Pompe disease is a rare, inherited, and progressive disorder also referred to as glycogen storage disease type II. Glycogen is a large, complex sugar stored in muscle cells that normally is broken down into smaller sugar molecules such as glucose. Cells then use glucose to produce energy.
Pompe disease is caused by mutations, or abnormalities, in the GAA gene, which encodes for an enzyme called acid alpha-glucosidase. Acid alpha-glucosidase breaks down glycogen into glucose. As a result of the mutations, glycogen builds up in tissues and causes damage.
Three main types of Pompe disease exist, each differing based on the age of onset and severity. They include classic infantile-onset Pompe disease, non-classic infantile-onset Pompe disease, and late-onset Pompe disease. The type of Pompe disease is partially associated with variants, or alleles, of the mutated GAA gene. For example, some variants result in a total loss of function of the GAA enzyme, leading to earlier onset disease and more severe symptoms. Other variants produce a GAA enzyme that works but is less effective, resulting in a milder and later onset disorder.
Classic infantile-onset Pompe disease
Symptoms of classic infantile-onset Pompe disease normally appear within a few months after birth. Early symptoms include difficulty gaining weight and failure to grow at a normal rate. This is known as “failure to thrive.” The condition can cause muscle weakness, decreased muscle tone (called hypotonia, or a lack of resistance in muscles), an enlarged liver, breathing problems, lung infections, feeding problems, hearing problems, and heart defects.
The condition is often fatal within the first year of life, but rapid treatment can reduce the risk of heart failure.
Non-classic infantile-onset Pompe disease
Non-classic infantile-onset Pompe disease begins later than the classic form but still appears within the child’s first year. Despite symptoms such as an abnormally enlarged heart, the chance of heart failure is lower with this form of the disease. Other symptoms include progressive muscle weakness leading to the delayed development of motor skills such as rolling over and sitting.
Infants with this type of Pompe disease often experience severe respiratory problems due to damage and weakness in the muscles involved in breathing. As a result, the condition has a poor prognosis and is often fatal.
Late-onset Pompe disease
About two-thirds of people with Pompe disease have the late-onset form. This type may appear in late childhood, or even in the teenage or adult years.
Late-onset Pompe disease is often milder and progresses more slowly than the infantile forms. In general, the later the disease appears, the slower the symptoms progress.
- Progressive muscle weakness, especially in the legs and trunk
- Mobility problems, including difficulty walking and exercising, and an increased chance of falls
- Breathing problems and shortness of breath
- Frequent lung infections
- Morning headaches
- Weight loss
- Difficulty swallowing
- Difficulty hearing
- Scoliosis, or a curved spine.
Heart involvement is reduced in most cases of late-onset Pompe disease, but some patients may experience an irregular heartbeat or an enlarged heart. As a result, the risk of heart failure is reduced compared to infantile-onset Pompe. However, as the disease progresses, breathing problems may increase and the most common cause of death is lung failure.
As the disease progresses, increased muscle weakness will often result in patients having to use mobility assistance, such as wheelchairs.
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