Glycogen storage disease type I (GSD I) is an inherited disease that prevents the liver from properly breaking down stored glycogen, which is necessary in maintain adequate blood sugar levels. GSD I is divided into two main types, GSD Ia and GSD Ib, which differ in cause, presentation, and treatment. There are also possibly rarer subtypes, the translocases for inorganic phosphate (GSD Ic) or glucose (GSD Id); however, a recent study suggests that the biochemical assays used to differentiate GSD Ic and GSD Id from GSD Ib are not reliable, and are therefore GSD Ib.
GSD Ia is caused by a deficiency in the enzyme glucose-6-phosphatase; GSD Ib, a deficiency in the transport protein glucose-6-phosphate translocase. Because glycogenolysis is the principal metabolic mechanism by which the liver supplies glucose to the body during fasting, both deficiencies cause severe hypoglycemia and, over time, excess glycogen storage in the liver and (in some cases) in the kidneys.
Because of the glycogen buildup, GSD I patients typically present with enlarged livers from non-alcoholic fatty liver disease. Other functions of the liver and kidneys are initially intact in GSD I, but are susceptible to other problems. Without proper treatment, GSD I causes chronic low blood sugar, which can lead to excessive lactic acid, and abnormally high lipids in the blood, and other problems. Frequent feedings of cornstarch or other carbohydrates are the principal treatment for all forms of GSD I.
GSD Ib also features chronic neutropenia due to a dysfunction in the production of neutrophils in the bone marrow. This immunodeficiency, if untreated, makes GSD Ib patients susceptible to infection. The principal treatment for this feature of GSD Ib is filgrastim; however, patients often still require treatment for frequent infections, and a chronically enlarged spleen is a common side effect. GSD Ib patients often present with inflammatory bowel disease.
It is the most common of the glycogen storage diseases.
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