Myxomatous mitral valve disease

Article written and reviewed by Dr Luca Moretti

Myxomatous mitral valve disease is a tissue-remodeling process in which the extracellular matrix of the leaflets and chordae loses its laminar organization. Proteoglycans and glycosaminoglycans increase, collagen and elastin become disrupted, and interstitial cells acquire an activated phenotype. The macroscopic result may be a thickened, redundant leaflet, elongated or fragile chordae, prolapse and mitral regurgitation, but histologic severity does not always correspond to hemodynamic severity.

The term “myxomatous” derives from the mucopolysaccharide-rich appearance observed microscopically and implies neither a myxoma nor a neoplasm. Even the word “degeneration” describes the process only partially, because the valve does not simply undergo passive wear: viable cells and matrix respond to genetic, developmental and mechanical signals. Contemporary interpretation therefore considers the disease a mechanobiological disorder of the valvular matrix.

Clinically, degenerative mitral valve disease is often divided into the phenotypes of Barlow disease and fibroelastic deficiency. Barlow disease represents diffuse disease with excess tissue; fibroelastic deficiency is often focal, with thin tissue and chordal rupture in older adults. Both can show myxomatous changes in the diseased segment, and histology demonstrates more continuity than the surgical labels suggest.

Normal structure and pathologic anatomy

The normal leaflet is not a homogeneous membrane. On the atrial side, the atrialis contains elastin and allows the leaflet to recover its shape; the central spongiosa, rich in proteoglycans, absorbs deformation; the ventricular fibrosa, organized in collagen bundles, withstands systolic tension. Endothelium covers both surfaces and communicates with interstitial cells embedded in the matrix.

In myxomatous disease, the spongiosa expands, with accumulation of hydrophilic glycosaminoglycans. The fibrosa loses collagen density and orientation, elastin becomes fragmented, and the interfaces between layers become indistinct. Superimposed tissue may accumulate on the surfaces and increase thickness beyond that produced by the spongiosa alone.

The leaflets become longer and more deformable, but not necessarily stronger. Excess hydrated material alters anisotropy and force distribution. The free edge may bow and move beyond the annulus; a larger surface area may initially preserve coaptation, until chordal elongation and annular dilatation disrupt the balance.

Normal chordae have a compact collagen core and an elastic covering. In degeneration, disorganized collagen, proteoglycan accumulation and changes in cellularity reduce the ability to withstand cyclic loads. Elongation causes prolapse; chordal rupture produces a flail leaflet and can transform mild chronic regurgitation into acute insufficiency.

The mitral annulus also participates in the phenotype. In diffuse forms it may become markedly dilated, flattened and hypermobile, sometimes with posterolateral disjunction; with age, disease duration and mechanical stress, calcification may also supervene. It is not yet clear whether every disjunction is a primary abnormality or is accentuated by leaflet motion, but its presence changes the forces acting on the valve and influences surgical planning.

Macroscopic examination in Barlow disease shows bulky leaflets, hooding and multiple elongated chordae; in fibroelastic deficiency, much of the leaflet is thin, with a focal lesion. Surgical specimens are inevitably selected: the surgeon resects the worst segments and preserves normal tissue, so histologic comparisons may overestimate similarities or differences.

Myxomatous disease must be distinguished from rheumatic fibrosis, which causes commissural fusion and retraction, and from annular calcification, which mainly stiffens the base. Endocarditis may produce rupture and vegetations on a myxomatous valve, but the infectious infiltrate is not part of the degeneration. Pathologic diagnosis must separate substrate from complication.

The distribution of the matrix is more informative than its total amount. A focal area of proteoglycan accumulation near the edge may have a greater mechanical effect than diffuse thickening that preserves load-bearing collagen. Different histologic stains highlight glycosaminoglycans, elastin and collagen; hematoxylin-eosin alone may not depict the entire architecture.

Thickness observed on echocardiography is an imperfect surrogate for histology because it reflects not only matrix but also superimposed tissue, edema, calcification and beam angle. A macroscopically redundant valve may also show substantial variability among segments, and a resected specimen does not necessarily represent the preserved portions. For this reason, clinical diagnosis is based on the anatomic-functional phenotype and does not require biopsy.

Valvular tissue is predominantly avascular under normal conditions, especially in the distal two thirds of the leaflets, and is nourished by diffusion; in advanced forms, neovessels and inflammatory cells may appear. Their presence does not make the disease primarily inflammatory. Instead, it may reflect remodeling, microinjury or an end-stage process, and the causal relationship remains under study.

Cell biology, signaling and biomechanics

Valvular interstitial cells are quiescent in normal tissue and maintain the matrix. In response to injury and tension, they can acquire myofibroblastic markers, increase synthesis and degradation, and remodel collagen and proteoglycans. In degeneration, this interstitial activation becomes persistent and contributes to disordered repair.

Valvular endothelium senses different shear stress on its two sides. Developmental signals may reactivate endothelial-to-mesenchymal transition, providing cells and mediators. Animal models and human tissues indicate involvement of TGF-β, BMP, Wnt, Notch and mechanosensitive pathways; however, their relative importance in the onset of human disease remains incompletely defined.

TGF-β signaling promotes myofibroblastic activation and matrix production. Its increase in syndromic and experimental forms has suggested a pharmacologic target, but the pathway also performs homeostatic functions and varies according to disease stage and cell type. Findings in the aorta in Marfan syndrome cannot be automatically transferred to the mitral valve.

Metalloproteinases and their inhibitors regulate collagen degradation; cathepsins and oxidative systems contribute to turnover. Serotonergic 5-HT2B receptors are of interest because serotonergic drugs can induce valvular disease, but spontaneous human myxomatous disease is not simply serotonin-induced valvulopathy. Molecular similarities do not amount to etiologic identity.

Each heartbeat applies tension to the leaflets and chordae. An initial matrix defect concentrates stress, which activates additional signals and further alters the tissue: a mechanobiological loop. Coaptation and chordal insertion regions experience different loads, explaining the nonuniform distribution. Regurgitation changes flows and forces and may accelerate disease in an already vulnerable valve.

Observations in murine and canine models have clarified pathways and progression, but require caution. Canine myxomatous disease is very common and differs in species characteristics, distribution and inheritance. A signal that is effective in an animal does not prove human therapeutic efficacy; to date, no approved drug normalizes the matrix.

Aging is not simply wear either. Cellular senescence, low-grade inflammation, glycation and changes in collagen can modify the response to stress. In fibroelastic deficiency in older adults, tissue loss and chordal fragility may predominate over the excess matrix observed in Barlow disease.

Valves are subjected to multidirectional deformation: radial and circumferential stretch, bending and shear. Normal properties are anisotropic and nonlinear because crimped collagen fibers progressively align with loading. Degeneration alters this curve and transfers stress to the chordae and adjacent segments; static thickness measurements do not describe this behavior.

Biomechanics also helps explain repair durability. An excessively small ring or neochordae of incorrect length can concentrate forces despite producing a good immediate result, whereas techniques that restore broad coaptation while distributing tension have greater plausibility for long-term durability. Cellular research and surgical research therefore converge on the same objective: reconstructing a geometry capable of normalizing mechanical load.

The relationship between stimulus and response is probably stage-specific. In early phases, surface adaptation may preserve competence; in intermediate phases, activation increases redundancy; in late phases, fibrosis and rupture predominate. A future biologic therapy will need to identify the phase, because indiscriminately inhibiting turnover could further weaken the chordae.

Genetics, development and phenotypic spectrum

Familial aggregation is well documented, but the genetic architecture is heterogeneous. Rare high-penetrance variants explain some families; numerous common variants confer small increases in risk; environment and loading influence expression. A negative genetic test therefore does not exclude predisposition, and a positive test does not accurately predict when regurgitation will develop.

Variants in FLNA, the gene encoding filamin A on the X chromosome, cause a familial form with often multivalvular abnormalities and sex-specific features. DCHS1, involved in cell adhesion and polarity, and DZIP1, with ciliary and developmental roles, have been identified in autosomal dominant families. PLD1 is associated mainly with recessive congenital valve defects.

Association studies have implicated loci such as LMCD1, TNS1 and GLIS1, supporting a polygenic nature. These associations illuminate biology and development but are not individual diagnostic tests. Broad panels generate variants of uncertain significance that must not be used to indicate surgery or restrict activity.

In Marfan and Loeys-Dietz syndromes, pathways related to fibrillin and TGF-β affect multiple tissues; in Ehlers-Danlos syndrome, collagen defects predominate. Evaluation includes the aorta, skeleton, eyes, skin and family history. Overall risk may be determined more by aortopathy than by the valve, so management is not the same as for isolated prolapse.

Echocardiographic screening of first-degree relatives is reasonable in families with significant disease, early onset or multiple affected members. Genetic counseling reconstructs the pedigree and determines whether testing is appropriate. In a single older person with a focal lesion typical of fibroelastic deficiency, the yield of a panel is very low.

The myxomatous phenotype is not binary. Barlow disease, non-Barlow myxomatous forms, FLNA-related prolapse and fibroelastic deficiency have different distributions of tissue, chordae, annulus and age at onset, and intermediate phenotypes may exist. A clinically useful classification should therefore integrate biology, echocardiography and surgical findings without requiring every patient to fit a pure category.

Degeneration can also involve the tricuspid valve and, more rarely, other valves. In Barlow disease, tricuspid prolapse or annular dilatation deserves attention; in syndromic forms, involvement is systemic. Imaging should not stop at the main mitral jet.

Penetrance is age-dependent: a young relative with a normal examination may not yet have expressed the phenotype. Intervals for any reassessment are individualized according to the gene, family and clinical findings; there is no single schedule. In minors, the usefulness of predictive testing must be balanced against the absence of a specific preventive therapy and the psychological implications.

The presence of a variant of uncertain significance must not be communicated as a diagnosis. Segregation within the family, population frequency, functional data and phenotype determine classification. Periodic laboratory reassessment may reclassify a variant; testing should therefore remain within a genetics pathway rather than as an isolated report.

In some families, prolapse accompanies cardiomyopathy or arrhythmias, raising the hypothesis of a shared substrate. The evidence does not allow every premature beat to be attributed to a valvular gene. When dysfunction is disproportionate to regurgitation, an independent cardiomyopathy should be sought by magnetic resonance imaging, clinical history and, when indicated, broader genetic testing.

Imaging, progression and anatomic-clinical correlation

Echocardiography describes thickness, length, redundancy, billowing, prolapse, flail, segments, chordae, annulus and regurgitation. The term myxomatous should not be assigned solely because a leaflet appears bright: gain and resolution alter apparent thickness. A diffuse pattern, with excess tissue and multisegmental prolapse, is more convincing.

Three-dimensional echocardiography reconstructs leaflet surface and its relationship with the annulus. In the diffuse phenotype, it measures prolapse height and volume and identifies clefts or commissures. Transesophageal echocardiography is reserved for planning or inconclusive cases, not for routine confirmation of every thickened leaflet.

Quantification of mitral regurgitation determines clinical relevance. Markedly redundant tissue with a small jet may require surveillance but not intervention; a focal rupture in minimally thickened tissue may cause severe regurgitation. EROA, regurgitant volume, pulmonary venous flow and remodeling are integrated without confusing the histologic phenotype with the hemodynamic stage.

Magnetic resonance imaging quantifies flow and chamber size and characterizes the myocardium. Papillary or inferolateral fibrosis may reflect traction; diffuse interstitial fibrosis may result from volume overload. These findings contribute to arrhythmic risk assessment, but do not replace ECG monitoring and do not constitute an isolated indication for repair.

Progression manifests as increasing prolapse, chordal elongation or rupture, annular dilatation and increasing regurgitation. The atrium and ventricle adapt for years. Late symptoms may appear when damage is already significant; follow-up intervals depend on regurgitation, not on a static histologic diagnosis.

After surgical resection, histology may confirm proteoglycan expansion and disorganization. It is not required in typical cases and rarely changes postoperative management, but it is useful when endocarditis, inflammatory disease, carcinoid disease or an unusual lesion is suspected. The report should be correlated with the site of the specimen.

Circulating biomarkers of turnover, microRNAs and genetic signatures are under investigation. None has sufficient validation for screening or surgical timing. The promise of a valvular “liquid biopsy” should not be confused with a clinical tool that is already available.

Echocardiographic surveillance should distinguish biologic variation from error. A true increase in regurgitation is supported by concordant parameters and chamber remodeling; a different single PISA value may depend on pressure or geometry. Archiving three-dimensional images and indexed measurements makes long-term comparison more reliable.

Ventricular strain may decline before ejection fraction in volume overload, but depends on vendor and loading conditions. It is an additional element for detecting subclinical dysfunction, not a stand-alone threshold for surgery. Likewise, persistently elevated natriuretic peptides in an apparently asymptomatic patient suggest assessment of exercise and pressures, without diagnosing tissue progression.

Matrix imaging with molecular tracers or quantitative magnetic resonance remains experimental. Clinical practice observes macroscopic effects, not real-time cellular activity. This gap explains why it is not possible to predict precisely which thickened leaflet will sustain chordal rupture.

The tissue differential diagnosis includes valvular diseases induced by serotonergic drugs, carcinoid disease, lupus and radiation therapy. These conditions can alter matrix and thickness, but produce different patterns of retraction, plaque or inflammation. Medication history, multivalvular involvement and histologic findings prevent every degenerated valve from being labeled myxomatous.

Chordal rupture alone does not prove myxomatous degeneration: endocarditis, trauma and fibroelastic fragility are alternatives. Likewise, prolapse without thickening may belong to fibroelastic deficiency or a specific familial form. Report language should describe what is visible and reserve etiology for the complete clinical picture.

Management, repair and prospects for biologic therapy

There is no proven drug therapy capable of stopping degeneration. Treatment addresses consequences: blood pressure control, management of atrial fibrillation, diuretics for congestion and heart failure therapy if dysfunction develops. No drug compensates for a ruptured chord or eliminates severe primary regurgitation.

Follow-up is guided by prolapse and severity. In mild disease, intervals may be several years; in moderate regurgitation they are shorter; in asymptomatic severe disease, six-monthly assessments at a valve center allow evaluation of symptoms, ejection fraction, end-systolic diameter, volumes, atrial fibrillation and pulmonary pressure.

When intervention is indicated, surgical repair is preferred because it preserves the apparatus and function. The technique depends on the phenotype: limited resection or neochordae for a focal lesion, multisegmental combinations and a large ring in diffuse disease. Annuloplasty always stabilizes geometry, but does not replace correction of the leaflets.

The likelihood of durable repair should be estimated before proposing early surgery to an asymptomatic patient. At expert centers, degenerative repair has low mortality and excellent durability; outcomes at a general center cannot be inferred from those of reference series. Replacement remains necessary when tissue, calcification or infection prevents reliable reconstruction.

TEER reduces regurgitation by bringing the leaflets together and may be appropriate in symptomatic patients at high surgical risk. It does not correct matrix biology, annular dilatation or every multisegmental lesion. In a young patient with repairable anatomy, surgery generally offers more complete correction and a more rational lifetime strategy.

Biologic prospects include modulation of TGF-β, mechanosensors, serotonin, metalloproteinases and cell differentiation. The risk is interfering with essential processes or treating a late pathway that is not the initiating cause. Human models, activity biomarkers and trials with structural endpoints are needed, not merely molecular changes in vitro.

Organoids, pluripotent cells and cultures under load can reconstruct interactions that are impossible in static culture. To become clinically relevant they will need to reproduce layers, endothelium on both surfaces and cyclic forces. Studies of surgical tissue must also control for age, phenotype and medications, avoiding comparison of an end-stage valve with a young normal valve.

Prevention currently means recognizing consequences before they become irreversible. There is no evidence that supplements, dietary restrictions or moderate physical activity accelerate or halt matrix changes. Smoking, hypertension and physical inactivity should be addressed for overall cardiovascular risk, without promising “regeneration” of the valve.

In future registries, separating Barlow disease, fibroelastic deficiency and syndromic forms will be essential. Grouping every prolapse as “degenerative” dilutes genetic and therapeutic signals. Longitudinal samples, quantitative imaging and surgical tissue linked to the same patient can distinguish causal markers from simple consequences of regurgitation.

Precision medicine in this field does not currently mean prescribing a genetic drug, but defining phenotype, family history, mechanics, rhythm and stage. This classification guides follow-up intervals, the surgical center and the technique, and creates the basis for testing a biologic therapy in the correct population in the future.

The most precise management separates three levels: tissue substrate, mechanical lesion and hemodynamic consequence. Matrix science explains why the valve becomes vulnerable; echocardiography shows how it fails; clinical assessment and guidelines establish when to correct it. Confusing these levels leads to operating on histology without regurgitation or, conversely, waiting until severe mechanical disease becomes irreversible.