Glycogen Storage Diseases
What is Glycogen Storage Diseases ?
Glycogen storage diseases consist of at least 8, and possibly 12, distinct errors of metabolism. The diseases alter the synthesis or degradation of glycogen, the form in which glucose is stored in the body. All such metabolic errors are inherited.
Normally, muscle and hepatic cells store glycogen. Muscle glycogen is used in muscle contraction; liver glycogen can be converted into free glucose, which can then diffuse out of the hepatic cells to increase blood glucose levels.Glycogen storage diseases manifest primarily as dysfunctions of the liver, heart, or musculoskeletal system. Signs and symptoms vary from mild and easily controlled hypoglycemia to severe organ involvement.
Causes of Glycogen Storage Diseases
Almost all glycogen storage diseases (types 1 through 5 and type 7) are transmitted as autosomal recessive traits. The precise mode of transmission of type 6 isn't known. Phosphorylase b kinase deficiency (type 8) may be an X-linked recessive trait.The most common type of glycogen storage disease is type I, glucose-6-phosphatase deficiency or von Gierke's disease, resulting from a deficiency of the liver enzyme glucose-6-phosphatase. This enzyme converts glucose-6-phosphatase into free glucose and is necessary for the release of stored glucagon and glucose in the bloodstream to relieve hypoglycemia.
Signs & Symptoms of Glycogen Storage Diseases
Most patients experience the following symptoms:
Liver biopsy is used to confirm diagnosis of type IA disease by showing normal glycogen synthetase and phosphorylase enzyme activities but reduced or absent glucose-6-phosphatase activity. Glycogen structure is normal, but amounts are elevated.
Laboratory studies of plasma demonstrate low glucose levels but high levels of free fatty acids, triglycerides, cholesterol, and uric acid in type IA disease.
Serum analysis reveals elevated pyruvic acid levels and elevated lactic acid levels. Prenatal diagnoses are available for types 2, 3, and 4.Injection of glucagon or epinephrine increases pyruvic and lactic acid levels but doesn't increase blood glucose levels in patients with type IA disease. A glucose tolerance test curve typically shows depletion hypoglycemia and reduced insulin output.
For patients with type I glycogen storage disease, the goals of treatment include maintaining glucose homeostasis and preventing secondary consequences of hypoglycemia through frequent feedings and constant nocturnal nasogastric (NG) tube feedings. Dietary treatment calls for a regimen of about 60% carbohydrate intake, with low fat and normal amounts of protein and calories; carbohydrates should contain glucose or glucose polymers only, such as raw cornstarch.
Therapy for type 3 disease includes frequent feedings and a high-protein diet. Type 4 disease necessitates a high-protein, high-calorie diet, bed rest, diuretics, sodium restriction, and paracentesis, necessary to relieve ascites.
Patients with types 5 and 6 diseases require no treatment except avoidance of strenuous exercise. No treatment may be necessary for types 4 and 7 diseases.There is no effective treatment for type 2 glycogen storage disease.
Prospective parents may seek genetic counseling and testing to determine their risk for passing on von Gierke disease. Treatment is effective at preventing many of the complications.
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