The concept of myxomatous mitral valve disease (MMVD) emerged in the mid-20th century, in a world dominated by rheumatic valve diseases, after the observations of astute surgeons attempting the first repairs of mitral valves (MV), of pathologists describing these atypical MV observed at autopsy, and of master clinicians intrigued by the peculiarities of systolic clicks and late systolic murmurs. The observation of the myxomatous degeneration histologically involving mitral tissue led to a “unification” of the condition under 1 central functional feature, the mitral valve prolapse (MVP).
The mitral annulus is a component of the MV apparatus and consists of a cord-like ring of collagen and elastic fibers distributed along the atrioventricular junction and giving support to the MV leaflets. It is conventionally divided into anterior and posterior portions, although the real annulus is the one that serves as hinge point for the mural posterior leaflet of the MV. The motion of the mitral annulus is passive and determined by the contraction and relaxation of adjacent atrial and ventricular musculature. As consequence, in normal condition, the posterior mitral ring and its adjacent myocardium move downward and anteriorly in systole, in synchrony with the remainder of the LV. (Figure: Annular Disjunction, Fawaz Alenezi 2018)
Mitral annular disjunction (MAD) is characterized by detachment of the roots of the annulus from the ventricular myocardium to which it would normally be attached. Because the fibrous trigones are densely fibrotic, the base of the anterior leaflet is unaffected and only the area under the posterior leaflet, specifically under P1 and P2 scallops, is affected. There is no detachment of the annulus from leaflets or from atrial wall. This ventriculoannular detachment (disjunction) is difficult to diagnose and requires a keen eye to be noticed, which explains the “forgotten” nature of this component of MMVD to which it is quite specific. Indeed, in diastole the diagnosis cannot be made because the ventricular myocardium is appropriately situated under the annulus. Over systole, as the posterolateral myocardium contracts, the annulus “slides” and becomes detached from the ventricular myocardium by a variable distance, few millimeters to sometimes more than 1 cm. Often, few fibers are visible between posterolateral ventricular myocardium and annulus. A frame-by-frame analysis of high-resolution and high-frequency 2-dimensional imaging is best to visualize the defect but not the optimum technique.
The dynamic nature of MAD explains the paucity of pathological reports using flaccid hearts and of surgical reports whereby the annulus is observed from the left atrium and its disjunction from the ventricular myocardium is unnoticeable unless the posterior leaflet is separated from its implantation. The disjunction of the mitral annulus fibrous could play a role in the development of the pathological features of myxomatous valve disease through the mechanical stress incited by the excessive mobility of the mitral apparatus and the source of multiple arrhythmias.
In fact, using only echocardiography we know very little about MAD, its prevalence and consequences, with disagreements on prevalence, significance and association with MVP. Lists studies that have compared cardiac magnetic resonance (CMR) and echocardiography using quantitative methods. It details the agreement between echocardiography and CMR as well as the agreement between echocardiography and CMR among studies that were considered to have severe mitral regurgitation. In general, there is a significant degree of discordance between CMR and echocardiography when quantifying mitral regurgitation. Based on the available data, there appears to be significant discordance between 2D echocardiography and CMR. This discordance is highlighted among patients who are considered to have severe mitral regurgitation and who are likely to be referred for mitral valve surgery.
CMR has become an established noninvasive imaging modality to assess mitral regurgitation severity. Quantification of mitral regurgitant volume by CMR does not rely upon the characteristics of the regurgitant jet. Instead, the assessment of mitral regurgitation by CMR relies upon the difference between the left ventricular stroke volume and forward stroke volume, both of which are quantified using already established accurate and reproducible techniques.
In summary, the clinical and physiological understanding of MAD and MVP is a work in progress. MVP heterogeneity has been demonstrated with regard to clinical outcomes, but the analysis of MMVD morphological and physiological heterogeneity, particularly incorporating MAD description, is crucial to appropriate phenotyping of MVP and MMVD, with the hope of linking this heterogeneity to the genotyping, molecular mechanism and progression of the disease.
Dr. Fawaz Abdulaziz M Alenezi is a Clinical Imaging Fellow at the Duke University Health Systems. He conducts medical research on the derivation and validation of novel echocardiographic approaches to myocardial deformation and a new echocardiographic technique which assists patients with heart ventricular function.