Spontaneous upper extremity deep vein thrombosis as initial manifestation of a malignant mediastinal teratoma.

Upper extremity deep vein thrombosis (UEDVT) is an uncommon clinical condition, accounting for fewer than 10% of all deep vein thromboses. While catheter-associated thrombosis and anatomical abnormalities are well-recognized causes, spontaneous UEDVT may occasionally serve as the first manifestation of an underlying malignancy. We report the case of a 43-year-old previously healthy male who developed spontaneous thrombosis of the left subclavian vein. He tested heterozygous for the Factor V Leiden mutation. Imaging revealed a large anterior mediastinal mass compressing the brachiocephalic vein. Histopathology confirmed a teratoma with enteric-type adenocarcinomatous differentiation. The patient underwent neoadjuvant chemotherapy followed by complete tumor resection and remained recurrence-free at one-year follow-up. This case underscores the importance of thorough diagnostic evaluation in patients with idiopathic UEDVT, particularly in the absence of catheterization or mechanical triggers. We review the spectrum of malignancy-associated UEDVT, including mechanisms such as venous compression, invasion, and paraneoplastic hypercoagulability. Spontaneous UEDVT can be the initial clinical sign of an occult malignancy. Early imaging and hemostatic assessment are crucial for timely diagnosis and treatment.