Isolated fourth cranial nerve palsy in a patient treated with pembrolizumab: a case report.

: Pembrolizumab is widely used in melanoma treatment and may rarely be associated with neuro-ophthalmic immune-related adverse events. Isolated ocular motor nerve palsies are exceptional. : We report a 76-year-old man who presented with sudden vertical binocular diplopia six weeks after starting adjuvant pembrolizumab following resection of cutaneous melanoma. A comprehensive neurological, ophthalmological, laboratory, and neuroimaging assessment was performed. : Ocular motility examination revealed an isolated right superior oblique palsy. Brain and orbital MRI showed no abnormalities. Serum creatine kinase was elevated, without clinical evidence of myocarditis or generalized myopathy. After discontinuation of pembrolizumab and initiation of low-dose corticosteroids, diplopia progressively resolved and creatine kinase levels normalized. : This case describes a rare isolated fourth cranial nerve palsy temporally associated with pembrolizumab, highlighting the importance of considering uncommon neuro-ophthalmic presentations during immune checkpoint inhibitor therapy.