The mitral valve has two leaflets: the anterior leaflet and the posterior leaflet. Together, they separate the left atrium from the left ventricle. During systole the valve closes, which means blood cannot do anything but be ejected out of the aortic valve and into circulation.
If the mitral valve doesn’t completely shut, blood can leak back into the left atrium; this is called mitral valve regurgitation. During diastole, the mitral valve opens and lets blood fill into the ventricle. If the mitral valve doesn’t open enough, it gets harder to fill the left ventricle; this is called mitral valve stenosis.
Let’s start with mitral valve regurgitation. The leading cause of mitral valve regurgitation, and the most common of all valvular conditions, is mitral valve prolapse. When the left ventricle contracts during systole, a ton of pressure is generated so that the blood can be pumped out of the aortic valve; therefore, a lot of pressure pushes on that closed mitral valve. Normally, the papillary muscles and connective tissue, called chordae tendineae or heart strings, keep the valve from prolapsing, or falling back into the atrium.
With mitral valve prolapse, the connective tissue of the leaflets and surrounding tissue are weakened; this is called myxomatous degeneration. Why this happens isn’t well understood, but it is sometimes associated with connective tissue disorders, such as Marfan syndrome and Ehlers-Danlos syndrome. Myxomatous degeneration results in a larger valve leaflet area and elongation of the chordae tendineae, which can sometimes rupture; this rupture typically happens to the chordae tendineae on the posterior leaflet, and can cause the posterior leaflet to fold up into the left atrium.
Patients with a mitral valve prolapse are usually asymptomatic, but often have a classic heart murmur that includes a mid-systolic click, which is sometimes followed by a systolic murmur.