Clinical Reasoning: A 66-Year-Old Female Patient With Rapidly Progressive Memory Changes.

An otherwise healthy 66-year-old female patient presented with 3 months of memory loss prompting evaluation for rapidly progressive dementia. A broad systemic evaluation for vascular, infectious, toxic metabolic, autoimmune/paraneoplastic, and neoplastic diseases was performed. Neuroimaging revealed symmetric bilateral fluid-attenuated inversion recovery hyperintensities of the thalami, paramedian frontal regions, and mesial temporal lobes without contrast enhancement on MRI. Lumbar puncture did not uncover an inflammatory profile. The patient was treated for presumed seronegative autoimmune encephalitis and showed mild initial improvement but subsequently developed obstructive hydrocephalus and status epilepticus during the corticosteroid taper. The patient's atypical course and imaging presented a challenging differential diagnosis and provide an instructive approach to establishing a definitive diagnosis.