Background: A growing body of evidence has shown that the occurrence of cardiovascular and cerebrovascular events exhibits a definite seasonal variation. As for venous thromboembolism (VTE), not univocal results are available in literature and studies are mainly retrospective. Material/Methods: The aim of this study was to verify the hypothesis of the existence of a seasonal pattern in the occurrence of VTE on a large prospective population. The analysis considered consecutive cases of VTE enrolled into the MASTER Registry in 25 Italian hospitals, between January 2002 and November 2004. The total population consisted of 2119 subjects (1056 males and 1063 females, mean age 59.3 ± 18.1 yrs). The total sample was divided into subgroups by gender, age (<40, 41-60, 61-80, ≥80 years), type of event (first episode, proximal or distal, upper or lower limb, idiopathic or secondary, DVT or PE or both), and underlying risk factors, e.g., cancer, previous VTE, estroprogestinic therapy, lack of prophylaxis, immobilization, surgery, pregnancy or puerperium, and medical diseases. Cases were first grouped according to season of occurrence, and the data were analyzed by the 2 test for goodness of fit. Then, inferential chronobiologic (Cosinor and partial Fourier) analysis was applied to monthly data, and the best-fitting curve for the annual variation was derived. Results: VTE was most frequent in Autumn (n = 719, 32.9%) and less frequent in Spring (n = 414, 19%, goodness of fit 2 = 90.62, p < 0.001). The same pattern was shown for rather all subgroups by gender, age, type of event, and risk factors. Inferential chronobiological analysis identified a significant rhythmic annual pattern for VTE, with a main September-October peak for several subgroups (males, age 41-60 and 61-80 yrs, secondary event, previous VTE, immobilization, home therapy), and a trend for several others (total cases, age >80 yrs, proximal or distal DVT, lower limb, first episode, medical diseases). Conclusions: These observations might have implications in the adoption of preventive measures. It is possible that subjects at increased risk could perhaps deserve appropriate or potentiated VTE prophylaxis in certain periods of the year.
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