A Prospective Study to Observe & Describe Clinical Outcomes of Alglucosidase Alfa Treatment in Patients ≤6 Months of Age With Infantile-onset Pompe Disease (IOPD)

Inclusion Criteria: * At the time of informed consent, participants must be ≤6 months of age, corrected for gestation if necessary. Gestational age \<40 weeks will be adjusted to a full-term gestational age of 40 weeks. * Participants must have alglucosidase alfa enzyme replacement therapy (ERT) planned or initiated for IOPD treatment irrespective of study participation, according to the treating physician's decision regarding participants' routine disease management. * Participants must have available and accessible medical records from the time of IOPD diagnosis and from subsequent follow-up. * Participants must have a confirmed diagnosis of IOPD, defined as presence of 2 pathogenic acid alpha glucosidase (GAA) variants and documented GAA deficiency in blood (dried blood spot \[DBS\] accepted), skin, or muscle tissue, or presence of 1 pathogenic GAA variant and documented GAA deficiency in blood, skin, or muscle tissue from separate samples (either from 2 different tissues or from the same tissue but at 2 different sampling dates.) (DBS and leukocytes are acceptable as 2 different samples from blood). * Participants must have established cross-reacting immunologic material (CRIM) status available prior to enrollment. CRIM status may be provided by historical CRIM testing results or prediction of CRIM status based on genotyping performed at a Clinical Laboratory Improvement Amendments (CLIA) or other appropriately certified genetic laboratory. * Participants must have cardi…