What Is Glycogen Storage Disease Type Ib?
Glycogen Storage Disease Type Ib (GSDIb), also called von Gierke disease, is an inherited disorder in which the body lacks an enzyme called glucose-6-phosphate translocase. GSDIb is caused by mutations in the SLC37A4 gene. A deficiency of glucose-6-phosphate translocase impairs the body's ability to breakdown a stored form of sugar, called glycogen, into glucose. As a result, the body cannot maintain normal blood-sugar levels between meals, leading to low blood sugar (hypoglycemia). Also, glycogen builds up in the body and impairs the function of the liver, the kidneys, and other organs.
Children with GSDIb appear normal at birth but usually begin to show symptoms when they start to sleep longer through the night. Low blood sugar can cause tiredness, irritability, and seizures. Children with GSDIb typically have abnormal levels of certain metabolic substances, such as increased blood levels of lactic acid (lactic acidosis), fats (hyperlipidemia), and a waste product called uric acid (hyperuricemia). If not properly diagnosed, these children will likely experience a medical crisis within the first few months of life. Children with GSDIb have delayed or stunted growth and the appearance of a swollen abdomen due to an enlarged liver. Other potential symptoms or complications include delayed puberty, thinning of the bones (osteoporosis), and a form of arthritis due to uric acid crystals in joints (gout). Mental function is not affected by GSDIb. Non-cancerous (benign) tumors in the liver are often seen around the time of puberty. Rarely, these can become cancerous. Changes in kidney function may occur as the individual reaches his or her twenties, and may include kidney stones and a decreased ability to filter waste products. In advanced cases, dialysis and/or a kidney transplant may be needed.
Individuals with GSDIb are also prone to frequent bacterial and fungal infections, due to the impaired function and/or decreased levels of a type of white blood cell called neutrophils. They are also more likely to develop chronic inflammation of the pancreas, chronic inflammatory bowel disease, and Crohn's disease.
Individuals with glycogen storage disease type Ia (GSDIa) lack a different component of glucose-6-phosphatase and experience similar symptoms. For this reason, GSDIa and GSDIb are often spoken about as one disease: GSD type I.
How Common Is Glycogen Storage Disease Type Ib?
The incidence of glycogen storage disease type I (both GSDIa and GSDIb) is 1 in 100,000 live births. Approximately 20% of glycogen storage disease type I is GSDIb.
How Is Glycogen Storage Disease Type Ib Treated?
The treatment of GSDIb involves careful monitoring of the affected individual's diet, both in the frequency of meals and type of foods eaten. Individuals with GSDIb should avoid foods with sucrose (table sugar), fructose (sugar from fruits), and lactose and galactose (sugars found in milk). They need to eat around the clock, typically every one to three hours during the day and every three to four hours at night, to maintain healthy blood sugar levels.
Infants and young children often need a feeding tube in order to tolerate frequent eating. They may also need to use a feeding pump at night and for emergency feedings should their blood sugar drop dangerously low. Because they must eat so frequently, children with GSDIb frequently develop problems eating and swallowing food orally and may need therapy to re-learn sucking, swallowing, and sometimes speech.
Physicians recommend that individuals with GSDIb drink cornstarch mixed with water, soy formula, or soy milk. Cornstarch is digested slowly and therefore releases its glucose gradually, helping to safely extend the time between meals. Due to the restricted nature of the diet, multivitamins, calcium, and vitamin D are necessary.
Individuals affected by GSDIb also frequently take medication to increase the number of neutrophils, a type of white blood cell that fights infection. They must be vigilant in treating any infection in the body as it arises.
Individuals with GSDIb should be followed by a team of specialists who are familiar with the long-term management of GSD, to ensure appropriate monitoring and treatment for potential complications of the condition.
What Is the Prognosis for an Individual with Glycogen Storage Disease Type Ib?
With careful monitoring of diet and blood-sugar levels, individuals with GSDIb can improve their metabolic abnormalities and live into adulthood. Without close monitoring of the diet, however, extremely low blood sugar can be fatal. In adolescence and adulthood, individuals with GSDIb must be alert to infections, kidney complications, high blood pressure, and/or cancerous liver tumors. Long-term complications can include kidney damage, thinning bones (osteoporosis), and benign tumors of the liver (adenomas).