What is HOCM (Hypertrophic Obstructive Cardiomyopathy)?
HOCM is an outdated term that should no longer be used; the preferred nomenclature is HCM (hypertrophic cardiomyopathy) with or without left ventricular outflow tract obstruction, as the term HOCM incorrectly implies that obstruction is an invariable feature of the disease when in fact only one-third of patients have resting obstruction. 1
Definition and Core Pathophysiology
Hypertrophic cardiomyopathy (HCM) is a genetic cardiovascular disease characterized by unexplained left ventricular hypertrophy in the absence of another cardiac or systemic disease capable of producing the magnitude of hypertrophy evident 1. The disease affects approximately 1 in 500 people in the general population 1, 2.
Diagnostic Criteria
The clinical diagnosis is established by imaging demonstrating:
- Maximal LV wall thickness ≥15 mm in adults on echocardiography or CMR 1, 3
- Wall thickness of 13-14 mm is considered borderline and may be diagnostic in family members of HCM patients or with positive genetic testing 1
- In children, wall thickness >2 standard deviations above the mean for age, sex, or body size 1, 3
- A nondilated, hyperdynamic left ventricle (often with systolic cavity obliteration) 1
Obstruction Patterns
The disease presents in three hemodynamic forms 1:
- One-third have resting obstruction (gradients ≥30 mmHg at rest) 1
- One-third have labile, provocable obstruction (gradients <30 mmHg at rest but ≥30 mmHg with provocation) 1
- One-third remain nonobstructive (gradients <30 mmHg at rest and with provocation) 1
Mechanism of Obstruction
When obstruction occurs, it is caused by systolic anterior motion (SAM) of the mitral valve with mid-systolic contact against the ventricular septum, creating mechanical impedance to outflow 1. This occurs due to drag force on the mitral valve leaflets during ventricular systole, pushing them into the outflow tract 1. Less commonly, mid-cavity obstruction can occur from hypertrophied papillary muscles abutting the septum 1.
The obstruction is dynamic and varies with loading conditions 1:
- Increased by: enhanced contractility, decreased ventricular volume, decreased afterload 1
- Worsened by: exercise, Valsalva maneuver, postprandial state, alcohol intake 1
Genetic Basis
HCM is caused by autosomal dominant mutations in genes encoding sarcomere proteins 1. The eight definitively causative genes include beta-myosin heavy chain (MYH7), myosin-binding protein C (MYBPC3), troponin T, troponin I, alpha-tropomyosin, actin, and regulatory and essential light chains 1. These two most common genes (MYH7 and MYBPC3) account for approximately 50% of cases 4.
Clinical Consequences
The pathophysiology creates multiple adverse effects 1:
- Elevated LV systolic and diastolic pressures from obstruction 1
- Diastolic dysfunction from impaired relaxation and increased chamber stiffness 1
- Myocardial ischemia from supply-demand mismatch, even without coronary artery disease 1
- Mitral regurgitation from incomplete leaflet apposition during SAM 1
Key Clinical Pitfall
The threshold for septal reduction therapy is a gradient ≥50 mmHg (either at rest or with provocation) in severely symptomatic patients who fail medical management 1. Gradients ≥30 mmHg define the presence of obstruction but do not automatically mandate intervention 1.