Symptoms can also begin later in life, during childhood or adulthood, and the disease is then known as late-onset Pompe. This form can also be called juvenile/adult-onset Pompe disease.
Causes of late-onset Pompe disease
Pompe is a type of glycogen storage disease, meaning patients have an excessive build-up of a complex sugar molecule known as glycogen within their cells, especially in muscle cells. This is because of a defect in the GAA gene that causes low levels of the acid alpha-glucosidase enzyme to be produced, an enzyme that is crucial in breaking glycogen down into glucose.
Different types of mutations in the GAA gene can affect how much of a functional acid alpha-glucosidase enzyme exists in cells. The type and severity of Pompe disease depends on the levels of the working acid alpha-glucosidase enzyme.
People with late-onset Pompe have higher GAA enzyme levels than are found in the infantile-onset forms of this disease, but generally less than 40 percent of “normal” levels. In comparison, infantile-onset Pompe patients typically have less than 1 percent of GAA enzyme activity, while those with non-classic forms usually have less than 10 percent.
Symptoms of late-onset Pompe disease
Muscle weakness, especially in the torso and legs, are often the first symptoms of late-onset Pompe disease. Patients may develop a waddling or swaying gait, and may find it difficult to stand after sitting, running, or climbing stairs.
As muscle weakness is progressive, facial muscles, muscles supporting the spine, and those involved in breathing and eating can also be affected. Spinal muscular weakness may lead to scoliosis, or an abnormal curvature of the spine, especially in adolescents. It is not uncommon for patients to experience muscle aches and cramps, as well as headaches and fatigue.
The severity of respiratory problems can vary greatly in late-onset Pompe patients, and is due to weakness in the diaphragm and intercostal muscles, or those between the ribs.
Diagnosing late-onset Pompe disease
Late-onset Pompe disease can be diagnosed with a test that checks the levels of working acid alpha-glucosidase enzyme, or by genetic analysis to determine if mutations exist in the GAA gene.
Muscle weakness is usually evaluated through a physical examination; by checking the levels of creatine kinase, which are elevated in Pompe disease patients; via electromyography, a test of the electrical activity in muscles; and by evaluating a patient’s breathing ability through tests likespirometry, a measure of lung function.
Prognosis and management
While late-onset Pompe disease is incurable, levels of the acid alpha-glucosidase enzyme can be replenished with enzyme replacement therapy, generally given patients every two weeks throughout their lives. Potential therapies are also being evaluated in tests in patients and in preclinical, or laboratory, work.
Life expectancy for late-onset Pompe disease is currently estimated to be age 30 when it first appears in children or teenagers, and 50 years of age for adults.
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