[Bickerstaff brainstem encephalitis preceding central nervous system relapse of diffuse large B-cell lymphoma].

We report the case of a 65-year-old man who was diagnosed with Bickerstaff brainstem encephalitis prior to central nervous system (CNS) relapse of diffuse large B-cell lymphoma (DLBCL). He was diagnosed with DLBCL of the paranasal sinus in June 2022. After treatment with 6 cycles of R-GCVP (rituximab, gemcitabine, cyclophosphamide, vincristine, prednisolone) and 2 cycles of high-dose methotrexate (HD-MTX), complete remission was confirmed. In June 2023, the patient developed dizziness and was admitted to our hospital. MRI showed FLAIR high-intensity areas extending from the medial part of each thalamus to the pons. Cerebrospinal fluid (CSF) showed increased mononuclear cells, but they were predominantly T-cells. Impairment of consciousness progressed, and thus the patient was treated with HD-MTX based on a presumptive diagnosis of CNS relapse of DLBCL. However, there was no improvement. An anti-GQ1b IgG antibody test was positive, leading to a diagnosis of Bickerstaff encephalitis, and 6 sessions of plasma exchange were performed. Neurological symptoms improved after these treatments, but 2 months later, the left blepharoptosis and ophthalmoparesis reappeared. CSF analysis revealed B-cell clonality, and the patient was diagnosed with CNS relapse. Autoimmune encephalitis should be considered as a differential diagnosis even in patients with lymphoma who are at high risk of CNS invasion.