McArdle’s Disease Overview & Facts
Definition of McArdle’s Disease
McArdle’s disease is a metabolic disorder in which skeletal muscle cells are unable to break down glycogen into glucose. Also known as myophosphorylase deficiency or Type V glycogen storage disease. It was first described in 1951 by Dr. Brian McArdle of Guy’s Hospital.
This disease is caused by a missing or non-functioning enzyme, myophosphorylase C, which breaks down glycogen into glucose within muscle cells during exercise. Because this metabolic pathway is interrupted, the energy demands of the cell cannot be met.
People with McArdle’s disease have had the disease from birth, although many people aren’t formally diagnosed until much later in life. Because most individuals who have the condition appear physically healthy but experience premature fatigue, weakness and pain during exercise, McArdle’s disease has historically been confused with being out of shape or unwilling to exercise.
Symptoms of McArdle’s Disease
Symptoms include difficulty during physical activities like jogging, walking up hills or stairs, swimming, or anything moderately intense. Episodes of premature muscle fatigue and pain may be accompanied by muscle contractures known as rhabdomyolysis, which indicates muscle damage require medical attention. Rhabdomyolysis can cause muscle proteins like creatine kinase and myoglobin to enter the bloodstream where they pass in urine, sometimes causing darkened urine (myoglobinuria) which is a warning sign of injury. A “second wind” phenomenon has been observed clinically and anecdotally, in which individuals with McArdle’s disease are able to sustain moderate activities like walking or even jogging following a brief period of warm-up, the duration of which seems to depend on a number of factors including diet and fitness level.
There are few long-term studies published that document the long-term effects of McArdle’s disease on general health. The condition is not by itself life-threatening and people with McArdle’s disease are often otherwise in excellent health.
The presently identified genetic mutations associated with McArdle’s disease follow an autosomal recessive inheritance pattern, requiring two defective copies of the gene for the condition to manifest. Carriers are not known to have symptoms of the disease.
There is no consensus effective therapy for McArdle’s disease. Some physicians recommend that individuals with McArdle’s disease ingest high-sugar beverages or snacks prior to exercise, but this is subject to considerable debate. There have been some clinical studies which show that a diligent regimen of careful aerobic exercise to improve cardiovascular conditioning can yield some relief from symptoms and improve quality of life for people with the disease.