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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Anmol Pitliya, M.B.B.S. M.D.[2]
Pathophysiology
Pathophysiology
- GSD type 1 results due to defects in either hydrolysis or transport of glucose-6-phosphate
- GSD type 1a is due to the deficiency of enzyme glucose-6-phosphatase (G6Pase).
- GDS type 1b is due to defect in glucose-6-phosphate translocase (T1 deficiency).
- G6Pase is primarily expressed in expressed primarily in the gluconeogenic the liver and kidney. It is also expressed to a lesser extent in the intestine and pancreas.
- Glucose-6-phosphatase catalyzes the conversion of glucose-6-phosphate to glucose during glycogenolysis and gluconeogenesis.
- This defects hinders the conversion of glucose-6 phosphate to glucose in organs.
- This leads to accumulation of glycogen in organs including liver, kidney, and intestine.
- The inability of glucose-6-phosphate to leave cells leads to severe fasting hypoglycemia.
- This also results in the development of various secondary metabolic and biochemical abnormalities including hyperlactacidemia, hyperuricemia, and hyperlipidemia.
Genetics
- 80% Cases of GSD 1 are of GSD type 1a.
- G6Pase gene is located on chromosome locus 17q21.
- Glucose-6-phosphate translocase is located on chromosome locus 11q23.
- GSD type 1 follows an autosomal recessive pattern.
Hypoglycemia
Hypoglycemia is the central clinical problem, the one that is most damaging, and the one that most often prompts the initial diagnosis. Maternal glucose transferred across the placenta prevents hypoglycemia in a fetus with GSD I, but the liver is enlarged with glycogen at birth. The inability to generate and release glucose soon results in hypoglycemia, and occasionally in lactic acidosis fulminant enough to appear as a primary respiratory problem in the newborn period. Neurological manifestations are less severe than if the hypoglycemia were more acute. The brain's habituation to mild hypoglycemia is at least partly explained by use of alternative fuels, primarily lactate.
More commonly, infants with GSD I tolerate without obvious symptoms a chronic, mild hypoglycemia and compensated lactic acidosis between feedings. Blood glucose levels are typically 25 to 50 mg/dl (1.4-2.8 mM). These infants continue to need oral carbohydrates every few hours. Many never sleep through the night even in the second year of life. They may be pale, clammy, and irritable a few hours after a meal. Developmental delay is not an intrinsic or inevitable effect of glucose-6-phosphatase deficiency but is common if the diagnosis is not made in early infancy.
Although mild hypoglycemia for much of the day may go unsuspected, the metabolic adaptations described above make severe hypoglycemic episodes, with unconsciousness or seizure, uncommon before treatment. Episodes which occur are likely to happen in the morning before breakfast. GSD I is therefore a potential cause of ketotic hypoglycemia in young children.
Once the diagnosis has been made, the principal goal of treatment is to maintain an adequate glucose level and prevent hypoglycemia.
Glycogen also accumulates in kidneys and small intestine. Hepatomegaly, usually without splenomegaly, begins to develop in fetal life and is usually noticeable in the first few months of life. By the time the child is standing and walking, the hepatomegaly may be severe enough to cause the abdomen to protrude. The liver edge is often at or below the level of the umbilicus. Other liver functions are usually spared, and liver enzymes and bilirubin are usually normal.
However, there is a risk of developing tumors of the liver by adolescence or adult ages, and periodic ultrasound examinations of the liver are recommended from late childhood onward. Occasional cases of various types of liver disease and failure have been reported in children and adults with GSD I.
Lactic acidosis
Impaired gluconeogenesis results in elevations of lactic acid (4-10 mM) even when the child is well. In an episode of metabolic decompensation, lactic acid levels abruptly rise and can exceed 15 mM, producing severe metabolic acidosis. Uric acid, ketoacids, and free fatty acids further increase the anion gap. Manifestations of severe metabolic acidosis include vomiting and hyperpnea, which can exacerbate hypoglycemia by reducing oral intake. Repeated episodes of vomiting with hypoglycemia and dehydration may occur in infancy and childhood, precipitated by (or mimicking) infections such as gastroenteritis or pneumonia.
Growth failure
Without treatment, growth failure is common, due to chronically low insulin levels, persistent acidosis, chronic elevation of catabolic hormones, calorie insufficiency, and/or malabsorption.
Hyperlipidemia and blood vessel effects
A secondary effect of low insulin levels is hypertriglyceridemia. Triglycerides in the 400–800 mg/dl range may produce visible lipemia, and even a mild pseudohyponatremia due to a reduced aqueous fraction of the serum. Cholesterol is only mildly elevated.
Hyperuricemia and joint problems
A further effect of chronic lactic acidosis in GSD I is hyperuricemia, as lactic acid and uric acid compete for the same renal tubular transport mechanism. Increased purine catabolism is an additional contributing factor. Uric acid levels of 6-12 mg/dl are typical of GSD I.
Kidney effects
Kidneys are usually 10 to 20% enlarged with stored glycogen. This does not usually cause clinical problems in childhood, with the occasional exception of a Fanconi syndrome with multiple derangements of renal tubular reabsorption, including proximal renal tubular acidosis with bicarbonate and phosphate wasting. However, prolonged hyperuricemia can cause uric acid nephropathy. In adults with GSD I, chronic glomerular damage similar to diabetic nephropathy may lead to renal failure.
Bowel effects
Intestinal involvement can cause mild malabsorption with sloppy stools but usually requires no treatment.
Infection risk
Blood clotting problems
Impaired platelet aggregation is an uncommon effect of chronic hypoglycemia. It may cause clinically significant bleeding, especially epistaxis.
Neurodevelopmental effects
Developmental delay is a potential secondary effect of chronic or recurrent hypoglycemia, but is at least theoretically preventable. Because normal brain and muscle cells contain no glucose-6-phosphatase, GSD I causes no other neuromuscular effects.