Expanding the Genetic and Phenotypic Spectrum of Kearns-Sayre Syndrome: A Case Report.

Chronic progressive external ophthalmoplegia (CPEO) is a mitochondrial disorder characterized by progressive bilateral ptosis and symmetric ophthalmoparesis. When CPEO is associated with pigmentary retinopathy, cardiac conduction defects, endocrine abnormalities, muscle weakness, and other neurological impairments, it defines Kearns-Sayre syndrome (KSS), most commonly caused by a single large-scale mitochondrial DNA (mtDNA) deletion. We report a case of a 34-year-old man with a three-year history of progressive bilateral ptosis. A muscle biopsy from the left vastus lateralis revealed cytochrome c oxidase-deficient fibers (COX-negative). mtDNA analysis revealed a novel single large-scale deletion detected in muscle tissue. This deletion has not been previously reported in the scientific literature and led to a diagnosis of KSS. Additionally, cerebrospinal fluid analysis revealed the presence of oligoclonal bands, a finding not previously described in KSS. The deleted mtDNA region includes ND4, ND5, and ND6 genes, which encode subunits of NADH dehydrogenase. These genes are implicated in various biological functions, including mitochondrial energy production, seizure susceptibility, and inflammatory processes.