What Is Glycogen Storage Disease Type Ia?
Glycogen Storage Disease Type Ia (GSDIa), also called von Gierke disease, is an inherited disorder in which the body lacks an enzyme called glucose-6-phosphatase. GSDIa is caused by mutations in the G6PC gene. A deficiency of glucose-6-phosphatase 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 organs, leading to an enlarged liver (hepatomegaly) and enlarged kidneys (nephromegaly) with impaired function.
Children with GSDIa 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 GSDIa 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.
Additionally, children diagnosed with GSDIa have the appearance of a swollen abdomen due to an enlarged liver, as well as delayed or stunted growth. 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 patient 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. Other symptoms or complications that may develop 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 GSDIa.
Individuals with glycogen storage disease type Ib lack a different component of glucose-6-phosphatase and experience similar symptoms. For this reason, type Ia and Ib are often spoken about as one disease: glycogen storage disease type I.
How Common Is Glycogen Storage Disease Type Ia?
The incidence of glycogen storage disease type I (both Ia and Ib) is 1 in 100,000 live births. Approximately 80% of glycogen storage disease type I cases are GSDIa.
How Is Glycogen Storage Disease Type Ia Treated?
The treatment of GSDIa involves a careful monitoring of the patient's diet, both in the frequency of meals and the type of foods eaten. Individuals with GSDIa 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 to dangerously low levels. Because they must eat so frequently, children with GSDIa often develop problems eating and swallowing food orally and may need therapy to relearn sucking, swallowing, and sometimes speech.
Physicians recommend that individuals with GSDIa 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 with GSDIa should be followed by a team of specialists who are familiar with the long-term management of glycogen storage disease, to ensure appropriate monitoring and treatment for potential complications of the condition.
What Is the Prognosis for an Individual with Glycogen Storage Disease Type Ia?
With careful monitoring of diet and blood sugar levels, individuals with glycogen storage disease have normal growth and live into adulthood. Without close monitoring of the diet, extremely low blood sugar levels can be fatal. In adolescence and adulthood, individuals with the disease must be alert to kidney complications, high blood pressure, and/or cancerous liver tumors.