Lung cancer associated with seronegative myasthenia gravis

A 64-year-old man presented with diplopia, muscle weakness, a pulmonary nodule and mediastinal widening on a chest radiograph. He was diagnosed with clinical stage IIIA (T2aN2M0) lung cancer. His neurological symptoms worsened following the initiation of thoracic radiation therapy (60 Gy) and chemotherapy. A diagnosis of myasthenia gravis (MG) was confirmed with a repetitive nerve stimulation test that showed a waning pattern, and a positive edrophonium test, although neither anti-acetylcholine receptor antibodies nor anti-muscle-specific tyrosine kinase antibodies were detected. The ptosis and limb muscle weakness improved with prednisolone and acetylcholinesterase inhibitor treatment, and a partial response of the lung cancer to chemoradiotherapy was obtained. However, the ptosis and limb muscle weakness worsened again following a recurrence of the lung cancer. The herein described case, in which lung cancer and MG occurred and recurred simultaneously, suggests that MG can develop as a paraneoplastic syndrome of lung cancer.