Aortic Stenosis (AS)

Aortic stenosis is rarer in infancy. It is present in 0.004-0.34% of live births. It ranks 16th among critical congenital heart diseases in infants (2%). Incidence increased with age to become the second most common congenital heart disease after ventricular septal defect in the third decade of life.

Outflow tract septum:  The cardiac outflow tract include the ventricular outflow tract, the semilunar valves and the aortopulmonary septum.  There has been much debate regarding this process, the following is a summary of various theories [6, 36, 40].

 Kramer (1942) suggested that there are three embryological areas, the conus, the truncus and the pulmonary arterial segments.  Each segment develop two opposing ridges of endocardial tissue, the opposing pair of ridges and those from various segments meet to form the septum separating the two outflow tracts and the aortopulmonary trunks.  The aortopulmonary septum is formed by ridges separating the fourth (future aortic arch) and the sixth (future pulmonary arteries) aortic arches.  The truncus ridges are formed at the area where the semilunar valves are destined to be formed, therefore forming the septum between the ascending aorta and the main pulmonary artery.  The conus ridges form just below the semilunar valves and from the septation between the right and left ventricular outflow tracts.

 Van Mierop (1979), agreed that there are three pairs of ridges forming in the aortopulmonary, truncus and conus regions, however, he stated that the pairs of ridges fuse independently and later on fuse with each other to complete the septation.  His theory indicate that the truncus ridges form  first, and as they fuse they form a truncal septum which then fuses with the aortopulmonary septum which is formed by invagination of the dorsal wall of the aortic sac between the fourth and the sixth aortic arch arteries.  Figure 11

 Asami(1980),  followed Van Mierop’s theory, however, he stated that these ridge fuse in the opposite direction of what Van Mierop has indicated, i.e. from the outflow tract to the aortopulmonary region.

 Pexieder(1978, 1984) and Orts Llorca et al (1982), stated that there are only tow septa, a conotruncal (or bulbar) and an aortopulmonary septum.

 Bartlings et al (1989), introduced a new theory.  They stated that the septation process of the ventricular outflow tracts, pulmonary and aortic valves and the great vessels is mostly caused by a single septation complex, which they termed aortopulmonary septum.  This septation complex develops at the junction of the muscular ventricular outflow tract with the aortopulmonary vessel.  This junction has a saddle shape, i.e. not in one plane which would allow the right ventricular outflow tract to be long with a short main pulmonary artery, while the left ventricular outflow tract become short with a long ascending aorta.  (Figure 12) The ventricular outflow septation is formed by condensed mesenchyme, embedded in the endocardial cushion tissue just proximal to the level of the aorto-pulmonary valves.  The condensed mesenchyme will come in close contact with the outflow tract myocardium, from the area just above the bulboventricular fold, and participate in the septation of the outflow tract by providing an analogue to muscle tissue.  [6-9].  Myocardium in contact with the mesenchymal arch grow rapidly and form the bulk of the outflow septum and is continuous with the primary fold on the parietal  wall of the right ventricle and the myocardium on the right side of the primary septum.

 The commissures may be fused, the valve ring is occasionally hypoplastic. The orifice of the aortic valve could be eccentric. The valve is commonly bicuspid in aortic stenosis and the leaflets are asymmetric in 40% of cases. There is occasionally more than one level of obstruction in the left ventricular outflow tract or supravalvar area together with aortic stenosis. 10% of patients with aortic stenosis have subvalvar aortic stenosis. Supravalvar aortic stenosis is rarely associated with aortic stenosis. 50% of valvar aortic stenosis are associated with aortic insufficiency. Aortic stenosis most probably begets aortic stenosis rather than being a result of bicuspid aortic valve since bicuspid aortic valve is 10-20 times as common as aortic stenosis. Therefore, a small number of bicuspid valve will lead to aortic stenosis and the majority will not develop into aortic stenosis. Right-to-left commissures have worse prognosis in bicuspid aortic than anteroposterior commissures in developing aortic stenosis. Also eccentric orifices have worse prognosis than central orifices in developing aortic stenosis.

The left ventricle will become hypertrophied to overcome aortic stenosis. In severe cases contractility will diminish resulting in heart failure, affecting the left atrial pressure causing pulmonary edema and right heart failure.

The ascending aorta may become dilated due to the forceful jet of blood as it cross the aortic valve.

If significant aortic regurgitation is associated with stenosis, left ventricular failure and dilation will develop faster.

75% of patients are males. They are typically asymptomatic. The murmur detected is typically at a routine physical examination. In the minority of patients they complain of chest pain which is angina-like with syncope particularly during exercise.

Physical examination is benign except for a palpable thrill particularly over the suprasternal notch as well as the right second intercostal space. On auscultation the first heart sound is normal. There is an ejection click followed by a harsh systolic crescendo-decrescendo type murmur. The second heart sound is normal except with severe aortic stenosis where the aortic valve closure (A2) will be delayed secondary to prolonged left ventricular systole causing reversed splitting or single S2. 50% of patients with aortic stenosis have a decrescendo early diastolic murmur due to aortic insufficiency.

LVH with possibly ST and T wave changes particularly in the inferolateral leads. The changes on the electrocardiogram reflects the daily strain (i.e. chronic)of the myocardium rather than the severity at rest as may be measured by ECHO or cardiac catheterization.

The short axis reveals the aortic valve ring which should be assessed. Fusion between the right coronary and left coronary cusps will result in a right-to-left commissure and fusion of right coronary and non coronary cusps results in antero-posterior commissure. A fusion of the left coronary and non coronary is extremely rare. In addition, aortic insufficiency should also be evaluated, this is best seen with color Doppler or Doppler flow in the apical four chamber and parasternal long axis views. The severity of aortic insufficiency is assessed by the width of the aortic insufficiency jet and flow in the descending aorta (diminished and severe aortic insufficiency with reversal flow in diastole).

 

Parasternal short axis, showing bicuspid aortic valve	Apical 5 chamber view, showing thick aortic valve
Apical view with zoom showing thick aortic valve cusps	Parasternal long axis, showing thick aortic valve cusps

In infants with critical aortic stenosis the aim is to improve cardiac output. In older children the aim is to preserve the myocardium against irreversible damage.

The following criteria are used to decide intervention:

• Angina
• Premature ventricular contractions
• ST and T changes
• Echocardiographic evidence of left ventricular dysfunction

More than 50 mm Hg pressure gradient with no aortic insufficiency may benefit from balloon valvuloplasty. Balloon valvuloplasty and surgical valvotomy results are comparable.

Aortic valve replacement with a prosthetic valve is indicated with aortic stenosis and aortic insufficiency when the aortic insufficiency appears to be problematic as much as aortic stenosis. Aortic valve replacement is also indicated in severely deformed aortic valve. Aortic valve replacement is performed in older children.

Younger children they might benefit from the Ross procedure.

Normal ST segments and T waves changing into ST elevation or depression with T wave changes is an indication that aortic stenosis is severe.

Natural History

Mild cases usually do not progress.

Moderate and severe cases usually do progress.

Follow-up should be every year with electrocardiography. A 24-hour Holter monitor should be performed once every five years or more frequently if ectopy are encountered.

Exercise testing should be performed once every two years and echocardiography once every two years.

Subacute bacterial endocarditis accounts for 0.3% of patients.

Sudden death may occur to any patient with aortic stenosis but mainly symptomatic patients.

Competitive sports should be prohibited in those with pressure gradients of 40 mm Hg or more.

Repeat surgical procedure is typically not helpful. Repeat valvoplasty may be more helpful.

25% of patients with surgical repair develop aortic valve regurgitation.

Infants With Aortic Stenosis

These infants present early in life with congestive heart failure and aortic stenosis murmur.

The more severe cases result in decreased cardiac output presenting with an ashen color and decreased pulses.

The aortic valve leaflets are typically thick and gelatinous with small aortic valve ring. The left ventricle may be small and in some instances smaller than what could be compatible with adequate cardiac output (20 ml in newborns).

Patent foramen ovale will allow left-to-right shunting therefore decreasing the cardiac output.

PDA allows right-to-left shunting and improves cardiac output.

Balloon valvoplasty is helpful and is less risky than surgery but with risk of aortic insufficiency.

Surgical morbidity is acceptable.

Rarely, evolve to normal looking valves as the child grows.

Usually residual aortic stenosis is left.