Publications on the temporal pattern of the occurrence of subarachnoid hemorrhage (SAH) have produced conflicting results. Variations between studies may relate to the relatively small numbers of SAH cases analyzed, including those in meta-analyses. We identified all cases of SAH from 3 well-designed population-based studies in Australia (Adelaide, Hobart, and Perth) and New Zealand (Auckland) during 3 periods between 1981 and 1997. The diagnosis of SAH was confirmed with CT, cerebral angiography, cerebrospinal fluid analysis, or autopsy in all cases. Information on the time of occurrence of each event was obtained. Risk ratios (RRs) and 95% CIs were calculated using Poisson regression, with age, sex, smoking status, and history of hypertension entered in the model as covariates. A total of 783 cases of SAH were registered. Age- and sex-adjusted RRs of SAH occurrence were highest in the period between 6 AM and 12 MIDNIGHT (RR 3.2, 95% CI 2.4-4.3) and in winter and spring (RR 1.3, 95% CI 1.1-1.5; RR 1.3, 95% CI 1.1-1.5; respectively). No particular pattern of SAH occurrence was observed according to the day of the week. Restriction of the analyses to proved aneurysmal SAH did not substantially change the point estimates. Circadian and circaseptan (weekly) fluctuations of SAH occurrence in the southern hemisphere are similar to those in the northern hemisphere, but the occurrence of SAH in Australasia exhibits clear seasonal (winter and spring) peaks.
