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Lambert Eaton myasthenic syndrome (LEMS) is a rare autoimmune neuromuscular junction disorder involving loss of functional pre-synaptic P/Q-type voltage-gated calcium channels. Many cases occur as a paraneoplastic disorder, often in small cell lung cancer (SCLC). Recently, immune checkpoint inhibitors (ICI) have emerged as treatment of choice for various malignancies. While generally well tolerated, certain ICI-treated patients experience neurologic immune-related adverse events (irAEs). Here, we explore therapeutic and diagnostic conundrums from the unclear etiology (paraneoplastic vs. irAE) of a case of LEMS in a patient with SCLC treated with ICI therapy. A 62-year-old female patient with SCLC was referred to EMG laboratory with 7 weeks of progressive weakness, shortness of breath and dysphagia. Due to tumor progression, immunotherapy with pembrolizumab was initiated five months prior to presentation. On examination, she had mild non-fatigable right-sided ptosis and diplopia, normal bulbar strength, and proximal greater than distal weakness of lower greater than upper extremities. Her reflexes were 2-/4 throughout, with left biceps reflex facilitating after 30 seconds of exercise. On nerve conduction studies (NCS), there was an amplitude increase in multiple nerves including the left median nerve (160%) and left ulnar nerve (370%) after 10 seconds of exercise. Paraneoplastic panel came back with elevated LEMS-related anti-P/Q-type voltage gated calcium channel antibodies at 0.19nmol/L (normal: ≤0.02nmol/L). This case illustrates the diagnostic and therapeutic challenges that surround LEMS in SCLC patients on immunotherapy. Diagnosis hinges on clinical presentation, motor NCS, and antibody testing while determination of the etiology (paraneoplastic vs ICI related LEMS) is more complex and may affect selection of the correct treatment. Therapy for ICI-related neuromuscular irAEs depends on symptom severity, but typically should include holding immunotherapy and administration of high dose corticosteroids as first line treatment with possible addition of IVIg and plasmapheresis. This differs from the common first line treatment for paraneoplastic LEMS, highlighting the importance of understanding of the etiology. Further research is needed to better understand optimal management.

Case Report of Lambert Eaton Myasthenic Syndrome in a Patient with Small Cell Lung Cancer on Immune Checkpoint Inhibitor Therapy