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 buildup of abnormal amounts or types of glycogen in tissues.

The main types of GSDs include:

  • Type 0, Liver glycogen synthase deficiency
  • Type 1, Von Gierke disease, defect in glucose-6-phosphatase enzyme, the most common type of GSD accounting for 25% of all cases
  • Type 2 , Pompes disease, acid maltase deficiency
  • Type 3, Coris disease, debrancher enzyme deficiency
  • Type 4, Andersens disease, brancher enzyme deficiency
  • Type 5, McArdles disease, muscle phosphorylase deficiency
  • Type 6, Hers disease, liver phosphorylase deficiency
  • Type 7, Taruis disease, muscle phosphofructokinase deficiency
  • Type 9, 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.

Causes

GSDs are caused by a genetic enzyme defect. It is inherited from one or 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 builds up in the liver and/or muscle tissues.

Risk

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

Symptoms

Each type of GSD has specific symptoms.

Type 0:

Type 0 symptoms commonly appear in late infancy when night feedings stop. In the early morning the child may have low blood sugar which could cause:

  • Paleness
  • Vomiting
  • Extreme fatigue
  • Convulsions

The children may also have a mild growth delay. They also may have poor exercise tolerance.

Type 1:

Type 1 may appear as early as 3 months old. Common symptoms include:

  • Protruding abdomen from an enlarged liver and seizures from low blood sugar are common in infants
  • Signs of low blood sugar including
    • Paleness
    • Irritability
    • Fatigue
    • Seizures
  • Growth delay

Enlarged Liver

enlarged liver
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Type 2:

Type 2 is broken into the time of symptom onset. Infants generally show symptoms at 4-8 months of age. Type 2 can also form in childhood, adolescence, or adulthood.

Infant-onset Type 2 GSD symptoms include:

  • Muscle weakness and floppiness
  • Delayed motor skills
  • Weak breathing
  • Progressive heart muscle enlargement and reduced pumping may lead to fatal heart failure within first year of life

Late-onset Type 2 GSD symptoms include:

  • Progressive muscle weakness that spreads from the legs into the arms
  • Difficulty breathing

Type 3:

Type 3 may appear in childhood. Common symptoms include:

  • Muscle weakness
  • Growth delay during childhood
  • Swollen abdomen due to an enlarged liver
  • Signs of low blood sugar including
    • Paleness
    • Irritability
    • Fatigue

Type 4:

Type 4 appears during infancy. Common symptoms include:

  • Physical and mental development stops at a certain point and does not continue as normal
  • Progressive muscle weakness
  • Poor muscle development
  • Progressive cirrhosis of the liver, which may lead to liver failure

Type 5:

Common symptoms of Type 5 include:

  • Muscle cramps during exercise
  • Extreme muscle fatigue during exercise
  • Potential fainting or seizure after exercise
  • Red or brown-colored urine after exercise

Type 6, 9:

Symptoms may not appear in people with Types 6 or 9. When they do occur, symptoms include:

  • Enlarged liver in infancy
  • Mild growth delay
  • Anxiety, sweating, confusion, or seizures associated with low blood sugar

Type 7:

Common symptoms of Type 7 include:

  • Muscle cramps and tenderness with exercise
  • Muscle fatigue with exercise
  • Red or brown-colored urine after exercise

Diagnosis

You will be asked about your child's 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:

Your child's bodily fluids and tissues may be tested. This can be done with:

  • Biopsy of the affected organs
  • Blood tests
  • Urine tests
  • Genetic testing

Images may be need of your child's bodily structures. This can be done with an MRI scan.

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.

Treatment

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

Type 0

People with Type 0 need to manage their blood sugar throughout the day. This means frequent meals or snacks every few hours. Uncooked cornstarch provides a steady, slow release of glucose and may help prevent low blood sugar in the overnight hours. Other considerations includes eating high amounts of protein.

Type 1

Treatment for Type 1 includes eating small, frequent meals throughout the day. People with Type 1 may have to restrict certain sugars. This may include sugar found in fruits, juice, and candy. Eating uncooked cornstarch is also necessary. Many people will need to take a multivitamin.

Medications and procedure may be needed to control complications of Type 1 GSD.

Type 2

Type 2 is treated with enzyme replacement therapy. Alglucosidase alpha is given by IV infusion.

Type 3

People with Type 3 need to manage their blood sugar throughout the day. This means frequent meals or snacks every few hours. Diet for Type 3 includes high amounts of carbohydrates and cornstarch. Those with weak muscle involvement are generally put on a diet with high protein intake.

Liver transplant surgery may be necessary for some people who develop liver cancer or have liver failure.

Type 4

Type 4 treatment focuses on treating progressive liver disease and its complications. Ultimately, a liver transplant may be necessary to survive. This does not improve damage to the muscles from the condition though.

Type 5

There is no specific treatment for people with Type 5 GSD. Changing exercise patterns and avoiding certain activities may be important to help manage this condition. Exercise tolerance may be improved by ingestion of some sugars prior to starting exercise.

Type 6 and type 9

Treatment includes cornstarch to avoid low blood sugars and eating adequate amounts of protein. Frequent meals may also be needed to help avoid low blood sugars. These changes can help promote normal growth and development.

Type 7

Type 7 treatment focuses on avoiding high muscle stress. This can be accomplished by reducing strenuous exercise to manageable levels. In some people, high protein intake has helped with muscle weakness.

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.