Glycogen storage disease: One of the multiple inherited disorders of metabolism that interfere with glycogen synthesis or breakdown, leading to the storage of carbohydrates as glycogen in the body. The conditions may affect the liver or the skeletal (striated) muscle, both primary glycogen storage sites. Symptoms and signs depend upon the exact type but can include enlargement of the liver (hepatomegaly), hypoglycemia, and muscle weakness or cramps eventually accompanied by muscle wasting. McArdle disease (glycogen storage disease type V) is the most common type of glycogen storage disease. McArdle disease is an autosomal recessive disorder caused by mutations in the gene that encodes myophosphorylase, an enzyme that is essential for glycogenolysis. Other types of glycogen storage disease include von Gierke disease and Pompe disease.