What is HOCM (Hypertrophic Obstructive Cardiomyopathy) with obstruction?

Hypertrophic Obstructive Cardiomyopathy (HOCM) with Obstruction

HOCM with obstruction is a specific subtype of hypertrophic cardiomyopathy characterized by left ventricular outflow tract obstruction (LVOTO) with a peak instantaneous gradient of ≥30 mm Hg at rest or with physiologic provocation, causing impaired cardiac output, diastolic dysfunction, and symptoms of heart failure.

Classification of HOCM with Obstruction

HOCM with obstruction can be further classified into two main types based on the presence and timing of obstruction:

1. Basal (Resting) Obstruction:

   • Defined by LVOT gradient ≥30 mm Hg at rest
   • Represents approximately one-third of all HCM cases 1
   • Continuous obstruction present under normal conditions

2. Labile (Provocable) Obstruction:

   • Defined by LVOT gradient <30 mm Hg at rest but ≥30 mm Hg with physiologic provocation
   • Represents another one-third of HCM cases 1
   • Obstruction occurs during exercise, Valsalva maneuver, or other provocative conditions

Pathophysiologic Mechanisms

The obstruction in HOCM occurs through several mechanisms:

• Primary mechanism: Systolic anterior motion (SAM) of the mitral valve leaflets causing mid-systolic contact with the hypertrophied ventricular septum 1
• Secondary mechanisms:
  • Drag effect on mitral valve leaflets during ventricular systole 1
  • Abnormally positioned mitral valve apparatus 1
  • Hypertrophied basal ventricular septum encroaching on the LVOT
  • Occasionally, mid-cavity obstruction due to hypertrophied papillary muscles abutting the septum 1

Hemodynamic Consequences

Obstruction leads to a complex cascade of hemodynamic abnormalities:

• Increased LV systolic pressure
• Prolonged ventricular relaxation
• Elevated LV diastolic pressure
• Mitral regurgitation (typically posteriorly directed)
• Myocardial ischemia due to supply-demand mismatch
• Decreased forward cardiac output 1, 2

Clinical Significance

The presence of obstruction has significant clinical implications:

• Associated with more severe symptoms (dyspnea, angina, syncope)
• Contributes to debilitating heart failure symptoms 1
• Serves as a major determinant of clinical outcome 1
• Marked gradients ≥50 mm Hg (at rest or with provocation) represent the threshold for considering invasive septal reduction therapy in severely symptomatic patients 1, 2

Diagnostic Considerations

• Echocardiography with Doppler is the primary diagnostic tool
• Important to assess for both resting and provocable gradients
• Exercise stress echocardiography is particularly valuable for identifying physiologically provocable obstruction in symptomatic patients without resting gradients 1, 2
• Cardiac MRI provides additional structural assessment

Treatment Approach

Treatment is guided by symptom severity and degree of obstruction:

1. First-line therapy: Non-vasodilating beta-blockers (e.g., propranolol) titrated to maximum tolerated dose 2

2. Second-line options:

   • Non-dihydropyridine calcium channel blockers (verapamil, diltiazem)
   • Disopyramide added to beta-blockers 2

3. Invasive treatment (for drug-refractory symptoms and LVOTO ≥50 mm Hg):

   • Surgical septal myectomy (preferred for younger patients with greater septal thickness)
   • Alcohol septal ablation (alternative for patients with comorbidities or advanced age) 2, 3

Important Caveats

• Avoid dehydration, excess alcohol consumption, and vasodilators (nitrates, phosphodiesterase inhibitors)
• Careful monitoring required when using verapamil in patients with severe obstruction (≥100 mm Hg)
• Assess symptom response rather than measured gradient to determine medication effectiveness 2
• Both surgical myectomy and alcohol septal ablation are highly operator-dependent procedures 3

HOCM with obstruction represents a distinct hemodynamic state requiring careful assessment and targeted management strategies to improve symptoms and outcomes.