Metabolic myopathy
Metabolic myopathies are myopathies that result from defects in biochemical metabolism that primarily affect muscle. They are generally genetic defects that interfere with muscles ability to create energy. At the cellular level, metabolic myopathies lack some kind of enzyme that prevent the chemical reactions necessary to create adenosine triphosphate (ATP). This prevents the muscle cells from being able to function properly and disrupts communication between joints and bones. Some people with a metabolic myopathy never develop symptoms due to the bodys ability to produce enough ATP through alternative pathways. In the event more ATP is needed from the affected pathway, the lack of it becomes an issue and symptoms develop. People with a metabolic myopathy often experience symptoms such as progressive muscle weakness, fatigue, pain and cramping after exercise, and considerable breakdown of muscle tissue. The degree of symptoms varies greatly from person to person and is dependent on the severity of enzymatic defect. In extreme cases it can lead to Rhabdomyolysis.

Types
Metabolic myopathies are generally caused by an inherited genetic mutation. This mutation has an autosomal recessive inheritance pattern making it fairly rare to inherit, but it can also be caused by a random genetic mutation. Metabolic myopathies are named after the pathway in which the defective enzyme or a lack of enzymes is present. This causes the underproduction of adenosine triphosphate (ATP) and affects the muscles in different parts of the body. 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