What Causes Obstructive Hypertrophic Cardiomyopathy
Obstructive hypertrophic cardiomyopathy is primarily caused by autosomal dominant mutations in genes encoding cardiac sarcomere proteins, with beta myosin heavy chain (MYH7) and myosin-binding protein C (MYBPC3) accounting for 70% of genetically confirmed cases. 1
Genetic Etiology
Primary Sarcomeric Gene Mutations
The disease follows an autosomal dominant inheritance pattern, meaning each offspring of an affected parent has a 50% chance of inheriting the pathogenic variant 1. The key causative genes include:
- MYH7 and MYBPC3: Account for 70% of variant-positive patients 1
- Minor contributors (1-5% each): TNNI3, TNNT2, TPM1, MYL2, MYL3, ACTC1 1
- Over 1,500 variants identified, most being "private" (unique to individual families) 1
Important caveat: Up to 40% of patients have no identifiable genetic etiology or affected family members, suggesting other novel pathophysiologic mechanisms may contribute 1. Genetic testing identifies pathogenic mutations in approximately 60-70% of patients with positive family history and only 10-50% without family history 2.
Non-Sarcomeric Genetic Causes (5-10% of cases)
While less common in adults, these must be excluded as they require different management 3:
- Metabolic disorders: Anderson-Fabry disease (0.5-1% in patients >35-40 years), PRKAG2 mutations (~1%), Danon disease (LAMP-2 mutations, 0.7-2.7%) 3
- Mitochondrial cardiomyopathies: Mutations in respiratory chain protein complexes 3
- RASopathies: Variants in RAS-MAPK signaling genes 1, 4
Pathophysiologic Mechanisms Leading to Obstruction
Structural Abnormalities Creating LVOT Obstruction
The obstruction develops through two principal mechanisms 1:
- Septal hypertrophy with LVOT narrowing: Creates abnormal blood flow vectors that dynamically displace mitral valve leaflets anteriorly
- Mitral valve apparatus abnormalities:
- Elongated mitral valve leaflets
- Anterior displacement of papillary muscles
- Hypertrophied papillary muscles
- These make the valve more susceptible to abnormal flow vectors
This results in systolic anterior motion (SAM) of the mitral valve, causing LVOTO and mitral regurgitation from loss of leaflet coaptation 1.
Molecular and Cellular Changes
Mutant sarcomere genes trigger a cascade of myocardial changes 1:
- Myocyte hypertrophy and disarray
- Interstitial fibrosis
- Hypercontractility from increased actin-myosin cross-bridges 5
- Abnormal intramural coronary arteries causing microvascular dysfunction
- Results in a small, stiff ventricle with impaired diastolic function despite preserved ejection fraction
Critical distinction: These abnormal sarcomeric proteins may not be solely responsible for all clinical features. Elongated mitral valve leaflets, abnormal coronary arteries, and congenital sub-mitral valve apparatus anomalies appear to have no direct association with sarcomere variants, suggesting additional developmental factors 1.
Dynamic Nature of Obstruction
LVOT obstruction is present or develops over time in approximately 75% of HCM patients 1. The obstruction is dynamic and worsens with:
- Increased myocardial contractility
- Decreased ventricular volume (dehydration, Valsalva)
- Decreased afterload
- Exercise
- Food or alcohol intake 6, 2
Key Clinical Pitfall
Do not confuse HCM with secondary causes of left ventricular hypertrophy 1, 4. Systemic disorders (amyloidosis, sarcoidosis, hemochromatosis), athlete's heart, and hypertensive cardiomyopathy can mimic HCM but have different pathophysiology, natural history, and treatment strategies. These should NOT be labeled as HCM 1.