Functional or secondary mitral regurgitation (FMR) is defined as impaired mitral valve competence resulting from dysfunction of the valvular apparatus in the absence of intrinsic leaflet structural abnormalities. The underlying pathophysiologic mechanism is an imbalance between tethering forces and closing forces acting on the mitral leaflets, leading to ineffective coaptation.

From a mechanistic standpoint, two clearly distinct entities can be identified:

• Ventricular functional mitral regurgitation (VFMR):secondary to global or regional left ventricular dysfunction and remodelling, resulting in apical displacement of one or both mitral leaflets. It may be ischaemic or non-ischaemic in origin and produces distortion of leaflet coaptation with either symmetric tethering (typically due to global ventricular dilatation and papillary muscle displacement) or asymmetric tethering (commonly related to regional dysfunction of the posterior papillary muscle and/or its subtended ventricular segment, leading to tethering at the posterolateral commissure and/or P3 segment).
• Atrial functional mitral regurgitation (AFMR):secondary to progressive left atrial enlargement associated with mitral annular dilatation and elongation, resulting in annulus–leaflet mismatch and impaired coaptation in the presence of preserved left ventricular function.

Unlike VFMR, which has been extensively investigated and is associated with worse prognosis and higher rates of adverse events, AFMR has historically been less well characterized. Data regarding its pathophysiology, clinical course, prognosis, and—particularly, optimal therapeutic strategies, remain limited.

The present study aimed to evaluate baseline characteristics, surgical strategies, and early postoperative outcomes in a cohort of patients undergoing minimally invasive mitral surgery for AFMR. This was an international, multicentre study including 17 specialized centres participating in the Mini Mitral International Registry (MMIR), a registry specifically designed to collect contemporary data on minimally invasive mitral surgery across a broad spectrum of indications.

AFMR was defined according to current consensus recommendations and required the simultaneous presence of specific structural and functional features. Patients were required to have a normal-sized left ventricle with preserved systolic function (LVEF ≥50%), in association with left atrial enlargement and mitral annular dilatation. Importantly, leaflet morphology and motion had to be normal, excluding intrinsic valvular disease. In addition, the coaptation point was required to be located at the annular plane or only mildly displaced in an apical direction.

Patients with organic leaflet disease, VFMR, abnormal leaflet motion, absence of annular dilatation, impaired left ventricular function, or previous mitral procedures were excluded. Data were collected using standardized definitions and assessment criteria in accordance with ESC/EACTS and ACC/AHA/HRS guidelines. Clinical events were defined according to the EuroSCORE II model and the Mitral Valve Academic Research Consortium (MVARC) criteria

Among 7957 patients enrolled in the MMIR between 2015 and 2023, 430 patients (5.4%) fulfilled strict diagnostic criteria for AFMR. The cohort had a median age of 73 years, was predominantly female (67.7%), and showed a high prevalence of atrial fibrillation (69.7%). Anterior mitral leaflet (AML) pseudo-prolapse was identified in 6 patients (1.7%). Mitral repair was achieved in 91.4% of cases, whereas 8.6% required valve replacement. In 0.7% of patients (n = 3), intraoperative conversion to valve replacement was necessary due to unsuccessful repair. Surgical access was direct vision in 24.7%, video-assisted in 34.2%, and totally endoscopic in 41.2%. The surgical strategy consisted almost exclusively of isolated mitral annuloplasty, except in 3 cases. A complete ring was used in 97.1% of repairs.

On multivariable analysis, AML pseudo-prolapse was independently associated with an increased likelihood of valve replacement (OR 5.3; 95% CI 1.07–9.12). Concomitant procedures were frequently performed: tricuspid repair in 190 patients (44.2%), surgical AF ablation in 174 patients (40.5%), and left atrial appendage (LAA) closure in 110 patients (25.6%). In-hospital mortality was 2.3% (10 patients). Median ICU stay was 2 days (IQR 1–3), and median hospital stay was 9 days (IQR 7–13). At discharge, 99% of patients had none or mild MR, with a median transmitral gradient of 3.9 mmHg.

COMMENTARY:

Recent guidelines for valvular heart disease recommend, in AFMR, an initial strategy based on optimal medical therapy targeting underlying conditions, including heart failure management and atrial fibrillation control, according to disease-specific recommendations. Although evidence remains limited, several studies suggest that rhythm control strategies may reduce MR severity and promote reverse atrial remodelling.

From an interventional perspective, surgical treatment has been associated with reductions in heart failure hospitalizations and mortality, even in patients with elevated risk profiles. Available observational data indicate that mitral annuloplasty is a safe and effective technique in AFMR, as it directly addresses the principal pathophysiologic driver of disease progression—annular dilatation.

The present study confirms that minimally invasive mitral repair is a safe and highly effective strategy in AFMR, achieving 99% none or mild residual MR and low transmitral gradients, with isolated annuloplasty as the sole technique in the vast majority of patients.

Furthermore, the surgical approach enables concomitant treatment of atrial and right-sided pathology, including AF ablation, tricuspid repair, and LAA closure—interventions that may contribute to reverse remodelling and sustained functional improvement.

Of particular interest is the association between AML pseudo-prolapse and reduced repair feasibility. This entity is characterized by progressive enlargement of the posterior left atrial wall extending toward the basal posterior left ventricular wall, displacing the posterior mitral annulus outward toward the ventricular crest. This anatomical distortion generates “atriogenic” tethering of the posterior leaflet, alters mitral apparatus geometry, increases posterior leaflet angulation, and compromises coaptation. It represents an intermediate phenotype between Carpentier type I and type IIIb lesions. In such cases, conventional annuloplasty may be insufficient, requiring advanced repair techniques or valve replacement.

In conclusion, this study represents the largest multicentre experience to date on surgical treatment of AFMR and the first specifically focused on the minimally invasive approach. Its findings support mitral annuloplasty as a safe and effective strategy in carefully selected patients and highlight the need for future studies providing robust evidence to optimize therapeutic decision-making in this emerging clinical entity.

REFERENCE:

Berretta P, Nakamura M, Fiore A, Lamelas J, Bonaros N, Kempfert J, et al. Surgical techniques and outcomes for atrial functional mitral regurgitation: insights from the Mini Mitral International Registry. Eur J Cardiothorac Surg. 2025;67(12):ezaf438.