Progressive Multifocal Leukoencephalopathy Mimicking Relapse of Anti‐ GABA B Receptor Encephalitis After Rituximab‐Containing Immunotherapy: A Case Report

ABSTRACT Background Rituximab is widely used as second‐line therapy for autoimmune encephalitis; however, progressive multifocal leukoencephalopathy (PML) after rituximab‐containing immunotherapy for autoimmune encephalitis has not been reported. Case Presentation An 82‐year‐old woman with anti‐GABA B receptor encephalitis was treated with immunotherapy including rituximab. Six months later, she developed progressive neurological deterioration. MRI showed new white matter lesions suggestive of PML, and CSF JC virus (JCV) PCR was positive. Retrospective review revealed that a small right frontal lesion had already appeared 1 month after rituximab, when ultrasensitive CSF JCV PCR remained below the detection limit. Conclusion To our knowledge, this is the first reported case of PML after immunotherapy for autoimmune encephalitis. PML should be considered when neurologic deterioration or new white matter lesions emerge after rituximab‐containing immunotherapy, even when initial CSF JCV PCR is negative.