What Is Glycogen Storage Disease?

Overview

Every day we consume carbohydrates in our daily diet. It is one of the macromolecules that provide glucose as an instant energy source to the body. The glucose is stored as glycogen in the body, particularly in the liver and skeletal muscle. The liver is the main organ that helps in maintaining normal blood glucose levels. That is to store excess glucose as glycogen which happens after the intake of food. These glycogen stores are released during periods of fasting i.e., during overnight sleep or between meals. In short, this whole process is nothing but glycogen metabolism. 

Glycogen metabolism is a series of steps involved in glycogen synthesis and glycogen breakdown with the help of certain specific enzymes such as glycogen synthase and glycogen branching enzymes under the influence of hormones and body condition.

Any hindrance in glycogen metabolism results in Glycogen storage disease (GSD). Also called glycogenosis. Glycogen storage disease is a rare disease affecting  neonates. It is caused due to a defect in glycogen metabolism. As a result, there is an accumulation of glycogen and lipid in the liver and other organs¹

Types of glycogen storage disease

Glycogen storage disease is classified further based on defective enzyme and tissue. The various glycogen storage disease types are gsd type 0, I, iii, iv, vi, and ix. While there are many other several subtypes as well.

GSD type 0

They are of two types. gsd type 0a and 0b caused due to the malfunctioning of the enzyme liver glycogen synthase and muscle glycogen synthase. As a result, glycogen is not synthesized in the body. The main clinical feature observed by the patient is hypoglycemia (low blood sugar). The symptoms start appearing before 3.5 years of age.

Glycogen storage disease type i

Also referred to as Von Gierke disease. It is a rare disease and severe form of glycogen storage disease affecting 1 in 100,000 live births. Glycogen and lipids are accumulated within the liver and kidney manifesting hepatomegaly and splenomegaly in patients. Further, it can transform into liver cancer as well. The life expectancy of the patient is until adulthood. There are two variants of glycogen storage disease type i:

Gsd ia and Gsd ib

The defective enzyme in gsd ia is Glucose-6-phosphatase and Glucose-6-phosphate translocase in gsd ib.The patient experiences clinical features such as short stature, enlarged liver, low blood sugar, increased triglycerides, lactate, uric acid, and abnormal retention of fat.

Gsd ii also referred to as Pompe disease

is a rare disease and patients start showing the symptoms at around 4 months of infancy period in the case of infantile Pompe disease and late-onset Pompe disease is also seen goes beyond 12 months of age.

Gsd iii (Cori disease or Forbes disease)

The defective enzyme is the Debranching enzyme. The main features observed in gsd iii are an enlarged liver, low blood sugar, short height, high lipid, and cardiomyopathy.

Gsd IV also called (Andersen disease) 

The branching enzyme is defective and as a result, an abnormal form of glycogen is formed. These abnormal structures of glycogen is called  polyglucosan bodies and gets accumulated in the liver and muscle. The common features are enlarged liver and spleen, liver cirrhosis, low blood sugar, Neuropathy, Cardiomyopathy, and Myopathy(skeletal muscle disorder).

Gsd v (McArdle’s disease)

McArdle’s disease is characterized by disorders associated with skeletal muscle often showing symptoms of muscle weakness and exercise intolerance. It is caused due to defective glycogen phosphorylase enzyme specific to muscle.

Gsd VI also known as Hers disease

Defective enzyme is glycogen phosphorylase. Similar symptoms such as low blood sugar, hepatomegaly, and growth failure are observed.

Gsd ix

Due to the defective enzyme glycogen phosphorylase kinase.²

Causes of glycogen storage disease

Glycogen storage disorder is an inherited condition caused due to defects in the enzyme helping in glycogen metabolism. Glycogen is a carbohydrate made of sugar or glucose units. Their main role is to maintain normal blood glucose levels and to provide energy during periods of fasting or starvation as well as for muscle exercise. When these physiological needs are unmet the body starts experiencing GSD symptoms.

Signs and symptoms of glycogen storage disease

Since the skeletal muscle and liver stores glycogen, the main patient symptoms are pertaining to these organs. Glycogen stores in the liver (hepatic gsd) lead to hepatomegaly and frequently low blood sugar is observed due to the lack of free glucose in the liver. Glycogen storage disease of the muscle (muscle GSD) causes weakness of muscle or rhabdomyolysis and cardiomyopathy since the skeletal muscles are present around the heart.³

The general symptoms observed are:

• Delayed growth
• Bruising tendency
• Hypoglycemia
• Enlarged liver
• Swollen belly
• Muscle weakness
• Muscle cramps
• Cardiomyopathy
• Hyperlipidemia
• Acidosis

Management and treatment for glycogen storage disease

There is no specific treatments for glycogen storage disease. Most of the treatments available are to alleviate the symptoms. Management of nutrition lies in the foremost therapy for treating GSD patients. The main strategy is to prevent low blood sugar levels or hypoglycemia by consuming corn starch; a modified form is available commercially. Corn starch is a complex carbohydrate and slow release can maintain normal blood glucose levels. Along with corn starch therapy frequent meals are given. 

Increased uric acid levels can be treated by allopurinol and statins for high lipids.

Enzyme replacement therapy and gene therapy are also available for treating GSD patients.

Liver transplantation is considered for patients with hepatic GSD in a critical state.

With proper early detection and diagnosis, glycogen storage disease is manageable⁴

Diagnosis of glycogen storage disease

Diagnosis is primarily through physical examination of the patient. Followed by various biochemical blood tests to identify the glycogen storage disease type. The following tests can help with the diagnosis however they are not specific to the disease and thus are not reliable to arrive at a specific conclusion. They are:

• Fasting glucose levels 
• Creatine kinase-muscle/brain (CK-MB)
• Liver function test
• Uric acid
• lactic acid
• Lipid profile

A genetic test is specific to detect glycogen storage disease. Molecular genetic testing can help in ruling out most of genetic disorders. While in some cases liver biopsy is required. Swollen liver cells can be observed after staining. In case of an abnormal liver profile, liver biopsy helps in diagnosis as they show glycogen deposits in liver cells. They are still considered the gold standard for diagnosing hepatic gsd.⁵

Risk factors

Most of the glycogen storage disorders are inherited in an autosomal recessive manner, which means if both the genes of the parent are defective then only the child is affected. Only gsd ix is X linked, that is the mother serves a carrier of the disease every time. However, the carrier of the defective gene does not show any symptoms.

Complications

Over a period of time the glycogen deposits in the liver can lead to cirrhosis and increased chances of transforming into a cancerous condition

FAQs

How can I prevent glycogen storage disease

There is no prevention as such but early diagnosis and treatment can improve the condition

How common is glycogen storage disease

It is a very rare disease affecting 1 in 100,000 newborns.

When should I see a doctor

As soon as the discomfort or irritation seen in the baby or any mentioned symptoms consult a doctor.

Summary

Glycogen storage disease is a rare disease affecting neonates and can hold up to adulthood. It is caused due to the defect in the synthesis and breakdown of glycogen as the enzymes for doing it are absent. Hence there is a buildup of intermediates resulting in serious complications and symptoms of hepatomegaly, hypoglycemia, muscle weakness, and delayed growth in babies. There are various glycogen storage disease types depending on the affected enzyme. Diagnosis is mainly through blood tests combined with genetic tests. Maintaining the symptoms through diet and medication can help in a better prognosis for the patient.