Seasonal variation in the occurrence of venous thromboembolism: data from the MASTER Registry.

Many studies showed that the occurrence of cardiovascular and cerebrovascular events exhibits a seasonal variation. As for venous thromboembolism (VTE), not univocal results are available, and studies are mainly retrospective. We aimed to confirm 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 men, mean age 59 +/- 18 years). The total sample was divided into subgroups by gender, age (<40, 41-60, 61-80, > or = 80 years), type of event (first episode; proximal or distal; upper or lower limb; idiopathic or secondary deep vein thrombosis or pulmonary embolism, or both), and underlying risk factors, eg, cancer, previous VTE, estroprogestinic therapy, lack of prophylaxis, immobilization, surgery, pregnancy or puerperium, and medical diseases. Cases were grouped according to season and month of occurrence, and the data were analyzed by either the chi(2) test for goodness of fit and chronobiological analysis. VTE was most frequent in Autumn and less frequent in Spring (32.9% vs 19%, respectively, chi(2) = 90.62; P < .001). This pattern was shown for most subgroups. Chronobiological analysis identified a significant rhythmic annual pattern, with a main September-October peak for several subgroups (men, age 41-60 and 61-80 years, secondary event, previous VTE, immobilization), and a trend for most of the others. It is possible that subjects at increased risk could perhaps deserve appropriate or potentiated VTE prophylaxis in certain periods of the year.