INCIDENCE OF SUBARACHNOID HEMORRHAGE - ROLE OF REGION, YEAR, AND RATEOF COMPUTED-TOMOGRAPHY - A METAANALYSIS

Background and Purpose The incidence of subarachnoid hemorrhage (SAH) has been estimated for many years al 10 to 15 per 100 000 person-years , but the most recent studies yield lower figures, of 6 to 8 per 100 0 00 person-years. To investigate the cause of this apparent decline, we studied the influence of year of study, rate of CT, and region. Metho ds We performed a meta-analysis of SAH incidence studies from 1960 onw ard and assessed the influence of the three factors by means of univar iate and multivariate Poisson regression analyses. Results Eighteen st udies fulfilled predefined inclusion criteria. In three Finnish studie s, the pooled incidence was 21.4 per 100 000 person-years (95% confide nce interval [CI], 19.5 to 23.4); in 15 non-Finnish studies, it was 7. 8 per 100 000 person-years (95% CI, 7.2 to 8.4). With univariate analy sis, in non-Finnish studies the incidence decreased 0.96% for each per centage point increase of patients investigated with CT (rate ratio, 0 .9904; 95% CI, 0.9878 to 0.9930). With 100% CT scanning, the incidence of SAH outside Finland is estimated at 6 per 100 000 person-years. Th e rate ratio for year of study was 0.952 (95% CI, 0.935 to 0.969) for each later year in the period 1960 to 1994. In multivariate analysis, only the use of CT was independently related to SAH incidence. For the Finnish studies. the rate ratios for use of CT and year of study were not statistically significant. We also found in six studies that inci dence for women was 1.6 (95% CI, 1.1 to 2.3) times higher than that fo r men (7.1 [95% CI, 5.4 to 8.7] and 4.5 [95% CI, 3.1 to 5.8], respecti vely). Conclusions The actual incidence of SAH has remained stable ove r the last three decades; the apparent decline in incidence is entirel y explained by the greater proportion of patients investigated with CT . The incidence of SAH in Finland is almost three times as high as in other parts of the world.