eMedicine Specialties > Pediatrics: Cardiac Disease and Critical Care Medicine > Cardiology

Aortic Stenosis, Subaortic

Author: Gautam K Singh, MD, DCh, MRCP, FACC, Associate Professor of Pediatrics, Division of Cardiology, Director of Noninvasive Imaging Research, Co-director of Echocardiography Laboratory, Washington University School of Medicine; Attending Faculty, Department of Pediatrics, Division of Cardiology, St Louis Children's Hospital
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Updated: Oct 27, 2009

Introduction

Background

Subvalvular aortic stenosis (SAS) is a fixed form of anatomic obstruction to egress of blood across the left ventricular outflow tract (LVOT).¹ Although classified as a congenital heart defect, its rarity at birth and during infancy, its progressive course, and its high rate of postoperative recurrence suggest that it may be an acquired condition.²

Subaortic stenosis has a spectrum of anatomic variants and usually has a variable, progressive course. Associated congenital heart defects are found in 25-50% of patients with subvalvular aortic stenosis, the most common defects include ventricular septal defect (VSD), patent ductus arteriosus, coarctation of aorta, bicuspid aortic valve, abnormal left ventricular (LV) papillary muscle, atrioventricular septal defect, Shone complex, interrupted aortic arch, and persistent superior left vena cava.

Pathophysiology

Boundaries of the LVOT are formed posterolaterally by the anterior leaflet of the mitral valve, superiorly by mitral valve leaflets, and anteromedially by the interventricular septum.

Fixed lesions of the LVOT that cause subvalvular aortic stenosis have a spectrum of morphologies. The 4 basic anatomic variants are (1) a thin discrete membrane consisting of endocardial fold and fibrous tissue, (2) a fibromuscular ridge consisting of a thickened membrane with a muscular base at the crest of the interventricular septum, and (3) a diffuse fibromuscular tunnel-like narrowing of the LVOT.^3,4

Types 1 and 2 account for 70-80% of all cases of subvalvular aortic stenosis. Located 0.5-1.5 cm beneath the aortic valve, types 1 and 2 involve a variable extent of LVOT. A steep (>130°) aortoventricular septal angle, increased mitral-aortic separation, and an exaggerated aortic override are present in children who later develop subvalvular aortic stenosis.

LVOT obstructions secondary to accessory tissue, an anomalous basal attachment of the anterior mitral leaflet, and an anomalous chordal attachment of mitral valve occur but are uncommon. Abnormalities of the aortic and mitral valves frequently develop during the natural course of subvalvular aortic stenosis because of tethering by encroaching fibroelastic tissue of the membrane and fibromuscular ridge. Thickening of the aortic valve and mild asymmetric poststenotic dilatation of the ascending aorta result from repetitive trauma and vibrations from a subaortic jet of blood. In this setting, aortic regurgitation often develops and may persist, even after the subvalvular aortic stenosis is removed. Clinically significant obstruction to ejection due to subvalvular aortic stenosis results in concentric LV hypertrophy, often with an excessive septal bulge. This effect leads to a cycle of further obstruction and localized fibromuscular growth.

Frequency

United States

The overall prevalence of isolated subvalvular aortic stenosis is low. Subvalvular aortic stenosis is estimated to occur in as many as 0.08% of the general population. No genetic inheritance is known for subvalvular aortic stenosis, and few familial incidences are reported. Subvalvular aortic stenosis accounts for 15-20% of all fixed LVOTs and approximately 1% of all congenital heart defects.

Mortality/Morbidity

Sudden cardiac death is possible because subvalvular aortic stenosis is a progressive disorder. Although natural history studies have not delineated the annual mortality rate, 2-10% of sudden deaths are reported in untreated individuals who have severe LVOT obstruction, including subvalvular aortic stenosis, valvular aortic stenosis, and supravalvular aortic stenosis. Close follow-up care is important to detect the progressive course of the disease and to prevent sudden death.

Aortic regurgitation develops in nearly 65% of patients in the clinical course of the disease, and its detection is a function of timely follow-up care. Although the aortic regurgitation is usually mild, its incidence and severity increases with an increasing LVOT pressure gradient. This finding reflects progressive damage to the aortic valve by the high-velocity jet of blood that subvalvular aortic stenosis produces. Aortic regurgitation adds volume overload to an already pressure-overloaded LV and predisposes the patient to myocardial damage. In some patients with subvalvular aortic stenosis, progressive aortic regurgitation requires aortic valve repair or replacement at the time of surgical intervention. Subvalvular aortic stenosis may recur even after surgical resection appears to be complete.

Bacterial endocarditis is uncommon and most likely to occur in patients with a damaged aortic valve. Bacterial endocarditis can also result in hemodynamically significant aortic regurgitation and congestive heart failure (CHF) in patients with subvalvular aortic stenosis.

Sex

The male-to-female ratio of subvalvular aortic stenosis is 2:1 to 3:1.⁵ Distinctions in the natural histories and postoperative courses of subvalvular aortic stenosis between male and female patients have not been clearly defined. However, more male than female patients require repeat operation.

Age

Isolated subvalvular aortic stenosis is rarely seen at birth or during infancy. Subvalvular aortic stenosis may develop in some patients after they undergo repair of associated congenital heart defects (eg, VSD), usually by age 2 years. Exceptions include patients with Shone complex and interrupted aortic arch who have subvalvular aortic stenosis in the first year of life. Subvalvular aortic stenosis has variable and unpredictable rates of progression in children, whereas the rate of progression in adults is slow.

Clinical

History

Symptoms due to severe subvalvular aortic stenosis (SAS), even with severe stenosis, are rare in infancy and uncommon in early childhood.

• Symptoms of associated congenital heart defects frequently mask those of subvalvular aortic stenosis.
• Isolated subvalvular aortic stenosis may be diagnosed relatively late in life because of the progressive nature of the lesion and because patients lack symptoms that prompt evaluation.
• In most patients, subvalvular aortic stenosis is detected in the course of follow-up care for associated congenital heart defects or during evaluation of a heart murmur. When present, symptoms include dyspnea on exertion, effort syncope and presyncope, angina, orthopnea, congestive heart failure (CHF), and sudden cardiac death. Most of these symptoms occur in children, adolescents, and young adults aged 10-21 years with moderate or severe left ventricular (LV) outflow tract (LVOT) obstruction and peak-to-peak pressure gradients of more than 50 mm Hg.
• Exertional dyspnea is the most common symptom, occurring in as many as 40% of symptomatic patients. Exertional dyspnea with orthopnea reflects various degrees of pulmonary venous hypertension due to elevated LV filling pressure resulting from impaired diastolic compliance of the hypertrophied LV.
• Effort syncope and presyncope occur more frequently in subvalvular aortic stenosis than they do in stenosis of the aortic valve. Syncope during exertion occurs because cerebral perfusion decreases when arterial pressure declines consequent to systemic vasodilation in the presence of a fixed cardiac output. Presyncope may manifest as a graying-out spell or as dizziness on effort because of exertional hypotension. In pediatric patients, syncope and presyncope at rest rarely occur, and their presence may indicate a cardiac arrhythmia, notably transient ventricular arrhythmia.
• Angina may occur in as many as 25% of symptomatic patients with more-than-mild LVOT obstruction. Exertion commonly precipitates the condition, and rest relieves it. Angina occurs in the absence of coronary artery disease. It results from the combination of increased oxygen demand by hypertrophied myocardium and reduced oxygen delivery secondary to excessive compression of coronary vessels.
• CHF only occasionally occurs in pediatric patients. When present early in life, CHF usually results from associated congenital heart defects. Cardiac output usually is well maintained, and systolic function is well preserved in children with isolated subvalvular aortic stenosis until severe obstruction develops.
• Sudden cardiac death, unlike hypertrophic cardiomyopathy, is reported in a small percentage of patients with subvalvular aortic stenosis. Sudden cardiac death is usually not the first clinical manifestation of the disease. It almost always occurs in previously symptomatic patients who are likely to have an LVOT pressure gradient of more than 50 mm Hg.

Physical

• The physical growth of the child with SAS is usually normal.
• Peripheral pulses are symmetric and rarely of small volume unless severe LVOT obstruction is present.
• A prominent a wave in the jugular venous pulse occasionally occurs in children with SAS. This a wave reflects reduced right ventricular compliance consequent to hypertrophied ventricular septum.
• A palpable carotid thrill and a left parasternal thrill are present in one third of patients with mild subvalvular aortic stenosis (pressure gradient <50 mm Hg) and in approximately one half of patients with more-than-mild subvalvular aortic stenosis.
• A forceful LV apical impulse is present in most patients with moderate or severe subvalvular aortic stenosis.
• The first heart sound is normal.
• The second heart sound can be narrowly split or single because of prolonged LV systole. Paradoxic splitting of the second heart sound, which suggests associated LV dysfunction, may occur in severe subvalvular aortic stenosis.
• An ejection click opening the ejection murmur is absent in isolated subvalvular aortic stenosis. This is an important clue for differentiating this murmur from that of aortic valve stenosis.
• A low-pitched ejection systolic murmur of 2-4/6 intensity is best appreciated in second and third left parasternal spaces, with radiation to suprasternal notch. This murmur is typically present in all cases of isolated subvalvular aortic stenosis. The length of the murmur is proportional to the degree of obstruction.
• A high-pitched early diastolic murmur of aortic regurgitation in the same area is present in 30-50% of patients.
• A pansystolic murmur of mitral regurgitation due to dysfunction of the papillary muscle is sometimes heard.

Causes

The etiology of subvalvular aortic stenosis still is not fully understood. Fixed subvalvular aortic stenosis may be the postnatal expression of a latent congenital lesion brought out by many mechanisms, such as genetic predisposition, certain anatomic characteristics of LVOT, hemodynamic abnormalities associated with other cardiac lesions, or surgical interventions that result in chronic flow disturbance in the outflow tract.

• Polygenic inheritance
  • Studies of hearts of Newfoundland puppies, which are predisposed to develop subvalvular aortic stenosis in early life, showed striking similarities with the clinical progression observed in humans.⁶ The puppies had persistent embryonic tissue in LVOT that was capable of proliferation. However, no such tissue is found in human lesions.
  • Because instances of subvalvular aortic stenosis among several members of human families are few, the role of genetic inheritance in humans is unclear.
• Morphogenetic basis of subvalvular aortic stenosis^7,3,8
  • Certain anatomic characteristics of LVOT promote a chronic flow disturbance in the outflow tract. LVOT is longest and narrowest in patients who have subvalvular aortic stenosis. A steep (>130°) aortoventricular septal angle, increased mitral-aortic separation, and exaggerated aortic override are present in children who later develop subvalvular aortic stenosis.
  • Focal abnormal myocardium similar to that found in hypertrophic cardiomyopathy may be present in patients with some congenital heart defects, including subvalvular aortic stenosis.
  • Some have hypothesized that such LVOT morphology inherently increases fluid shear stress on the interventricular septum and induces an abnormal endothelial and muscle-proliferative response in the outflow tract with eventual formation of a fibromuscular ridge.
• Hemodynamic basis of subvalvular aortic stenosis: Alternation in left-sided flow before and after repair of associated congenital heart defects likely causes turbulence in LVOT and adds to fluid shear stress on the interventricular septum, as discussed above. Therefore, both morphologic abnormalities and flow disturbance may be synergistically instrumental in the formation of subvalvular aortic stenosis.

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