The effects of mitochondrial disease can be quite varied. Since the distribution of defective DNA may vary from organ to organ within the body, the mutation that in one person may cause liver disease might in another person cause a brain disorder. In addition, the severity of the defect may be great or small. Some minor defects cause only "exercise intolerance", with no serious illness or disability. Other defects can more severely affect the operation of the mitochondria and can cause severe body-wide impacts.
As a general rule, mitochondrial diseases are worst when the defective mitochondria are present in the muscles, cerebrum, or nerves, because these are the most energy-hungry cells of the body.
However, even though mitochondrial disease varies greatly in presentation from person to person, several major categories of the disease have been defined, based on the most common symptoms and the particular mutations that tend to cause them.
