What is SAM (Systolic Anterior Motion) on echocardiography?

Systolic Anterior Motion (SAM) on Echocardiography

Systolic Anterior Motion (SAM) is defined as the abnormal anterior motion of the mitral valve cusp/leaflet toward the interventricular septum during left ventricular contraction, which can cause left ventricular outflow tract obstruction (LVOTO) and mitral regurgitation. 1

Definition and Mechanism

SAM is characterized by:

• Abnormal displacement of the mitral valve leaflets (typically anterior leaflet) toward the interventricular septum during systole
• Can involve the leaflets themselves or the subchordal apparatus
• Results in narrowing of the left ventricular outflow tract (LVOT)
• Commonly associated with hypertrophic cardiomyopathy (HCM)

The mechanism of SAM involves:

• Septal hypertrophy creating a narrowed LVOT
• Abnormal blood flow vectors that dynamically displace mitral valve leaflets anteriorly
• Anatomical alterations in the mitral valve and apparatus, including:
  • Longer leaflets
  • Anterior displacement of papillary muscles
  • Abnormal mitral valve apparatus 1

Severity Classification

SAM severity is classified according to the 2019 ACC/AHA/ASE guidelines 1:

• None: Normal mitral valve cusp/leaflet motion
• Mild: Mild abnormal anterior motion toward the septum during LV contraction
• Moderate: Moderate abnormal anterior motion toward the septum during LV contraction
• Severe: Severe abnormal anterior motion toward the septum during LV contraction

Clinical Significance

SAM has several important clinical implications:

1. LVOT Obstruction: SAM can cause dynamic LVOTO with gradients ≥30 mmHg considered obstructive. Gradients ≥50 mmHg are generally considered the threshold for septal reduction therapy in symptomatic patients 1

2. Mitral Regurgitation: SAM leads to incomplete mitral leaflet coaptation, resulting in mitral regurgitation that is:

   • Typically mid-to-late systolic
   • Usually posteriorly or laterally oriented
   • Dynamic in nature, varying with the degree of LVOTO 1

3. Hemodynamic Effects:

   • Increased LV systolic pressure
   • Exacerbation of LV hypertrophy
   • Myocardial ischemia
   • Prolonged ventricular relaxation
   • Impaired stroke volume 1

Echocardiographic Assessment

Key aspects of SAM assessment on echocardiography include:

• Direct visualization of mitral leaflet motion toward the septum during systole
• Measurement of LVOT gradient using continuous wave Doppler
• Assessment of mitral regurgitation jet direction and timing
• Evaluation with provocative maneuvers when resting gradients are <30 mmHg:
  • Valsalva maneuver
  • Standing
  • Exercise 1

Associated Conditions

While SAM is most commonly associated with HCM, it can occur in other settings:

• Primary HCM (most common)
• Post-surgical repair of mitral valve
• Hypovolemic states with hyperdynamic LV function
• Structurally normal hearts under certain conditions (rare) 2, 3

Clinical Implications

The presence of SAM should prompt:

• Assessment of LVOTO severity
• Evaluation of mitral regurgitation
• Consideration of HCM diagnosis if not previously established
• Risk stratification in patients with HCM
• Determination of appropriate management strategies based on symptom severity and degree of obstruction 1

Pitfalls and Caveats

• SAM-related mitral regurgitation is inherently dynamic and varies with LVOTO
• Central or anteriorly directed mitral regurgitation jets may suggest primary mitral valve abnormalities rather than SAM-related regurgitation
• Dobutamine should not be used to identify latent LVOTO due to lack of specificity 1
• SAM can occasionally involve the posterior mitral leaflet rather than the anterior leaflet 4
• In rare cases, SAM may regress with extensive cardiac remodeling 5

Understanding SAM on echocardiography is crucial for proper diagnosis, risk stratification, and management of patients with HCM and other conditions that can cause dynamic LVOTO.