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 th e possible influence of vascular risk factors. Methods Of a consecutiv e series of 217 cases of SAH, precise determination (within 30 minutes ) of the time of symptom onset was possible in 199 (91.7%). Partial Fo urier series with up to six harmonics were applied to hourly and month ly data, and the best-fitting curves for circadian and annual rhythmic ity were calculated. The amplitude-MESOR (rhythm-adjusted mean over th e time period analyzed) ratio was used as a measure of temporal variab ility. Results In the total population, a significant circadian patter n of occurrence was demonstrated with major peaks in the morning (appr oximate to 9 AM) and evening (approximate to 9 PM) hours and a nocturn al trough (approximate to 3 AM). Younger, male,and hypertensive subjec ts had lower amplitude-MESOR ratios; smokers had no significant rhythm icity. The annual pattern showed a 6-month periodicity with two major peaks in March and September and minor differences in the subgroups st udied. Conclusions Our study indicates that the temporal distribution in onset of SAH may be influenced by variable combinations of environm ental and vascular risk factors.