Pediatric Subvalvar Aortic Stenosis

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Background

Subvalvar aortic stenosis (SAS), also called subaortic stenosis, 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. Images of SAS appear below.[2] (See Etiology.) 



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Echocardiogram of membranous subaortic stenosis. AO = aortic; LA = left atrium; LVOT = left ventricular outflow tract.



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Tunnel-type of subaortic stenosis (subvalvular aortic stenosis [SAS]). MV = mitral valve.

SAS 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 SAS; 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. (See Diagnosis.)

In most patients, SAS is detected in the course of follow-up care for associated congenital heart defects following previous cardiac surgery or during evaluation of a heart murmur. Many patients are asymptomatic. Among symptomatic patients, exertional dyspnea is the most common manifestation. (See Clinical Presentation.)

Two-dimensional (2D) echocardiography with color Doppler imaging is the current modality of choice to establish the diagnosis of SAS (see Workup). The surgery of choice for discrete fibromuscular SAS is complete resection with myotomy, with or without myomectomy through an aortotomy. Children, adolescents, and young adults with clinically significant aortic regurgitation, which is usually a consequence of the subaortic stenosis, may require aortic valve repair or replacement. (See Pathophysiology and Treatment and Management.)

Go to Aortic Stenosis for more complete information on this topic.

Anatomy

The boundaries of the left ventricular outflow tract (LVOT) are formed posterolaterally by the anterior leaflet of the mitral valve and intravalvar fibrosa and anteromedially by the muscular and membranous portions of the interventricular septum.

Anatomic variants in SAS

Fixed lesions of the LVOT that cause subvalvar aortic stenosis (SAS) have a spectrum of morphologies. The 4 basic anatomic variants are as follows: (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, (3) a diffuse, fibromuscular, tunnel-like narrowing of the LVOT,[3, 4] and (4) accessory or anomalous mitral valve tissue.

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

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 they are uncommon.[5]

Pathophysiology

Abnormalities of the aortic and mitral valves frequently develop during the natural course of subvalvar aortic stenosis (SAS) because of tethering by encroaching fibroelastic tissue of the membrane and fibromuscular ridge.

Clinically significant obstruction to ejection due to SAS results in concentric left ventricular (LV)  hypertrophy, often with an excessive septal bulge. This effect leads to a cycle of further obstruction and localized fibromuscular growth.

SAS has variable and unpredictable rates of progression in children, whereas the rate of progression in adults is slow.

Heart failure occurs only occasionally in pediatric patients. When present early in life, heart failure usually results from associated congenital heart defects. Cardiac output usually is well maintained, and systolic function is well preserved in children with isolated SAS, until severe obstruction develops.

Aortic regurgitation

Thickening of the aortic valve and mild, asymmetrical, poststenotic dilatation of the ascending aorta may be, in part, due to repetitive trauma and vibrations from the high-velocity jet of blood flow through the site of stenosis. Aortic regurgitation, which is usually progressive, develops in nearly 65% of patients in the course of SAS, and it usually persists even after the SAS is removed.

The detection of aortic regurgitation is a function of timely follow-up care. Although the aortic regurgitation is usually mild, its incidence and severity increases with an increasing LV outflow tract (LVOT) pressure gradient. This finding may reflect progressive damage to the aortic valve by the high-velocity jet of blood that SAS produces.

Aortic regurgitation adds volume overload to an already pressure-overloaded LV. The resultant decreased aortic diastolic pressure leads to diminished coronary perfusion and, in combination with increased left ventricular oxygen demand from pressure and volume overload, predisposes the left ventricular myocardium to ischemic injury. In some patients, progressive aortic regurgitation requires aortic valve repair or replacement at the time of surgical intervention for SAS. SAS may recur even after surgical resection appears to be complete and is usually a consequence of abnormal left ventricular outflow tract geometry.

Etiology

The etiology of subvalvar aortic stenosis (SAS) still is not fully understood. Fixed SAS may be the postnatal expression of a latent congenital lesion brought out by many mechanisms, such as genetic predisposition, certain anatomic characteristics of the left ventricular outflow tract (LVOT), hemodynamic abnormalities associated with other cardiac lesions, or surgical interventions that result in chronic flow disturbance in the outflow tract.[6] No genetic inheritance is known for SAS, and few familial incidences are reported.[7]

SAS may have a principally morphogenetic basis, whereby anatomical variants lead to abnormal cell proliferation and morphologic changes due to abnormal flow patterns.[3, 8, 9] Anatomic characteristics that may promote a chronic flow disturbance in the LVOT include a long, narrow LVOT; a steep (>130°) aortoventricular septal angle; increased mitral-aortic separation; and exaggerated aortic override. A focal myocardial abnormality similar to that found in hypertrophic cardiomyopathy may also play a role. The effect of abnormal flow patterns on a genetically vulnerable myocardium may account for the development of SAS in some cases.

Such LVOT morphology may inherently increase fluid shear stress on the interventricular septum and induce an abnormal endothelial and muscle-proliferative response in the outflow tract, with eventual formation of a fibromuscular ridge.

A possible hemodynamic basis of SAS is that alternation in left-sided flow before and after repair of associated congenital heart defects likely causes turbulence in the LVOT and adds to fluid shear stress on the interventricular septum. Therefore, morphologic abnormalities may result in flow disturbance that may be instrumental in the formation of SAS.

A possible genetic etiology in humans has not been identified. However, the Newfoundland dog has been shown to experience an increased incidence of subaortic membranes, which may in part be secondary to inbreeding and is consistent with an autosomal inheritance.[10]

Epidemiology

The approximate incidence of congenital heart defects is approximately 8 per 1000 live births. Subvalvar aortic stenosis (SAS) accounts for approximately 1% of all congenital heart defects (8 in 10,000 births) and for 15-20% of all fixed left ventricular outflow tract (LVOT) obstructive lesions.

The male-to-female ratio of SAS is 2:1 to 3:1.[11] Distinctions in the natural history and postoperative course of SAS between male and female patients have not been clearly defined. However, more male than female patients require repeat surgery. Isolated SAS is rarely seen at birth or during infancy. SAS may develop in some patients after they undergo repair of associated congenital heart defects (eg, ventricular septal defect), usually by age 2 years. Exceptions include patients with Shone complex and interrupted aortic arch, who can have SAS in early infancy.

Prognosis

Although natural history studies have not delineated the annual mortality rate, 2-10% of sudden deaths are reported in untreated individuals with severe left ventricular outflow tract (LVOT) obstruction, including subvalvar aortic stenosis (SAS), valvar aortic stenosis, and supravalvar aortic stenosis.

Postoperative survival rates are at least 85-95% at 15 years.[12] Late mortality is mostly related to residual LVOT obstruction and repeat operation.

The high postoperative recurrence rate (10-50% on >10-y follow-up) has been associated with a high preoperative LVOT pressure gradient (>50 mm Hg), tunnel-like lesions, incomplete removal of discrete SAS, and age younger than 10 years at surgery. The Ross-Konno procedure seems to reduce the postoperative recurrence rate for tunnel-like SAS.[13]

Overall surgical mortality for SAS is now less than 1% in most centers.[14, 15, 16, 17]

Complications

Preoperative complications include the following:

Sudden cardiac death, unlike in hypertrophic cardiomyopathy, is reported in only a small percentage of patients with SAS. Sudden cardiac death is usually not the first clinical manifestation of the disease. It almost always occurs in previously symptomatic patients, typically those who have an echo Doppler LVOT pressure gradient of more than 50 mm Hg.

Postoperative complications include the following:

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 in patients with SAS.

History

Symptoms of subvalvar aortic stenosis (SAS ) are rare in infancy and uncommon in early childhood, even if the stenosis is severe.

Isolated SAS may be diagnosed relatively late in life because of the progressive nature of the lesion and because patients lack symptoms that prompt evaluation. The diagnosis is frequently made during an evaluation for an asymptomatic heart murmur.

When present, symptoms include dyspnea on exertion, effort syncope and presyncope, angina, orthopnea, heart failure, 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 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 SAS 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 are rare. Their presence may indicate a cardiac arrhythmia, notably transient ventricular arrhythmia.

Angina may occur in as many as 25% of symptomatic patients with an LVOT obstruction that is more than mild. 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 decreased cardiac output and compression of the coronary vessels.

Physical Examination

Symptoms of associated congenital heart defects frequently mask those of subvalvar aortic stenosis (SAS). In most patients, in fact, SAS is detected in the course of follow-up care for associated congenital heart defects or during evaluation of a heart murmur.

The physical growth of the child with SAS is usually normal. Peripheral pulses are symmetrical and rarely of small volume, unless severe left ventricular outflow tract (LVOT) obstruction is present.

A prominent a wave in the jugular venous pulse occasionally occurs in children with SAS. This wave reflects reduced right ventricular compliance consequent to a hypertrophied ventricular septum.

A palpable carotid thrill and a left parasternal thrill are present in one third of patients with mild SAS (pressure gradient < 50 mm Hg) and in approximately one half of patients with more-than-mild SAS. A forceful LV apical impulse is present in most patients with moderate or severe SAS.

The first heart sound is normal. The second heart sound can be narrowly split or single because of prolonged LV systole. Paradoxical splitting of the second heart sound, which suggests associated LV dysfunction, may occur in severe SAS.

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 SAS. The length of the murmur is proportional to the degree of obstruction.

An ejection click opening the ejection murmur is absent in isolated SAS. This is an important clue for differentiating this murmur from that of aortic valve stenosis.

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.

Approach Considerations

No specific laboratory blood tests are required in the workup of SAS. Echocardiography is the principal diagnostic study.

Go to Imaging in Aortic Stenosis for more complete information on this topic.

Echocardiography

In subvalvar aortic stenosis (SAS), echocardiography enables evaluation of the following:

Echocardiography helps in defining and localizing SAS (see the image below). It reveals the extent of LVOT involvement, the degree of LV hypertrophy, the indices of LV performance, and the parameters of diastolic function of the LV. Secondary effects, such as the degree of aortic valve insufficiency, mitral valve regurgitation, or poststenotic dilatation of the aorta, may be assessed. Finally, associated congenital heart defects and their influence on the hemodynamic effects of SAS may be evaluated.



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Echocardiogram of membranous subaortic stenosis. AO = aortic; LA = left atrium; LVOT = left ventricular outflow tract.

Two-dimensional echocardiography with color Doppler imaging is the current modality of choice to establish the diagnosis of SAS. This noninvasive method allows for serial evaluation of the progression of the obstruction, the development of aortic valve insufficiency, and the results of surgical intervention.

M-mode echocardiography provides indirect evidence of SAS by revealing early closure (from the Venturi effect of the jet formed by the SAS) and the coarse flutter of the aortic valve leaflets.

Two-dimensional echocardiography, and now 3-dimensional (3D) echocardiography, reveals and defines the position of lesions, the extent of involvement of the LVOT (in tunnellike SAS), and the associated defects.[18] Apical views reveal the relationship of the SAS to surrounding structures (eg, mitral valve), and parasternal and subcostal long-axis views reveal the proximity of SAS to the aortic valve (see the image below).

Three-dimensional echocardiography can produce images of SAS with depictions of the morphology and extent remarkably similar to those observed during surgical visualization.

Multiplanar transesophageal echocardiography (TEE) provides superior definitions of the lesion, making it an ideal tool for intraoperative evaluation of the lesion to guide surgical resection and to evaluate the immediate results at the time of surgery.[19] TEE is also useful for diagnostic purposes in patients with a poor acoustic window in whom transthoracic imaging results are not definitive.

A peak instantaneous and a mean pressure gradient across the LVOT estimated during continuous wave Doppler interrogation provide measures of the severity of LVOT obstruction.

When used as a guide to cardiac intervention, Doppler interrogation does not permit the clinician to accurately estimate the pressure gradient in the presence of multiple obstructive LVOT lesions in series, a large ventricular septal defect (VSD), or a tunnel-like obstruction.

Color Doppler evaluation reveals the presence and severity of aortic and mitral regurgitation.

Cardiac Catheterization

Cardiac catheterization is not routinely indicated in isolated subvalvar aortic stenosis (SAS) but can be utilized for preoperative hemodynamic evaluation when associated with other congenital heart defects.

If multiple levels of left ventricular outflow tract obstruction are present, careful pullback pressure measurements performed with an end-hole or high-fidelity manometer-tipped catheter from the left ventricle to the aorta may allow delineation of the pressure gradient and the exact site of obstruction.

Electrocardiography

In subvalvar aortic stenosis (SAS), even if it is mild, electrocardiography (ECG) reveals a variable degree of left ventricular hypertrophy in 50-80% of patients. ECG findings are occasionally normal in patients with severe SAS. A prominent Q wave in the left precordial leads may be present from septal hypertrophy. Strain pattern is visible on the ECG in approximately 25% of patients and indicates severe obstruction.

Cineangiography

Cineangiography is usually not necessary to define the anatomy of subvalvar aortic stenosis (SAS), but it may be helpful if catheterization is being performed to evaluate other associated cardiac defects. A left ventriculogram obtained in a angulated orientation (70º left anterior oblique/20º cranial angulation) delineates the  left ventricular outflow tract (LVOT) and the anatomy of the SAS. The degree of mitral valve regurgitation and anatomy of any ventricular septal defects, if present, are also well demonstrated.

Histologic Findings

Histologic findings in subvalvar aortic stenosis (SAS) are the same in lesions of the fibromuscular ridge or collar and in tunnellike lesions. A composite of different tissue cells, which varies from patient to patient, is present.

Abundant amounts of irregularly oriented and dense collagen fibers and thin, short elastic fibers are visible. Also visible are sparsely scattered fibroblasts with elongated nuclei and smooth muscle cells. Vascularity is generally absent.

Approach Considerations

Most pediatric patients with subvalvar aortic stenosis (SAS) are asymptomatic. Therefore, medical therapy has no role for such patients. SAS is progressive, however, and intervention is often required at some point in the clinical course of the disease to relieve left ventricular outflow tract (LVOT) obstruction or prevent progressive aortic valve insufficiency. Diagnostic evaluation can be performed on an outpatient basis.

Old reviews reported that heart failure occurred in small percentages of patients. However, surgical intervention is currently undertaken before heart failure develops. If SAS progresses to the point that heart failure or clinically significant LV dysfunction develops, standard medical therapy (except the use of systemic vasodilators such as angiotensin-converting enzyme [ACE] inhibitors) is indicated until surgery can be performed.

Consultation with a pediatric cardiologist and a pediatric cardiac surgeon, as needed, is advisable.

Indications for Intervention

The criteria and timing of intervention for discrete subvalvar aortic stenosis (SAS ) have been controversial. Rationale for early intervention in patients with these lesions, based on rapid progression and eventual aortic valve injury in patients with unrelieved SAS, is offset by the problem of high postoperative incidence of recurrence, late reoperation, and development of aortic regurgitation even after successful early relief of the obstruction.[14, 15, 16]

Based on published data, an approach to the management of SAS after risk-benefit stratification is nonintervention and medical follow-up in children and adolescents with Doppler mean gradients of less than 30 mm Hg and no left ventricular (LV) hypertrophy.[14, 16]

Children and adolescents with Doppler mean gradients of 50 mm Hg or more should have surgical intervention, similar to what is indicated in patients with valvar aortic stenosis.

Children and adolescents with Doppler mean gradients of 30-50 mm Hg may be considered for surgical intervention if they are symptomatic with angina, syncope, or dyspnea on exertion (class I), if they are asymptomatic but develop ST/T-wave changes over left precordium on ECG at rest or with exercise (class I), or if they have substrate for a progressive course, a Doppler peak systolic gradient of more than 50 mm Hg, or an older age at diagnosis.[14, 15, 16, 17]

Prevention of aortic regurgitation is usually not an indication for surgical intervention in those with mild left ventricular outflow tract (LVOT) obstruction.[16]  Progression from no or trace aortic valve insufficency to a more significant grade is an indication for surgical intervention in an effort to prevent worsening insufficiency in the future. Tunnel-like SAS should have an early surgical intervention.

Interventional Procedures

Percutaneous balloon dilation of discrete subvalvar aortic stenosis (SAS) has been infrequently reported as successful in reducing the left ventricular outflow tract (LVOT) pressure gradient and, usually, relief is relatively brief. Balloon dilation is not typically considered to be useful in SAS.

The surgery of choice for discrete fibromuscular SAS is complete resection with myotomy, with or without myomectomy through an aortotomy. Children, adolescents, and young adults with clinically significant aortic regurgitation may require aortic valve repair or replacement.

For tunnel-type SAS (seen in the image below) with a small LV-aortic junction, an aortoventriculoplasty (Konno procedure) may be required. This involves excision and replacement of the aortic valve with a prosthesis, patch augmentation of ventricular septum to enlarge the LVOT, and pericardial patch closure of the right ventriculotomy, which is used to gain access to the LVOT.



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Tunnel-type of subaortic stenosis (subvalvular aortic stenosis [SAS]). MV = mitral valve.

For recurrent SAS and for tunnel-type SAS with normal LV-aortic junction and aortic valve, a modified Konno procedure (ie, without aortic valve excision and replacement) may be performed.

For complex SAS or tunnel stenosis, current surgical practices are individualized but may employ an aortoventriculoplasty in combination with aortic root replacement by using a prosthetic aortic valve, an aortic valve allograft, or a pulmonary valve autograft (Ross-Konno procedure).[20]

Activity

Impose activity restrictions, as indicated, on the basis of the degree of residual hemodynamic abnormality. Advise patients with subvalvar aortic stenosis (SAS) to avoid participation in strenuous activities and in competitive sports.

Competitive sports and games, weight training, and strenuous exercises should not be permitted if patients have any of the following:

Endocarditis Prophylaxis

According to American College of Cardiology/American Heart Association (ACC/AHA) guidelines, no antibiotic prophylaxis is required for subvalvar aortic stenosis when no surgery for the condition has been performed.[21] If a patch is used for outflow reconstruction (eg, for the Konno procedure), then subsequent antibiotic prophylaxis against bacterial endocarditis is recommended for 6 months postoperatively.

Long Term Monitoring

Sudden cardiac death is possible because subvalvar aortic stenosis (SAS) is a progressive disorder. Close follow-up care is important to detect the progressive course of the disease and to prevent sudden death. Routine follow-up care is required for patients who undergo surgery and for those who do not.

Because of the possibility of rapid progression, follow-up care for infants and young children should be more frequent (eg, every 4-6 mo) until the rate of progression is understood. Other children may be followed up at intervals of 3-12 months, depending on the severity of their obstruction and associated lesions.

Long-term postoperative follow-up care is indicated for all patients with SAS. About 10-25% of patients may have a residual gradient in excess of 50 mm Hg. The known high incidence of recurrence and the possible progressive worsening of aortic regurgitation after surgery underscore the need for long-term surveillance. Repeat surgery may be indicated.

More-than-mild SAS poses a high risk during pregnancy.[22] Therefore, close monitoring for heart failure, follow-up by a multidisciplinary team, and delivery at a center experienced in dealing with such problems are warranted.

Medication Summary

SAS is treated surgically.

Author

Douglas J Schneider, MD, Associate Professor of Pediatrics, Clinical Tract, Division of Pediatric Cardiology, Department of Pediatrics, University of Kentucky College of Medicine; Director, Pediatric Cardiac Catheterization Laboratory, Kentucky Children’s Hospital

Disclosure: Nothing to disclose.

Coauthor(s)

Gautam K Singh, MD, , MRCP, Professor of Pediatrics, Division of Cardiology, Director of Noninvasive Imaging Research, Co-Director of Echocardiography Laboratory, Washington University in St Louis School of Medicine; Attending Faculty, Department of Pediatrics, Division of Cardiology, St Louis Children's Hospital

Disclosure: Nothing to disclose.

Specialty Editors

Mary L Windle, PharmD, Adjunct Associate Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Nothing to disclose.

John W Moore, MD, MPH, Professor of Clinical Pediatrics, Section of Pediatic Cardiology, Department of Pediatrics, University of California San Diego School of Medicine; Director of Cardiology, Rady Children's Hospital

Disclosure: Nothing to disclose.

Chief Editor

Howard S Weber, MD, FSCAI, Professor of Pediatrics, Section of Pediatric Cardiology, Pennsylvania State University College of Medicine; Director of Interventional Pediatric Cardiology, Penn State Hershey Children's Hospital

Disclosure: Received income in an amount equal to or greater than $250 from: Abbott Medical .

Additional Contributors

Paul M Seib, MD, Associate Professor of Pediatrics, University of Arkansas for Medical Sciences; Medical Director, Cardiac Catheterization Laboratory, Co-Medical Director, Cardiovascular Intensive Care Unit, Arkansas Children's Hospital

Disclosure: Nothing to disclose.

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Echocardiogram of membranous subaortic stenosis. AO = aortic; LA = left atrium; LVOT = left ventricular outflow tract.

Tunnel-type of subaortic stenosis (subvalvular aortic stenosis [SAS]). MV = mitral valve.

Echocardiogram of membranous subaortic stenosis. AO = aortic; LA = left atrium; LVOT = left ventricular outflow tract.

Tunnel-type of subaortic stenosis (subvalvular aortic stenosis [SAS]). MV = mitral valve.

Echocardiogram of membranous subaortic stenosis. AO = aortic; LA = left atrium; LVOT = left ventricular outflow tract.

Tunnel-type of subaortic stenosis (subvalvular aortic stenosis [SAS]). MV = mitral valve.