An autosomal recessively inherited glycogen storage disease caused by glucan 1,4-alpha-glucosidase deficiency. Large amounts of glycogen accumulate in the lysosomes of skeletal muscle (skeletal muscle); heart; liver; spinal cord; and brain. Three forms have been described: infantile, childhood, and adult. The infantile form is fatal in infancy and presents with hypotonia and a hypertrophic cardiomyopathy (hypertrophic cardiomyopathy). The childhood form usually presents in the second year of life with proximal weakness and respiratory symptoms. The adult form consists of a slowly progressive proximal myopathy.