What else is it called?
- Glycogen Storage Disease Type II
- Acid Maltase Deficiency Disease
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What causes it?
Pompe Disease is caused by a problem in the GAA gene. This gene provides instructions for the enzyme acid alpha-glucosidase (acid maltase) which breaks down glycogen. This process is needed for the conversion of Glycogen to Glucose to produce energy within the body. Glycogen is stored in the body for when blood glucose levels run low between meals and the body needs energy.
If you have a problem with the GAA gene, you will not produce enough of the acid alpha-glucosidase enzyme. Therefore, glycogen cannot be broken down and begins to build up in lysosomes; these are the garbage collectors of the cell where glycogen is broken down ready to be converted into glucose. Because there is a fault in the enzyme, glycogen cannot be broken down and begins to build up in lysosomes. This then inhibits lysosome function and damages cells and tissues within the body.
There are three types of Pompe disease based on the progression of symptoms, severity and age of onset; infantile Pompe disease begins before 12 months of age, with symptoms appearing at around 2 months old and is split into classic and non-classic. Non-classic Pompe disease is in general less serious than classic Pompe disease as it is less likely to affect the heart. Non-classic infantile Pompe disease appears within the first year of life but later than classic infantile Pompe which occurs during the early months. Late-onset Pompe typically occurs later in life, during your teenage years or going into adulthood and does not affect the heart as much.
How common is it?
It is estimated that 1 in 40,000 people live with Pompe disease, with 200 people diagnosed in the UK. It is believed that there are ethnic factors that influence the incidence of the disease; for Caucasians, the frequency of the infantile form of Pompe disease is estimated to be 1 in 150,000 and 1 in 60,000 in the late onset form. It is reported to be more common in patients of Dutch, African American and Chinese descent.
What are the signs and symptoms?
There are different symptoms depending on the type of Pompe disease the patient developments. Symptoms progress as time goes on and the progression of the disease is linked to the activity of the GAA gene. Late-onset Pompe disease progresses at a slower pace compared to the infantile form of Pompe disease. The symptoms of infantile Pompe disease begin to develop at around 2 months old and are:
- Severe muscle weakness (myopathy); the baby may miss certain milestones such as holding up their head and sitting up on their own.
- Diminished muscle tone without muscle wasting
If the symptoms go untreated, the baby may become more serious and have a greater strain on the heart as it will begin to struggle pumping blood. In the most serious of cases, this can unfortunately lead to the death of the child.
Sometimes the symptoms can include:
- Enlargement of the heart, liver and tongue
- Baby is not feeding properly
- Difficulty in gaining weight
- Slow rate of growth.
The life expectancy of a child with untreated infantile Pompe disease is at most 2 years old but with treatment it can be prolonged.
Symptoms of late-onset Pompe disease are:
- Delayed motor skills
- Muscle weakness (myopathy); patient may struggle to walk or climb up stairs
- Difficulty breathing (pulmonary insufficiency)
In some cases, the patient may develop an enlarged heart (cardiomegaly). The life expectancy of someone with late-onset Pompe disease is age 30 when onset occurs as a child or teenager, and age 50 when it develops during adulthood.
How is it diagnosed?
There are different ways that Pompe disease can be diagnosed and distinguished from other diseases with similar symptoms.
Enzyme activity tests measure the levels and activity of acid alpha-glucosidase in the body. A skin biopsy one the ways this can be done as it is one of the least invasive techniques. A muscle biopsy can be sometimes to measure the enzyme activity, but this technique is more invasive and therefore less preferred. Blood tests can also be done to test for enzyme activity and screen for Pompe disease.
Genetic testing is another effective method for diagnosis. A blood or spit sample can be taken and examination of the GAA gene can be done to identify the faulty gene.
Can it be treated?
Enzyme replacement therapy (ERT) is the main treatment method for Pompe disease; it involves fortnightly intravenous infusions of the deficient acid alpha-glucosidase enzyme. This enzyme allows glycogen to be broken down properly and is shown to slow muscle wastage and development of heart problems. ERT greatly improves survival rates and increases strength levels. However, ERT can not enter the brain due to the blood-brain barrier, so it has no effect on any neurological symptoms caused by Pompe disease.
Physical therapy can sometimes be needed alongside ERT to help combat muscle weakness. Respiratory support can be needed in the later stages of Pompe disease, alongside a feeding tube in infants but rarely in late onset patients. Late onset patients may need to switch to a soft diet.
Do my family need to be tested?
Pompe Disease is an autosomal recessive disease, meaning that a fault copy of the gene must be inherited from both parents, giving a 25% chance of the child being born with the disorder. There is a 50% chance that the child will be born carrier, which means that the child will only inherit one copy of the gene and cannot develop Pompe disease.
As Pompe disease is an autosomal recessive disorder, members of your family may be carriers, or they may have Pompe disease and do not know. Genetic testing can test for Pompe disease within your family and can catch it early, increasing the success of any treatments given. Identifying carriers within your family can also assist in any family planning.
Image 1: Picture showing the inheritance pattern for an autosomal recessive disorder like Pompe disease.