Mitochondrial diseases are a genetically and clinically heterogeneous group of disorders that arise as a consequence of dysfunction of the mitochondrial respiratory chain. The estimate for the prevalence of all mitochondrial disorders 1:8500, but they are thought to be greatly under-diagnosed. Mitochondrial disorders can be caused by mutations of nuclear or mitochondrial DNA (mtDNA). If nuclear gene defects may be inherited in an autosomal recessive or autosomal dominant manner, mtDNA defects are transmitted only maternally. As the female could have heteroplasmic mtDNA mutations, which could be transmitted unequally to her offspring, the sibs could exhibit considerable clinical variability.
Symptoms of the mitochondrial disease can begin at any age. Mitochondrial disorders may affect a single organ (e.g. Leber hereditary optic neuropathy, LHON) or involve multiple organ systems (e.g. Myoclonic epilepsy with ragged-red fibers, MERRF). Common clinical features of mitochondrial disorder include, for example muscle weakness, exercise intolerance, trouble with balance and coordination, sensorineural deafness, impaired vision, seizures and learning deficits, cardiomyopathy, diabetes mellitus, stunted growth, and a high incidence of mid- and late pregnancy loss.
Indications for genetic testing:
1. Diagnosis of patients with phenotype characteristic for mitochondrial disease
2. Diagnosis of patients with family history suggestive for mitochondrial disease
3. Genetic counseling of individuals with mitochondrial disease and affected family members
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