Venous anomalies: an underrecognized but important cause of venous thromboembolism.

Anatomical variants, such as May-Thurner syndrome (MTS) and inferior vena cava (IVC) variants, are underrecognized causes of deep venous thrombosis (DVT), despite affecting management. We aimed to identify the proportion of anatomical variants in proximal lower limb DVT. A retrospective cohort study was performed with cases of acute proximal DVT from 2014 to 2021 identified from ICD-10 codes. We identified 4731 DVTs and included 1268 proximal DVTs. Thirty-six (2.84%) had an anatomical variant (25 MTS and 11 IVC variants), with a rate of 14.39% in females <50 years old. Compared with nonvariant DVTs, they were more likely to be unprovoked (81% vs 23%), younger (median age, 37 vs 63 years), female (67% vs 37%), and have postthrombotic syndrome (22% vs 9%). Variants frequently received thrombolysis (58% vs 1%) or angioplasty (47% vs 0%) and indefinite anticoagulation (83% vs 40%). Further investigation for variants should be considered for high-risk patients, as variants affect management.