Paraneoplastic Lambert-Eaton Myasthenic Syndrome Unveiling Occult Small-Cell Lung Carcinoma: A Case Report

Lambert–Eaton myasthenic syndrome (LEMS) is a rare autoimmune disorder of the neuromuscular junction, characterized by proximal muscle weakness, hyporeflexia, and autonomic dysfunction. Approximately 50–70% of adult LEMS cases are paraneoplastic, most commonly associated with small-cell lung carcinoma (SCLC). Although early recognition is critical, as LEMS may precede the diagnosis of the underlying malignancy, its initial presentation may be subtle, leading to a diagnosis delay. We report the case of a 68-year-old male with progressive proximal muscle weakness, fatigability, and autonomic symptoms. Electromyography revealed low-amplitude compound muscle action potentials with facilitation on repetitive stimulation. Anti–P/Q-type voltage-gated calcium channel (VGCC) antibodies were elevated. Chest imaging demonstrated a left apical mass, confirmed as SCLC on histopathology with positive VGCC immunostaining. After tumor resection and adjuvant chemotherapy, the patient experienced marked improvement in neuromuscular symptoms. This case highlights the importance of considering paraneoplastic LEMS as an early manifestation of SCLC, illustrating the role of combined clinical, electrophysiological, serological, and histopathological assessments in timely diagnosis.