Mitral valve prolapse

Mitral valve prolapse (MVP) is a valvular heart disease characterized by the displacement of an abnormally thickened mitral valve leaflet into the left atrium during systole. In its nonclassic form, MVP carries a low risk of complications. In severe cases of classic MVP, complications include mitral regurgitation, infective endocarditis, and — in rare circumstances — cardiac arrest, usually resulting in sudden death.

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Overview

  The mitral valve, so named because of its resemblance to a bishop's miter, is the heart valve that prevents the backflow of blood from the left ventricle into the left atrium. It is composed of two leaflets (one anterior, one posterior) that close when the left ventricle contracts.^[1]

Each leaflet is composed of three layers of tissue: the atrialis, fibrosa, and spongiosa. Patients with classic mitral valve prolapse have excess connective tissue that thickens the spongiosa and separates collagen bundles in the fibrosa. This is due to an excess of dermatan sulfate, a glycosaminoglycan. This weakens the leaflets and adjacent tissue, resulting in increased leaflet area and elongation of the chordae tendineae. Elongation of the chordae often causes rupture, and is commonly found in the chordae tendineae attached to the posterior leaflet. Advanced lesions — also commonly involving the posterior leaflet — lead to leaflet folding, inversion, and displacement toward the left atrium.

History

The term mitral valve prolapse was coined by J. Michael Criley in 1966 and gained acceptance over the other descriptor of "billowing" of the mitral valve, as described by JB Barlow.^[2]

For many years, mitral valve prolapse was a poorly understood anomaly associated with a wide variety of both related and seemingly unrelated signs and symptoms, including late systolic murmurs, inexplicable panic attacks, and polythelia (extra nipples). Recent studies suggest that these symptoms were incorrectly linked to MVP because the disorder was simply over-diagnosed at the time. Continuously-evolving criteria for diagnosis of MVP with echocardiography made proper diagnosis difficult, and hence many subjects without MVP were included in studies of the disorder and its prevalence. In fact, some modern studies report that as many as 55% of the population would be diagnosed with MVP if older, less reliable methods of MVP diagnosis—notably M-mode echocardiography—were used today.

In recent years, new criteria have been proposed as an objective measure for diagnosis of MVP using more reliable two- and three-dimensional echocardiography. The disorder has also been classified into a number of subtypes with respect to these criteria.

Subtypes

Prolapsed mitral valves are classified into several subtypes, based on leaflet thickness, concavity, and type of connection to the mitral annulus. Subtypes can be described as classic, nonclassic, symmetric, asymmetric, flail, or non-flail.

Note: all measurements below refer to adult patients and applying them to children may be misleading.

Classic versus nonclassic

Prolapse occurs when the mitral valve leaflets are displaced more than 2 mm above the mitral annulus high points. The condition can be further divided into classic and nonclassic subtypes based on the thickness of the mitral valve leaflets: up to 5 mm is considered nonclassic, while anything beyond 5 mm is considered classic MVP.

Symmetric versus asymmetric

Classical prolapse may be subdivided into symmetric and asymmetric, referring to the point at which leaflet tips join the mitral annulus. In symmetric coaptation, leaflet tips meet at a common point on the annulus. Asymmetric coaptation is marked by one leaflet displaced toward the atrium with respect to the other. Patients with asymmetric prolapse are susceptible to severe deterioration of the mitral valve, with the possible rupture of the chordae tendineae and the development of a flail leaflet.

Flail versus non-flail

Asymmetric prolapse is further subdivided into flail and non-flail. Flail prolapse occurs when a leaflet tip turns outward, becoming concave toward the left atrium, causing the deterioration of the mitral valve. The severity of flail leaflet varies, ranging from tip eversion to chordal rupture. Dissociation of leaflet and chordae tendineae provides for unrestricted motion of the leaflet (hence "flail leaflet"). Thus patients with flail leaflets have a higher prevalence of mitral regurgitation than those with the non-flail subtype.

Diagnosis

Echocardiography is the most useful method of diagnosing a prolapsed mitral valve. Two- and three-dimensional echocardiography are particularly valuable as they allow visualization of the mitral leaflets relative to the mitral annulus. This allows measurement of the leaflet thickness and their displacement relative to the annulus. Thickening of the mitral leaflets >5 mm and leaflet displacement >2 mm indicates classic mitral valve prolapse.

Prevalence

Prior to the strict criteria for the diagnosis of mitral valve prolapse, as described above, the incidence of mitral valve prolapse in the general population varied greatly. Some studies estimated the incidence of mitral valve prolapse at 5 to 15 percent or even higher.^[3]

As part of the Framingham Heart Study, the prevalence of mitral valve prolapse in Framingham, MA was estimated at 2.4%. There was a near-even split between classic and nonclassic MVP, with no significant age or sex discrimination.^[4] Based on data gathered in the United States, MVP is prevalent in 7% of autopsies.^[5]

Signs and symptoms

Some patients with MVP experience heart palpitations, atrial fibrillation, or syncope, though the prevalence of these symptoms does not differ significantly from the general population. Between 11 and 15% of patients experience moderate chest pain and shortness of breath. These symptoms are most likely not caused directly by the prolapsing mitral valve, but rather by the mitral regurgitation that often results from prolapse. In addition, the American Heart Association has linked anxiety and panic attack disorders to Mitral Valve Prolapse.

For unknown reasons, MVP patients tend to have a low body mass index (BMI) and are typically leaner than individuals without MVP.^[4] MVP is a frequent occurrence in individuals with the Marfan syndrome.^[6]

Auscultation

Upon auscultation of an individual with mitral valve prolapse, a mid-systolic click, followed by a late systolic murmur heard best at the apex is common.

Mitral valve prolapse syndrome

Mitral valve prolapse syndrome (MVP Syndrome), also referred to as mitral valve prolapse dysautonomia, is an imbalance of the autonomic nervous system that appears to be associated with mitral valve prolapse. It is unclear what the underlying etiology is that causes both autonomic dysregulation and the structural abnormalities present in mitral valve prolapse.

Symptoms generally attributed to MVP syndrome include palpitations, shortness of breath, and syncope. Because of the low specificity of these symptoms, and the fact that there is significant overlap in the causes of these symptoms with sequelae of significant mitral regurgitation often seen with mitral valve prolapse, MVP syndrome is often over-diagnosed.^[7] This is made more difficult because there is no consensus criteria to diagnose MVP syndrome. Although Mitral Valve Prolapse Syndrome is often associated to Mitral Valve Prolapse it is possible for the two to exist independently from each other.

Most patients who suffer from mitral valve prolapse syndrome will have dysautonomia as the cause of their symptoms. In particular, supraventricular arrhythmias are associated with increased parasympathetic tone.^[8]

Complications

Mitral regurgitation

Mitral valve prolapse is frequently associated with mild mitral regurgitation,^[9] where blood aberrantly flows from the left ventricle into the left atrium during systole. Occasionally MVP patients experience severe regurgitation, often due to chordae tendineae rupture.^[10]

Sudden death

Severe mitral valve prolapse is associated with arrhythmias and atrial fibrillation that may progress and lead to sudden death.^{[citation needed]} As there is no evidence that a prolapsed valve itself contributes to such arrythmias,^[7] these complications are more likely due to mitral regurgitation and congestive heart failure.

Prognosis

The major predictors of mortality are the severity of mitral regurgitation and the ejection fraction.^[11] Generally, MVP is a benign disorder. However, MVP patients with a murmur, not just an isolated click, have a general mortality rate that is increased by 15-20%.^[5]

Treatment

Most patients only need reassurance. Those with mitral valve prolapse and symptoms of dysautonomia (palpitations, chest pain) may often benefit from beta-blockers (e.g., propranalol). Patients with prior stroke and/or atrial fibrillation may require blood thinners, such as aspirin or warfarin.

Mitral valve prolapse associated with severe mitral regurgitation can be treated with repair or surgical replacement of the mitral valve. Repair of the mitral valve is always preferable to replacement and should be performed by surgeons that are skilled in the procedure. Current ACC/AHA guidelines suggest that early repair of mitral valve, performed in centers of surgical excellence, should be considered even in patients without symptoms of heart failure. Symptomatic patients, those with evidence of diminished left ventricular function or left ventricular dilatation need urgent attention.

Prevention of infective endocarditis

People with mitral valve prolapse are at higher risk of infective endocarditis (bacterial infection of the heart tissue), as a result of certain non-sterile procedures such as teeth cleaning and biopsy during colonoscopy. However, an April 2007 study by the American Heart Association has determined that the risks of prescribing antibiotics outweigh the benefits of antibiotic prophylaxis before an invasive procedure (such as dental surgery). Therefore, MVP patients who have taken prophylactic antibiotics routinely in the past may no longer need them.^[12]

References