Do you know if adult pompe disease is caused by a sudden mutation or a long build up of glycogen?

Long build up: I'm not a geneticist; and, there are good references on this subject easily available on the net. However, a quick answer is long build-up. The disease is viewed as constitutional (born-in) and not a later development of a "sudden mutation". If you are really interested, do your homework on the subject by looking at NIH and other sites.

Hereditary: Pompe Disease is a rare genetic / hereditary disorder (1 in 40,000 births) in which an enzyme called acid alpha-glucosidase either doesn't function correctly or there is too little of it in lysosomes. There can be gradations -so that glycogen may build up more quickly or more slowly & it may show up in a baby or later in adulthood.