Clinical Characteristics and Impact of May-Thurner Syndrome in Patients With Proximal Deep Vein Thrombosis: Insights From the RIETE Registry

May-Thurner syndrome (MTS) is an anatomical variant involving compression of the left iliac vein, predisposing to proximal deep vein thrombosis (DVT). Despite its clinical significance, population-level data remain limited. Using the RIETE (Registro Informatizado Enfermedad Trombo-Embólica) registry (2009-2024), we analyzed proximal DVT patients who underwent advanced imaging (CT-venography, contrast-venography, or MRI). Among 2872 patients with advanced imaging-confirmed proximal DVT, 124 (4.3%) had MTS. MTS patients were more likely to be female (78% vs 52%), younger (mean age 42 vs 62 years), and to present with left-sided DVT (92% vs 46%). They had fewer comorbidities, but greater exposure to estrogen or pregnancy. Endovascular therapy was more frequently used in MTS patients (44% vs 3.5%), who also received longer median anticoagulation (365 vs 164 days). Despite this, MTS patients had a significantly higher rate of recurrent DVT (rate ratio: 2.37; 95% CI 1.09-4.70). Multivariable analysis confirmed MTS as an independent predictor of recurrent DVT (adjusted hazard ratio: 2.26; 95% CI: 1.02-5.01). Major bleeding was non-significantly less frequent (rate ratio: 0.42; 95% CI: 0.13-1.04), though retroperitoneal bleeding was more common. MTS is underdiagnosed, has distinct clinical features and is associated with increased DVT recurrence despite aggressive therapy. Improved recognition and tailored management strategies are needed.