Muscle tends to be particularly affected as it requires a lot of energy, but other tissues such as the brain may also be involved. Under the microscope, the mitochondria in muscle from people with mitochondrial myopathies look abnormal, and they often accumulate around the edges of muscle fibres. This gives a particular staining pattern, known as a 'ragged red' appearance, and this is usually how mitochondrial myopathies are diagnosed. Mitochondria use a shuttle system of enzymes in what is called the respiratory chain to produce energy. If mitochondria from people with mitochondrial myopathies are studied biochemically, a block in this shuttle system can usually be demonstrated.
Mitochondrial myopathies affect people in many ways.
The most common problem is a combination of mild muscle weakness in the arms and legs together with droopy eyelids and difficulty in moving the eyes. The latter is known as chronic progressive external ophthalmoplegia. Other people do not have problems with their eye muscles, but have weakness of the arms and legs which gets worse after exertion. This may be associated with nausea and headache. Sometimes muscle weakness is obvious in small babies if the illness is severe, and they may have difficulties in feeding and swallowing.