This calculation is from estimated gene frequencies in normal individuals from various ethnic groups. As the weakness progresses, patients develop feeding difficulties and respiratory insufficiency. Enlargement of the left ventricle leads to outflow tract obstruction and ventricular failure. Death results from cardiopulmonary failure. Patients generally do not survive beyond the second or third decades of life.
All patients have involvement of respiratory muscles and most die of respiratory failure.
Several patients are reported to have died of basilar artery aneurysm. All were found to have abnormal storage within the lysosomes of arterial smooth muscle fibers. Age of onset does not predict age of death. Patients with the adult form may survive for decades following diagnosis. Muscle weakness may interfere with normal daily activities, and respiratory insufficiency is often associated with sleep apnea.