Paraneoplastic neurological syndrome presenting as difficulty in mouth opening in a patient with small cell lung cancer

A 60-year-old man was admitted to our hospital experiencing difficulty in opening his mouth, and diplopia. Chest computed tomography showed a mass on the right lung. Transbronchial biopsy and scrutiny of the whole body resulted in the diagnosis of small cell lung cancer, cT2N2M1a stage IV. With chemotherapy including carboplatin and etoposide, the primary mass of the right lung shrank, and the patient no longer had diplopia or trouble opening his mouth. After the first-line chemotherapy, itching, ptosis, and dysphagia appeared, accompanied by tumor relapse. These symptoms improved with chemotherapy for lung cancer. After four lines of chemotherapy, the lung cancer was not controlled. The patient died 19 months after the first chemotherapy had started. Difficulty in opening the mouth is a rare symptom for lung cancer patients, but chemotherapy for lung cancer clearly improved this symptom. Therefore, we diagnosed this rare symptom as paraneoplastic neurological syndrome.