Types of Glycogen Storage Disease
There are at least 10 different types of GSDs. The types are put into groups based on the enzyme that is missing. The most common forms of GSD are types I (one), III (three) and IV (four). About one in 20,000 people have a type of GSD.
GSD I, also known as von Gierke disease: Results from a lack of the enzyme Glucose-6-Phosphatase.
GSD III, also known as Cori disease: Results from a lack of the debrancher enzyme. This causes the body to form glycogen molecules that have an abnormal structure which prevents the glycogen from being broken down into free glucose.
GSD IV, also known as amylopectinosis: There is not an increased amount of glycogen in the tissues. Instead, the glycogen that does build up in the tissues has very long outer branches. With this type of GSD, there is lack of the branching enzyme. This abnormal glycogen is thought to stimulate the immune system. The result is a great deal of scarring (cirrhosis) of the liver as well as other organs, such as muscle and heart.
Causes of Glycogen Storage Disease
When glucose is changed into glycogen, a different enzyme is required at each step. If one of these enzymes is defective (not normal) and fails to complete its step, the process stops. These enzyme defects cause glycogen storage diseases.
GSD is passed down through families (genetic) and occurs because of an inherited gene change from both parents. We normally have two copies of each gene. In order for people to have GSD both of their gene copies must not work properly. When people are carriers, it means that only one of their genes is not working properly. If both parents carry the defective gene, there is:
- A 25 percent chance that their child will develop the disorder
- A 50 percent chance that their child will receive a gene change from one of the parents, which means the child will not show symptoms of the disorder but is a "carrier"
- A 25 percent chance their child will have two working copies and will not have a GSD
Signs and Symptoms
Symptoms vary based on the enzyme that is missing. They usually result from the buildup of glycogen or from not being able to produce glucose when needed. Because GSD occurs mainly in muscles and the liver, those areas show the most symptoms.
Symptoms may include:
- Poor growth
- Muscle cramps
- Low blood sugar
- A greatly enlarged liver
- A swollen belly
- Abnormal blood test
The age when symptoms begin and how severe they are depends on the type of GSD. Children with GSD I rarely develop cirrhosis (liver disease), but they are at an increased risk for developing liver tumors.
In some ways, GSD III is a milder version of GSD I. It also is a very rare cause of liver failure, but it may cause fibrosis (early scarring of the liver, which may be caused by a healing response to injury, infection or inflammation). GSD II is a muscle disease and does not affect the liver.
Glycogen storage disease IV causes cirrhosis; it may also cause heart or muscle dysfunction. Often, infants born with GSD IV are diagnosed with enlarged livers and failure to thrive within their first year of life. They develop cirrhosis of the liver by age 3-5.
Treatment of Glycogen Storage Disease (GSD)
Treatment depends on the type of GSD. Some GSD types cannot be treated; others are fairly easy to control by treating the symptoms.
For the types of GSD that can be treated, patients must carefully follow a special diet.
- Frequent high carbohydrate meals during the day. For some children, eating several small meals rich in sugars and starches every day helps prevent blood sugar levels from dropping.
- Cornstarch. For some young children, giving uncooked cornstarch every four to six hours – including during overnight hours – also can help keep blood sugar levels from getting low. A doctor would know how much cornstarch a child would need.
- Continuous nighttime feeding. Some children will need a special feeding tube placed into their stomach in order to maintain the blood glucose level. The feeding tube is then used to give formula with a high concentration of glucose. This helps control the blood sugar level.
- Medicine: GSD tends to cause uric acid (a waste product) to build up in the body. This buildup of uric acid can cause gout (painful inflammation of the joints) and kidney stones. Medication is often necessary.
In some types of this disease, children must limit their amount of exercise to reduce muscle cramps.
Some GSD types cannot be treated, while others are fairly easy to control by treating the symptoms. Patients with treatable GSD do very well if the blood glucose level is maintained within the normal range. Maintaining a healthy blood glucose level can reverse all of the signs of this disease, allowing the child to lead a long life.
In the more severe cases of GSD, infection and other complications are likely to occur. These include liver, heart and respiratory failure. If liver failure occurs, receiving a liver transplant is the only option. Transplants have been effective in reversing the symptoms of GSD.