Glycogen storage disease type V (GSD-V), also called McArdle disease, is an inherited disease in which the body lacks an enzyme called myophosphorylase. This deficiency prevents an affected person from turning glycogen, a stored form of sugar, into glucose, which can be used for energy. People with GSD-V have difficulty doing exercise of any kind because the body lacks its normal source of energy, glucose, and instead breaks down its own muscles. For people with GSD-V, exercise will cause pain, cramping, fatigue, and soreness to the muscle tissue. Some people with GSD-V find that after an initial difficulty with exercise, they get a “second wind” and are able to continue. Muscle strength is often normal.
In half of people with GSD-V, the breakdown of muscle also causes the release of a protein called myoglobin into the bloodstream, resulting in dark red urine. If a person with GSD-V exercises excessively, large amounts of myoglobin may overwhelm the kidneys and cause temporary kidney failure.
Symptoms of GSD-V usually begin in one's 20s or 30s, but can begin anywhere from infancy to later adulthood. The severity of the symptoms can vary, but in many people symptoms do not get significantly worse over time. About a third of adults with the disease experience progressive muscle weakening later in life. The degree of enzyme deficiency can determine the severity of the disease.
GSD-V is rare, but its exact frequency is unknown. One study showed the disease incidence to be 1 in 100,000 in the Dallas-Fort Worth area of Texas in the United States.
There is no treatment for GSD-V other than to restrict the frequency and intensity of exercise. Too much physical exertion will result in muscle breakdown and severe kidney problems. The results from a study of 12 people affected by GSD-V suggested that eating the sugar sucrose before exercise may minimize muscle pain and injury.
If people with GSD-V properly manage their physical activity, the disease should not significantly impact their lifespan. Those who exercise only moderately can lead normal lives.
A parent and patient-oriented support group that shares information about the disease, pushes for research, and maintains a listserv to contact other members.
P.O. Box 896
Durant, IA 52747
Phone: (563) 785-6038
A summary of glycogen storage disease type V written by a physician.