Myophosphorylase deficiency

Myophosphorylase deficiency, also known as Glycogen storage disease type V (GSD V) or McArdle disease, is a rare genetic disorder that affects the body's ability to break down glycogen in muscle cells. This condition is caused by a deficiency in the enzyme myophosphorylase, which is essential for glycogenolysis, the process of converting glycogen into glucose for energy.

Causes

Myophosphorylase deficiency is an autosomal recessive disorder, meaning that an individual must inherit two copies of the defective gene, one from each parent, to develop the disease. The condition is caused by mutations in the PYGM gene, which provides instructions for producing the myophosphorylase enzyme.

Symptoms

The symptoms of myophosphorylase deficiency typically begin in childhood or adolescence, although they can appear at any age. They are primarily related to muscle function and include muscle weakness, fatigue, and pain during exercise (exercise intolerance). In severe cases, individuals may experience myoglobinuria, a condition in which muscle proteins are released into the urine, leading to kidney damage.

Diagnosis

Diagnosis of myophosphorylase deficiency typically involves a combination of clinical examination, family history, and laboratory tests. A definitive diagnosis can be made through a muscle biopsy, which can reveal a lack of the myophosphorylase enzyme. Genetic testing can also be used to identify mutations in the PYGM gene.

Treatment

There is currently no cure for myophosphorylase deficiency. Treatment is focused on managing symptoms and preventing complications. This may include physical therapy to improve muscle strength and endurance, dietary modifications to provide an alternative energy source, and avoidance of strenuous exercise to prevent muscle damage.

Prognosis

The prognosis for individuals with myophosphorylase deficiency varies. Some individuals may lead relatively normal lives with few symptoms, while others may experience significant disability due to muscle weakness and fatigue. The condition does not typically affect life expectancy.

See also

• Glycogen storage disease
• Metabolic disorder
• Genetic disorder

NIH genetic and rare disease info

• NIH info on Myophosphorylase deficiency

Myophosphorylase deficiency is a rare disease.