What happened, in summary

This 80-year-old man had no psychiatric history and was independent in daily activities before becoming ill. About 10 days before hospitalization, he developed flu-like symptoms, including fever, headache, and fatigue. About 3 days before admission, he developed pressured speech, agitation, short-term memory impairment, loud shouting, and unusual hand movements. He was admitted to a psychiatric department because dementia or a primary psychiatric disorder was suspected. On the first hospital day, neurological examination showed mild neck stiffness and impaired orientation and memory, while brain MRI did not show a typical medial temporal lobe lesion. Cerebrospinal fluid testing showed mild white-cell elevation, and whole-body CT unexpectedly found a right upper-lung mass with mediastinal and right hilar lymphadenopathy. Autoimmune encephalitis was suspected, so high-dose IV methylprednisolone 1000 mg/day was started immediately for 3 days. Because only partial improvement followed the first course, a second steroid pulse was given on hospital days 8 to 10, followed by oral methylprednisolone taper. His orientation improved early, pressured speech and agitation settled by day 8, and abnormal behavior was gone by day 14. Anti-NMDAR antibodies were confirmed in the CSF that same day. Bronchoscopy biopsy of the lung mass on hospital day 24 confirmed small-cell lung cancer, linking the encephalitis to the cancer. He was discharged in stable condition on hospital day 30 without significant neurologic deficits. No chemotherapy was given during the first hospitalization, but he was later readmitted for carboplatin plus paclitaxel. No recurrence of neuropsychiatric symptoms was reported after that during later follow-up.