A 56-year-old woman with a background of neuromyelitis optica spectrum disorder associated with aquaporin-4 antibodies (AQ4-NMOSD) treated with azathioprine, presented with a 2-month history of declining mobility, weight loss and hoarse voice. She had a history of autoimmune thyrotoxicosis and treated hypertension. Given her smoking history, the initial clinical concern was of malignancy. Neurophysiological examination identified severely attenuated responses from the upper and lower limb motor units, with significant neurophysiological incrementation postexercise, indicating a presynaptic neuromuscular junction disorder. Antibody testing showed markedly raised antibodies to the p/q subtype voltage-gated calcium channels, and she was diagnosed with Lambert-Eaton myasthenic syndrome (LEMS). She was treated with 3,4-diaminopyridine and rituximab to maintain remission. Follow-up serial fluorodeoxyglucose positron emission tomography (FDG-PET) CT scans have yet to identify an underlying malignancy. LEMS is associated with additional autoimmune diseases in about half of cases. This case illustrates the importance of recognising other treatable autoimmune conditions in the context of NMOSD.