• Glycogen Storage Diseases

    (Glycogenoses; GSD)

    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 0:

    • Fasting low blood sugar
    • Increased lactate levels in the blood
    • Fasting ketone bodies in the blood
    • Affected children with morning fatigue

    Type I:

    • Large and fatty liver and kidneys
    • Low blood sugar
    • High levels of lactate, fats, and uric acid in the blood
    • Impaired growth and delayed puberty
    • Osteoporosis
    • Increased mouth ulcers and infection

    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
    enlarged liver
    Copyright © Nucleus Medical Media, Inc.

    Type III:

    • Swollen abdomen due to an enlarged liver
    • Growth delay during childhood
    • Low blood sugar
    • Elevated fat levels in blood
    • Possible muscle weakness

    Type IV:

    • Growth delay in childhood
    • Enlarged liver
    • Progressive cirrhosis of the liver (which may lead to liver failure)
    • May affect muscles and heart in late-onset type

    Type V:

    • Muscle cramps during exercise
    • Extreme fatigue after exercise
    • Burgundy-colored urine after exercise

    Type VI, IX:

    • Liver enlargement occurs, but diminishes with age
    • Low blood sugar

    Type VII:

    • Muscle cramps with exercise
    • Anemia

    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

    Pre-implantation Genetic Diagnosis

    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.
    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.

    Treatment of GSDs That Affect the Liver

    These general guidelines apply to people with types I, III, IV, VI, and IX. The goal of treatment is to maintain normal blood glucose levels. This may be done with:
      Dietary changes:
      • For children over the age of two years, small, frequent carbohydrate feeding may be given throughout the day. This may include uncooked cornstarch, which provides a steady, slow-release form of glucose.
      • People with type 1 will need to eliminate foods that are high in fructose or lactose.
      Medicines:
      • Allopurinol (Aloprim, Zyloprim)—reduces uric acid levels in the blood to prevent gout and kidney stones
      • Alglucosidase alfa (Lumizyme)—may work by replacing the enzyme that is needed for muscles to function properly; for use in children aged eight years and older who have type II
    • Procedure—For babies under age two, glucose can be given through a tube that goes into the nose and down into the stomach.
    • Surgery—For those with type IV, a liver transplant is needed in some cases.

    Treatment of GSDs That Affect the Muscles

    These general guidelines apply to people with types V and VII:
    The goal of treatment is to avoid muscle fatigue and/or cramps induced by exercise. This is done by:
    • Regulating or limiting strenuous exercise to avoid fatigue symptoms
    • Improving exercise tolerance by oral intake of glucose or fructose (fructose must be avoided in people with type I), or an injection of glucagon
    • Eating a high protein diet

    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.

    RESOURCES

    Association for Glycogen Storage Disease http://www.agsdus.org/

    Genetic Alliance http://www.geneticalliance.org/

    CANADIAN RESOURCES

    Canadian Health Network http://www.canadian-health-network.ca/

    Canadian Institute for Health Information http://www.cihi.ca/

    References

    Harrison’s Principles of Internal Medicine . 14th ed. New York, McGraw-Hill; 1998.

    Merck Manual of Diagnosis & Therapy . 17th ed. Simon and Schuster; 1999.

    5/28/2010 DynaMed's Systematic Literature Surveillance DynaMed's Systematic Literature Surveillance : FDA approves new treatment for late-onset Pompe disease. US Food and Drug Administration website. Available at: http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm213282.htm . Published May 25, 2010. Accessed May 28, 2010.

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