Lambert-Eaton Myasthenic Syndrome During Immunotherapy in Extensive-Stage Small-Cell Lung Cancer: A Case Report

AIMS/BACKGROUND: Lambert-Eaton myasthenic syndrome (LEMS) is a rare autoimmune disorder of the neuromuscular junction that is frequently associated with small-cell lung cancer (SCLC). It is characterized by proximal muscle weakness, reduced deep tendon reflexes, and autonomic symptoms, all of which can significantly impair quality of life and complicate oncologic management. Early recognition and timely intervention are crucial to improving clinical outcomes. CASE PRESENTATION: We report a case of a 76-year-old man with extensive-stage SCLC and chronic renal insufficiency whose LEMS worsened during durvalumab immunotherapy. Despite continued chemotherapy and immunotherapy, his symptoms progressively worsened. Chemotherapy failed to improve his neurological condition, with subsequent recurrent episodes of muscle weakness. Subsequent administration of intravenous immunoglobulin (IVIG) combined with pyridostigmine led to marked neurological improvement and restoration of muscle strength to grade 5, enabling continuation of antitumor therapy. The patient subsequently received three cycles of second-line lurbinectedin chemotherapy, during which inflammatory responses, anemia, and venous thrombosis developed but were effectively managed with supportive treatment. RESULTS: Following chemotherapy and immunotherapy, the patient's limb muscle strength recovered to grade 5, followed by recurrent muscle weakness symptoms. Hematologic and biochemical parameters gradually normalized, and inflammatory markers returned to baseline. At discharge, his clinical condition was stable. CONCLUSION: This case highlights the importance of early recognition and standardized management of paraneoplastic LEMS in enhancing functional recovery and enabling the safe continuation of antitumor therapy.