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Metabolic myopathies are genetic diseases, usually inherited, that affect the body’s muscles. [Metabolism refers to chemical reactions that provide energy and nutrients, or substances necessary for health and growth.] Some people with a metabolic myopathy (muscle disease) develop weakness; others tire easily with exercise or physical activity, suffer muscle pain after physical effort, and/or experience severely swollen and tender muscles. These symptoms are caused by the muscle cells’ not getting enough energy. Without enough energy, the muscle lacks enough “fuel” to work properly.

Each of these disorders causes symptoms when the muscle cells cannot make enough energy to move the muscle.

Not all people with a metabolic myopathy experience the same symptoms.

Diet, levels of physical activity, and dietary supplements recommended for treatment will vary depending on the myopathy

Metabolic myopathies are genetic defects that interfere with the energy-generating processes in skeletal muscles. When these defects interfere with the muscle functioning and block energy production, muscle cells cannot work properly. Then the muscles that help the body’s movement are unable to communicate (“talk”) between bones and joints. These myopathies can cause progressive muscle weakness, fatigue, episodes of pain and cramps after exercise, and/or extensive death and breakdown of muscle tissue.

Some myopathies can generate a condition called rhabdomyolysis which breaks the muscles down, leaving them very weak, sore, swollen and tender. As portions of muscle fiber resulting from breakdown enter the circulation, they can lead to kidney failure.

What causes metabolic myopathies

Muscles require chemical energy to work properly. This energy comes from a chemical called ATP (adenosine triphosphate). ATP is made in cells from sugars or fats by series of chemical reactions called pathways. Processing these nutrients for energy begins inside the cells (in the sarcoplasm). It is completed in the cells, in small compartments (organelles) called mitochondria. Under normal conditions, most ATP is produced in mitochondria.

Specific proteins called enzymes are necessary for each chemical reaction. A metabolic myopathy is caused when enough of a particular enzyme is not present to cause the necessary reactions.

The name of each metabolic myopathy is based on which pathway has the deficient enzyme. For instance, diseases due to a defect in sugar metabolism are called glycogen storage diseases. The most common defect in sugar metabolism is McArdle’s disease. Lipid storage diseases are due to abnormal fat processing. Finally, the term mitochondrial myopathy is used when the enzyme present in mitochondria is deficient.

Some people with metabolic myopathies live normal lives and never experience significant symptoms. That is because our cells have several ways (pathways) to make ATP. Under usual conditions, normal pathways can produce enough ATP to make up for the deficient pathway. However, when the body needs to make more energy from the deficient pathway, the deficit in ATP can become severe, so symptoms develop. When cells are moderately low in ATP, then the major problem may be fatigue or exercise intolerance. [Think of a car engine. If the gas in the tank is a good fuel, the engine runs well. If the gas has water or sand in it, the car runs poorly.] Rhabdomyolysis (muscle cell death) occurs when the cells are severely depleted of ATP (that is, the car runs out of gas). Constant or fixed (on-going) weakness may develop as a result of repeated low grade rhabdomyolysis.

Most metabolic myopathies tend to run in families, appearing at any age in those who inherit the disease. Either parent or another relative may have been symptomatic. However, metabolic myopathies can develop in a person with no family history of the condition.

How metabolic myopathies are diagnosed

Most often, metabolic myopathies are diagnosed through testing of a sample of muscle tissue (biopsy), which is taken through a needle or with a small incision under local anesthesia. However, more recent techniques enable diagnosis through genetic testing of blood samples.

How metabolic myopathies are treated

Treatment varies by myopathy, but is focused on changes in physical activities, aerobic exercise training, changes in diet, and the use of various vitamins or supplements.

Living with the illness

Everyone has run out of energy, become tired when doing physical activities, or experienced muscle pains and cramps after exercising. These are common symptoms. However, for some, these are the symptoms coming from metabolic myopathy. Metabolic myopathies are rare, so many physicians are not familiar with this type of muscle disease. Patients often go undiagnosed.

How do we know when these commons symptoms mean that patients have one of these diseases? First, patients, family and friends realize there is a reason for their limitations. Second, because these are inherited diseases, genetic counseling will help people understand the potential risks for future children. Finally, with appropriate activity, changes, regular aerobic exercise, diet and use of supplements, the symptoms often can be controlled.

Metabolic myopathies are rare genetic diseases that cause muscle problems.

Diagnosis often requires a muscle biopsy.

Despite the fact that metabolic myopathies are inherited, treatments can help many people with these diseases live normal lives.

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