Glycogen storage disease type II, also called Pompe disease, is an autosomal recessive metabolic disorder which damages muscle and nerve cells throughout the body. It is caused by an accumulation of glycogen in the lysosome due to deficiency of the lysosomal acid alpha-glucosidase enzyme. It is the only glycogen storage disease with a defect in lysosomal metabolism, and the first glycogen storage disease to be identified, in 1932 by the Dutch pathologist J. C. Pompe. The build-up of glycogen causes progressive muscle weakness (myopathy) throughout the body and affects various body tissues, particularly in the heart, skeletal muscles, liver and the nervous system. Signs and symptoms Newborn The infantile form usually comes to medical attention within the first few months of life. The usual presenting features are cardiomegaly (92%), hypotonia (88%), cardiomyopathy (88%), respiratory distress (78%), muscle weakness (63%), feeding difficulties (57%) and failure to thrive (50%). The main clinical findings include floppy baby appearance, delayed motor milestones, and feeding difficulties. Moderate hepatomegaly may or may not be present. Facial features include macroglossia, wide open mouth, wide open eyes, nasal flaring (due to respiratory distress), and poor facial muscle tone. Cardiopulmonary involvement is manifested by increased respiratory rate, use of accessory muscles for respiration, recurrent chest infections, decreased air entry in the left lower zone (due to cardiomegaly), arrhythmias, and evidence of heart failure. Before developing a treatment, median age at death in untreated cases was 8.7 months, usually due to cardiorespiratory failure; However, this outcome is drastically changed since treatment has been available, improving with early access to treatment. Late onset form This form differs from the infantile principally in the relative lack of cardiac involvement. The onset is more insidious and has a slower progression. Cardiac involvement may occur but is milder than in the infantile form. Skeletal involvement is more prominent with a predilection for the lower limbs. Late onset features include impaired cough, recurrent chest infections, hypotonia, progressive muscle weakness, delayed motor milestones, difficulty swallowing or chewing and reduced vital capacity. One of the difficulties with attributing the illness solely to genetic deficiencies is that, even between people as genetically similar as identical twins, the symptoms may differ For example, one may be in pain, whilst the other may not. Similarly, one's rate of muscle deterioration may be faster than the other's. Prognosis depends on the age of onset of symptoms with a better prognosis being