Immune Checkpoint Inhibitor-Induced Miller Fisher Syndrome: A Case of Relapsing Symptoms Requiring a Slow Corticosteroid Taper.

Immune checkpoint inhibitors (ICIs) have transformed oncology and are used across multiple malignancies, including Merkel cell carcinoma. However, ICIs have been increasingly recognized to trigger neurological immune-related adverse events. Miller Fisher syndrome is a rare variant of Guillain-Barré syndrome characterized by ophthalmoplegia, ataxia, and areflexia. We report the first fully described case of avelumab (a programmed death-ligand 1 (PD-L1) inhibitor)-associated MFS, highlighting its rarity and relapsing course. A 76-year-old man with metastatic Merkel cell carcinoma developed diplopia, ptosis, and ataxia shortly after two doses of avelumab. On examination, he had the classic clinical triad of ophthalmoplegia, ataxia, and areflexia. Cerebrospinal fluid analysis revealed albuminocytologic dissociation, anti-GQ1b antibodies were negative, and investigations for differential diagnoses such as myasthenia and paraneoplastic antibodies were negative. Nerve conduction studies demonstrated only mild sensory abnormalities without demyelination. He made substantial improvements with high-dose oral corticosteroids and plasma exchange but relapsed twice after steroid withdrawal (once after four months, and again after 14 months), requiring a slow taper of over one year. He has remained symptom-free on low-dose maintenance corticosteroids after two years of follow-up, and his Merkel cell carcinoma remains in complete remission after permanently discontinuing his immunotherapy and starting radiotherapy. Similar MFS presentations have been described with programmed cell death protein 1 inhibitors (such as pembrolizumab) but not previously with PD-L1 blockers. This case expands the spectrum of ICI-associated neuropathies, emphasizing that MFS due to PD-L1 inhibition may occur despite negative antibodies and subtle neurophysiological findings. Early immunosuppression, cautious corticosteroid tapering, and close multidisciplinary collaboration are essential for achieving sustained neurological and oncologic recovery.