Lambert–Eaton myasthenic syndrome presenting with occult mediastinal small cell carcinoma and positivity for anti-CV2/CRMP5 and anti-SOX1 antibodies: a case report

Lambert-Eaton myasthenic syndrome (LEMS) is a rare autoimmune neuromuscular disorder characterized by proximal muscle weakness, autonomic dysfunction and hyporeflexia. Numerous studies have reported the association of LEMS with anti-SOX1 antibodies and small cell carcinoma, but LEMS with double antibody positivity and cerebellar ataxia accompanied by small cell carcinoma is relatively rare. A 56-year-old female patient developed progressive limb weakness in November 2023, with symptoms gradually worsening over six months to include gait instability, impaired grip, chest tightness, dyspnea, dysphagia, and aspiration. Electrodiagnostic testing confirmed Lambert-Eaton myasthenic syndrome (LEMS) with positive VGCC antibodies. Paraneoplastic antibody testing showed positive anti-CV2/CRMP5 antibodies (24 AU) and borderline anti-SOX1 antibodies (9 AU), while other antibodies were negative. Initial imaging (contrast-enhanced chest CT and PET-CT) revealed no abnormalities, but follow-up one year later detected mediastinal small cell lung cancer. The patient underwent three plasma exchange sessions with corticosteroid therapy with limited response, then showed significant improvement after switching to subcutaneous Telitacicept (80 mg) combined with Fampyra Comprimidos and Mycophenolate Mofetil capsules, achieving independent ambulation and normal grip strength with only mild residual limb weakness. The patient has since been transferred to a local hospital for chemotherapy. We find that LEMS with concurrent anti-SOX1 and anti-CV2/CRMP5 positivity manifests as rapid progression, prominent proximal weakness, and potential cerebellar and autonomic involvement. Early and sustained oncological surveillance—particularly of mediastinal lymph nodes—and combined immunotherapeutic strategies can significantly improve neurological outcomes.