Antiphospholipid Syndrome-Associated Lemierre's Syndrome With Extensive Cervicothoracic Venous Thrombosis Mimicking Thoracic Outlet Syndrome.

Lemierre's syndrome is a rare postanginal septic thrombophlebitis involving the internal jugular vein in younger adults. Extension into the thoracic inlet and upper extremity venous system is uncommon, and the role of underlying autoimmune disease in amplifying thromboinflammatory responses remains poorly characterized. A 29-year-old man with systemic lupus erythematosus (SLE), intermittently compliant with hydroxychloroquine, presented with two weeks of progressive right-sided neck pain and swelling, meeting sepsis criteria on arrival with marked systemic inflammation, acute kidney injury, and proteinuria. Initial contrast-enhanced computed tomography demonstrated diffuse cervicothoracic cellulitis without abscess. Within 24 hours, he developed worsening neck pain, dysphagia, and right upper extremity swelling with C5-C8 distribution numbness, and hand weakness. Repeat imaging revealed extensive septic thrombophlebitis involving the right internal jugular, subclavian, brachiocephalic, axillary veins, and superior vena cava, producing thoracic outlet-like neurovascular compression. Blood cultures remained persistently negative. Autoimmune evaluation demonstrated high-titer antinuclear antibody, elevated anti-double-stranded DNA antibodies, positive anti-Smith and anti-SSA antibodies, and preserved complement levels. Coagulation studies confirmed lupus anticoagulant positivity and concurrent antiphospholipid syndrome. The patient was treated with broad-spectrum antibiotics, therapeutic anticoagulation, and corticosteroids, resulting in clinical improvement with C-reactive protein decline and resolution of upper extremity swelling. No surgical intervention was required. While anticoagulation remains debated in Lemierre's syndrome, therapeutic anticoagulation was strongly indicated given the multifactorial nature of presentation. The culture-negative presentation, absence of pharyngitis, potential environmental exposure, age, and suboptimally controlled SLE with active lupus nephritis distinguish this from classic Lemierre's syndrome and highlight diagnostic challenges in atypical presentations.