Glycogen Storage Disease Type VII
What is Glycogen Storage disease?
Glycogen storage diseases (GSDs) are a group of inherited genetic disorders that cause glycogen to be improperly formed or released in the body. They are characterized by the accumulation of abnormal amounts or types of glycogen in tissues.
Glucose is a simple sugar, which is a form of carbohydrate. It is found in many foods and is the main source of energy for our bodies. Glycogen is the storage form of glucose in our bodies. Glycogen storage diseases are due to either a deficiency or blockage of an enzyme that is important in converting glucose to glycogen so it can be stored in the body for later use. The main places glycogen is stored in the body includes the liver and muscle cells.
The main types of GSDs are categorized by number and name. They include:
- Type I (Von Gierke disease, defect in glucose-6-phosphatase) – this is the most common type of GSD, and accounts for 90% of all GSD cases
- Type II (Pompe’s disease, acid maltase deficiency)
- Type III (Cori’s disease, debrancher enzyme deficiency)
- Type IV(Andersen’s disease, brancher enzyme deficiency)
- Type V (McArdle’s disease, muscle glycogen phosphorylase deficiency)
- Type VI (Hers’ disease, liver phosphorylase deficiency)
- Type VII (Tarui’s disease, muscle phosphofructokinase deficiency)
- Type IX (liver glycogen phosphorylase kinase deficiency)
Since glycogen is primarily stored in the liver or muscle tissue, GSDs usually affect functioning of the liver, the muscles, or both.
What is Tarui’s disease?
The phosphofructokinase enzyme is needed to facilitate the breakdown of glycogen into energy in muscle. Patients with this type of GSD have a deficient amount of this enzyme in their muscle tissue. With this deficiency, effective glycogen breakdown (glycolysis) during muscle stress cannot be accomplished, resulting in pain, weakness, and cramping in the exercising muscle.
The clinical features of Type VII Glycogen Storage Disease are similar to those of Type V GSD. Patients experience early onset of fatigue and muscle pain with exercise. The body breaks down muscle when trying to attain energy, which causes many symptoms such as muscle pain, muscle cramping, muscle fatigue, and muscle tenderness. With the breakdown of muscle (rhabdomyolysis) and the release of the red protein myoglobin, myoglobinuria may develop, as evidenced by dark-red or red-brown urine. However, exercise intolerance is evident in childhood and symptoms are more severe than in Type V GSD. Patients may also display a hemolytic anaemia.
Does it have any other name?
Yes, other names that Tarui’s disease may be called include:
- Phophofructokinase Deficiency
- Glycogenosis type VII
- Glycogen storage disease VII
- Tarui disease
How is it diagnosed?
Several specialized tests are used to confirm a suspected diagnosis of metabolic disease:
- Blood tests can be used to detect the presence of certain chemicals in the blood that may indicate some metabolic diseases.
- An exercise test is used to monitor a person’s response to intense or moderate exercise. Blood samples are taken during exercise for testing.
- Electromyography (EMG) uses small needle electrodes to measure the electrical currents in a muscle as it contracts. While an EMG can’t definitively diagnose metabolic disease, it can be used to rule out a number of other types of neuromuscular disease that cause similar patterns of weakness.
- A muscle biopsy requires the removal of a small piece of muscle tissue for microscopic analysis. The procedure is done either surgically, with an incision to expose the target muscle, or with a needle. A skin biopsy is also sometimes performed. A muscle biopsy will show a deficiency of the muscle phosphofructokinase enzyme and a modest accumulation of glycogen found
- Other tests that may be needed include an electrocardiogram to test heart function, and brain imaging studies such as CT or MRI scans.
- Genetic tests, using a blood sample, can analyse the person’s genes for particular defects that cause metabolic disease, but these tests often aren’t necessary for diagnosis or for determining treatment.
Is there any treatment?
Unfortunately there is not a cure for Tarui’s disease. Treatment at this stage manages the symptoms. It is known, however, that it is important to avoid strenuous exercise to prevent muscle cramps and pain.
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