Kearns- Sayre syndrome is a mitochondrial disease characterized by a triad of features including onset in persons younger than 20 years, chronic progressive external ophthalmoplegia and pigmentary degeneration of retina. It may affect many organ systems and a wide range of complications may develop. Cardiac conduction defects are significant and preventable cause of mortality.

We present a 13-year-old boy with Kearns-Sayre syndrome. His complaints were short stature, ptosis and weakness. Clinical findings consisted of failure to thrive, external ophthalmoplegia, bilateral ptosis, mild hypotonia, retinitis pigmentosa and complete heart block. Cerebrospinal fluid protein and lactate levels were increased. Cranial magnetic resonance imaging showed bilateral symmetric hyper intensities. The ragged red fibers were not shown by modified Gomori's trichrome staining but succinate dehydrogenase and cytochrome oxidase combined enzyme staining demonstrated the correlation of COX negative fibers with succinate dehydrogenasepositive ones. In addition, an increased number of mitochondria on electron microscopic examination of the skeletal muscle was determined. No mutations were found in the DNA from blood samples.

Most patients with Kearns-Sayre syndrome have deleted mitochondrial DNA. However, mutations are organ specific and could not be demonstrated in blood samples. Electron microscopy is also of limited use in the diagnosis of mitochondrial disease as there are abnormal fibers in normal muscle and it is the percentage of these fibers that is important. The ragged red fibers can be noticed with modified Gomori's trichome staining in mitochondrial diseases of long duration but ragged red fibers may not be shown with histochemical stains in muscle biopsies of children with mitochondrial myopathies. Enzyme activity revealed on muscle biopsy is therefore important for the diagnosis of mitochondrial diseases, especially in early childhood.