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The metabolic myopathies comprise a diverse group of genetic disorders that cause impaired energy production in muscle. These disorders are organized into three major groups:
- Glycogen metabolism disorders
- Lipid metabolism disorders
- Mitochondrial myopathies
However, there are some metabolic myopathies that do not clearly fit into any of these groups. Unlike the muscular dystrophies, these disorders often cause dynamic muscle symptoms that fluctuate in relation to exercise or other stress on the muscles. Patients with these disorders frequently report episodes of muscle pain, poor endurance, or weakness in response to exercise, cold, prolonged fasting, and other illness. In between these episodes, the symptoms may be mild or absent.
Glycogen Metabolism Disorders:
Glycogen metabolism disorders are caused by impairment in the body's ability to convert glucose stores into usable energy. As most glucose stores in the body are found in the liver and the skeletal muscle, these are the organs that are most commonly affected in these disorders.
There are 14 named glycogen metabolism disorders, the most common of which are: McArdle disease (phosphorylase deficiency), Pompe disease (acid maltase deficiency), Tarui disease (phosphofructokinase deficiency), and phosphorylase b kinase deficiency.
Lipid Metabolism Disorders:
Skeletal muscle breaks down lipids in the form of fatty acids in order to produce energy at rest and with exercise. Impairment in the ability to break down these fatty acids can therefore produce muscle weakness and pain during periods of prolonged exercise.
The most common lipid metabolism disorders presenting in adulthood include carnitine palmitoyltransferase II deficiency (CPT II), trifunctional protein deficiency (TFP), and very long-chain acyl-CoA dehydrogenase deficiency (VLCAD).
The mitochondria are small organs inside cells whose primary purpose is energy production. They are unique because they contain their own separate set of DNA that is passed on only from mother to child. Because they are found in almost all human cells, dysfunction in the mitochondria can cause problems in multiple organ systems. In most patients, this includes the musculoskeletal system.
Some of the more common mitochondrial disorders include MELAS, MERRF, chronic progressive external ophthalmoplegia, and Kearns-Sayre syndrome.
The diagnosis of metabolic myopathies begins with a detailed history and physical examination. Other diagnostic tests that can be used to confirm or rule out specific metabolic myopathies include blood tests, urine tests, electromyography (EMG), muscle biopsy, exercise testing, and genetic testing.
The treatment for metabolic myopathies varies widely based on the nature of the disorder and the symptoms that it produces. For the various metabolic myopathies, medications, modifications in activity, dietary changes, and nutritional supplements may be recommended to reduce symptoms and prevent complications. Rehabilitation specialists can help determine appropriate exercise and stretching regimens to prevent muscle loss and maximize activity. Genetic counseling can help to determine the risk of inheritance in other family members, as well as the risks and benefits of genetic testing. Social, mental health and financial needs will be addressed by social workers and other members of the medical team. If needed, other physicians, such as pulmonologists, orthopedic surgeons, and cardiologists, will be consulted to provide expert advice on management of extra-muscular symptoms and complications.
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