“What is Mitochondrial Disease?”

Basis of the Disease
Mitochondrial diseases result from failures of the mitochondria, specialized compartments present in every cell of the body except red blood cells. Mitochondria are responsible for creating more than 90% of the energy needed by the body to sustain life and support growth. When they fail, less and less energy is generated within the cell. Cell injury and even cell death follow. If this process is repeated throughout the body, whole systems begin to fail, and the life of the person in whom this is happening is severely compromised. The disease primarily affects children, but adult onset is becoming more and more common.

Diseases of the mitochondria appear to cause the most damage to cells of the brain, heart, liver, skeletal muscles, kidney and the endocrine and respiratory systems.

Depending on which cells are affected, symptoms may include loss of motor control, muscle weakness and pain, gastro-intestinal disorders and swallowing difficulties, poor growth, cardiac disease, liver disease, diabetes, respiratory complications, seizures, visual/hearing problems, lactic acidosis, developmental delays and susceptibility to infection.

Laboratory Evaluation for Disorders of Energy Metabolism 
Laboratory testing is the usual method physicians go about evaluating patients for disorders of energy metabolism (which include mitochondrial disorders, disorders of oxidative phosphorylation and b -oxidation). Most hospitals do not have a metabolic laboratory and therefore can run only the most basic tests. However, most hospitals will send specimens to any laboratory in the country. Not all laboratory tests are required for all patients, and your physician may decide that some of these tests are not necessary. The lists are authoritative, but are meant to serve as a general guide for evaluation. Not all metabolic disorders primarily affect energy metabolism, but the clinical features may overlap. Testing for these metabolic are listed in a separate table. There is no substitute for good clinical judgment.

The initial laboratory evaluation is generally used as a non- invasive screening for inborn errors of energy metabolism. If the results of this evaluation are suggestive of a specific disorder, a direct test for the disease in question may be able to be performed. If the results of the initial evaluation are normal and there is a strong suspicion of a disorder of a mitochondrial disease, a more intensive evaluation is performed.

The secondary tests are more invasive (and may include a spinal tap) and because some of the tests may require urine specimens collected over time, a bladder catheter may be required in young children. Many of these tests require the specimens to be sent to a special laboratory. Abnormalities found on the secondary tests will guide the physician as to the direction of further testing. However, as with the initial testing, normal results do no eliminate the possibility of a mitochondrial disease, but make it less likely.

The tertiary tests are invasive and/ or expensive, and may carry with them some risks, such as metabolic decompensation during a fast. However, if the physician strongly suspects a metabolic illness, these tests may be diagnostic. The muscle biopsy is a tertiary test, but is listed separately because it is the most complicated and invasive of all tests, and in children requires a general anesthesia. Although a muscle biopsy can be performed at any medical center, very few centers have the ability to do all the testing necessary to make a diagnosis. Therefore, the physician must be very conscientious in planning before the biopsy is done.

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