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

Glycogen Storage Disease Type III

Glycogen storage disease type III (GSDIII) is an inherited disorder caused by the accumulation of structurally abnormal glycogen, which impairs the function of the liver and muscles.

Overview

Glycogen storage disease type III, also known as Cori disease or GSDIII, is an inherited condition resulting from the buildup of a complex sugar called glycogen in the body's cells. The accumulated glycogen is structurally abnormal and impairs the function of certain organs and tissues, particularly the liver and muscles.

GSDIII is divided into four types—IIIa, IIIb, IIIc, and IIId—distinguished by their specific patterns of signs and symptoms:

  • GSD types IIIa and IIIc mainly affect the liver and muscles.
  • GSD types IIIb and IIId typically affect only the liver.

It is very difficult to distinguish between the types of GSDIII that affect the same tissues. GSD types IIIa and IIIb are the most common forms of this condition.

Symptoms

Beginning in infancy, individuals with any type of GSDIII may experience:

  • Low blood glucose (hypoglycemia)
  • Excess amounts of fats in the blood (hyperlipidemia)
  • Elevated blood levels of liver enzymes

As children with this condition get older, they typically develop an enlarged liver (hepatomegaly). Liver size usually returns to normal during adolescence, but some affected individuals develop chronic liver disease (cirrhosis) and liver failure later in life. People with GSDIII often have slow growth due to liver problems, which can lead to short stature. In a small percentage of cases, noncancerous (benign) tumors called adenomas may form in the liver.

Muscle Involvement

Individuals with GSDIIIa may develop muscle weakness (myopathy) later in life. These muscle problems can affect both the heart and the muscles used for movement. Muscle involvement varies greatly among affected individuals:

  • The first signs are typically poor muscle tone (hypotonia) and mild myopathy in early childhood.
  • The myopathy may become severe by early to mid-adulthood.
  • Some people with GSDIIIa have a weakened heart muscle (cardiomyopathy), though affected individuals usually do not experience heart failure.
  • Other individuals affected with GSDIIIa have no cardiac muscle problems.

Frequency

The incidence of GSDIII in the United States is 1 in 100,000 individuals. This condition is seen more frequently in people of North African Jewish ancestry; in this population, an estimated 1 in 5,400 individuals are affected.

GSDIIIa is the most common form, accounting for about 85 percent of all cases. GSDIIIb accounts for about 15 percent of cases. GSD types IIIc and IIId are very rare, with poorly defined signs and symptoms; only a small number of affected individuals have been suspected to have these specific types.

Causes

Mutations in the AGL gene cause GSDIII. The AGL gene provides instructions for making the glycogen debranching enzyme. This enzyme is involved in the breakdown of glycogen, which is a major source of stored energy in the body. Between meals, the body breaks down stores of energy, such as glycogen, to use for fuel.

Most AGL gene mutations lead to the production of a nonfunctional glycogen debranching enzyme. These mutations typically cause GSD types IIIa and IIIb. The mutations that cause GSD types IIIc and IIId are thought to lead to the production of an enzyme with reduced function. All AGL gene mutations lead to the storage of abnormal, partially broken-down glycogen molecules within cells. A buildup of abnormal glycogen damages organs and tissues throughout the body, particularly the liver and muscles, leading to the signs and symptoms of GSDIII.

Inheritance

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Other Names for This Condition

  • AGL deficiency
  • Cori disease
  • Cori's disease
  • Debrancher deficiency
  • Forbes disease
  • Glycogen debrancher deficiency
  • GSD III
  • GSD3
  • Limit dextrinosis

Frequently Asked Questions

Baseline abdominal ultrasound is obtained at diagnosis. After that, imaging is typically repeated every 1–2 years in children and every 6–12 months in adults, especially if there are abnormal liver enzymes, rapid growth of a known lesion, or clinical signs of liver dysfunction. MRI with contrast may be used for detailed assessment if a lesion is detected.
Frequent, balanced meals are essential. Offer a carbohydrate‑rich snack every 3–4 hours, including complex carbs (e.g., oatmeal, rice) and a small amount of protein. At night, a continuous overnight feed of uncooked cornstarch (1–2 g/kg) provides a slow‑release glucose source. Monitor blood glucose before meals and during illness; have glucagon or a fast‑acting glucose gel readily available.
high‑protein, moderate‑carbohydrate, low‑fat diet is recommended. Protein should provide 15–20% of total calories to supply amino acids for muscle maintenance. Complex carbohydrates are preferred over simple sugars to avoid rapid glucose spikes. Limit long‑chain fats, especially saturated fats, to reduce hepatic steatosis. Some clinicians add a controlled amount of uncooked cornstarch or a slow‑release carbohydrate supplement to maintain steady glucose between meals.
Watch for increasing difficulty with activities such as climbing stairs, rising from a seated position, or lifting objects. New onset of muscle cramps, fatigue after minimal exertion, or a measurable drop in strength on manual testing (e.g., hand‑grip dynamometer) are red flags. If CK (creatine kinase) levels rise markedly or electromyography shows worsening myopathy, refer to a neuromuscular specialist for evaluation and possible physiotherapy or cardiac monitoring.
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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