Creutzfeldt-Jakob disease mimicking Hashimoto's encephalopathy: steroid response followed by decline.

The diagnosis of Creutzfeldt-Jakob disease (CJD) is particularly challenging because its heterogeneous clinical presentations mimic other rapidly progressive dementias and neurodegenerative disorders (e.g., Hashimoto's encephalopathy, autoimmune encephalitis, atypical Alzheimer's disease). Diffusion-weighted imaging (DWI) is the most sensitive neuroimaging sequence for diagnosing CJD. However, early magnetic resonance imaging (MRI) findings may be subtle or evolving, and autoimmune etiologies often remain in the differential. Therefore, empiric corticosteroids are reserved for cases in which an autoimmune etiology is under consideration while definitive tests are pending. A 67-year-old woman presented with rapidly progressive cognitive decline, ataxia, and visual symptoms. Short-course glucocorticoids produced transient improvement for three days, followed by rapid deterioration within a week. Serial MRI evolved from cortical ribboning to basal ganglia involvement. Electroencephalogram (EEG) showed non-convulsive status epilepticus that responded to diazepam and valproate. Cerebrospinal fluid (CSF) 14-3-3 protein (14-3-3) and RT-QuIC were positive, confirming prion disease. CJD can present with features resembling HE, and brief improvement after a short course of glucocorticoids, even in the presence of markedly elevated thyroid antibodies, does not exclude CJD. To avoid diagnostic delay, obtain CSF RT-QuIC and 14-3-3 at presentation before or in parallel with glucocorticoids, and use serial MRI and EEG to arbitrate.