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)