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 (muscle, skeletal); 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 (cardiomyopathy, hypertrophic). 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. (From Muscle Nerve 1995;3:S61-9; Menkes, Textbook of child neurology, 5th ed, pp73-4)