opening snap

The opening snap occurs during early diastole when the fused, calcified mitral valve leaflets suddenly reach their maximal opening excursion, causing a rapid deceleration and vibration of the valve apparatus. It is best heard at the apex or left lower sternal border using the diaphragm, and is typically followed by a low-pitched, rumbling mid-diastolic murmur (also heard best in the left lateral decubitus position).

A key clinical pearl is the relationship between the S2-OS interval and the severity of mitral stenosis: a shorter S2-OS interval (<80 ms) indicates more severe stenosis (higher left atrial pressure forces the valve open faster and earlier), while a longer interval suggests milder disease. This is analogous to the inverse relationship seen with the Austin Flint murmur timing in aortic regurgitation.

Opening snaps are most commonly caused by rheumatic mitral stenosis, where chronic inflammation leads to commissural fusion and leaflet thickening. As the mitral valve becomes more calcified and immobile in advanced disease, the opening snap may become softer or disappear entirely, and it may be replaced by a single S2 if the valve becomes completely immobile.

Differentiating an opening snap from a widely split S2 or an S3 gallop is important: OS is higher-pitched than S3 and occurs earlier in diastole, while a split S2 occurs around the same time in the cardiac cycle but is heard best at the pulmonic area with respiratory variation. The OS does not vary significantly with respiration, unlike a split S2.

Clinically, the presence of an opening snap in a patient with a diastolic rumble strongly suggests rheumatic mitral stenosis, often with a history of rheumatic fever. This finding should prompt echocardiographic evaluation to assess valve area, gradient, and left atrial size, and to guide decisions regarding balloon valvuloplasty or surgical intervention.