Glycogen Storage Disease
Glycogen storage disease is an inherited condition that changes the way your body uses glycogen, a form of glucose, or sugar. There are at least nine known forms of glycogen storage disease (GSD). GSDs are categorized according to the specific gene mutation that is present. The types are:
Type I. Also known as von Gierke’s disease, this is the most common form of GSD, although only one person in 100,000 has it. If you have type I, you lack the enzyme you need to turn glycogen into glucose.
Type II. This form of GSD is also known as Pompe disease. If you have this disorder, you have a genetic mutation that reduces or eliminates your levels of an enzyme called GAA, which helps process glycogen.
Type III. Also called Cori disease, type III involves a gene defect that causes you to make a nonworking form of an enzyme known as a glycogen debranching enzyme. There are four subclasses of GSD type III: IIIa, IIIb, IIIc, and IIId.
Type IV. Mutations in your GBE1 gene, the gene responsible for making the glycogen branching enzyme, cause GSD type IV. This condition is commonly called Andersen disease. Glycogen cannot form properly without this enzyme.
Type V. Also known as McArdle disease, type V involves a genetic defect that prevents an enzyme called myophosphorylase from breaking down glycogen. Difficulty when exercising is a typical symptom of GSD type V, because your body is unable to effectively process glycogen in your muscle cells.
Type VI. Also known as Hers disease, this genetic mutation reduces the effectiveness of a liver enzyme, called liver glycogen phosphorylase. Because your symptoms can be mild and grow milder as you age, this form of GSD may be underdiagnosed.
Type VII. This is also called Tarui disease. A genetic mutation affects your production of a glycogen-processing enzyme called phosphofructokinase. If you have this form of GSD, you probably experience muscle cramps and pain when you try to exercise.
Type IX. This form of GSD is also called type VIII. It signals a shortage of the enzyme called phophorylase kinase. Low levels of this enzyme can cause high levels of glycogen in your liver and muscles.
Type 0. This GSD stems from a deficiency in the enzyme glycogen synthase, which your body uses to make glycogen. Because your liver cannot store much glycogen, your fasting blood sugar levels can get very low. A baby will get symptoms once nighttime feeding stops. The symptoms occur early in the morning before breakfast. Because they are associated with low blood glucose, the symptoms can include: drowsiness, nausea, vomiting, a pale appearance, and sometimes convulsions.
The absence, or low levels, of certain enzymes that help process glycogen cause the symptoms of GSD. They can vary, depending on the form of GSD you have, but may include:
With many forms of GSD, symptoms appear in the first few months of life.
Who’s at risk
You inherit glycogen storage diseases. This means your parents pass them on to you. GSDs are autosomal recessive traits. This means that both of your parents must have the abnormal gene for the disease to affect you.
The procedure for diagnosing glycogen storage disease depends on the type of GSD your doctor suspects you have. Your doctor will probably perform a physical examination to check for symptoms, such as an enlarged liver or muscle weakness. Your doctor may also give you blood tests, a liver biopsy, and a muscle biopsy. Your doctor may also conduct prenatal tests (before your baby is born) to diagnose types II, III, and IV.
Treatment for GSD varies. Experts recommend special diets to control the symptoms of some forms of GSD. These include types I, III IV, VI, IX, and 0. Your doctor may advise you to avoid strenuous exercise if you have types V and VII. Enzyme replacement therapy may help with type II.
Damage to your liver and muscles, which glycogen buildup causes, can create other problems if you have certain types of GSD. If you have type III, you may develop benign tumors in your liver. If you have type IV, it is likely that you will eventually develop cirrhosis of the liver, which leads to liver failure. If you have type V, you may experience a breakdown of muscle tissue. You will be at risk for a condition called myoglobinuria, which can cause kidney failure.
Because you inherit glycogen storage diseases, there is no known prevention method. If you're concerned that you and your partner may carry the gene mutations for these disorders, genetic counseling before pregnancy can determine your chances of having a child with a glycogen storage disease.
When to call the doctor
Some forms of glycogen storage disease show up in early childhood. In infants, notify your doctor if your baby’s behavior changes after you stop nighttime feedings. If your child has symptoms, such as slow growth, chronic hunger, or a swollen abdomen, seek medical attention. Adults and teens should watch for symptoms, such as muscle weakness, and pain and cramps during exercise.
How to manage or live with the condition
If you have GSD, you may have special needs, in addition to medications and dietary changes. It is important to receive regular medical care so your doctor can monitor your condition. Regular medical visits will also allow you to keep up with new treatment options. Online or in-person support groups may also be helpful to you or your family.