Background: Chronic progressive external ophthalmoplegia (CPEO) is a mitochond rial cytopathy presenting with ptosis and external ophthalmoparesis. Mitochondri al disorders are characterized by a broad clinical spectrum and genetic backgrou nd with marked genotype/phenotype variability. The routine diagnostic work-up u sually includes clinical and laboratory examinations as well as histological and histochemical analysis of skeletal muscle biopsy. In our case only an additiona l molecular biological examination allowed the diagnosis of CPEO. Patient and Me thods: We report a 35-year-old woman with a 7-years history of slowly progres sive diplopia due to impaired ocular motility and bilateral ptosis. We performed ophthalmological and neurological examinations, laboratory testing, lower limb skeletal muscle biopsy including histo logical and histochemical investigations, biochemical analysis of respiratory ch ain enzymes in skeletal muscle homogenate and molecular genetic testing of skele tal muscle DNA. Results: Although clinical, laboratory, histological and biochem ical analyses did not give any hints suggesting a mitochondrial cytopathy, molec ular genetic testing of total DNA from skeletal muscle tissue by Southern blot a nalysis finally revealed a 3.8 kb mitochondrial DNA deletion with a degree of he teroplasmy of 45%. Conclusions: In patients with unexplained binocular diplopia , oculomotor deficits and/or acquired ptosis, an underlying mitochondrial cytopa thy should be considered. Even in the case of inconspicuous skeletal muscle hist ology and biochemistry, molecular genetic testing of skeletal muscle DNA is advi sable in order to establish the diagnosis.
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