Seasonal variation in the occurrence of deep vein thrombosis.

Backround: The occurrence of several adverse cardiovascular events, eg, myocardial infarction and stroke, is not randomly distributed over time, but shows definite seasonal patterns. The temporal pattern for pulmonary embolism (PE) is known, whereas the data concerning deep vein thrombosis (DVT) are not conclusive. The aim of our study was to investigate whether a seasonal variation exists also for DVT, together with the possible influence of different comorbid risk factors. Materials/Methods: We observed 1164 consecutive cases of DVT at the General Hospital of Ferrara, Italy, from 1998 to 2002. The total sample was divided into subgroups by gender, and by the most common comorbid conditions. For statistical analysis, the distribution of symptom onset was tested for uniformity by the X-2 test for goodness of fit, and partial Fourier series were used to assess rhythmicity. Results: Seasonal analysis showed a significantly reduced frequency of DVT events in summer and increased in winter, for the total population (p<0.0001), men (p=0.003), women (p=0.007), subjects with PE (p=0.001), and hypertension (p=0.001). Similarly, chronobiological analysis yielded a significant seasonal variation, with a peak in December for the total population (p<0.001), men and women (p<0.001 and p=0.02, respectively), PE (p=0.006), and hypertension (p=0.028). Conclusions: This study shows the existence of a highly significant seasonal pattern in the occurrence of DVT, characterized by a winter peak. Thus, colder months and relative hypercoagulability could be considered a supplementary high risk condition when prescribing an anticoagulant regimen.