Discrete sub-aortic stenosis (DSS) can develop at any age, but in the vast majority of cases, is detected in childhood.  It comprises approximately 6.5% of adult congenital heart disease.  Although it is frequently categorised as congenital heart disease, it is considered to be an acquired condition.

The hypothesised pathogenesis of DSS centres on abnormal LVOT geometry that predisposes to turbulent flow within the LVOT. It is this turbulent flow that is thought to increase local sheer stress and consequently lead to local reactive cellular proliferation and progressive fibro-muscular changes.

In the majority of cases (90%) the sub-valvular membrane takes the form of a fibromuscular ridge that encircles the LVOT, but it can also be composed of a diffuse tunnel-like narrowing. Occasionally the defect can involve the base of the aortic cusp or the anterior leaflet of the mitral valve.

Progression of DSS is unpredictable. Risk factors for progression of LVOT gradient include: initial mean gradient >30 mmHg, initial aortic valve thickening and attachment of the subaortic membrane to the mitral valve. The reported rates of progression (peak instantaneous gradient) vary from centre to centre, but are in the range of 1-3 mmHg/year.

There is a high prevalence of aortic regurgitation among patients with DSS (up to 80%). When detected, the severity of aortic regurgitation is ≤ 2+ in in the vast majority of patients, and progresses slowly. Risk factors for progression of aortic regurgitation include: higher peak gradient (>50 mmHg), and longer distance between DSS and aortic valve. The longer the distance between the DSS and the aortic valve allows for a greater distortion of flow in the LVOT, creating high velocity turbulent jets that strike and damage the underside of the aortic leaflets, leading to aortic regurgitation.

Guidelines for intervention vary from centre to centre, but surgical intervention is generally recommended if the peak instantaneous gradient is >50-60 mmHg, with a lower threshold in the setting of aortic regurgitation or left ventricular hypertrophy. The timing of surgery is an important consideration, especially in children, due to the risk of recurrence. Reported rates regarding need for re-operation are based on small retrospective studies and range between 15-26% during median follow-up periods of 10-15 years.