Mitochondrial diseases are whole body diseases but the exact features of the disease vary from one individual to another. Some patients will have predominantly brain disease or nerve disease. Others will have muscle disease (mitochondrial myopathies), cardiac disease (cardiomyopathies), endocrine, renal or bone marrow disease or a mixture of these and other features. Many mitochondrial diseases result in the accumulation of organic acids in the body. These are usually normal metabolic intermediates but when present in excess the acidosis itself may he damaging or even life threatening. Lactic acid accumulation is a common problem in mitochondrial diseases.
Mitochondria are small complex structures which produce energy in every cell of the body (except red blood cells). Those organs in the body which require a lot of energy to work properly are particularly dependent on well functioning mitochondria. The most energy dependent organs are brain, heart, skeletal muscle, kidney, endocrine glands and bone marrow and these are the organ systems commonly affected in mitochondrial disease.
Mitochondrial diseases used to be considered rare childhood disorders. Recently it has been discovered that many commoner diseases such as diabetes and ischaemic heart disease have, in some cases, a mitochondrial basis. Also, diseases of aging such as Parkinson's disease and Alzheimer's disease may result in part from mitochondrial failure. The aging process itself may be due to a lifetime of damage in mitochondria through oxidative stress and accumulated damage in mitochondrial proteins and mitochondrial DNA.
Most childhood onset mitochondrial diseases are inherited although in some cases the affected child seems to be the only affected family member. Mitochondria can be damaged by some drugs producing mitochondrial disease and it seems likely that high fat diets and perhaps environmental toxins contribute to the mitochondrial injury of old age. In the case of late onset adult diseases involving mitochondria, the current theory is that there are multiple contributing causes.