205 An unusual cause of rapid cognitive decline in multiple sclerosis patient on natalizumab

However, it’s associated with increased risk of progressive multifocal leukoencephalopathy (PML) caused by John Cunningham virus (JC virus). The risk increases with presence of JC virus antibodies, treatment duration of more than 2 years and previous use of immunosuppressive agents. PML should be suspected whenever MS patients on natalizumab develop rapid cognitive decline and encephalopathy. Here we present a case of RRMS patient on natalizumab (had 18 infusions) for who presented with acute encephalopathy and fluctuant level of consciousness. He was admitted for assessment for PML. Investigations ruled out JC virus related PML and showed unexpected sCJD based on MRI, EEG changes and CSF analysis. To the best of our knowledge this is the first reported case of sCJD in MS patient on natalizumab in the UK.