echocardiographic assessment is parallel to that of aortic stenosis Careful interrogation of the left ventricular outflow tract in the parasternal long axis imaging plane will demonstrate the mechanism of obstruction (Figs 44.25 and 44.26; Videos 44.16 and 44.17) Full evaluation in apical, subcostal, and suprasternal notch views will also allow for optimal spectral Doppler evaluation Color Doppler is necessary to localize the flow turbulence and to make an assessment of aortic insufficiency The severity of the subvalvar stenosis is generally made by similar criteria as valvar aortic stenosis FIG 44.25 Fibromuscular subaortic stenosis (arrows) Note the distance of the fibromuscular ridge from the leaflets of the valve There is extension of the shelf onto the aortic leaflet of the mitral leaflet The color Doppler trace shows that the turbulence starts at the level of the subaortic obstruction AO, Aorta; LA, left atrium; LV, left ventricle FIG 44.26 Transesophageal echocardiogram from a patient with subaortic stenosis produced by accessory leaflet tissue derived from the mitral valve (arrow) AO, Aorta; LA, left atrium; LV, left ventricle; RV, right ventricle Evaluation of the aortic valve and its motion should also be evaluated in the parasternal long and short axis imaging plane The aortic valve leaflets may be noted to flutter due to the turbulence from the subaortic membrane There has also been increasing use of three-dimensional echocardiography to demonstrate the nature of the lesion and to aid in surgical management (Fig 44.27) Finally, echocardiogram provides important quantification of left ventricular function and hypertrophy FIG 44.27 Three-dimensional echocardiogram showing fibromuscular subaortic stenosis (arrow), as seen from below the left ventricular outflow tract Note that in comparison to the cross-sectional counterpart (shown in Fig 44.25), the three-dimensional reconstruction reveals the full extent of the obstruction LV, Left ventricle; MV, mitral valve; RV, right ventricle Cardiac Catheterization Left heart catheterization is not typically used for diagnosis of subaortic stenosis, unless there is a discrepancy between noninvasive imaging findings and clinical symptoms, or to better clarify the level of obstruction in patients with multiple outflow tract lesions A gradient is obtained across the left ventricular outflow tract by direct pull-back pressure measurements using an end-hold or micromanometer-tipped catheter Management Intervention to address subaortic stenosis is indicated in the setting of left ventricular systolic dysfunction and/or associated symptoms such as syncope, angina, and diminished exercise tolerance Timing of intervention in asymptomatic patients with normal cardiac function is less clear Due to the progressive nature of the disorder and the risk for development of aortic regurgitation, surgery is usually advised at gradients lower than those used to determine intervention for valvar aortic stenosis Surgery should be considered in asymptomatic patients with discrete subaortic membrane when the peak Doppler gradient across the left ventricular outflow tract is 50 mm Hg or greater or mean gradient is 30 mm Hg or greater.179 The risk for developing moderate to severe aortic regurgitation is significantly greater when the peak gradient exceeds 50 mm Hg.180–182 Surgery is also advocated with lower gradients if there is evidence of progressive aortic valve regurgitation during serial follow-up imaging Patients with a peak gradient less than 30 mm Hg and no significant left ventricle hypertrophy are followed closely for progression, especially in the first several years of life In asymptomatic patients with tunnel-like left ventricular outflow tract obstruction, surgery should be considered when the gradient is 60 mm Hg or greater In the setting of hypertrophic obstructive cardiomyopathy, indication for operation is driven by the progression of symptoms despite the maximum medical therapy Although transcatheter balloon dilation has been reported for discrete subaortic stenosis,183 intervention is primarily surgical The surgical approach for discrete subaortic stenosis varies depending on size and function of the aortic valve at the time of intervention (Fig 44.28)