Sir William Osler was one of the first to recognize the bicuspid aortic valve as a common congenital anomaly of the heart.  Leonardo da Vinci recognized the superior engineering advantages of the normal trileaflet valve.  However, bicuspid aortic valve is mentioned only briefly in many pediatric and cardiology textbooks.
The normal aortic valve has 3 equal-sized leaflets or cusps with 3 lines of coaptation. A congenitally bicuspid aortic valve has 2 functional leaflets. Most have one complete line of coaptation. Approximately half of cases have a low raphe. Stenotic or partially fused valves caused by inflammatory processes, such as rheumatic fever, are not included.
The embryonic truncus arteriosus is divided by the spiral conotruncal septum during development. The normal right and left aortic leaflets form at the junction of the ventricular and arterial ends of the conotruncal channel. The nonseptal leaflet (posterior) cusp normally forms from additional conotruncal channel tissue. Abnormalities in this area lead to the development of a bicuspid valve, often through incomplete separation (or fusion) of valve tissue. 
Bicuspid aortic valve is often observed with other left-sided obstructive lesions such as coarctation of the aorta or interrupted aortic arch, suggesting a common developmental mechanism.  Specific gene mutations have been isolated. [5, 6]
The larger leaflet is referred to as the conjoined leaflet. Two commissures (or hinge points) are present; usually, neither is partially fused. The presence of a partially fused commissure, which has also been called a high raphe, probably predisposes toward eventual stenosis. At least half of all congenitally bicuspid valves have a low raphe, which never attains the plane of the attachments of the two commissures and never extends to the free margin of the conjoined cusp. Redundancy of a conjoined leaflet may lead to prolapse and insufficiency. 
Valve leaflet orientation and morphology can vary. A recent surgical study showed conjoined leaflets in 76% of specimens.  Of these, fusion of the raphe was noted between the right and left cusps in 86%, and fusion was noted between the left and noncoronary cusps in only 3%. Of the valves without raphes, more than 30% of the leaflets were unequal in size.
Coronary arteries may be abnormal.  A left-dominant coronary system (ie, posterior-descending coronary artery arising from the left coronary artery) is more commonly observed with bicuspid aortic valve. Rarely, the left coronary artery may arise anomalously from the pulmonary artery. The left main coronary artery may be up to 50% shorter in patients with a bicuspid aortic valve. Occasionally, the coronary ostium may be congenitally stenotic in association with bicuspid aortic valve.
The aortic root may be dilated.  This dilatation has some similarities to the dilatation of the aorta seen in Marfan syndrome. [12, 13] The dilatation may involve the ascending aorta (most commonly) but may also involve the aortic root or transverse aortic arch. [14, 15, 16, 17] A recent study compared aortic dilation in children who had bicuspid aortic valve with and without coarctation of the aorta; the conclusion was that valve morphologic characteristics and function and age at the time of coarctation of the aorta repair had no impact to minimal impact on aortic dimensions. 
With degeneration of aging valves, sclerosis and calcification can occur.  The changes are similar to those in atherosclerotic coronary arteries. The bicuspid valve may also be completely competent, producing no regurgitant flow. However, redundancy and prolapse of cusp tissue can lead to valve regurgitation. Complications arise in as many as one third of patients over their lifetimes;  this disorder, therefore, deserves close attention and medical follow-up.
Valve morphology may be predictive of problems of stenosis, insufficiency, or both. Fusion along the right or left leaflets is less commonly associated with stenosis or insufficiency in children. This arrangement is much more common in patients with coarctation of the aorta, whose valves function adequately. Fusion along the right and noncoronary leaflets is more frequently associated with pathologic changes of stenosis or insufficiency in the pediatric population. 
Bicuspid aortic valves may be present in as many as 1-2% of the population. Because the bicuspid valve may be entirely silent during infancy, childhood, and adolescence, these incidence figures may be underestimated and are not generally included in the overall incidence of congenital heart disease.
Incidence does not appear to be affected by geography.
A recent report suggests much lower than expected prevalence in African-Americans. 
The male-to-female ratio is 2:1 or greater. Sex is not a predictive variable in the natural history of bicuspid aortic valve. A recent prospective echocardiographic study in newborn infants showed a prevalence of bicuspid aortic valve in 7.1 per 1,000 male newborns versus 1.9 per 1,000 female newborns. 
Bicuspid aortic valve may be identified in patients of any age, from birth through the 11th decade of life. It may be only an incidental finding at autopsy. Bicuspid aortic valve may remain silent and be discovered as an incidental finding on echocardiographic examination of the heart.
Critical aortic stenosis and infective endocarditis may be considered relatively early sources of morbidity for patients with bicuspid aortic valve. Critical aortic stenosis may occur in infancy and may be associated with a bicuspid valve.
Occasionally, bicuspid aortic valve is diagnosed after a patient has developed infective endocarditis with systemic embolization.
Stenosis of a bicuspid aortic valve is more likely to develop in persons older than 20 years and is caused by progressive sclerosis and calcification. High levels of serum cholesterol have been associated with more rapidly progressive sclerosis of the congenitally bicuspid aortic valve. 
Children who develop early progressive, pathologic changes in the bicuspid aortic valve are more likely to develop valve regurgitation than stenosis. Bicuspid aortic valve was identified in 167 (0.8%) of 20,946 young Italian military conscripts. Of these, 110 were found to have either mild or moderate aortic insufficiency.