High risk of recurrent venous thrombosis in patients with lower‐leg cast immobilization

Unlabelled Box Essentials • The risk of recurrent venous thrombosis (VT) after leg‐cast in patients with prior VT is unknown. • In a nested case‐control study within the MEGA follow‐up study we aimed to estimate this risk. • Patients with a history of VT who require lower‐leg cast have a 4.5‐fold risk for recurrence. • This relative risk translates to an absolute risk for recurrent VT of about 3.2% within 3 months. Acknowledgements We gratefully thank all the participants of the MEGA follow‐up study. Summary: Background Patients with lower‐leg cast immobilization have a substantially increased risk of developing a first venous thrombosis (VT), whereas the risk in patients with a history of VT is as yet unknown. Aims To estimate the risk of recurrent VT after lower‐leg cast immobilization in patients with a history of VT. Methods A case–control study nested within a cohort of 4597 patients with a first VT who were followed over time for recurrence from 1999 to 2010 (MEGA follow‐up study). Participants completed a questionnaire on risk factors for recurrent thrombosis, including having a cast in the first 3 months before a recurrence (cases) or a random 3‐month period during follow‐up for participants without recurrence (controls). In total, 2723/4597 (59%) participants returned the questionnaire. Odds ratios (ORs), adjusted for age and sex, were calculated to compare risks of recurrence between subjects with and without a cast. Results A total of 2525/2723 participants (93%) filled out information on cast immobilization and were included in the analysis (451 cases; 2074 controls). Twenty (1.0%) controls and 10 (2.2%) cases reported having had a lower‐leg cast in the 3 months before the control or recurrence date (adjusted OR, 2.4; 95% confidence Interval [CI], 1.1–5.3). We cross‐checked the data with these patients’ medical records. Cast application within 3 months was verified in seven (0.3%) controls vs. six (1.3%) cases, leading to an adjusted OR of 4.5 (95% CI, 1.5–14.0) and corresponding cumulative incidence of 3.2%. Conclusions Lower‐leg cast immobilization increases the risk of recurrent VT in the 3 months after its application in patients with a history of VT.