The purpose of this study is to better understand the long-term health effects of Pompe disease and to determine if there are any abnormal changes in the brain and peripheral nerves. Additionally, the investigators will study the relationship between the abnormal changes in brain, nervous system findings, and developmental outcomes. The investigators will collect clinical information from clinic visits as well as assessments such as neuroimaging (magnetic resonance imaging (MRI), magnetic resonance spectroscopy (MRS), and diffusion tensor imaging (DTI)), cognition, academic skills, speech and language function, physical therapy and quantitative muscle ultrasound. Subjects will be in this study for at least 3 years and up to 6 years.