Definition:
Glycogen storage diseases (GSDs) are the result of deficiency of enzymes that cause the alteration of glycogen metabolism.
Glycogen stored in the liver and muscle cells of animals, is broken down into glucose to provide immediate energy or to maintain blood glucose levels during times of need.
The liver forms of GSDs (type I, III, IV and VI) are due to an increase in liver glycogen and hypoglycemia, which is caused by the inability to convert glycogen to glucose.
The muscle forms of GSDs (type II, IIIA, V and VII) have mild symptoms appearing during strenuous exercises owing to the inability to provide energy for muscle contraction.
Type 0 (Glycogen synthase deficiency)
There is hypoglycemia; hyperketonemia and early death.
Type I (Glucose-6-phosphatase deficiency)-Von Gierke’s disease
This is the most common disease.
Characterized by severe hypoglycemia (Decreased blood sugar levels) leading to metabolic acidosis, ketonemia and elevated lactate (due to excess glycolysis) and alanine.
Hypoglycemia occurs because glycogen cannot be converted back to glucose.
A glycogen build up is found in the liver causing hepatomegaly.
Type II (Lysosomal α1->4 and α1->6 Glucosidase deficiency)- Pompe's disease
It affects predominantly the heart and skeletal muscle, producing muscle weakness and cardiomegaly.
Liver function is normal and patients do not have hypoglycemia.
Type III (Amylo-1,6-Glucosidase deficiency)-Forbe’s or Cori’s disease
Deficiency of glycogen debranching enzymes results in storage of an abnormal form of glycogen.
Both liver and muscle are affected (type IIIA), producing hepatomegaly (Enlargement of liver) and muscle weakness.
Type IV (Branching Enzyme deficiency)-Andersons disease of Amylopectinosis
It is an extremely rare disorder caused by production of an abnormal form of unbranched glycogen in all tissue.
Patients exhibit hepatosplenomegaly with ascites and liver failure.
There is death from heart or liver failure before 5 years of age.
Type V (Muscle Phosphorylase deficiency)-McArdle’s disease
Characterized with muscle cramps after exercise.
Type VI (Liver phosphorylase deficiency)- Hers’ disease
It manifests as hepatomegaly caused by increased deposits of normal glycogen in the liver or in red or white blood cells.
Type VII (Muscle and erythrocyte phosphofructokinase deficiency)-Taruis’ disease
Patients have abnormal glycogen in muscle.
Exercise intolerance, unresponsiveness to glucose administration, and hemolysis (caused by decreased glycolysis in RBC).
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