Hypertrophied obstructive cardiomyopathy

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I now have comprehensive material from multiple authoritative sources. Here is a complete overview of Hypertrophic Obstructive Cardiomyopathy (HOCM):

Hypertrophic Obstructive Cardiomyopathy (HOCM)

Definition

Hypertrophic cardiomyopathy (HCM) is defined as unexplained left ventricular (LV) hypertrophy in the absence of abnormal loading conditions (valve disease, hypertension, congenital defects) that would suffice to explain the degree of hypertrophy. When this hypertrophy causes dynamic obstruction of the LV outflow tract (LVOT), the condition is specifically termed hypertrophic obstructive cardiomyopathy (HOCM). Prevalence is 0.2-0.5% worldwide, affecting all racial groups.

Goldman-Cecil Medicine, p. 494
Robbins & Kumar Basic Pathology, p. 373

Genetics and Pathogenesis

HCM is usually autosomal dominant with variable penetrance and expression. Sarcomeric contractile protein gene mutations account for 50-60% of cases, with over 1,400 distinct mutations identified. These are uniformly gain-of-function mutations that enhance myofilament activity, leading to:

Myocyte hypercontractility
Increased energy consumption
Net negative energy balance

The most commonly mutated genes:

Gene	Protein	Frequency
MYH7	Beta-myosin heavy chain	Most frequent
MYBPC3	Myosin-binding protein C	Second most
TNNT2	Troponin T	Third most

Together, mutations in these three genes account for 70-80% of all HCM cases.

Note: the same genes (e.g., beta-myosin) can be mutated in dilated cardiomyopathy (DCM), but those are loss-of-function mutations, in contrast to the gain-of-function mutations in HCM.

Robbins & Kumar Basic Pathology, p. 373-374

Pathology / Morphology

Gross:

Massive myocardial hypertrophy without ventricular dilation
90% of cases: Asymmetric septal hypertrophy - disproportionate thickening of the interventricular septum relative to the LV free wall
10% of cases: Concentric hypertrophy
The ventricular cavity is compressed into a characteristic "banana-shaped" configuration
The anterior mitral leaflet contacts the septum during systole, producing a fibrous endocardial plaque in the LVOT

FIG. 9.27 - Hypertrophic cardiomyopathy: (A) Septal muscle bulging into LVOT forming "banana-shaped" lumen, with fibrous endocardial plaque (arrow). (B) Histology showing myocyte disarray, extreme hypertrophy, branching, and interstitial fibrosis.

Histology (pathognomonic triad):

Marked myocyte hypertrophy
Haphazard myocyte and myofiber disarray (whorled pattern)
Interstitial fibrosis

Robbins & Kumar Basic Pathology, p. 373-374

Mechanism of LVOT Obstruction

The obstruction in HOCM is dynamic (not fixed), caused by two interacting mechanisms:

Asymmetric septal hypertrophy - the bulging septum narrows the LVOT
Systolic Anterior Motion (SAM) of the mitral valve - ventricular septal hypertrophy creates a Venturi effect pulling the anterior mitral leaflet forward into the outflow tract during systole

This produces:

A dynamic gradient across the LVOT that worsens with reduced preload, reduced afterload, or increased contractility
Functional mitral regurgitation from incomplete coaptation of the mitral leaflets

Conditions that worsen the gradient (increase obstruction):

Standing, Valsalva maneuver, dehydration, tachycardia, amyl nitrite inhalation, exercise

Conditions that reduce the gradient (decrease obstruction):

Squatting, handgrip exercise, leg raising, increased preload/afterload, beta-blockers

Clinical Features

Most patients are asymptomatic at initial diagnosis, often discovered incidentally on family screening, routine ECG, or murmur. Symptoms can develop at any age.

Classic symptom triad:

Exertional dyspnea (most common - diastolic dysfunction + LVOT obstruction)
Angina/chest pain (in 20-30%; due to microvascular ischemia even without CAD)
Syncope (in ~20%; often exertional, serious warning sign)

Murmur: Harsh systolic ejection murmur, loudest at the left sternal edge, radiates to the right upper sternal edge and apex. Accompanied by functional mitral regurgitation murmur. The murmur is dynamic - it increases with Valsalva/standing and decreases with squatting/handgrip (distinguishing it from fixed obstruction like aortic stenosis).

Major complications:

Atrial fibrillation with mural thrombus and embolic stroke
Ventricular fibrillation causing sudden cardiac death (SCD)
Infectious endocarditis of the mitral valve
Progressive congestive heart failure (<5% of patients)

HCM is responsible for ~1/3 of sudden cardiac deaths in athletes under age 35.

Harrison's Principles of Internal Medicine 22E, p. 2051
Goldman-Cecil Medicine, p. 494-495
Robbins & Kumar Basic Pathology, p. 374

Diagnosis

Initial evaluation should include:

12-lead ECG - typically abnormal in 90%+ of symptomatic patients; shows LV hypertrophy, deep Q waves (especially II, III, aVF, V5-V6), ST changes
2D Echocardiography (gold standard) - demonstrates asymmetric septal hypertrophy, SAM of the mitral valve, LVOT gradient, diastolic dysfunction
Cardiac MRI - provides superior tissue characterization; late gadolinium enhancement (LGE) represents fibrosis and is an important risk factor for SCD
Holter monitoring / exercise stress testing - detects NSVT, assesses exercise-induced gradient
Genetic testing - next-generation sequencing panels; pathogenic variants found in <50% of probands
Family history (3-4 generation) - all first-degree relatives should be screened

Differential diagnosis to exclude:

Aortic stenosis
Hypertensive heart disease
Cardiac amyloidosis (Fabry disease, Danon disease, Pompe disease)
Noonan syndrome, LEOPARD syndrome

Risk Stratification for Sudden Cardiac Death

Major risk factors:

Risk Factor	Notes
Prior cardiac arrest or sustained VT	Strongest predictor
Unexplained syncope	Especially exertional
Family history of SCD	In first-degree relatives
LV wall thickness >30 mm	Present in <10%
LV systolic dysfunction (EF <50%)	Present in <10%
LV apical aneurysm	Especially with apical HCM
LVOT gradient, LA diameter, age	Used in ESC risk calculator
Late gadolinium enhancement on MRI	Modifying risk factor
Nonsustained VT on Holter	>3 beats at rate >120

The ESC HCM Risk Calculator (doc2do.com/hcm/webHCM.html) provides a 5-year estimated SCD risk:

Risk ≥6%: ICD generally recommended
Risk 4-6%: ICD can be considered
Risk <4%: ICD not routinely advised
Harrison's Principles of Internal Medicine 22E, p. 2051

Management

Treatment Algorithm

FIGURE 267-5 (Harrison's 22E) - HCM treatment algorithm: all patients evaluated for SCD risk; symptomatic patients treated with beta-blockers/CCBs, then mavacamten or disopyramide for outflow gradient, and septal reduction for refractory cases.

Medical Therapy

Step 1 - First-line agents (reduce heart rate, increase diastolic filling, reduce contractility):

Beta-blockers (first choice): metoprolol (target 150 mg/day) or propranolol (target 120-480 mg/day); titrate to resting HR 50-70 bpm. Effective in 60-70% with LVOT obstruction.
Non-dihydropyridine calcium channel blockers: verapamil (120 mg/day, up to >480 mg/day) or diltiazem (180 mg/day, up to >360 mg/day) - alternatives, especially for refractory chest pain

Step 2 - Add-on for persistent LVOT obstruction:

Disopyramide: negative inotropic antiarrhythmic agent; dose 400-600 mg/day; effective in up to 2/3 of patients. Side effects include anticholinergic effects (dry eyes/mouth, urinary retention). Should always be combined with a beta-blocker or verapamil to prevent excessive AV nodal conduction from the anticholinergic effect.
Mavacamten (FDA-approved): small-molecule cardiac myosin inhibitor - directly reduces myosin cross-bridge formation, reducing hypercontractility and LVOT gradient. High efficacy in symptomatic obstructive HCM, including in patients with persistent symptoms on beta-blockers.
Aficamten (under investigation): another cardiac myosin inhibitor

Diuretics: Used cautiously for fluid overload symptoms; avoid hypovolemia, especially in the presence of outflow gradient.

Drugs to AVOID in HOCM:

Vasodilators (nitrates, ACE inhibitors, hydralazine) - reduce preload/afterload and worsen obstruction
Digoxin/cardiac glycosides - increase contractility and worsen obstruction
Dihydropyridine CCBs

Atrial Fibrillation Management

Beta-blockers or CCBs (verapamil, diltiazem) for rate control
Disopyramide + beta-blocker or verapamil/diltiazem for rhythm control in HCM-setting AF
Amiodarone as alternative
Anticoagulation for all HCM patients with AF (regardless of CHA₂DS₂-VASc score)
Cardiac glycosides should be avoided (worsen obstruction)

Invasive / Interventional Therapy (for medically refractory cases, ~5%)

Both procedures reduce the gradient but neither has been shown to improve survival - only symptoms.

Procedure	Mechanism	Notes
Surgical septal myectomy	Excision of proximal septal myocardium	Gold standard; 60+ year track record; very low perioperative mortality at expert centers; durable long-term relief
Alcohol septal ablation	Controlled infarction of proximal septum via injection of alcohol into septal perforators	Percutaneous; reserved for patients wishing to avoid surgery or with prohibitive comorbidities; requires suitable coronary anatomy

Both carry risk of complete heart block requiring permanent pacing
Mitral valve repair/replacement usually unnecessary after myectomy (MR resolves with septal relief)
Dual-chamber pacing alone is not effective as primary therapy for LVOT obstruction

Cardiac transplantation: Rarely considered for patients with refractory HF and severe progressive LV dysfunction.

ICD Implantation

Primary prevention: based on risk stratification (ESC risk calculator ≥6%)
Secondary prevention: all patients with prior cardiac arrest or sustained VT (regardless of other factors)
Long-term device-related complications must be weighed, especially in young patients

Genetic Counseling and Family Screening

Offered to all newly diagnosed patients
First-degree relatives screened with ECG and echo every 12-18 months from age 12 until adulthood (18-21 years)
Adults with normal screening evaluated every few years (late-onset LVH possible)

Lifestyle Advice

Competitive sports and vigorous exercise were historically restricted; however, recent studies have failed to show a clear link between exertion and ventricular arrhythmias in HCM, leading to more individualized, shared decision-making regarding exercise
Avoid dehydration and excessive alcohol consumption
Harrison's Principles of Internal Medicine 22E, p. 2051
Goldman-Cecil Medicine, p. 495-496

Recent Evidence (2024-2026)

A systematic review on obstructive HCM management (Butzner et al., 2024) analyzed evidence gaps in HOCM therapy, highlighting the expanding role of cardiac myosin inhibitors. A 2025 meta-analysis (Lee et al.) compared outcomes in apical vs. nonapical HCM subtypes.

The most notable recent advance is mavacamten (FDA-approved), which represents the first disease-targeted medical therapy for obstructive HCM by directly inhibiting cardiac myosin. It has been shown to reduce LVOT gradients and improve symptoms in patients who remain symptomatic on traditional beta-blockers/CCBs.

Summary Table

Feature	HOCM
Inheritance	Autosomal dominant
Key genes	MYH7, MYBPC3, TNNT2
Mutation type	Gain-of-function (sarcomeric proteins)
LV morphology	Asymmetric septal hypertrophy (90%)
Obstruction mechanism	SAM of mitral valve + septal bulge
Murmur	Systolic ejection; increases with Valsalva/standing
Histology	Myocyte disarray + hypertrophy + interstitial fibrosis
First-line medical Rx	Beta-blockers, verapamil/diltiazem
Add-on for gradient	Disopyramide, mavacamten
Invasive Rx	Surgical myectomy or alcohol septal ablation
SCD prevention	ICD (risk-stratified)
SCD statistic	~1/3 of SCD in athletes <35 years

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