Why is the mitral valve often involved in Hypertrophic Cardiomyopathy (HCM)?

Mitral Valve Involvement in Hypertrophic Cardiomyopathy

The mitral valve is involved in hypertrophic cardiomyopathy (HCM) primarily through systolic anterior motion (SAM) of the mitral valve, which causes distortion of the mitral valve apparatus and leads to both left ventricular outflow tract obstruction (LVOTO) and mitral regurgitation. 1

Mechanisms of Mitral Valve Involvement in HCM

Primary Mitral Valve Abnormalities

• Structural abnormalities: Many HCM patients have intrinsic mitral valve abnormalities including:

  • Elongated mitral leaflets (particularly anterior leaflet) 2
  • Excessive leaflet length 1
  • Anomalous papillary muscle insertion directly into the anterior mitral leaflet 2
  • Anteriorly displaced papillary muscles 1, 3
  • Thickened leaflets 3

• Prevalence: Approximately 66% of HCM patients have structural mitral valve abnormalities that are unlikely to be acquired or secondary to mechanical factors 2

Secondary Mitral Valve Dysfunction

1. Systolic Anterior Motion (SAM)

   • The hypertrophied septum narrows the left ventricular outflow tract
   • During systole, high-velocity blood flow through the narrowed LVOT creates Venturi forces that pull the mitral valve leaflets toward the septum
   • This results in SAM of the mitral valve, further obstructing the LVOT 1

2. Mitral Regurgitation

   • SAM distorts the mitral valve apparatus, compromising leaflet coaptation 1
   • This creates a "eject-obstruct-leak" temporal sequence 1
   • The mitral regurgitation is typically:
     • Mid-to-late systolic in timing
     • Directed laterally and posteriorly
     • Proportional to the degree of LVOT obstruction 1

Diagnostic Characteristics

• Jet direction: MR caused by SAM typically produces a posteriorly or laterally directed jet 1
• Warning sign: An anteriorly directed jet suggests intrinsic mitral valve abnormality rather than SAM-related regurgitation 1
• Timing: Mid-to-late systolic predominance 1
• Variability: The severity of MR often varies with factors that affect LVOT obstruction (exercise, medications, volume status) 1

Clinical Significance

1. Symptom contribution: Mitral regurgitation often plays a primary role in producing symptoms of dyspnea in HCM patients 1

2. Treatment implications:

   • Identifying intrinsic mitral valve abnormalities is crucial as it influences treatment options 1
   • Patients with primary mitral valve abnormalities may require specific surgical approaches beyond septal reduction alone 4
   • Alcohol septal ablation cannot address primary mitral valve abnormalities and is not preferred if mitral valve abnormalities make dominant contributions to LVOTO 4

3. Diagnostic considerations:

   • Echocardiography should assess both the degree of obstruction and mitral valve morphology
   • Imaging should be performed at rest and with provocation 1
   • Transesophageal echocardiography may be needed in selected cases to define the mechanism and severity of mitral valve regurgitation 1

Clinical Pitfalls and Caveats

• Misdiagnosis risk: The assessment of intrinsic mitral valve abnormalities can be challenging in the presence of LVOTO 1
• Measurement limitations: Conventional quantitative Doppler parameters for MR are not validated in patients with LVOTO 1
• Treatment planning: Close examination of the mitral valve is required before septal reduction therapy to determine optimal approach and potential need for concomitant mitral valve intervention 1
• Endocarditis risk: Infective endocarditis in HCM is virtually confined to patients with LVOT obstruction, with endocardial lesions most commonly occurring on the thickened anterior mitral leaflet 1

Understanding the complex interplay between the mitral valve and septal hypertrophy in HCM is essential for proper diagnosis, risk assessment, and treatment planning.