Pompe disease is a rare (estimated at 1 in every 40,000 births), inherited and often fatal disorder that disables the heart and skeletal muscles. It is caused by mutations in a gene that makes an enzyme called acid alpha-glucosidase (GAA). Normally, the body uses GAA to break down glycogen, a stored form of sugar used for energy. The enzyme performs its function in intracellular compartments called lysosomes. Lysosomes are known to function as cellular clearinghouses; they ingest multiple substances including glycogen, which is converted by the GAA into glucose, a sugar that fuels muscles. In Pompe disease, mutations in the GAA gene reduce or completely eliminate this essential enzyme. Excessive amounts of lysosomal glycogen accumulate everywhere in the body, but the cells of the heart and skeletal muscles are the most seriously affected. The severity of the disease and the age of onset are related to the degree of enzyme deficiency. 
Types of Pompe Disease :
Early onset (or the infantile form) is the result of complete or near complete deficiency of GAA. Symptoms begin in the first months of life, with feeding problems, poor weight gain, muscle weakness, floppiness, and head lag. Respiratory difficulties are often complicated by lung infections. The heart is grossly enlarged. Many infants with pompe disease also have enlarged tongues. Most babies die from cardiac or respiratory complications before their first birthday. 
Late onset (or juvenile/adult) Pompe disease is the result of a partial deficiency of GAA. The onset can be as early as the first decade of childhood or as late as the sixth decade of adulthood. The primary symptom is muscle weakness progressing to respiratory weakness and death from respiratory failure after a course lasting several years. The heart is usually not involved. A diagnosis of pompe disease can be confirmed by screening for the common genetic mutations or measuring the level of GAA enzyme activity in a blood sample. Once pompe disease is diagnosed, testing of all family members and a consultation with a professional geneticist are recommended. Carriers are most reliably identified via genetic mutation analysis. 
Genetic Changes :
Mutations in the GAA gene cause Pompe disease. The GAA gene provides instructions for producing an enzyme called acid alpha-glucosidase (also known as acid maltase). This enzyme is active in lysosomes, which are structures that serve as recycling centers within cells. The enzyme normally breaks down glycogen into a simpler sugar called glucose, which is the main energy source for most cells.
Mutations in the GAA gene prevent acid alpha-glucosidase from breaking down glycogen effectively, which allows this sugar to build up to toxic levels in lysosomes. This build up damages organs and tissues throughout the body, particularly the muscles, leading to the progressive signs and symptoms of Pompe disease.
Flow Chart: 
The discovery of the GAA gene has led to rapid progress in understanding the biological mechanisms and properties of the GAA enzyme. As a result, an enzyme replacement therapy has been developed that has shown, in clinical trials with infantile-onset patients, to decrease heart size, maintain normal heart function, improve muscle function, tone, and strength, and reduce glycogen accumulation. A drug called alglucosidase alfa (Myozyme©), has received FDA approval for the treatment of infants and children with Pompe disease. Another algluosidase alfa drug, Lumizyme©, has been approved for late-onset (non-infantile) Pompe disease.
Sign & Symptoms :
Pompe disease is a genetic disorder that is always present at birth for those who are affected. However, symptoms may show up at any time from infancy through adulthood. Pompe disease is a single disease, but it affects people differently.Historically, Pompe disease had been described by physicians as either early-onset or late-onset, depending on when the patient’s signs and symptoms first appear. However, Pompe disease may be best understood
as a spectrum. When symptoms occur during the first few months of life, Pompe disease progresses very rapidly and is almost always fatal by the age of 1 year, usually due to heart failure. When symptoms occur after infancy, Pompe disease progresses more variably but can cause great difficulties as muscles throughout the body become weaker and weaker. The muscles most often affected are those used for breathing and mobility (the ability to move around).
Pompe Disease in Infants :
Pompe disease in infants is a life-threatening condition that affects all of the major body organs. Without disease-specific treatment, the disease progresses rapidly. The infant may quickly become gravely ill. Without treatment, infants with Pompe disease are not likely to survive past the age of 1 year. The chart below lists the major symptoms. 
Pompe Disease in Children and Adults :
Pompe disease in children and adults tends to progress more variably than in infants. Symptoms and severity can vary widely from one person to another. Major breathing problems, such as respiratory failure, can shorten the life span of people with Pompe disease. However, many are able to adapt to the challenges that the disease presents and continue with their lives. The chart below describes the major symptoms. 
Diagnosing Pompe disease can be challenging because many of the symptoms are similar to those of other diseases. In addition, symptoms often develop slowly and may not present themselves at the same time. It may be easier to diagnose infants with Pompe disease because the rapid progression and more pronounced symptoms may prompt healthcare providers to perform more extensive testing. 
Tests that confirm the diagnosis :
A number of tests may be done to help diagnose Pompe disease and determine the extent of muscle weakness or how far the disease has progressed. However, an enzyme assay is commonly used to confirm a diagnosis of Pompe disease. This biochemical test measures the activity of acid alpha-glucosidase enzyme in a small sample of skin, muscle, or blood. The enzyme assay may be performed using different samples, which include:
⦁ Dried blood spot Lymphocyte or leukocyte (blood)
⦁ Cultured skin fibroblasts
A diagnosis of Pompe disease is confirmed if the test shows there is less than normal or no enzyme activity. 
1- https://www.ninds.nih.gov/Disorders/All-Disorders/Pompe-Disease- Information-Page