The three main categories of diseases are listed below:
Glycogen storage diseases- defect in sugar metabolism
Lipid storage disorder- defect in fat processing
Mitochondrial myopathy- defect in mitochondrial enzyme

Diagnosis
The symptoms of a metabolic myopathy can be easily confused with the symptoms of another disease. In most cases, a Muscle biopsy is necessary for an accurate diagnosis of the cause of muscle weakness. A blood test can be done under normal circumstances to test for genetic differences and signs of tissue breakdown, or with an added cardio portion that can indicate if muscle breakdown is occurring. An electromyography is sometimes taken in order to rule out other disorders if the cause of fatigue is unknown. Differentiating between different types of metabolic myopathies can be difficult due to the similar symptoms of each type such as Myoglobinuria and exercise intolerance. It has to be determined whether the patient has fixed or exercise induced manifestations, and if exercise related what kind of exercise, before extensive exercise related lab testing is done to determine the underlying cause. Treatment
Metabolic Myopathies have varying levels of symptoms, being most severe when developed during infancy. Those who do not develop a form of a metabolic myopathy until they are in their young adult or adult life tend to have more treatable symptoms that can be helped with a change in diet and exercise. Depending on what enzyme is affected, a high-protein or low-fat diet may be recommended along with mild exercise.
It is important for people with metabolic myopathies to consult with their doctors for a treatment plan in order to prevent acute muscle breakdowns while exercising that lead to the release of muscle proteins into the bloodstream that can cause kidney damage. References
External links
Metabolic Myopathies - eMedicine