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

Mitral Valve Involvement in Hypertrophic Cardiomyopathy (HCM)

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

Pathophysiological Mechanisms

The involvement of the mitral valve in HCM occurs through several interrelated mechanisms:

1. Primary Structural Abnormalities:

   • Elongated mitral leaflets (particularly the anterior leaflet)
   • Enlarged mitral valve area (present in 58% of HCM patients)
   • Thickened leaflets
   • Anomalous papillary muscle insertion directly into the anterior mitral leaflet 1, 2

2. Dynamic Obstruction Process:

   • The hypertrophied septum narrows the left ventricular outflow tract
   • During systole, high-velocity blood flow through the narrowed LVOT creates Venturi forces
   • These forces pull the mitral valve leaflets toward the septum (SAM)
   • This creates an "eject-obstruct-leak" temporal sequence 1

Mitral Regurgitation in HCM

Mitral regurgitation in HCM has distinctive characteristics:

• Typically mid-to-late systolic in timing
• Directed laterally and posteriorly
• Proportional to the degree of LVOT obstruction
• Varies with factors affecting LVOT obstruction (exercise, medications, volume status)
• Often plays a primary role in producing symptoms of dyspnea 1

Clinical Implications

Understanding mitral valve involvement in HCM has important clinical implications:

1. Diagnostic Considerations:

   • Echocardiography should assess both obstruction and mitral valve morphology
   • Imaging should be performed at rest and with provocation
   • Transesophageal echocardiography may be needed in selected cases 1

2. Treatment Implications:

   • Identifying intrinsic mitral valve abnormalities is crucial as it influences treatment options
   • Patients with primary mitral valve abnormalities may require specific surgical approaches beyond septal reduction alone
   • Alcohol septal ablation cannot address primary mitral valve abnormalities 1, 2

Important Distinctions

It's critical to recognize that mitral valve abnormalities in HCM:

• Are not merely secondary to mechanical factors but may be primary structural abnormalities
• Expand the morphological definition of HCM beyond just cardiac muscle disease 3
• May be present even in the absence of significant hypertrophy 4

Complications

A notable complication related to mitral valve involvement in HCM is infective endocarditis, which is virtually confined to patients with LVOT obstruction. Endocardial lesions most commonly occur on the thickened anterior mitral leaflet 1.