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Fatal Familial Insomnia (FFI) is a rare prionopathy with autosomal dominant inheritance. Although it owes its name because insomnia is one of the most frequent and core symptoms, its clinical phenotype can be wide and heterogeneous. This usually makes it necessary to rule out other clinical processes, such as limbic encephalitis or Creutzfeldt Jakob disease, whose symptoms can sometimes overlap. We present the case of a 46-year-old male with a rapidly progressive multidomain cognitive impairment, associated with instability in gait, myoclonus and persistent and progressive insomnia. His mother had died from a genetically determined FFI (D178N mutation). Due to clinical course, an immunomediated encephalopathy was suspected, and immunosuppressive treatment with steroids and immunoglobulins was initiated. The patient showed initial improvement, but later rapidly progressive deterioration until his death 7 months after clinical onset. Cranial magnetic resonance, electroencephalogram and cerebrospinal fluid (CSF) did not show any findings. The antiTPO and antineuronal antibodies were negative. The genetiic study demonstrated the missense mutation c.532G > A (p.Asp178Asn) compatible with FFI. Postmortem study showed synaptic deposits of PrPsc in the entorhinal cortex and in thalamus, which confirmed FFI diagnosis. Unlabelled Image • It is important to rule out autoimmune disorders in patients with rapidly progressive diffuse encephalopathies. • Agrypnia excitata is related to fatal familial insomnia, but it has also been described in some autoimmune encephalitis. • Early immunotherapy initiation should be evaluated on a good risk-benefit balance. • Initial response to immunotherapy should be interpreted with caution because neurodegenerative disorders might also improve. [ABSTRACT FROM AUTHOR]