Deletions of mitochondrial DNA (mtDNA) are reported in 19 of 56 patients with mitochondrial myopathy examined in the Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, MN. All 19 patients had progressive external ophthalmoplegia and 12 had complete or partial Kearns-Sayre syndrome. The age at onset varied from 4 to 48 years, 10 presenting in childhood. Patients with more than 50% deleted mtDNA had an earlier onset of symptoms and a higher proportion of ragged red fibers and cytochrome c oxidase negative fibers than patients with less than 50% deleted mtDNA. 
COMMENT. This paper confirms that large scale mtDNA deletions are present in a high proportion of patients with mitochondrial myopathy associated with progressive external ophthalmoplegia and that these deletions are a hallmark of Kearns-Sayre syndrome. Kearns-Sayre syndrome consists of progressive external ophthalmoplegia, pigmentary retinopathy, cardiac conduction abnormalities, and mitochondrial myopathy involving facial, cervical and limb muscles and increased CSF protein.