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The other common causes of left anterior fascicular block include coronary artery disease, and certain types of congenital heart disease, most notably ostium primum atrial septal defects where it occurs in association with right bundle branch block. Isolated left anterior fascicular block occurs quite frequently in older patients with no other manifestation of heart disease and is probably caused by fibrosis of the conduction system or sclerosis of the cardiac skeleton. It may also occur in younger patients with no cardiac disease.