Subvalvular and Supravalvular Aortic Stenosis

Definition and Morphology

SAS comprises 8% to 30% of all forms of congenital LVOT obstruction. It spans a spectrum of anomalies ranging from a simple fibrous membrane to a tunnel-like fibromuscular band. The lesion itself is produced by an accumulation of fibroelastic tissue. The most common clinical presentation, in approximately 84% of cases, is as a fibrous crescent or ring that completely encircles the LVOT to produce a discrete obstructive lesion. In its more severe form, a fibromuscular band encircles the complete length of the LVOT, producing a diffuse, tunnel-like narrowing. This tunnel-like obstruction commonly occurs in association with a small aortic root. The resultant outflow obstruction causes myocardial hypertrophy, which may in turn add to the severity of the obstruction.

Subaortic stenosis may also result as a consequence of abnormal mitral valve insertion, accessory mitral apparatus tissue, abnormal insertion of papillary muscles, abnormal muscular bands in the LVOT, or posterior displacement of the infundibular septum. Subaortic stenosis can develop after surgical repair of atrioventricular septal defects, ventricular septal defects, double-outlet right ventricle, or after the arterial switch operation.

Associated Lesions

SAS can be isolated or found in association with other heart defects (∼60% of cases), particularly multilevel LVOT obstruction. Other associated congenital cardiac anomalies include ventricular septal defect, coarctation of the aorta, Shone syndrome (aortic coarctation, parachute mitral valve, supramitral valve ring, subaortic stenosis), patent ductus arteriosus, left superior vena cava, and valvular aortic stenosis. There may be an association between SAS and familial hypertrophic cardiomyopathy.

Genetics and Etiology

There is clinical and experimental evidence that isolated discrete SAS is an acquired lesion. There have also been reports of familial occurrence implying a genetic predisposition. There are, however, no antenatal reports of this lesion and it has never been described in neonates. Furthermore, no SAS has been described in experimental genetic mouse models. The pathological initiator of SAS is likely to reside in the myocardium, but the mechanism by which the abnormal hypertrophic response within the LVOT is generated is as yet unclear. Subtle morphologic abnormalities of the LVOT (a steeper aortoseptal angle) may result in altered shear stress on the outflow septum, triggering cell proliferation in this region in the genetically predisposed individual.

Early Presentation and Management

SAS is usually a progressive lesion. The rate of progression is variable, but it tends to be more rapid in those with tunnel-type obstruction. Progression of subaortic stenosis in children may be quite rapid, particularly in patients with a higher LVOT gradient at baseline and those diagnosed at a younger age. In contrast, the rate of progression of obstruction in patients diagnosed in adulthood tends to be slower, with an annual increase in LVOT gradient of less than 1 mm Hg and a median intervention-free survival of 16 years. The presence of associated congenital lesions may identify those at risk of more rapid progression, but neither age at diagnosis or baseline LVOT gradient appear to be predictive in adults. Campbell reported a 1.4% annual mortality and 0.9% sudden death rate per year in a review of 2816 nonsurgically treated cases of valvar or subvalvar aortic obstruction from six separate series.

The predominant pathophysiologic features of SAS are progressive left ventricular hypertrophy and a variable degree of aortic valve regurgitation. It is believed that the “jet lesion” through the obstructed outflow tract causes shear stress on the aortic valve cusps, initiating a secondary fibrous thickening of the valve endothelium. More rarely, there can be fibrous attachments from the subaortic membrane to the valve cusps, which impair valve function. Mild to moderate aortic regurgitation is therefore common (60% of cases).

Outpatient Assessment

Clinical presentation depends on the severity of outflow tract obstruction and whether there are associated lesions. Those patients presenting for the first time in adulthood are often referred for evaluation of a heart murmur. Symptoms are rare if the obstruction is mild, but exertional breathlessness, chest pain, or syncope may occur if there is moderate or severe obstruction.

On physical examination, the pulse volume may be reduced if the outflow obstruction is severe or increased if there is moderate to severe aortic regurgitation. There may be a left ventricular heave if there is left ventricular hypertrophy (LVH) and/or a palpable systolic thrill over the mid-left (tunnel stenosis) or upper right sternal edge (discrete stenosis). The first heart sound is normal. The second heart sound may be normal or diminished (reduced intensity of A ₂ ) depending on the severity of the stenosis. A crescendo-decrescendo systolic ejection murmur is audible either at the left mid-sternal border (tunnel) or right-upper sternal border (discrete). Transmission into the carotids is inconsistent. Unlike valvular aortic stenosis, no ejection click is heard. A blowing, decrescendo diastolic murmur is heard if there is aortic regurgitation.

On the electrocardiogram, LVH is seen in 65% to 85% of all patients and in up to 50% of those with even mild stenosis. Left atrial enlargement may be present. In postoperative patients, there may be left bundle-branch block.

The chest radiograph is often normal, or there may be prominence of the left ventricle with associated dilation of the ascending aorta. Left ventricular dilation may be seen if there is significant aortic regurgitation.

Transthoracic echocardiography will demonstrate a narrow LVOT, seen best in the parasternal long-axis (PLAX) view. A “membrane” or ridge is sometimes visualized (owing to limited acoustic window), or a long area of muscular thickening (tunnel type) may be seen. Fluttering or partial early closure of the aortic valve may be seen on two-dimensional (2D) or M-mode echocardiography.

Transesophageal echocardiography usually allows direct imaging of the subaortic “membrane” or ridge, especially if multiplanar imaging is used. The transverse and longitudinal views of the aortic valve and LVOT provide comprehensive definition of discrete membranes ( Fig. 36.1 ) and evaluation of aortic valve competence. The five-chamber transgastric view allows color flow demonstration of the level of obstruction and estimation of pressure gradients by spectral Doppler imaging. Advanced real-time three-dimensional transesophageal echocardiographic techniques have been increasingly used for spatial assessment of subaortic membranes and in quantification of the extent of subaortic stenosis for preoperative planning.

Continuous wave and color flow Doppler imaging quantifies the severity of subaortic obstruction. The severity of discrete stenosis can be estimated using the simplified Bernoulli equation (peak gradient = 4 V ² ), which calculates a peak instantaneous Doppler gradient. This gradient can be higher than the numerical figure of the peak-to-peak gradient recorded at cardiac catheterization and may vary with different loading conditions, heart rate, cardiac output, and circulating catecholamines, with beat-to-beat and respiratory variation.

The Doppler mean gradient is also useful, taking an average of all instantaneous gradients throughout systole (calculated by tracing the outside border around the continuous wave Doppler velocity profile, using commercially available computer software). Doppler gradient estimation is less accurate with the tunnel form of obstruction, because it neglects the pressure drop caused by viscous friction along its flow path and invalidates some of the physical assumptions in the Doppler gradient calculation. Three-dimensional (3D) echocardiography may also provide more accurate definition of these lesions.

Magnetic resonance imaging (MRI) provides an accurate noninvasive assessment of this lesion in both its forms. Spin-echo images define morphology, and gradient reversal images can be utilized to estimate the severity of obstruction. Associated anomalies can also be detected.

Right-sided and left-sided heart catheterization can assess the severity of outflow obstruction by recording pressure withdrawal gradients (peak-to-peak pressure gradients) across the respective outflow tracts. Left or right ventriculography can assess ventricular function and delineate the level of obstruction of both discrete and diffuse forms. End-hole or micromanometer-tipped catheters can be used to obtain accurate measurements. Aortography will confirm the presence and severity of aortic regurgitation and associated arch abnormalities.

A combination of investigations may be needed to confirm the diagnosis, define the anatomy, assess the severity of the lesion, and detect associated anomalies.