Circadian and circannual rhythmicity in the occurrence of subarachnoid hemorrhage

Background and Purpose Inconsistent data are available on the temporal pattern of onset of subarachnoid hemorrhage (SAH). We investigated the possible influence of vascular risk factors. Methods Of a consecutive series of 217 cases of SAH, precise determination (within 30 minutes) of the time of symptom onset was possible in 199 (91.7%). Partial Fourier series with up to six harmonics were applied to hourly and monthly data, and the best-fitting curves for circadian and annual rhythmicity were calculated. The amplitude-MESOR (rhythm-adjusted mean over the time period analyzed) ratio was used as a measure of temporal variability. Results In the total population, a significant circadian pattern of occurrence was demonstrated with major peaks in the morning (approximate to 9 AM) and evening (approximate to 9 PM) hours and a nocturnal trough (approximate to 3 AM). Younger, male,and hypertensive subjects had lower amplitude-MESOR ratios; smokers had no significant rhythmicity. The annual pattern showed a 6-month periodicity with two major peaks in March and September and minor differences in the subgroups studied. Conclusions Our study indicates that the temporal distribution in onset of SAH may be influenced by variable combinations of environmental and vascular risk factors.