Seronegative but Cerebrospinal Fluid-positive NMDA Receptor Encephalitis in an Alzheimer's Dementia Patient with Good Treatment Outcome: A Case Report.

We present a rare case of elderly-onset anti-N-methyl-D-aspartate (NMDA) receptor encephalitis combined with underlying Alzheimer dementia, characterized by seronegativity but cerebrospinal fluid (CSF) antibody positivity, which showed a significant improvement following intravenous immunoglobulin (IVIG) treatment. The case highlights serial electroencephalogram (EEG) changes. We focus the discussion on factors contributing to favorable recovery. A 78-year-old woman with Alzheimer's dementia and hypertension experienced rapid onset of further cognitive decline over 1 week, presenting with disorientation, irrelevant speech, and disorganized behaviors. Myoclonus of both upper limbs and focal seizures 10 days later which lead her into intensive care unit (ICU) admission. Test results revealed positive anti-NMDA antibodies in CSF but negative in serum. Serial electroencephalograms (EEG) exhibited extreme delta brush activity transitioning to excess beta activity. Initial treatment with a 5-day course of steroid pulse therapy showed limited efficacy. Subsequently, 5 days of IVIG therapy provided marked improvement in consciousness and cognitive function and also normalized the EEG. This patient presented with severe neurological dysfunction, the need for ICU management, and presence of extreme delta brush EEG pattern. Generally, the prognosis was unfavorable. However, advanced age onset and anti-NMDA antibody positivity in CSF with seronegativity in serum may imply better prognosis. Thus, testing anti-NMDA receptor antibody in both serum and CSF is mandatory and under this complicated situation, timely immunotherapy may be the influencing factors of good prognosis.