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Glycogen storage disease type IX
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Condition

Glycogen storage disease type IX

Glycogen storage disease type IX is an inherited metabolic disorder caused by an inability to break down glycogen, which can affect the liver, muscles, or both.

Overview

Glycogen storage disease type IX (GSD IX) is a condition caused by the inability to break down a complex sugar called glycogen. The different forms of the condition can affect glycogen breakdown in liver cells or muscle cells or sometimes both. A lack of glycogen breakdown interferes with the normal function of the affected tissue.

Symptoms

The symptoms of GSD IX vary depending on whether the condition affects the liver, muscles, or both.

Liver-Affecting Form

When GSD IX affects the liver, the signs and symptoms typically begin in early childhood. The initial features usually include:

  • An enlarged liver (hepatomegaly)
  • Slow growth and shorter than normal stature
  • Low blood sugar (hypoglycemia) or elevated levels of ketones in the blood (ketosis) during prolonged periods without food (fasting)
  • Delayed development of motor skills, such as sitting, standing, or walking
  • Mild muscle weakness
  • Delayed puberty in some adolescents

In the form of the condition that affects the liver, the signs and symptoms usually improve with age. Typically, individuals catch up developmentally, and adults reach normal height. However, some affected individuals have a buildup of scar tissue (fibrosis) in the liver, which can rarely progress to irreversible liver disease (cirrhosis).

Muscle-Affecting Form

GSD IX can affect muscle tissue, although this form of the condition is very rare and not well understood. The features of this form can appear anytime from childhood to adulthood and may include:

  • Fatigue, muscle pain, and cramps, especially during exercise (exercise intolerance)
  • Muscle weakness that worsens over time
  • Myoglobinuria, which occurs when muscle tissue breaks down abnormally and releases a protein called myoglobin that is excreted in the urine, causing the urine to be red or brown

Combined Form

In a small number of people with GSD IX, the liver and muscles are both affected. These individuals develop a combination of the features described above, although the muscle problems are usually mild.

Frequency

GSD IX that affects the liver is estimated to occur in 1 in 100,000 people. The forms of the disease that affect muscles or both muscles and liver are much less common, although the prevalence is unknown.

Causes

Mutations in the PHKA1, PHKA2, PHKB, or PHKG2 genes cause GSD IX. These genes provide instructions for making pieces (subunits) of an enzyme called phosphorylase b kinase. The enzyme is made up of 16 subunits, four each of the alpha, beta, gamma, and delta subunits. At least two different versions of phosphorylase b kinase are formed from the subunits: one is most abundant in liver cells and the other in muscle cells.

  • The PHKA1 and PHKA2 genes provide instructions for making alpha subunits. The protein produced from the PHKA1 gene is a subunit of the muscle enzyme, while the protein produced from the PHKA2 gene is part of the liver enzyme.
  • The PHKB gene provides instructions for making the beta subunit, which is found in both the muscle and the liver.
  • The PHKG2 gene provides instructions for making the gamma subunit of the liver enzyme.

Phosphorylase b kinase plays an important role in providing energy for cells. The main source of cellular energy is a simple sugar called glucose, which is stored in muscle and liver cells in a form called glycogen. Glycogen can be broken down rapidly when glucose is needed, such as to maintain normal levels of glucose in the blood between meals or for energy during exercise. Phosphorylase b kinase activates the enzyme that breaks down glycogen.

Mutations in these genes reduce the activity of phosphorylase b kinase in liver, muscle, and blood cells. This reduction of enzyme function impairs glycogen breakdown. As a result, glycogen accumulates in and damages cells, and glucose is not available for

Frequently Asked Questions

The muscle-affecting form of GSD IX is rare and presents with fatigue, muscle pain, and cramps, particularly after exercise. Diagnosis is challenging and usually requires genetic testing to identify mutations in the relevant genes, as enzyme activity in muscle tissue may be difficult to measure. Exercise guidelines are individualized, but generally, patients are advised to avoid intense, prolonged exertion that triggers cramps and to engage in moderate, regular activity to maintain muscle strength. Physical therapy and pacing strategies can help manage symptoms and improve quality of life.
lthough liver symptoms in GSD IX often improve with age, a subset of patients develops liver fibrosis, which can rarely progress to cirrhosis. Monitoring typically involves regular liver function tests and imaging studies, such as ultrasound or elastography, to assess liver stiffness and detect early signs of scarring. The frequency of monitoring depends on the severity of the initial presentation and the presence of any existing liver changes, but annual or bi-annual check-ups are common for affected individuals.
GSD IX is an autosomal recessive disorder, meaning that affected individuals have a 50% chance of passing the mutated gene to their children. If both parents carry a mutation, there is a 25% chance of having an affected child. Genetic counseling is recommended for families to understand inheritance patterns and discuss options for prenatal or preimplantation genetic testing. While the condition itself does not directly impair fertility, the metabolic demands of pregnancy may require careful monitoring of blood sugar and liver function, particularly in the liver-affecting form.
Unlike GSD I or III, which require strict, frequent feeding schedules and sometimes cornstarch to prevent hypoglycemia, dietary management for liver-type GSD IX is generally less restrictive. While avoiding prolonged fasting is important to prevent low blood sugar and ketosis, patients often do not need the same level of complex carbohydrate loading. As symptoms typically improve with age, many individuals can transition to a more standard diet in adulthood, though regular monitoring of liver function and growth remains essential.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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