RESUMEN
BACKGROUND: Diagnosis of rapidly progressive dementia (RPD) is very challenging. There are many conditions that fall into category of RPD ranging from autoimmune causes to neurodegenerative causes. Autoimmune encephalitis should be readily diagnosed and treated because of its response to immunomodulators. However there is no treatment available for conditions like Creutzfeldt-Jakob disease (CJD). CASE PRESENTATION: Here we present a case of anti-leucine-rich glioma inactivated 1 (LGI1) encephalitis who presented with only typical facio-brachial dystonic seizures at presentation. On follow up, patient had a rapid cognitive decline with development of myoclonic jerks, akinetic mute state and ultimately death. Neuroimaging showed presence of hyperintensities in two cortical regions namely parietal and occipital on fluid-attenuated inversion recovery (FLAIR) sequence. Electroencephalogram showed diffuse slowing with occasional periodic sharp wave complexes. Thus a diagnosis of probable CJD was made. CONCLUSION: Autoimmune encephalitis mimicking CJD or vice versa is not a very commonly encountered phenomenon. This case discusses the clinical overlap of these two conditions and its diagnostic dilemmas. This case presented with typical LGI1 encephalitis and in spite of therapy with immunomodulators had a rapid decline and ultimately turned out to be CJD. This has been rarely described in literature.