The role of imaging techniques in diagnosis and evaluation congenital aortic stenosis

 

Luc Mertens

Pediatric Cardiology, University Hospital Leuven

 

Introduction

Congenital aortic valve stenosis is one of the more common lesions in the paediatric population, comprising about 6-7% of all children born with congenital heart disease. Especially the bicuspid aortic valve has a prevalence of up to 2% of the population.  The presentation is variable and depends on the severity of the valvar stenosis as well as on the associated LV hypertrophy or hypoplasia. Symptoms vary with age: usually children and adolescents are asymptomatic and are sent to a pediatric cardiologist because of a cardiac murmur. Symptomatic infants or neonates present with cardiogenic shock of heart failure when critical stenosis is present. Echocardiography has become the most important technique for diagnosis of the disorder. The need for cardiac catheterization for diagnosis or evaluation of the severity of the stenosis has largely been eliminated. Catheterization is currently only performed with for treatment by balloon valvuloplasty. Apart from echocardiography, also other imaging techniques can be used for evaluation of aortic stenosis. Especially magnetic resonance imaging can be useful in some indications.

 

Echocardiographic evaluation of aortic stenosis

When evaluating children with aortic stenosis, different questions must be answered during the echocardiographic examination:

1.       The exact location and the nature of the stenosis must be evaluated: valvar stenosis must be distinguished from obstruction at the subvalvar or supravalvar level. Subvalvar stenosis is usually caused by a fibrous membrane immediately beneath or a few millimetres below the valve cusps. Supravalvar stenosis is caused by a narrowing on the ascending aorta above the valve. This is frequently associated with genetic disorders in which the elastin gene is involved such as the Williams’syndrome.

2.       The morphology of the aortic valve must be described. The most common form is a bicuspid aortic valve which usually forms by fusion of two cusps1. This fusion occurs either between the left and right coronary cusp (70%) or between the right coronary and noncoronary cusp (28%). The form where the left and noncoronary cusp are fused is very rare (<2%). The first form is more commonly associated with moderate to severe aortic stenosis and insufficiency while the second form tends to be associated with less severe stenosis and insufficiency but is more commonly associated with aortic coarctation. Unicommisural valves have been described especially with the severe/critical aortic stenosis in infants. In these infants the valve is frequently dysplastic and markedly thickened.

3.       The severity of valve stenosis must be evaluated using continuous wave Doppler ultrasound. This technique allows the measurement of peak blood velocity in the jet beyond the stenosis. Based on the Bernouilli principle, the instantaneous pressure gradient (P) can be calculated from the formula P=4v² where v is the peak velocity measured with continuous Doppler. This peak instantaneous pressure gradient calculated using Doppler echocardiography generally overestimates the gradient measured using cardiac catheterisation. This is explained by the fact that with cardiac catheterisation the peak-to-peak pressure difference between the left ventricle and the aorta is measured which may not occur at the same moment. Moreover recently the phenomenon of pressure recovery was described to partially account for the differences between the two techniques. This is an increase in pressure just distal to a stenosis due to a conversion from kinetic into potential energy. To partially correct for these problems the measurement of mean gradient was proposed. This mean gradient correlates better with the gradient measured during cardiac catheterisation. A mean gradient >50 mmHg is considered as a severe stenosis. As Doppler techniques are angle-dependent techniques, measuring the maximal velocity of a jet may be very challenging due to the variability in direction of the jet. The examiner should try to align as good as possible with the orientation of the jet which may require trying multiple angles. Another limitation which should be taken into account is that the peak velocity does only reflect the severity of the stenosis in case the left ventricular function is preserved. When the left ventricle fails, the pressure generated by the LV drops with concurrent fall in gradient. Thus Doppler technology may result in severe underestimation of the real gradient. This is especially true in neonates where severe aortic stenosis giving rise to heart failure can be associated with low gradients due to low output. Thus it is very important not to make treatment decisions based on pressure gradients only

4.       Evaluation of the effect of aortic stenosis on left ventricular (LV) size and function.  Assessment of ventricular size is extremely important in newborn infants, because when the ventricle is hypoplastic, the decision will have to be made whether it is likely to be capable of maintaining adequate systemic blood flow when the stenosis is relieved. If the ventricle is considered to be ‘too small’ a Norwood procedure is the only option. The Rhodes’ score 2 is a generally accepted method for assessing adequacy of LV size. It is based on calculating the aortic root diameter indexed to body surface area, the ratio of the long axis dimension of the left ventricle to the long axis dimension of the heart and the mitral valve area (cm²) indexed to body surface area.  A Rhodes score <-0.35 is considered not to be compatible with survival after two-ventricle repair. A modified score is proposed by Lofland et al. 3 In this approach a regression equation can be solved for characteristics of an individual patient to give the predicted 5-year survival benefit of Norwood versus biventricular repair. Parameters which have to be assessed during the echocardiographic examination are: z-score of the aortic valve at the sinuses, grade of endocardial fibroelastosis, ascending aorta diameter, presence of moderate or severe tricuspid regurgitation, z-score of the left ventricular length. This estimated risk can be calculated online in an aortic stenosis calculator (www.ctsnet.org/aortic_stenosis_calc). As mentioned in the previous paragraph, evaluation of cardiac function is very important in patients with aortic stenosis.   This is usually done by calculating ejection fraction or fractional shortening. The interpretation of these parameters is however not easy since both are load-dependent parameters which are decreased when afterload is increased. The function and the Doppler gradient should both be taken into account when assessing the severity of the valvar stenosis. There is a need for better techniques to evaluate cardiac function in this condition in order to detect these ventricles which do not tolerate the high afterload and are beginning to fail. Hopefully new echocardiographic techniques such as tissue Doppler gives new opportunities for evaluating ventricular function in variable loading conditions.

5.       Evaluation of left ventricular hypertrophy and detection of the presence of endocardial fibroelastosis. Left ventricular hypertrophy can be assessed using two-dimensional echocardiography by measuring wall thickness and calculating left ventricular wall mass using well-established formulae. In infants it is very important to assess the presence of endocardial fibroelastosis. This is an abnormal thickened fibrous layer in the inner part of the heart caused by fibrous degeneration of the subendocardial layers. This is thought to be caused by ischemic damage to the subendocardial layers due to inadequate coronary perfusion. The degree of fibroelastosis is an important prognostic factor for neonatal severe/critical aortic stenosis.

6.       Exclusion of associated congenital lesions.  As in every patient with congenital heart disease a full segmental analysis must be performed. Especially coarctation of the aorta and mitral valve abnormalities must be excluded.

 

 

 

The role of magnetic resonance imaging

Cardiac magnetic resonance imaging (cMRI) is proposed during the last years as an alternative technique to evaluate patients with aortic stenosis. Recent papers have demonstrated that this technology is capable of:

  1. Reliably assessing valvar morphology

  2. Calculation of valve area either by planimetry or using the continuity equation

  3. Measuring gradients using flow mapping techniques

  4. Excluding associated lesions such as coarctation of the aorta and mitral valve anomalies

  5. Assessing ventricular size and function:  3-D  volumetric datasets allow accurate calculation of ejection fraction and ventricular mass index. Myocardial tagging techniques allow the assessment of regional myocardial function and the evaluation of myocardial mechanics giving more insight into the pathophysiology of aortic stenosis.

cMRI can be used when the echocardiographic examination is not conclusive for instance on the evaluation of severity of the stenosis or ventricular function. In these cases cMRI might add additional information. Obviously cMRI is not readily available in most centres and requires general anaesthesia in young children. Moreover the flow mapping techniques tend to underestimate the peak velocities although MRI technology is evolving quickly into faster acquisition techniques.

 

 

References 


1. Fernandes SM, Sanders SP, Khairy P, Jenkins KJ, Gauvreau K, Lang P, Simonds H,Colan SD. Morphology of bicuspid aortic valve in children and adolescents.  Am Coll Cardiol. 2004 44:1648-51

2. Rhodes LA, Colan SD, Perry SB, Jonas RA, Sanders SP. Predictors of survival in neonates with critical aortic stenosis. Circulation 1991;84:2325-35.

3. Lofland, Gary K., McCrindle, Brian W., Williams, William G., Blackstone, Eugene H., Tchervenkov, Christo I., Sittiwangkul, Rekwan, Jonas, Richard A. Critical aortic stenosis in the neonate: A multi-institutional study of management, outcomes, and risk factors. J Thorac Cardiovasc Surg 2001 121: 10-27

 

 

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