Introduction: Progressive multifocal leukoencephalopathy (PML) is a rare infection of the central nervous system due to the reactivation of the John Cunningham polyomavirus. It is commonly a progressive fatal disease with worldwide distribution. Case Report: We describe a rare case of PML, which was misdiagnosed as neuropsychiatric systemic lupus erythematosus (SLE) with a catastrophic outcome due to delay in diagnosis with superadded cyclophosphamide therapy. Conclusion: There are several lessons taught from our case. Firstly, in patients with autoimmune disorders who are strongly immunosuppressed, the new onset of cognitive impairment and seizures should alert the treating physician to look carefully for PML. Secondly, in cases of SLE where the diagnosis of the cause of cognitive impairment and seizures is not clear, we suggest that immunosuppression should not be intensified until PML has clearly been ruled out. Lastly, multidisciplinary care in patients with suspected neuropsychiatric SLE including a neurologist, an infectious diseases consultant, a neuroradiologist, and a rheumatologist is needed.

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