Glycogen Storage Diseases

Definition

Glycogen Storage Diseases (GSDs) are a group of inherited genetic disorders that cause glycogen to be improperly stored in the body. They are characterized by the accumulation of abnormal amounts or types of glycogen in tissues.
Glycogen is the storage form of glucose in our bodies. Glucose is a simple sugar, which is a form of carbohydrate. It is found in many foods and is the main source of energy in our bodies.
The main types of GSDs are categorized by number and name. They include:
  • Type I (Von Gierke disease) ? 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)
  • Type IV (Andersen?s disease)
  • Type V (McArdle?s disease)
  • Type VI (Hers? disease)
  • Type VII (Tarui?s disease)
Since glycogen is primarily stored in the liver or muscle tissue, GSDs usually affect functioning of the liver, the muscles, or both. Liver ? theGSDs that mainly affect the liver are: type I, III, IV, and VI
Muscles ? the GSDs that mainly affect muscles are: type V and VII
Type II affects nearly all organs including the heart.

Causes

GSDs are caused by a genetic enzyme defect that is inherited from both parents. Normally, enzymes help convert glucose into glycogen for storage. Other enzymes convert the glycogen back to glucose when quick energy is needed, as in exercise. In a person with a GSD, some of these enzymes are defective, deficient, or absent. This causes the buildup of abnormal amounts and types of glycogen in liver and/or muscle tissues.

Risk Factors

A risk factor is something that increases your chances of getting a disease or condition. The primary risk factor for GSDs is having a family member with this disease.

Symptoms

The most common symptoms of GSDs include:
  • Low blood sugar
  • Enlarged liver
  • Slow growth
  • Muscle cramps
Symptoms of specific types of GSDs include: 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
  • Bone thinning from 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.
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
Type VII:
  • Muscle weakness
  • Anemia
  • Increased levels of uric acid

Diagnosis

The doctor will ask about symptoms and medical history and perform a physical exam. Diagnosis of GSDs usually occurs in infancy or childhood as a result of the above symptoms. 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

Treatment

Treatment will depend on the type of GSD and the symptoms.
Treatment of GSDs that Affect the Liver
These general treatment guidelines apply to people with types I, III, IV, and VI. Your doctor will develop a treatment regimen based on your specific symptoms.
The goal of treatment is to maintain normal blood glucose levels. This may be done with:
  • A nasogastric infusion of glucose in infants and children under age two
  • Dietary changes, including:
    • In children over age two, frequent small carbohydrate feedings are given throughout the day. This may include uncooked cornstarch. (Uncooked cornstarch provides a steady slow-release form of glucose.)
    • Elimination of foods that are high in fructose or lactose (type I only)
  • Allopurinol (Aloprim, Zyloprim) may be prescribed to reduce uric acid levels in the blood. This is done to prevent gout and kidney stones.
  • Type IV is sometimes treated with liver transplantation.
Treatment of GSDs that Affect the Muscles
These general treatment guidelines apply to people with types V and VII. Your doctor will develop a treatment regimen based on your specific symptoms.
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 can talk to a genetic counselor when deciding to have children.