Abstract
Fatal familial insomnia (FFI) is a rare prion disease commonly inherited in an autosomal dominant pattern from a mutation in the PRioN Protein (PRNP) gene. Hashimoto's encephalopathy (HE) is characterised by encephalopathy associated with antithyroid peroxidase (TPO) or antithyroglobulin (Tg) antibodies. These two conditions characteristically have differing clinical presentations with dramatically different clinical course and outcomes. Here, we present a case of FFI mimicking HE. A woman in her 50s presented with worsening confusion, hallucinations, tremor and leg jerks. Several maternal relatives had been diagnosed with FFI, but the patient had had negative genetic testing for PRNP. MRI of brain, cervical and thoracic spine were unremarkable except for evidence of prior cervical transverse myelitis. Cerebrospinal fluid analysis was normal. Anti-TPO and anti-Tg antibodies were elevated. She was started on steroids for possible HE and showed improvement in symptoms. Following discharge, the results of her PRNP gene test returned positive for variant p.Asp178Asn.