (95% CI) Ri* P value Migraine (any) All studies 14 2.16 (1.89 to 2.48) 0.00 0.77 Case-control studies 11 2.18 (1.86 to 2.56) 0.00 0.51 Cohort studies 3 2.10 (1.61 to 2.75) 0.00 0.96 Migraine with aura Case-control studies 8 2.28 (1.89 to 4.39) 0.33 0.16 Migraine without aura Case-control studies 7 1.56 (1.03 to 2.36) 0.50 0.07 Migraine among oral contraceptive users Case-control studies 3 8.72 (5.05 to 15.05) 0.26 0.28 Migraine among men and women <45 years Case-control studies 9 2.36 (1.92 to 2.90) 0.07 0.38 Migraine among women <45 years Case-control studies 7 2.76 (2.17 to 3.52) 0.00 0.82

^* Proportion of the total variance due to between study variance. Large values (>0.75) indicate large heterogeneity between studies; small values (<0.4) indicate lack of heterogeneity.

DerSimonian and Laird Q statistic.