Pompe Disease: A genetic disorder affecting both children and adults that is characterized by progressive muscle weakness and breathing difficulty. Pompe disease is caused by a deficiency or dysfunction of the enzyme acid alpha-glucosidase (GAA) that is responsible for the breakdown of glycogen (a large complex carbodhydrate molecule) within a compartment of the cell, resulting in an excessive accumulation of glycogen, especially in the muscles. Also known as glycogen storage disease type II (GSD-II) and acid maltase deficiency (AMD).