Glycogen Storage Disease Type II (GSD II), also known as Pompe disease, is an inherited disorder caused by excessive accumulation of glycogen, a type of complex sugar, in cells. This is due to a defective enzyme called acid alpha-glucosidase, also known as acid maltase. Affected organs and tissues may include muscles, heart and liver. Intellect is not affected. Based on the severity and the age at which symptoms appear, GSD II is categorized into two types, infantile-onset or late-onset. The infantile-onset type of GSD II is the most common and usually begins within the first year of life, leading to death from heart failure or breathing problems in early childhood. The late-onset type of GSD II may not become apparent until later in childhood, adolescence, or early adulthood, and has similar but less severe symptoms. Persons with late-onset type of GSD II usually have progressive muscle weakness and breathing problems, and shortened life spans. It is not possible to predict the severity of the condition for any particular person.Enzyme replacement therapy as soon as the diagnosis is established could alleviate symptoms and improve survival. Therapies are also available to manage complications with heart, breathing and muscular problems. GSD II is caused by pathogenic variants in the GAA gene.