McArdle Disease

Excerpt

McArdle disease, also known as glycogen storage disorder (GSD) type V, is an inborn metabolic disorder characterized by a deficiency or complete absence of an enzyme called muscle glycogen phosphorylase (or myophosphorylase). This disease is inherited in an autosomal recessive pattern and mainly affects skeletal muscles. The enzyme responsible for this disease normally catalyzes reactions that cause the conversion of glycogen to glucose. The deficiency of this enzyme, in turn, results in the accumulation of glycogen in tissues. The clinical sequelae are usually systemic, but the defect is limited to particular tissues in some cases. Glycolysis is only partially hindered in McArdle disease, as muscle fibers are able to convert glucose to glucose-6-phosphate (G6P) downstream of the metabolic block.

Most patients with GSDs present in childhood; however, McArdle disease is one of those that have adult-onset forms as well. Unfortunately, there have not been any established treatment options, although diet therapy has been observed to be efficacious in reducing clinical manifestations.

McArdle disease was first reported in 1951 by Dr. Brian McArdle from London. In 1959, it was described that the enzyme responsible for the affected step was myophosphorylase. The underlying gene for myophosphorylase (PYGM) was first discovered in 1984.