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Pompe Disease
Monday, 21 July 2003
Wednesday, 06 July 2005
Glycogen Storage Disease II; Acid Maltase Deficiency

What

Glycogen Storage Disease II (GSD II) is a rare genetic disorder that is classified as both a glycogen storage disease and a lysosomal storage disease. The disorder was first described by Dr. Pompe in 1932. Lysosomes are the major digestive units in cells that contain enzymes to break down or “digest” nutrients. One of these enzymes is called alpha-1,4-glucosidase (GAA), or acid maltase. This enzyme is needed to break down glycogen. Glycogen is a stored form of energy. Children with GSD II are missing the enzyme or have decreased amounts of it and so they cannot break down glycogen. Thus, the glycogen builds up in the lysosomes and can no longer work properly. There is a lot of glycogen in muscle and heart tissue. When glycogen builds up in these tissues, the muscles get weak and the heart gets large.

Who

GSD II occurs in all ethnic groups and equally affects both males and females. The disease occurs in about 1 in 40,000 people. There are different forms of GSD II as described further below. The infantile-onset form has a higher incidence among African Americans and Chinese. The late-onset adult form has a higher incidence in the Netherlands.

Signs and Symptoms

There are different forms of GSD II: infantile-onset and late-onset. The infantile-onset form is characterized by symptoms that start before 12 months of age. These infants have virtually no enzyme activity. They have difficulty feeding, breathing, and gaining weight. They have very low tone and their muscles are weak. Thus, they are not able to perform motor skills and have developmental delay. They can have a large tongue, a large liver, and/or a large heart. Individuals with late-onset GSD II can start having symptoms anytime after one year of age until adulthood. The more enzyme they have, the later they start having symptoms. The symptoms usually involve muscle weakness that progresses. If the weakness starts in childhood, they can have developmental delay. The weakness starts in the muscles of the trunk and hip and then affects the legs. This can cause difficulty with walking. Also because the trunk muscles are weak, they can develop scoliosis, or curvature of the spine. The weakness can spread to the diaphragm and other muscles that are important for breathing. Individuals may feel tired easily, feel that they can’t catch their breath, or have problems breathing with exercise or during sleep. Deep tendon reflexes are often absent. The heart can be normal in the late-onset form.

Possible Causes

GSD II is caused by changes (mutations) in the alpha-1,4-glucosidase, or GAA, gene. The GAA gene is located on chromosome 17. GSD II is inherited in an autosomal recessive manner.

Diagnosis

If someone has symptoms that suggest GSD II, a blood test called a CK level will usually be high. This test shows that the muscle is abnormal. If the heart is affected, a chest x-ray may show a large heart. An electrocardiogram (EKG) or an echocardiogram (heart ultrasound) may also show heart abnormalities. To measure if the enzyme is missing or decreased, a skin biopsy or muscle biopsy can be performed. This procedure requires taking a small piece of skin or muscle to check how much GAA enzyme they have. Prenatal diagnosis is possible if there is a family history of GSD II and the pregnant mother wants to test her fetus. If needed to confirm the diagnosis, analysis of the GAA gene can look for the exact genetic change (mutation).

Treatment

There is no cure for GSD II. However, there are ongoing clinical research trials for enzyme replacement therapy. This gives patients the GAA enzyme that they are missing. Enzyme replacement therapy reduces the accumulation of glycogen in lysosomes. The care of individuals affected with GSD II involves a geneticist who understands the management of lysosomal storage disorders. Depending on the patients’ symptoms, they may need other specialists such as a cardiologist if there are heart problems. If there are breathing problems, a pulmonologist and/or a respiratory therapist may be needed. If breathing becomes too difficult, they may need a mechanical ventilator, or a machine to help them breathe. A physical therapist can help with the muscle weakness. An occupational therapist and speech therapist may assist a child who has developmental delay. A dietician who specializes in metabolic disorders may help to manage nutrition. Anesthesia is risky in children and adults with GSD II, so general anesthesia should be avoided when at all possible.

Prognosis

The prognosis for individuals with GSD II is highly variable. Prognosis depends on the severity of symptoms and the age when symptoms start. The infantile-onset form is the most serious. Without enzyme replacement therapy, infants may not survive beyond the first year of life. The late-onset form that starts in childhood progresses more slowly. Without enzyme replacement therapy, young patients can expect to live beyond their twenties and thirties. This is improved with enzyme replacement therapy. People with the late-onset form that starts in adulthood may live for decades after diagnosis. However, muscle weakness may interfere with day-to-day activities. For all patients, respiratory failure is often the greatest danger.

Connect with other parents

In the spirit of community and support, Madisons Foundation offers the unique service of connecting parents of children with rare diseases. If you would like to be connected to other parents of children with this disease, please fill out this brief form.

Weblinks

Association for Glycogen Storage Disease
A great organization for families affected with glycogen storage disorders including information about GSD II, a parent handbook, links, and an electronic mailing list to connect families together.

The Pompe’s Disease Page
A “guide for families” that explains the basics of enzymes, DNA mutations, and lysosomes, and how to care for a child with GSD II.

Pompe Registry
Information about GSD II and how to register into a database for patients to help increase the knowledge and research about this disorder.

Google Search for Pompe Disease

References and Sources

McKusick, V. OMIM www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=232300 Kishnani PS, Howell RR. Pompe disease in infants and children. Journal of Pediatrics. 2004 May;144(5 Suppl):S35-43. Gene Tests www.geneclinics.org/servlet/access?id=8888891&testtype=both&qry=pompe&submit=Search&showglossarylinks=yes&db=genetests&fcn=b&res=&key=he3axaZ-V6pE9&testtype=both&type=e&ls=c&grp=b&submit=Search