What is functional mitral regurgitation?

Definition of Functional Mitral Regurgitation

Functional mitral regurgitation is defined as systolic retrograde flow from the left ventricle into the left atrium due to impaired coaptation of structurally normal mitral valve leaflets, resulting from left ventricular and/or left atrial remodeling rather than from intrinsic valve disease.

Pathophysiology and Classification

Functional mitral regurgitation (FMR) is characterized by the absence of structural abnormalities of the mitral valve apparatus itself. Instead, the regurgitation occurs due to geometric alterations in the supporting structures. There are two main types:

1. Ventricular Functional MR

• Results from left ventricular remodeling and dysfunction
• Characterized by:
  • Papillary muscle displacement due to LV dilatation
  • Tethering of the mitral leaflets (restricted leaflet motion)
  • Reduced closing forces due to LV systolic dysfunction
  • Often presents with a Type IIIb dysfunction according to Carpentier's classification 1

2. Atrial Functional MR

• Results from left atrial enlargement and mitral annular dilatation
• Often associated with:
  • Chronic atrial fibrillation
  • Loss of atrial contraction
  • Annular dilatation without significant ventricular dysfunction 1

Mechanisms and Distinguishing Features

The key distinguishing feature of functional MR is that the valve leaflets themselves are structurally normal. According to the European Association of Cardiovascular Imaging:

• Primary MR: Involves intrinsic valvular disease or structural abnormalities of the valve components
• Secondary MR: Occurs without evident structural abnormalities of the mitral valve 1

In functional MR, the pathology involves:

1. Left ventricular remodeling: Global or regional wall motion abnormalities leading to:

   • Papillary muscle displacement
   • Increased tethering forces on the leaflets
   • Incomplete coaptation during systole 1

2. Annular dilatation: Enlargement of the mitral annulus reducing the coaptation surface area 1

3. Reduced closing forces: Decreased LV contractility leading to insufficient force to close the valve leaflets properly 1

Clinical Significance

Functional MR is associated with:

• Poor long-term survival
• Progressive heart failure symptoms
• Increased risk of hospitalization
• Potential for irreversible left ventricular dysfunction if left untreated 1

Diagnostic Approach

Echocardiography is the primary tool for diagnosing functional MR, which should assess:

1. Valve structure: Confirmation of normal leaflet morphology
2. Mechanism of regurgitation: Tethering, annular dilatation, or both
3. Severity quantification: EROA, regurgitant volume, and regurgitant fraction
4. Chamber dimensions: LV size, function, and LA dimensions 1

Management Considerations

Management of functional MR differs from primary MR:

• Guideline-directed medical therapy is the first-line treatment for ventricular functional MR
• Cardiac resynchronization therapy may reduce MR in patients with LV dyssynchrony
• Transcatheter edge-to-edge repair (MitraClip) may be considered in selected patients with persistent severe MR despite optimal medical therapy
• Surgical intervention has more complex indications compared to primary MR 1

Common Pitfalls

1. Misclassification: Failing to distinguish between primary and functional MR can lead to inappropriate treatment decisions
2. Underestimation of severity: The dynamic nature of functional MR requires assessment under various loading conditions
3. Overtreatment: Not allowing sufficient time for medical therapy to reduce MR before considering interventional approaches
4. Neglecting the underlying cause: Focusing on the valve without addressing the ventricular or atrial pathology 1

Functional MR represents a complex pathophysiological entity that requires comprehensive assessment and a pathophysiology-guided approach to management, with careful attention to the underlying cardiac remodeling that drives the valvular dysfunction.