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Glycogen Storage Disease Type V
Sunday, 14 August 2005
Sunday, 14 August 2005
Glycogenosis Type V, McArdle Disease, Muscle Phosphorylase Deficiency, Myophosphorylase Deficiency

What

Glycogen Storage Disease Type V (GSD-V) is an autosomal recessive metabolic disorder characterized by the loss of the enzyme that breaks down glycogen, a major source of energy for muscles. As a result, patients with GSD-V are not able to utilize their glycogen stores and will present clinically with muscle pain, cramps, and stiffness during exercise.

Who

Glycogen storage diseases as a whole affect about 1 in 20,000 live births, usually presenting in childhood. However, GSD-V is the most common type of glycogen storage disease that can also be diagnosed in adulthood.

Signs and Symptoms

Patients with GSD-V may have either childhood-onset or adult-onset forms, but in most cases, symptoms will begin in young adulthood (second or third decade of life). The initial symptoms are cramps, fatigue, and pain after exercise, along with muscle weakness and general exercise intolerance, especially for anaerobic exercise. The exercises that produce these symptoms are often either brief exercises at a high intensity (i.e. sprinting, lifting weights) or sustained exercises at a lower intensity (i.e. climbing stairs, walking uphill). Over time, the degree of muscle weakness on exertion may worsen, but it may also remain fixed—the severity of symptoms is highly variable. Another common symptom in about 50% of patients is dark, burgundy-colored urine following intense exercise; this is due to a condition called rhabdomyolysis, in which muscle breaks down and releases the protein myoglobin into the kidneys and urine.

Possible Causes

GSD-V has two autosomal recessive forms, one causing the childhood-onset form and the other causing the adult-onset form. There is also a rare autosomal dominant form. The mutated genes in GSD-V lead to a deficiency in the enzyme called muscle phosphorylase (also known as myophosphorylase), which is needed to break down glycogen for energy use in muscle tissues. When patients with GSD-V exercise, their muscles are not able to utilize the energy from glycogen, and thus they will tire easily.

Diagnosis

A suspicious patient history and laboratory findings including increased muscle enzymes in the blood and red blood cells in the urine help doctors consider the diagnosis of GSD-V. However, definitive diagnosis is made by DNA identification of the mutated enzyme muscle phosphorylase. The physical exam of patients with GSD-V is usually unremarkable, with normal muscle strength and reflexes. Symptoms of GSD-V may be induced with forced muscle exertion in the doctor’s office. During exertion-induced cramps, electromyography (EMG) may not detect any electrical activity in the muscles. A biopsy of the muscles may show abnormally structured glycogen, with normal to increased amounts of glycogen.

Treatment

At present, there is no treatment for GSD-V, but its symptoms may be controlled by avoidance of strenuous exercise. Another symptomatic treatment is diet therapy (high-protein diet with sucrose or creatine supplements), but it is controversial whether or not it actually aids muscle function in some patients. Research is still ongoing—studies of successful gene therapy and enzyme replacement therapy in mice suggest that such treatments for humans may be possible in the near future.

Prognosis

GSD-V is a chronic condition, but if patients are able to manage their physical activity, they can lead normal lives. If patients continue to exercise intensely, however, the accumulation of proteins in the kidneys could lead to temporary kidney failure. Thus, it is very important that patients with GSD-V exercise only moderately; for those who do, the prognosis is very good.

Connect with other parents

In the spirit of community and support, Madisons Foundation offers the unique service of connecting parents of children with rare diseases. If you would like to be connected to other parents of children with this disease, please fill out this brief form.

Weblinks

Association for Glycogen Storage Disease
Excellent website for basic information on glycogen storage diseases, as well as the option to join a mailing list to connect with others interested in the diseases.

Muscular Dystrophy Association
Voluntary health agency offering research news, an online library, a chatroom, and an “Ask the Experts” section on various muscular dystrophies.

CLIMB (Children Living with Inherited Metabolic Diseases)
Active organization offering support networks, grants, newsletters, children’s services, and a magazine for metabolic disease awareness and support.

Google Search for Glycogen Storage Disease Type V

References and Sources

Anderson WE (2003). Glycogen Storage Disease, Type V. http://www.emedicine.com/med/topic911.htm. Kasper DL, et al. (2005). Chapter 341 Glycogen Storage Diseases and Other Inherited Disorders of Carbohydrate Metabolism. Harrison’s Online. http://www.accessmedicine.com/content.aspx?aID=103337. Krauss C (2004). McArdle syndrome. http://www.nlm.nih.gov/medlineplus/ency/article/000329.htm