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

Glycogen storage disease type VI

Glycogen storage disease type VI is an inherited metabolic disorder characterized by the body's inability to break down glycogen in liver cells, often leading to an enlarged liver and low blood sugar in childhood.

Overview

Glycogen storage disease type VI (also known as GSDVI or Hers disease) is an inherited disorder caused by an inability to break down a complex sugar called glycogen in liver cells. A lack of glycogen breakdown interferes with normal function.

Symptoms

The signs and symptoms of GSDVI typically begin in infancy to early childhood. Common features include:

  • An enlarged liver (hepatomegaly), which is usually the first sign.
  • Low blood sugar (hypoglycemia) during prolonged periods without food (fasting).
  • Elevated levels of ketones in the blood (ketosis) during fasting. Ketones are molecules produced during the breakdown of fats, which occurs when stored sugars are unavailable.
  • Slower growth than peers during childhood, though affected individuals often achieve normal height as adults.
  • Mild delays in the development of motor skills, such as sitting, standing, or walking.

The signs and symptoms of GSDVI tend to improve with age; most adults with this condition do not have any related health problems.

Frequency

The exact prevalence of GSDVI is unknown. At least 11 cases have been reported in the medical literature, although this condition is likely to be underdiagnosed because it can be difficult to detect in children with mild symptoms or adults with no symptoms. GSDVI is more common in the Old Older Mennonite population, with an estimated incidence of 1 in 1,000 individuals.

Causes

Mutations in the PYGL gene cause GSDVI. The PYGL gene provides instructions for making an enzyme called liver glycogen phosphorylase. This enzyme is found only in liver cells, where it breaks down glycogen into a type of sugar called glucose-1-phosphate. Additional steps convert glucose-1-phosphate into glucose, a simple sugar that is the main energy source for most cells in the body.

PYGL gene mutations prevent liver glycogen phosphorylase from breaking down glycogen effectively. Because liver cells cannot break down glycogen into glucose, individuals with GSDVI can have hypoglycemia and may use fats for energy, resulting in ketosis. Glycogen accumulates within liver cells, causing these cells to become enlarged and dysfunctional.

Associated Genes

  • PYGL

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

  • GSD type VI
  • GSD VI
  • GSD6
  • Hepatic glycogen phosphorylase deficiency
  • Hers disease
  • Liver phosphorylase deficiency syndrome

Frequently Asked Questions

Typical signs include sweating, shakiness, irritability, confusion, and in severe cases seizures or loss of consciousness. If hypoglycemia is suspected, give the child a fast‑acting carbohydrate such as glucose gel, fruit juice, or a small amount of honey. After the initial glucose rise, follow up with a complex carbohydrate (e.g., a slice of bread or a small bowl of oatmeal) to maintain blood sugar. Monitor the child for 30–60 minutes to ensure symptoms resolve and repeat the glucose test if symptoms recur.
There is no specific drug that directly reduces glycogen accumulation in GSDVI. However, growth hormone therapy has shown benefits in some patients by improving growth velocity and reducing liver size. Additionally, some clinicians prescribe vitamin E or antioxidants to support liver health, though evidence is limited. The most effective strategy remains strict dietary management and regular follow‑up with a metabolic specialist.
The goal is to provide frequent, balanced meals every 3–4 hours, emphasizing complex carbohydrates (whole grains, legumes, starchy vegetables) and limiting simple sugars that can cause rapid spikes. A bedtime dose of uncooked cornstarch or a slow‑release carbohydrate supplement helps maintain glucose levels overnight. Regular monitoring of growth parameters and periodic blood glucose checks guide adjustments. Parents should work with a dietitian experienced in metabolic disorders to tailor the plan to the child’s age, activity level, and tolerance.
GSDVI follows an autosomal recessive inheritance pattern, meaning both parents must carry a mutation in the PYGL gene for a child to be affected. Each sibling has a 25 % chance of being affected, a 50 % chance of being a carrier, and a 25 % chance of being unaffected. Genetic testing of parents and at-risk siblings is recommended, especially in communities with higher prevalence. Early identification allows for prompt dietary intervention and monitoring of liver function.
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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