doi 10.17992/lbl.2026. 09.908

Correspondence: Olafur Sveinsson, olafursv@landspitali.is

Key words: Progressive multifocal leukoencephalopathy, PML, JC virus, rituximab

Progressive multifocal leukoencephalopathy (PML) is a rare and life-threatening demyelinating disease of the central nervous system that occurs almost exclusively in the setting of impaired cell-mediated immunity. An 86-year-old woman with a history of PR3-ANCA-associated vasculitis treated with rituximab presented to the emergency department with progressive left-sided weakness and gait instability. Brain MRI demonstrated an enlarging white matter lesion without contrast enhancement or mass effect. JC virus was detected in the cerebrospinal fluid, thereby confirming the diagnosis of PML. Rituximab therapy was discontinued and mirtazapine initiated. Other treatment options were not considered feasible in this case. The patient deteriorated rapidly and died within three months from initial presentation at the emergency department. PML occurring in association with rituximab therapy is exceedingly rare. The potential for disease attenuation with new immunomodulating therapies underscores the importance of considering PML in immunosuppressed patients presenting with rapidly progressive central nervous system deficits.