What is glycogen and glycogen storage disease (GSD)?

The body’s cells need a steady supply of fuel in order to function the right way. This fuel is a simple sugar called glucose. Glucose comes from breaking down the food we eat. The body uses as much glucose as it needs to function and stores the rest to use later.

Before it can be stored, the body must combine the simple glucose units into a new, complex sugar called glycogen. The glycogen is then stored in the liver and muscle cells. When the body needs extra fuel, it breaks down the glycogen stored in the liver back into the glucose units the cells can use. Special proteins called enzymes help both make and break down the glycogen in a process called glycogen metabolism.

Sometimes a person is born missing an enzyme needed for this process or it may not work right. Then the body is not able to store or break down the glycogen as it should. This can lead to very low blood glucose levels during periods of fasting. The muscles and organs need a certain level of glucose in the blood to work properly.

When the body is missing an enzyme or has a flawed enzyme and is not able to use glycogen the right way, it leads to a condition called glycogen storage disease (GSD). Many different enzymes are used by the body to process glycogen. And, as a result, there are several types of GSD.

What are the types of GSD?

Each type of GSD centers on a certain enzyme or set of enzymes involved in glycogen storage or break down. There are at least 13 types of glycogen storage disease. Doctors know more about some types than others. GSD mostly affects the liver and the muscles. Some types cause problems in other areas of the body as well. Types of GSD and the parts of the body they affect the most include:

  • Type 0 (Lewis' disease) – Liver.
  • Type I (von Gierke’s disease) Type Ia – Liver, kidneys, intestines; Type Ib – Liver, kidneys, intestines, blood cells.
  • Type II (Pompe’s disease) – Muscles, heart, liver, nervous system, blood vessels.
  • Type III (Forbes-Cori disease) – Liver, heart, skeletal muscles, blood cells.
  • Type IV (Andersen’s disease) – Liver, brain, heart, muscles, skin, nervous system.
  • Type V (McArdle’s disease) – Skeletal muscles.
  • Type VI (Hers’ disease) – Liver, blood cells.
  • Type VII (Tarui’s disease) – Skeletal muscles, blood cells.
  • Type IX – Liver.
  • Type XI (Fanconi-Bickel syndrome) – Liver, kidneys, intestines.

How common are they?

A glycogen storage disorder occurs in about one in 20,000 to 25,000 babies. The most common types of GSD are types I, II, III, and IV, with type I being the most common. It is believed that nearly 90% of all patients with GSD have types I through IV. About 25% of patients with GSD are thought to have type I. However, GSD types VI and IX can have very mild symptoms and may be underdiagnosed.

Most of the severe forms of GSD are diagnosed in babies and children. Some of the milder types might not be found until the person is an adult.

What are the symptoms of glycogen storage disease (GSD)?

Symptoms vary based on the type of GSD. Some GSDs affect mostly the liver. These include Types 0, I, III, IV, VI and IX. However, they may sometimes have overlapping symptoms affecting muscle and heart. These types (except for GSD type 0) may cause the liver to become enlarged. An enlarged liver is linked to low blood glucose levels because excess glycogen is stored in the liver instead of being released as glucose in the blood stream. Symptoms of low blood glucose, or hypoglycemia, include sweating, tremor, drowsiness, confusion and sometimes seizures. Some GSDs, such as types V and VII, mostly affect the skeletal muscles. Muscle weakness and muscle cramps are the most common symptoms of these types.

Other symptoms that may occur include:

  • Tiredness.
  • Very slow growth.
  • Obesity (being very overweight).
  • Problems with bleeding and blood clotting.
  • Kidney problems.
  • Low resistance to infections.
  • Breathing problems.
  • Heart problems.
  • Mouth sores.
  • Gout.

What causes GSDs?

GSDs occur when there is a problem with the gene that has the instructions for making the enzyme that is missing or not working right. The gene is passed down from parents to children. In most cases, in order to have the GSD, a child must get a bad gene from both parents. Just because both parents have the gene does not always mean they will both pass it on to their children.

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