Metabolic Myopathies

Metabolic myopathies are genetic defects that interfere with the processes in muscles that generate energy. When these defects interfere with muscle function and block energy production, the cells in the muscles 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 breakdown and death of muscle tissue.

Some myopathies can cause rhabdomyolysis. In this condition, muscles break down, leaving them very weak, sore, swollen and tender. As muscles break down, portions of muscle fiber enter the bloodstream, which can lead to kidney failure.

Muscles require chemical energy to work properly. This energy comes from a chemical called adenosine triphosphate (sometimes called ATP). ATP is made in cells from sugars or fats by chemical reactions called pathways. Proteins called enzymes are necessary for each chemical reaction. A metabolic myopathy happens when there isn’t enough of a particular enzyme to cause the necessary reactions.

The name of each metabolic myopathy is based on which pathway doesn’t have enough enzymes. For example, diseases caused by 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. 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 cells have several ways (pathways) to make ATP. Under usual conditions, normal pathways can produce enough ATP to make up for the pathway that is not working. However, when the body needs to make more energy from that pathway, the deficit in ATP can become severe and symptoms develop.

When cells are moderately low in ATP, the major problem may be fatigue or exercise intolerance. Think of a car engine. If the gas in the tank is 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 don’t have enough ATP. Think of this as a car running out of gas. Constant or on-going weakness may develop as a result of repeated low-grade rhabdomyolysis.

Many different diseases and conditions can cause muscle symptoms. Rheumatologists are trained to determine the diagnosis responsible for the symptoms in individual patients.

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

Treatment for metabolic myopathies 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. With appropriate activity changes, regular aerobic exercise, diet and use of supplements, the symptoms often can be controlled. The diet, levels of physical activity and dietary supplements recommended for treatment will vary depending on myopathy.

Everyone can run out of energy, become tired when doing physical activities, or experience muscle pains and cramps after exercising. However, for some, these are the symptoms coming from metabolic myopathy and evaluation should be considered. Metabolic myopathies are rare, so diagnosis can be delayed. However, getting a correct diagnosis is important for a number of reasons.

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, appropriate activity, regular aerobic exercise, proper diet and use of supplements can help control symptoms.