The Surgical Treatment of Subaortic Obstruction

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Abstract

Subaortic stenosis (SAS) is a consequence of left ventricular outflow tract obstruction (LVOTO). Fixed obstruction at the subvalvular level is caused by a specific anatomic alteration whereas the orifice size remains unchanged. Obstruction may either be the result of an actual ridge of fibromuscular origin or a fibrous tissue that coats and tethers the aortic valve leaflets so that their opening is limited and LVOTO is exacerbated.
SAS is a spectrum of anatomical dearrangements, therefore two main categories may be proposed ├óÔé¼ÔÇ£ dis-crete and tunnel type. Discrete type is more common and is caused by a discrete sub-valvular membrane or muscular band. Congenital aortic stenosis (AS) is a spectrum of defects involving the aortic root with some overlap of anomalies. In a recent report from Indianapolis, AS comprised 5% of congenital cases.

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Morphology
The localised form of discrete SAS may be fibrous or fibromuscular. When the fibrous ridge is firmly adherent to the hypertrophied septum the condition is termed discrete fibromuscular stenosis. An obstructing localised shelf may occur at any level but is more often located a few millimetres from the cusps. A distally located ridge tends to be narrow. Fibrous stenosis sufficiently long to be termed a tunnel comprises approximately 20% of the cases.

A majority of patients have abnormal aortic valve (AV). Apart from subaortic obstruction, concern has centred around secondary damage to the AV that includes aortic insufficiency (AI) and valve thickening, with or without commissural fusion. AI results from trauma to the leaflets by the abnormal jetflow pattern. LV hypertrophy and AI can occur due to AV damage because of the jet from the subaortic narrowing, which may also render the AV prone to infective endocarditis. AI is occasionally mild and non-progressive, although severe AI may occur after infective endocarditis. Interaction of genetic predisposition and morphologically deformed long and narrow LVOTOs cause rheological abnormalities and increase shear stress in the SAS region. This also seems to be the main aetiologic factor along with its subtle but extensive role in the LV endocardium.

The LV usually shows concentric hypertrophy. The extensive hypertrophy of the septum may cause difficulty in distinction between this clinical entity and hypertrophic obstructive cardiomyopathy (HOCM).

Early surgical repair of SAS has been advocated to prevent rapid progression of LVOTO. A significant LV-aortic mean gradient, ├óÔÇ░─ä30mmHg in children and ├óÔÇ░─ä50mmHg in adults, requires surgery. SAS has a progressive nature in adult patients. Oliver et al. revealed that the progression of obstruction was slow over several decades and might be accelerated after reaching the age of 50. Some reports have shown that fixed SAS gives good results and low recurrence if operated before a severe gradient (more than 40mmHg) develops.

Clinical Features
Although a congenital disorder, adult onset SAS has been reported in literature. Signs and symptoms are similar to any valvular lesion of the AV. There is a diastolic murmur in 65% due to cusp thickening or eddy currents. X-rays show that the ascending aorta is not dilated and calcification of the valve is absent, while echocardiography (ECG) reveals severe LV hypertrophy. Transthoracic echocardiography (TTE) is so sensitive that a discrete subvalvular lesion is visible before a gradient develops and an aortoseptal angle may have a prognostic significance. TTE is also useful in differentiating SAS from HOCM. In the catheterisation laboratory, LVOT gradients are measured and the normal post-ectopic pressure pulse and absence of accessory wave in the aortic pulse contour help to differentiate from HOCM.

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