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 (approximately 9 AM) and evening (approximately 9 PM) hours and a nocturnal trough (approximately 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.