A 62-year-old male smoker presents with central chest mass on imaging and progressive proximal leg weakness that paradoxically improves with repeated muscle use. Serum calcium is normal, and electrolytes are unremarkable. The tumor is confirmed to be the subtype marked **C** in the diagram. Which of the following paraneoplastic syndromes is most likely responsible for his neuromuscular presentation?

Explanation

## Why Lambert-Eaton Myasthenic Syndrome (LEMS) is right SCLC (the tumor marked **C**) is the malignancy most strongly associated with Lambert-Eaton Myasthenic Syndrome. LEMS is caused by autoantibodies against presynaptic voltage-gated calcium channels at the neuromuscular junction, resulting in impaired acetylcholine release. The pathognomonic clinical feature is **proximal muscle weakness that IMPROVES with repeated use** (post-tetanic potentiation), which directly contrasts with myasthenia gravis. LEMS often precedes SCLC diagnosis by months, making it a critical paraneoplastic marker. The normal calcium and electrolytes exclude ectopic ACTH-induced hypokalemia as the primary cause (Robbins 10e Ch 15; Harrison 21e Ch 78). ## Why each distractor is wrong - **Myasthenia Gravis**: While also an autoimmune neuromuscular disorder, MG is NOT a paraneoplastic syndrome of SCLC. MG weakness characteristically WORSENS with repeated use (fatigue), the opposite of the patient's presentation. MG is associated with thymoma, not lung cancer. - **Paraneoplastic Cerebellar Degeneration**: This SCLC-associated paraneoplastic syndrome presents with ataxia, dysarthria, and nystagmus (cerebellar signs), not proximal muscle weakness that improves with use. Anti-Hu antibodies are the serological marker, but the clinical phenotype does not match. - **Ectopic ACTH production**: While SCLC can produce ectopic ACTH causing atypical Cushing syndrome with severe hypokalemia and proximal weakness, the patient's electrolytes are normal. Additionally, ACTH-induced weakness results from hypokalemia-mediated dysfunction, not from a specific neuromuscular junction pathology. The improvement with repeated use is incompatible with this mechanism. **High-Yield:** LEMS = SCLC until proven otherwise; weakness improves with use (opposite of MG); caused by anti-voltage-gated calcium channel antibodies; often precedes cancer diagnosis. [cite: Robbins 10e Ch 15; Harrison 21e Ch 78]