What is Glycogen Storage Disease (GSD)?

Glucose is a major source of energy for the body. It is stored in the form of glycogen in both the liver and muscles and later released with the help of enzymes. Persons affected by GSD have an inherited defect in one of the enzymes responsible for forming or releasing glycogen as it is needed by the body during exercise and/or between meals.

There are about nine known types of GSD. They are classified by a number and by the name of the defective enzyme, and sometimes by the name of the doctor who first described the condition. For example, GSD I, a defect in the enzyme glucose-6-phosphatase, is also known as Von Gierke Disease. Some GSD Types have subtypes, such as 1a and 1b. There used to be a GSD Type VIII resulting from lack of phosphorylase-b-kinase activity but this has now be reclassified as a subtype of GSD VI.

GSD can affect the liver, the muscles or both. Diagnosis of the type of GSD is made on the basis of an individual's symptoms, the results of a physical examination and of biochemical tests. Occasionally, a muscle or liver biopsy is required to confirm the actual enzyme defect. Increasingly DNA testing is a help in diagnosis.

All forms of GSD, except some forms of Type IX (liver phosphorylase kinase deficiency), occur when a child inherits the affected gene from both parents (autosomal recessive inheritance) each of whom is a carrier but not affected themselves. This means that for each pregnancy there is a 1 in 4 chance that the child will inherit both defective genes and thereby be affected.

For more details of the various types of GSD please use the links in the panel on the left.