We report the case of a woman with natalizumab-treated multiple sclerosis (MS) and clinically silent progressive multifocal leukoencephalopathy (PML) with an unusually long preclinical phase, followed by acute symptoms due to development of immune reconstitution inflammatory syndrome (IRIS). Furthermore, the course of the IRIS was prolonged and continued to progress even five months after natalizumab treatment was ceased. This case shows that PML and IRIS can have a considerably variable course in natalizumab-treated MS patients and underlines the need for PML screening in JC virus antibody-positive patients in order to detect clinically silent cases.
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