A 55-year-old man with a 3-month history of numbness and tingling in his extremities presented at a hospital. After obtaining a diagnosis of seronegative arthritis, the patient underwent immunosuppressive therapy with prednisone (5 mg/day) and salazosulfapyridine (100 mg/day). However, the symptoms did not improve, and he later experienced double vision. After 10 days, the patient was referred to our university hospital. At the examination, his vital signs were stable and his general appearance was good.