Glycogenosis type V

Glycogenosis Type V

Glycogenosis Type V, also known as McArdle disease, is a rare genetic disorder that affects the metabolism of glycogen in muscle cells. It is one of the glycogen storage diseases, which are a group of inherited metabolic disorders characterized by the accumulation or deficiency of glycogen in tissues.

Pathophysiology

Glycogenosis Type V is caused by a deficiency of the enzyme myophosphorylase, which is responsible for breaking down glycogen into glucose-1-phosphate in muscle cells. This enzyme deficiency leads to an inability to properly mobilize glycogen during exercise, resulting in a lack of available energy for muscle contraction.

Genetics

The disorder is inherited in an autosomal recessive pattern, meaning that an individual must inherit two copies of the defective gene, one from each parent, to manifest the disease. The gene responsible for McArdle disease is the PYGM gene, located on chromosome 11.

Clinical Presentation

Patients with Glycogenosis Type V typically present with exercise intolerance, muscle cramps, and fatigue. Symptoms often appear in childhood or adolescence. A hallmark of the disease is the "second wind" phenomenon, where patients experience an improvement in exercise tolerance after a brief rest period.

Diagnosis

Diagnosis is based on clinical presentation, laboratory tests, and genetic testing. A muscle biopsy may show an accumulation of glycogen and a deficiency of myophosphorylase activity. Genetic testing can confirm mutations in the PYGM gene.

Management

There is no cure for Glycogenosis Type V, but management focuses on lifestyle modifications and supportive care. Patients are advised to avoid strenuous exercise and to engage in moderate, aerobic activities. A diet rich in carbohydrates and the use of oral sucrose before exercise may help improve exercise tolerance.

Prognosis

The prognosis for individuals with Glycogenosis Type V is generally good, as the disease does not typically affect life expectancy. However, it can significantly impact quality of life due to exercise limitations.

Also see

• Glycogen storage disease
• Metabolic myopathy
• Exercise intolerance
• Autosomal recessive disorder