Glycogen Storage Diseases

Glycogen storage diseases are carbohydrate metabolism disorders. There are many numbered and named types, all of which are caused by deficiencies of enzymes involved in glycogen synthesis or breakdown; the deficiencies may occur in the liver or muscles and cause hypoglycemia or deposition of abnormal amounts or types of glycogen (or its intermediate metabolites) in tissues.

Inheritance for glycogen storage diseases (GSDs) is autosomal recessive except for GSD type VIII/IX, which is X-linked. Incidence ranges from 1/20,000 to 1/43,000 births, depending on the population studied (1). For a more complete listing of glycogen storage diseases, see table Glycogen Storage Diseases and Disorders of Gluconeogenesis.

Age of onset, clinical manifestations, and severity vary by type, but symptoms and signs are most commonly those of hypoglycemia and myopathy.

Diagnosis of glycogen storage diseases is suspected by history, examination, and detection of glycogen and intermediate metabolites in tissues by MRI or biopsy. Diagnosis is confirmed by DNA analysis or less commonly by detecting a significant decrease of enzyme activity in liver (types I, III, VI, and VIII/IX), muscle (types IIb, III, VII, and VIII/IX), skin fibroblasts (types IIa and IV), or red blood cells (type VII) or by lack of an increase in venous lactate with forearm activity/ischemia (types V and VII). GSD II (Pompe disease) is now part of the newborn screening panel in many states in the United States. (See also testing for suspected inherited disorders of metabolism.)

Treatment typically includes dietary supplementation with cornstarch to provide a sustained source of glucose for the hepatic forms of GSD and exercise avoidance for the muscle forms.

Prognosis for and treatment of glycogen storage diseases vary by type.

Defects in glycolysis (rare) may cause syndromes similar to GSDs. Deficiencies of phosphoglycerate kinase, phosphoglycerate mutase, and lactate dehydrogenase mimic the myopathies of GSD types V and VII; deficiencies of glucose transport protein 2 (Fanconi-Bickel syndrome) mimic the hepatopathy of other GSD types (eg, I, III, IV, VI).