Posterior reversible encephalopathy syndrome masquerading as progressive multifocal leukoencephalopathy in rituximab treated neuromyelitis optica

•Both NMO and rituximab have been associated with PRES.•Distinguishing PRES from PML may be challenging.•Radiographic features of PRES may mimic those of PML.•Clinical features are very important in distinguishing between the two disorders.

Both progressive multifocal leukoencephalopathy (PML) and posterior reversible encephalopathy syndrome (PRES) have been reported as complications of rituximab therapy. These disorders may appear indistinguishable on magnetic resonance imaging (MRI). We report on a 42 year old woman with neuromyelitis optica (NMO) of 10 years duration who developed extensive white matter disease affecting chiefly both parietal lobes 6 months after her first and only dose of rituximab. The MRI findings suggested the diagnosis of PML, but her history was more consistent with PRES. Ultimately, a brain biopsy was performed which was consistent with the diagnosis of PRES. PRES and PML may have overlapping symptomatology and be indistinguishable on MRI. An approach to distinguishing between these two disorders is addressed.