Objective: Kearns-Sayre syndrome (KSS) is a mitochondrial DNA deletion syndrome that is characterized by the triad of onset commonly before age 20, pigmentary retinopathy, and chronic progressive external ophthalmoplegia. Here, we present a case of KSS masquerading as myasthenia gravis.

Methods: Case report.

Results: A 15-year-old boy with a presumed diagnosis of myasthenia gravis presented with blurry vision, ophthalmoplegia, and ptosis. He was found to have a mitochondrial pigmentary retinopathy and was eventually diagnosed with KSS after mitochondrial DNA sequencing revealed a novel large-scale deletion of 7.9 kb of mitochondrial DNA from nucleotides 6,578 to 14,460.

Conclusions: We report a case of KSS found to have a novel large-scale mitochondrial DNA deletion. The presence of a mitochondrial pigmentary retinopathy found on dilated examination led to reconsideration of the previous diagnosis of myasthenia gravis and ultimately led to the correct diagnosis of KSS.