LGI1 Encephalitis Masquerading as Creutzfeldt-Jakob Disease – A Diagnostic Challenge: Case Report
Rohan Joshi1, Prasanna Venkatesan Eswaradass1, Elana Goldenberg2, Yunxia Wang1
1University of Kansas Health System, 2University of Kansas School of Medicine
Objective:
To describe a diagnostically challenging case of LGI1 encephalitis in a patient who initially met “probable CJD” criteria.  
Background:
LGI1 encephalitis and Creutzfeldt-Jakob disease (CJD) share overlapping clinical features, including rapidly progressive dementia, neuropsychiatric symptoms, seizures, and movement disorders. Early in the disease course, this resemblance can lead to diagnostic uncertainty and potential misclassification as prion disease. 
Design/Methods:
NA
Results:

A 43-year-old software engineer with no prior neurologic history presented with a new-onset tonic-clonic seizure and was incidentally found to have a cavernoma. Within weeks, he developed speech arrest, cognitive decline (MoCA 21/30), personality changes, insomnia, and diffuse myoclonic jerks (prominent in the face and upper extremities). MRI brain revealed T2/FLAIR hyperintensity in the caudate and putamen (L>R). EEG captured no correlate for the jerks. CSF analysis showed WBC 1, protein 59; autoimmune and prion panels were sent. The patient’s rapid decline, myoclonus, right leg spasticity, and basal ganglia signal abnormalities satisfied “probable CJD” criteria.  

Empiric treatment with high-dose steroids and IVIG yielded no immediate improvement. Repeat imaging demonstrated multifocal cortical and bilateral basal ganglia FLAIR hyperintensities; reviewing repeat vEEG revealed that the observed myoclonic jerks clinically resembled faciobrachial dystonic seizures (FBDS). He was discharged to rehabilitation on a prednisone taper with neuroimmunology follow-up. Post-discharge, CSF returned positive for LGI1 antibodies and negative for RT-QuIC/14-3-3, confirming LGI1 encephalitis. He is currently awaiting rituximab initiation.

Conclusions:
This case underscores that LGI1 encephalitis can mimic CJD and fulfill “possible” or “probable” CJD diagnostic criteria. Awareness of this overlap is essential to avoid premature attribution to prion disease and to facilitate appropriate immunotherapy.
10.1212/WNL.0000000000213234
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