Glycogen Storage Diseases

Definition

Glucose is a simple sugar. It is a form of carbohydrate. It is the main source of energy for our bodies. Glycogen is the storage form of glucose in our bodies.

Glycogen storage diseases (GSDs) are a group of inherited genetic disorders. They cause glycogen to be improperly formed or released in the body. This results in a build up of abnormal amounts or types of glycogen in tissues.

The main types of GSDs are categorized by number and name and include:

• Type 0 (Liver glycogen synthase deficiency)
• Type I (Von Gierke disease, defect in glucose-6-phosphatase)—most common type of GSD; 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)

Glycogen is mainly stored in the liver or muscle tissue. As a result, GSDs usually affect functioning of the liver, the muscles, or both.

• Liver—The GSDs that mainly affect the liver are types I, III, IV, VI, and IX.
• Muscles—The GSDs that mainly affect muscles are types V and VII.
• Type II affects nearly all organs including the heart.

Causes

GSDs are caused by a genetic enzyme defect. It is inherited from both parents.

Normally, enzymes help convert glucose into glycogen for storage. Other enzymes convert the glycogen back to glucose when energy is needed. This happens during activity like exercise. With GSD, some of these enzymes are defective, deficient, or absent.

The abnormal glycogen build up in the liver and/or muscle tissues.

Risk Factors

The main risk factor for GSDs is having a family member with this disease. The risk varies with the type of GSD.

Parents with one child with GSD have a 25% of having another child with GSD. In a few of the GSD types, the risk rises to 50%. In this case only male children are affected.

Symptoms

The most common symptoms of GSDs include:

• Low blood sugar
• Enlarged liver
• Slow growth
• Muscle cramps

Signs and symptoms of specific types of GSDs include:

Type II:

• Enlarged liver and heart
• In severe cases, muscle weakness and heart problems develop.
• In severe cases, infants may suffer fatal heart failure by the age of 18 months.
• Milder forms of type II may not cause heart problems.

Enlarged Liver

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Diagnosis

The doctor will ask about symptoms and medical history. A physical exam will be done. Diagnosis of GSDs usually occurs in infancy or childhood. It is often done by the symptoms listed above. Tests may include:

• Biopsy of the affected organs
• Blood and urine samples
• MRI scan —a test that uses magnetic waves to make pictures of the inside of the body

This test may be used for an early diagnosis on some types of GSD. It is often done when there is a family history.

In this technique eggs and sperm are harvested from a couple who have a known risk. The egg is fertilized in the lab. The GSD free embryo is then implanted within the mother’s uterus. This technique allows parents to have additional unaffected children. This process may still pose ethical or religious concerns for some couples.

Treatment

Treatment will depend on the type of GSD and the symptoms. Your doctor will develop a plan based on your specific symptoms.

Prevention

There is no way to prevent GSDs. However, early treatment can help control the disease once a person has it. If you have a GSD or a family history of the disorder, you may want to consult a genetic counselor. They can help determine the risk for your children.