The best-known form of this disease is the severe infantile form, Pompe disease or generalised glycogenosis. Later onset forms are known as juvenile or adult GSD II or the muscular forms. In Pompe disease the most common presentation is marked cardiomegaly, hypotonia, rapidly progressive weakness and macroglossia within the first months of life. Death usually occurs by one year of age. Less commonly the presentation is one of profound muscular weakness within the first six months without cardiomegaly, and these patients may survive beyond 2 years. In the muscular or late-onset forms there is a slowly progressive weakness of the proximal muscles of the limbs, presenting in childhood or adulthood.
Deficiency of acid a-glucosidase (also called acid maltase or acid a-1,4-glucosidase) is the primary defect.