Subaortic Stenosis

Excerpt

Subvalvular aortic stenosis, also known as subaortic stenosis (SAS), is a significant form of left ventricular outflow tract (LVOT) obstruction characterized by fixed anatomical narrowing immediately below the aortic valve. Although valvular aortic stenosis remains the most common cause of LVOT obstruction, SAS represents a meaningful subset of congenital heart disease—particularly in infants, children, and young adults—and poses unique diagnostic, surgical, and long-term management challenges. The chronic pressure overload created by this fixed subvalvular obstruction promotes the development of concentric left ventricular hypertrophy (LVH), which, when untreated, increases the risk of myocardial ischemia, malignant arrhythmias, and ultimately congestive heart failure, adversely impacting long-term survival. As such, SAS requires ongoing surveillance and timely intervention to interrupt the progressive cascade of myocardial remodeling and its downstream complications.

Anatomically, SAS manifests in 3 principal morphologic forms. The discrete membranous subtype, accounting for approximately 70% of cases, consists of a thin fibrous membrane attached to the upper interventricular septum and extending toward the anterior mitral leaflet, typically forming a crescent-shaped ridge (see Image. Discrete Subaortic Stenosis on Computed Tomography). A related but thicker variant, the fibromuscular ridge, produces similar hemodynamic consequences. Less commonly, tunnel-type SAS (<30%) presents as diffuse fibromuscular narrowing extending from the left ventricular (LV) cavity to a hypoplastic aortic annulus, often accompanied by septal thickening and necessitating more extensive surgical approaches such as the Konno procedure. Rarely, accessory mitral valve tissue may protrude into the LVOT, contributing to obstruction. Importantly, the distance between the obstructive membrane and the aortic valve is prognostic; distances of less than 5 mm are associated with increased risk of aortic valve injury and higher recurrence rates following resection.

The natural history of SAS is one of gradual but persistent progression. Turbulent, high-velocity systolic flow is directed toward the structurally normal aortic valve cusps, leading over time to cusp thickening, fibrosis, and the development of secondary aortic regurgitation (AR)—a hallmark of disease evolution. This combination of fixed subvalvular obstruction and progressive AR perpetuates substantial LV pressure overload, fueling further LVH and raising the risk of adverse cardiac outcomes. Early recognition and strategic intervention are therefore crucial to limiting the long-term consequences of sustained pressure burden and preventing irreversible ventricular remodeling.