Regular ArticleSeasonal variation in the occurrence of venous thromboembolism: A report from the Korean Venous Thromboembolism Working Party
Introduction
In contrast to arterial thrombotic events, there have been conflicting results regarding venous thromboembolism (VTE) in terms of seasonal variation. Recently, a meta-analysis demonstrated a significantly increased incidence of VTE in the winter [relative risk (RR) for a VTE in winter = 1.143], and this significantly increased risk was most evident in January, with the RR for VTE in January being 1.194 [1]. Based on these results, the effect of weather on VTE has been regarded as a risk factor for VTE albeit a relatively weak one.
Even though the meta-analysis supplied confirmatory evidence for seasonal variation in VTE, it had two limitations. First, few Asian patients were enrolled in the study (approximately 3% of the patient population) [1]. In term of ethnic differences, the incidence of VTE in the Asian population has been generally found to be lower than in whites, possibly because of genetic mutations [2]. There is evidence that genetic mutations, particularly those for clotting factor V (factor V Leiden) and factor II G20210A, are highly prevalent in the Caucasian population but virtually absent in the Asian population [3], [4]. Second, the meta-analysis did not include a subgroup analysis of unprovoked VTE. Dentali et al. pointed out that no study has provided data regarding seasonal variation in the incidence of unprovoked VTE. To address these issues, we investigated whether seasonal variation affected the incidence of VTE in Koreans. The primary aim of the present study was to determine if there was any seasonal variation in the occurrence of VTE in the Korean population, and the secondary aim was to evaluate if such seasonal variation may be influenced by age, gender, and VTE risk factors.
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Study population
The study consisted of patients with VTE who were registered with the VTE Registry of the Korean VTE Working Party (KVTEWP) between January 2001 and December 2010. During this period, 11 participating centers consecutively enrolled Korean VTE patients. VTE was confirmed by venography or compression ultrasonography for deep vein thrombosis (DVT) and by ventilation-perfusion lung scan or computed tomography for pulmonary embolism (PE). VTE was defined as provoked if the patient was found to have
Baseline characteristics
A total of 1,495 cases of VTE were registered in the study period. The mean age of the patients was 60 (SD 16) years, and 43.2% were male. Of the VTE patients, 746 (49.9%) patients were diagnosed as having an isolated DVT, whereas 749 (50.1%) were diagnosed with PE with or without DVT. VTE was unprovoked in 412 (27.5%) patients and provoked in 1,083 (72.4%) patients. Among the provoked VTE cases, VTE was related to malignancy (n = 434), recent surgery (n = 292), immobilization (n = 231),
Discussion
The weather of South Korea is characterized by four distinct seasons, spring, summer, autumn and winter, and the contrast between winter and summer is striking. Winter is bitterly cold, whereas summer is hot and humid. The transitional seasons, spring and autumn, are sunny and generally dry. Based on these environmental conditions, we investigated seasonal variations in the VTE Registry of 11 Korean teaching hospitals. Our study demonstrated a significant seasonal variation in VTE in Korean
Conflict of Interest Statement
The author(s) declared no conflicts of interest with respect to the authorship and/or publication of this article.
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These authors contributed equally to this work.