Endocardial Fibroelastosis is a rare heart disorder that affects infants and children. It is characterized by an abnormal thickening of heart tissue, especially around the valves, causing abnormal enlargement of the heart (cardiac hypertrophy), especially affecting the left ventricle. Impaired heart and lung function can eventually lead to congestive heart failure. It can cause valve failure and sudden death. Endocardial Fibroelastosis may occur for no apparent reason (sporadic) or may be inherited as an X-linked or autosomal recessive genetic trait.