Anti-VGCC negative and Anti-SOX1 positive Lambert-Eaton myasthenic syndrome as the initial presentation of small cell lung cancer: a case report.

Lambert-Eaton myasthenic syndrome (LEMS) is a rare autoimmune disorder characterized by impaired acetylcholine release at the neuromuscular junction, leading to proximal muscle weakness, autonomic dysfunction, and diminished or absent tendon reflexes. Here, we present a rare case of a 73-year-old male smoker with small cell lung cancer (SCLC), whose initial manifestation was LEMS. Notably, the patient tested seronegative for anti-voltage-gated calcium channel (VGCC) antibodies but was seropositive for anti-SOX1 antibodies. The diagnosis of LEMS was supported by characteristic clinical symptoms and electromyography findings. Subsequent imaging (chest CT and PET-CT) identified a left upper lobe lung mass, which was pathologically confirmed as SCLC, clinically staged as T1cN3M0 (IIIB). Following standard antitumor therapy, the patient's muscle weakness markedly improved, and his overall survival has exceeded 36 months. This case underscores the critical importance of testing for anti-SOX1 antibodies and applying the DELTA-P score for malignancy screening in high-risk patients presenting with LEMS-like symptoms, even when seronegative for anti-VGCC antibodies.