Mitochondria are small organelles inside almost all the cells, responsible for generating energy. Mitochondrial diseases are caused by defects in mitochondria, and in most cases, affect more than one type of cell, tissue, or organ. As muscle and nerve cells have especially high needs of energy, muscular and neurological problems are common in mitochondrial diseases. Mitochondrial diseases with prominent muscular problems are called mitochondrial myopathy. If both muscular and neurological symptoms are prominent, they are called mitochondrial encephalomyopathy.
Mitochondrial myopathies are caused by genetic mutations. The genes involved normally make proteins that work in or reside in the mitochondria. The inheritance of mitochondrial myopathies is complex, as they can be caused by mutations in either nuclear DNA (nDNA) or mitochondrial DNA (mtDNA). Most mitochondrial diseases caused by nDNA mutations are autosomal recessive: it takes mutations in both copies of a gene to cause disease. Mitochondrial diseases caused by mtDNA mutations are inherited in a maternal pattern, as mtDNA passes only from mother to child. A mother can pass defective mtDNA to any of her children, but only her daughters-and not her sons-will pass it to the next generation. In addition, as a typical human cell contains only one nucleus but hundreds of mitochondria, a single cell can contain both normal and defective mitochondria. The balance between the two will determine the cell’s health and the severity of the disease.
None of the typical symptoms of mitochondrial myopathies, such as muscle weakness, exercise intolerance, heart problems, are unique. But a combination of several symptoms in one person, especially when involving more than one organ system, strongly points to mitochondrial diseases. To evaluate the extent of these symptoms, a physician usually begins by taking the patient’s medical history and detailed family history including at least three generations, and then proceeds with physical and neurological exams. Depending on the information obtained, more specialized tests may be ordered to detect abnormalities in muscles and other organs, such as a muscle biopsy. Finally, a genetic test can determine whether someone has a genetic mutation that causes mitochondrial disease. A positive result can confirm the diagnosis, but a negative result does not necessarily negate the diagnosis.
There is no cure for mitochondrial diseases, and the goal of treatment is to alleviate symptoms and preserve or improve functioning, mobility and strength. Many of the symptoms have highly effective treatments, including medications, dietary modifications and lifestyle changes. For specifics, please refer to https://www.mda.org/disease/mitochondrial-myopathies/medical-management.