Glycogen Storage Disease (GSD)

What is glycogen storage disease?

Glycogen storage diseases (GSDs) are a group of rare conditions in which your body can’t use or store glycogen properly. They’re types of inherited (passed from parent to child) metabolic disorders.

Glycogen is the stored form of glucose (sugar). Glucose is your body’s main source of energy. It comes from carbohydrates (a macronutrient) in certain foods and fluids you consume. When your body doesn’t immediately need glucose for energy, it stores glucose primarily in your skeletal muscles and liver as glycogen for later use.

Your body creates glycogen from glucose through a process called glycogenesis. When your body needs extra fuel, it breaks down glycogen for use through a process called glycogenolysis. Several enzymes are responsible for these two processes.

Glycogen storage diseases happen when you don’t have one or more of these enzymes. Your body can’t use stored glycogen for energy or maintain steady blood glucose levels. This can cause several issues, including frequent symptomatic low blood sugar (hypoglycemia), liver damage and muscle weakness.

Types of glycogen storage diseases

Your body uses many different enzymes to process glycogen, so there are several types of GSD — at least 19.

Researchers know more about some types than others. GSD mostly affects your liver or muscles. Some types cause problems in other areas of your body, as well.

For each type of GSD, there’s a lack (deficiency) of a certain enzyme involved in glycogen storage or breakdown. Healthcare providers may refer to each type based on the specific enzyme that’s missing or the scientist who discovered that type of GSD.

How common is glycogen storage disease?

Glycogen storage disease is rare. GSD type I (von Gierke disease), the most common type, occurs in approximately 1 in 100,000 births.

What are the symptoms of glycogen storage disease?

The symptoms of glycogen storage disease can vary based on the type and even from person to person with the same type. Symptoms of GSD type 1 (the most common type) usually begin at three to four months of age. But symptoms of other types can develop later in life.

The two most common symptoms are low blood sugar (hypoglycemia) and/or getting tired easily from physical activity (exercise intolerance).

Low blood sugar happens when your blood glucose is below 70 mg/dL. Symptoms include:

• Shaking or trembling.
• Sweating and chills.
• Dizziness or lightheadedness.
• Weakness.
• Faster heart rate.
• Intense hunger (hyperphagia).
• Difficulty thinking and concentrating.
• Anxiousness or irritability.
• Pale skin (pallor).
• Seizures (this can happen with severe low blood sugar).

Other symptoms of GSD may include:

• Muscle cramps or muscle weakness.
• Slowed growth and poor weight gain in children.
• Enlarged liver (hepatomegaly).
• Low muscle tone.
• High cholesterol (hyperlipidemia).

What causes glycogen storage disease?

Inherited genetic mutations (changes) that affect the function of enzymes necessary for glycogen storage and use cause GSD. A child inherits the mutations from their biological parents.

Most GSDs have autosomal recessive inheritance, meaning both parents have to pass on the mutation for the child to get the condition.

But a few types, like GSD type IX, have an X-linked inheritance. This means that your X chromosome carries the mutation. A man who has the mutation will have the condition because they carry only one X chromosome. A woman who has the mutation in one gene, with a normal gene on the other X chromosome, generally doesn’t have the condition.

How is glycogen storage disease diagnosed?

To diagnose glycogen storage disease, your child’s healthcare provider will likely recommend several tests. Since GSD is rare, it can take time to rule out other conditions and figure out the specific type of GSD. The tests may include:

• Fasting blood sugar test: Fasting low blood sugar can point to GSD.
• Ketone blood test: Your body produces ketones when it has to use fat for energy instead of glucose. Children with GSD often have ketosis.
• Basic metabolic panel: This gives insight into your child’s overall health.
• Lipid panel: Hyperlipidemia (high cholesterol) is common in GSD.
• Liver function tests: These can show the health of your child’s liver. Abnormalities can point to GSD.
• Urinalysis: Urine tests can measure creatinine levels, which determine kidney function and uric acid levels. GSD often causes high uric acid levels (hyperuricemia).
• Abdominal ultrasound: An ultrasound can check to see if your child has an enlarged liver.
• Genetic testing: This looks for problems with the genes for different enzymes.

Although specific genetic testing is available for diagnosing most types of GSD, your child’s provider may want to perform a muscle or liver biopsy to confirm the diagnosis.

How is glycogen storage disease treated?

There’s no cure for glycogen storage disease, so treatment is based on managing symptoms. Treatment varies based on the type of GSD you have. Examples of treatment include:

• Preventing low blood sugar: Consuming regular doses of uncooked cornstarch and/or similar nutrition supplements can help maintain stable blood sugar levels. Cornstarch is a complex carbohydrate that’s difficult for your body to digest. Because of this, it maintains blood sugar levels for a longer period than most carbohydrates in food. There’s now a commercial product that lasts longer in your body. It eliminates the need for overnight feeding.
• Treating low blood sugar: Low blood sugar episodes caused by GSD need immediate treatment by consuming carbohydrates. It you don’t treat hypoglycemia, it can become more severe and cause complications like seizures and coma.
• Managing high cholesterol: Medications called statins can help manage high cholesterol.
• Managing high uric acid levels: A medication called allopurinol can help reduce the production of uric acid in your body.

Some GSDs (like GSD type II) can be treated with enzyme replacement therapy (ERT). This is usually an IV infusion. There’s ongoing research on using ERT with other forms of GSDs.

If you have severe liver damage from GSD, a liver transplant may become necessary.

Can I prevent glycogen storage disease?

As glycogen storage diseases are genetic (inherited) conditions, there’s nothing you can do to prevent them.

If you have a biological family member with GSD, you may want to consider genetic counseling if you’re thinking of having a biological child to see if you’re a carrier of the genetic mutation.

What is the prognosis of glycogen storage disease?

With early diagnosis and proper management, the prognosis (outlook) for people with most GSDs is good. But some types of GSD are difficult to manage. Your healthcare provider can give you a better idea of what to expect.

What are the complications of glycogen storage disease?

GSDs can cause several complications depending on:

• The type.
• If you have a delayed diagnosis.
• If it’s not consistently managed well.

For example, possible complications of untreated GSD type 1 include:

• Low bone mass, bone fractures and osteoporosis.
• Delayed puberty.
• Gout.
• Kidney disease.
• Pulmonary hypertension.
• Noncancerous liver tumors (hepatic adenomas).
• Polycystic ovary syndrome (PCOS).
• Pancreatitis.
• Changes in brain function due to repeated episodes of low blood sugar.

What is the life expectancy of someone with glycogen storage disease?

The life expectancy for glycogen storage disease varies based on the type, how early it’s diagnosed and how well it’s managed. Some types are fatal in infancy while others have relatively normal life expectancies. Your healthcare provider will be able to give you the best idea of what to expect.

When should I see my healthcare provider?

Talk to your healthcare provider if you or your child experiences muscle weakness or persistent symptoms of low blood sugar.