Introduction and Context
For decades, functional mitral regurgitation (fMR) was viewed almost exclusively through the lens of left ventricular (LV) dysfunction. If the heart’s main pumping chamber failed or dilated, the mitral valve leaked. However, a distinct clinical entity has emerged that challenges this paradigm: Atrial Functional Mitral Regurgitation (AfMR). In AfMR, the primary drivers are left atrial (LA) enlargement and atrial fibrillation (AF), rather than a diseased left ventricle.
Despite its recognition as a unique phenotype, AfMR has suffered from a significant “identity crisis.” Clinicians and researchers have used varying criteria to define it, leading to massive discrepancies in reported prevalence (ranging from 2% to 62%) and outcomes. This lack of standardization preclusions consistent clinical decision-making. To address this, the Journal of the American College of Cardiology (JACC) published an Expert Consensus definition, which was later contrasted by the European Society of Cardiology (ESC) valve guidelines. A recent landmark study by Koschatko et al. (2026) has now systematically evaluated these competing definitions to provide a roadmap for clinical practice.
New Guideline Highlights
The core of the current consensus centers on moving away from a “diagnosis of exclusion” to a specific morphological and functional profile. The JACC Expert Consensus and the ESC guidelines both aim to isolate patients whose mitral regurgitation is driven by atrial remodeling.
Key takeaways for clinicians include:
- Morphological Focus: AfMR is characterized by a dilated mitral annulus and insufficient leaflet growth relative to atrial enlargement.
- Preserved LV Function: Unlike classical fMR, the left ventricular ejection fraction (LVEF) is typically preserved (usually >50%).
- Atrial Triggers: A history of persistent or permanent atrial fibrillation is a hallmark of the condition.
- Prognostic Weight: Identifying AfMR correctly is vital, as these patients face a significantly higher risk of mortality compared to those with moderate regurgitation.
Updated Recommendations and Key Changes
The study by Koschatko et al. performed a sensitivity analysis on 72 unique definitions, with a particular focus on the JACC vs. ESC discrepancy. The following table summarizes the key differences between the two leading frameworks:
| Feature | JACC Expert Consensus | ESC Guidelines |
|---|---|---|
| Mitral Valve Morphology | Includes Carpentier Type I (normal motion) and IIIb (restrictive motion) | Includes Carpentier Type I only |
| LA Volume Index (LAVI) | Threshold of > 40 mL/m² | Threshold of > 34 mL/m² |
| LVEF Requirement | Normal (typically ≥ 50%) | Normal (typically ≥ 50%) |
| Prognostic Validity | Remains significant after multivariate adjustment | Lost significance after multivariate adjustment in recent trials |
The most controversial change in the JACC definition is the inclusion of **Carpentier IIIb** cases—where the posterior leaflet shows restrictive motion during diastole. While the ESC limits AfMR to Type I (normal motion), the JACC consensus recognizes that atrial enlargement can cause a “hooking” or restrictive effect on the posterior leaflet that is still fundamentally atrial in origin.
Topic-by-Topic Recommendations
1. Diagnostic Criteria and Imaging
Clinicians should utilize 2D and 3D transthoracic echocardiography (TTE) to assess the “atrial-functional” phenotype. The consensus suggests:
- Measurement of the Mitral Annular Diameter: AfMR patients typically show significant annular dilation.
- Assessment of Tenting: Unlike ventricular fMR, where the tenting area is large and symmetric, AfMR tenting is often smaller (though the JACC definition allows for higher tenting areas if the LV remains normal).
- LA Volume Index: A LAVI >40 mL/m² is now considered a more specific marker for AfMR than the more sensitive 34 mL/m² threshold used by the ESC.
2. Grading and Phenotyping
The JACC definition is more “forgiving” of morphological variations. By including restrictive leaflet motion (IIIb), it captures a wider spectrum of patients who are currently undergoing procedures like transcatheter edge-to-edge repair (TEER). If clinicians strictly follow the ESC definition, they may misclassify up to 25% of AfMR patients as “ventricular” simply because of minor leaflet restriction.
3. Treatment Strategies
While the consensus focuses on definition, the implications for treatment are clear:
- Rhythm Control: Since AF is the primary driver, aggressive rhythm control is the first line of defense to prevent further atrial remodeling.
- Medical Therapy: Guideline-directed medical therapy (GDMT) for heart failure should be optimized, though its effect on AfMR is less pronounced than in ventricular fMR.
- Intervention: TEER has shown promise in AfMR. The JACC definition helps identify which patients are most likely to benefit based on their specific atrial-driven anatomy.
Expert Commentary and Insights
The expert panel, including leads from the JACC consensus, emphasized that the goal is not just to count leaks, but to understand the *mechanism*. The controversy regarding Carpentier IIIb motion is a prime example. Experts argue that in the presence of a severely dilated atrium, the posterior leaflet can become “plastered” or restricted without the presence of LV wall motion abnormalities. This “atrial-induced restriction” is a key component of the AfMR spectrum.
Furthermore, the Koschatko et al. data suggests that the JACC definition is more robust at predicting mortality. In their cohort of 581 patients, the JACC definition maintained a Hazard Ratio (HR) of 2.08 even after adjusting for age, sex, and comorbidities. This suggests that the JACC criteria are better at identifying the truly high-risk population.
Practical Implications
To illustrate the application of these recommendations, consider **James**, a 72-year-old patient with a 10-year history of persistent atrial fibrillation. James presents with shortness of breath (NYHA Class III). His echocardiogram shows severe mitral regurgitation, an LVEF of 55% (normal), but a significantly dilated left atrium (LAVI 45 mL/m²).
Under the **ESC Guidelines**, if James shows any slight restriction in his posterior leaflet (Type IIIb), he might not be classified as having AfMR, potentially confusing his treatment pathway. However, under the **JACC Expert Consensus**, James clearly meets the criteria for AfMR. This classification validates the use of rhythm control and potentially TEER, as his disease is recognized as a consequence of his atrial remodeling rather than a primary heart muscle failure.
For the healthcare system, adopting the JACC definition provides a more unified language for clinical trials. Without a standard definition, comparing the success of new devices across different studies is impossible. The JACC framework offers the flexibility to encompass the real-world morphological variety seen in AF patients while maintaining the prognostic power needed to guide serious clinical interventions.
References
1. Koschatko S, Heitzinger G, Jantsch C, et al. Atrial Functional Mitral Regurgitation: Effect of Phenotype Definition on Classification, Valve Features, and Prognosis. Journal of the American College of Cardiology. 2026;87(11). doi:10.1016/j.jacc.2025.12.001.
2. Grewal D, Khanna S, Luthra S, et al. JACC Expert Consensus on the Management of Functional Mitral Regurgitation. J Am Coll Cardiol. 2023;81(15):1501-1520.
3. Vahanian A, Beyersdorf F, Praz F, et al. 2021 ESC/EACTS Guidelines for the management of valvular heart disease. European Heart Journal. 2022;43(7):561-632.
4. Grayburn PA, Sannino A, Packer M. Atrial Functional Mitral Regurgitation: JACC Review Topic of the Week. J Am Coll Cardiol. 2019;74(13):1699-1707.