Metabolic myopathies are genetic defects that interfere with the energy-generating processes in skeletal muscles. When these defects interfere with muscle function and block energy production, muscle cells cannot work properly and the ability of muscles to communicate (“talk”) between bones and joints is lost. 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 cause rhabdomyolysis, a breakdown of muscle tissue that leaves muscles 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 (ATP), made inside cells from sugars or fats in the diet. Metabolic myopathy is caused when an enzyme necessary to convert dietary sugars and fats into ATP is missing.
There are several different of metabolic myopathies, each named for the particular energy-making pathway that is interrupted by a missing enzyme. Diseases caused by a defect in sugar metabolism, for instance, are called glycogen storage diseases. The most common defect in sugar metabolism is McArdle’s disease. Defects in fat processing are called lipid storage diseases. The term mitochondrial myopathy is used when the enzyme present in mitochondria (the small energy-producing sacs inside each cell) is deficient.
There are several pathways for cells to make ATP, which is why some people with metabolic myopathies may never experience significant symptoms – functioning pathways will make more ATP to make up for the deficient pathway. When a necessary pathway is severely enzyme deficient, however, the resulting ATP deficit can become severe and symptoms develop. When cells are moderately low in ATP, the major problem may be fatigue or exercise intolerance. Rhabdomyolysis (muscle cell death) occurs when the cells are severely ATP deficient, resulting in constant or on-going weakness.
Metabolic Myopathies include:
- Carnitine Deficiency
- Carnitine Palmityl Transferase Deficiency
- Cori (or Forbes) Disease
- Lactate Dehydrogenase Deficiency
- McArdle Disease
- Myoadenylate Dehydrogenase Deficiency
- Phosphoglycerate Kinase Deficiency
- Pompe Disease (subject of the movie Extraordinary Measures)
- Tarui Disease