Kearns-Sayre syndrome is a very rare mitochondrial DNA disorder in which there is marked heterogenicity along with various inheritance patterns. It is distinguished by three features: (1) onset before 20 years of age; (2) a chronic, progressive external ophthalmoplegia; and (3) pigmentary degeneration of the retina.

As of 1992, a total of 226 cases of Kearns-Sayre syndrome have been documented in all of medical literature. There is no observed predilection with regard to race or gender.

Kearns-Sayre syndrome occurs as the result of mitochondrial DNA (mtDNA) deletions that produce a particular, peculiar phenotype. Most of the mtDNA deletions in this disorder are sporadic but occur most commonly between positions 8469 and 13147.