Atezolizumab Immunotherapy-Induced Encephalitis in a Patient With Triple-Negative Breast Cancer: A Case Report.

Atezolizumab, a programmed death-ligand 1 inhibitor, is a cornerstone in the management of various advanced malignancies, including triple-negative breast cancer. While generally well-tolerated, it can induce severe immune-related adverse events, with central nervous system (CNS) encephalitis being a rare but potentially life-threatening complication. We present the case of a 33-year-old female patient with stage IV triple-negative breast cancer who developed acute progressive confusion and a fluctuating Glasgow Coma Scale (GCS) following atezolizumab therapy. Diagnostic workup, including brain MRI and EEG, revealed findings consistent with encephalitis, while extensive investigations ruled out infectious, metabolic, and other autoimmune etiologies. The presence of anti-glutamic acid decarboxylase antibodies was noted, suggesting a potential autoimmune predisposition. Prompt multidisciplinary management with high-dose intravenous methylprednisolone, plasmapheresis, and intravenous immunoglobulin led to a gradual and significant improvement in cognitive function and GCS, with no recurrence of encephalitis during follow-up. This case highlights the critical importance of maintaining a high index of suspicion for atezolizumab-induced CNS encephalitis, emphasizing the need for early diagnosis, comprehensive diagnostic evaluation, and aggressive immunosuppressive therapy to achieve favorable neurological outcomes in patients receiving immune checkpoint inhibitors.