Distinguishing between the causes of rapidly progressive dementia can be difficult, yet diagnostic clarity is important for treatment decisions and prognostication.

A 62-year-old woman from Minnesota presented with rapidly progressive gait instability. Two months prior to presentation, she had developed imbalance followed by lightheadedness, periodic tremulousness, nighttime headaches, and a marked decrease in energy and ability to walk. She was unable to continue her office job and stopped driving. Initial neuroimaging was unremarkable. During the next month, she developed binocular horizontal and vertical diplopia, somnolence, and worsening tremor and ataxia. Her husband noted memory decline and new violent jerking movements upon falling asleep. On exam, she had orthostatic hypotension, fine hand tremor bilaterally, square wave jerks, and an unsteady wide-based gait. Repeat MRI showed T2-hyperintensity in the right medial temporal lobe, right basal ganglia, and caudate. Cerebrospinal fluid (CSF) was non-inflammatory and infectious testing was negative. Autoimmune encephalitis panel in CSF and serum were unrevealing. Cancer screen with mammogram, colonoscopy and CT-PET scan was normal. Intravenous methylprednisolone was started for possible autoimmune limbic encephalitis, but she continued to worsen and was admitted to the general neurology service after a fall resulting in a wrist fracture. Repeat MRI showed cortical ribboning with T2-signal and diffusion imaging abnormalities in the caudate, thalamus, and hippocampus. CSF testing at the Prion Disease Laboratory revealed abnormally elevated RT-QuIC, total tau, and 14-3-3 protein highly suggestive of Creutzfeldt-Jakob Disease (CJD). Her husband disclosed that they occasionally hunt and eat wild game. She continued to rapidly decline and was transitioned to hospice care. 

We highlight a case of Creutzfeldt-Jakob Disease with mesial temporal T2-hyperintensity mimicking autoimmune encephalitis. Cerebrospinal fluid RT-QuIC is a useful highly specific confirmatory test for CJD.