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.

Cause

This disease is caused by a non-functioning enzyme, myophosphorylase C, which intiates the breakdown of glycogen into glucose within muscle cells during exercise. Because this metabolic pathway is interrupted, large deposits of glycogen accumulate within muscle cells.

Onset

People with McArdle’s disease have had the disease from birth, although many people aren’t formally diagnosed until much later in life. Due to the condition’s premature muscle fatigue and weakness and pain during exercise, McArdle’s disease may historically have been confused with simply 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 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.

Progression

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.

Inheritance

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.

Treatment

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.