Successful Early Treatment of Anti-N-Methyl-d-Aspartate Receptor Encephalitis Associated With Small Cell Lung Cancer in an Elderly Male Patient: A Case Report.

Background: Anti-N-methyl-d-aspartate receptor (NMDAR) encephalitis, a type of autoimmune encephalitis, characterized by acute onset neuropsychiatric symptoms, predominantly affects young females and is often associated with ovarian teratomas. Although small cell lung cancer (SCLC) is a known cause of paraneoplastic encephalitis, its association with anti-NMDAR encephalitis is rare and often carries a poor prognosis due to limited response to immunotherapy.

Case presentation: An 80-year-old male with no psychiatric history presented with flu-like symptoms, followed by the acute onset of neuropsychiatric symptoms, including pressured speech, agitation, memory impairment, and abnormal behavior. Autoimmune encephalitis was suspected due to mildly elevated cerebrospinal fluid (CSF) white cell count and a mass in the right upper lung detected by whole-body computed tomography (CT) on the first day of hospitalization. High-dose intravenous corticosteroids were administered on Day 1, resulting in prompt and sustained improvement in symptoms. CSF was later confirmed positive for anti-NMDAR antibodies, and a bronchoscopy biopsy of the pulmonary mass diagnosed SCLC. The patient recovered without neurological deficits and was discharged in stable condition on hospital Day 30.

Conclusion: This was a rare case of anti-NMDAR encephalitis associated with SCLC in an elderly male patient. Diagnosis in elderly individuals is often challenging because of the atypical presentations and lower tumor association. Nevertheless, timely intervention initiation may lead to favorable outcomes. Clinicians should consider autoimmune encephalitis, including anti-NMDAR encephalitis, when evaluating acute onset neuropsychiatric symptoms in elderly individuals and initiate early immunotherapy alongside tumor screening.