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Introduction: Muscle biopsy is used in the diagnosis of mitochondrial disorders. There are limited data on the normal ranges, so interpretation of findings can be difficult., Methods: We evaluated the percentage of fibers showing mitochondrial abnormalities using Gomori trichrome staining, succinate dehydrogenase, and cytochrome oxidase histochemistry in autopsy samples taken from 45 individuals without evidence of muscle disease and biopsies from 17 patients with mitochondrial disorders., Results: In controls, mitochondrial abnormalities were rare before the fifth decade, and most had <0.1% abnormal fibers. The proportion of abnormal fibers increased with age and was higher in deltoid than quadriceps. Most patients with mitochondrial disorders had >0.5% abnormal fibers., Conclusions: Although some patients with mitochondrial disease have very few muscle fibers that show mitochondrial abnormalities, a rate of abnormality of >0.5% fibers, in the absence of a primary muscle disease this should raise the possibility of a mitochondrial disorder., (Copyright © 2011 Wiley Periodicals, Inc.)

We have used in situ hybridization with synthetic oligonucleotide probes to the 16S sub-unit of mitochondrial ribosomal RNA to detect mitochondrial accumulations in biopsies of skeletal muscle from 16 patients with mitochondrial myopathies. In all cases, the distribution of the hybridization signal matched the pattern of succinate dehydrogenase activity in adjacent cryostat sections. In situ hybridization also allowed visualization of mitochondrial accumulations in paraffin-embedded sections of skeletal muscle that had been fixed in either De Castro's solution or formalin. DNase pre-treatment did not significantly diminish the intensity of hybridization signal, suggesting that the hybridization is predominantly to RNA. The advantages of this technique are that it allows the investigation of mitochondrial disorders when only paraffin-embedded tissue is available and should also facilitate the demonstration of mitochondrial accumulations in tissues other than skeletal muscle.