Neurologic Deterioration Following Immune Checkpoint Inhibitor Therapy in a Patient With HAM / TSP

Introduction HTLV‐1 is a human retrovirus that targets T lymphocytes and can cause human T‐cell lymphotropic virus type‐1‐associated myelopathy/tropical spastic paraparesis (HAM/TSP). HAM/TSP is a rare, chronic, neuroinflammatory disorder characterized by progressive lower‐extremity stiffness and weakness. Case Presentation We present a 50‐year‐old man with a history of HAM/TSP who developed Lynch syndrome with synchronous gastrointestinal cancers and experienced acute neurological worsening following pembrolizumab therapy. Conclusion This case highlights the potential for immune checkpoint inhibitor–related neuroinflammatory exacerbation in a patient with HTLV‐1 infection.