Hypertrophic obstructive cardiomyopathy (HOCM) is an inherited, genetically transmitted myocardial disease defined by excessive cardiac hypertrophy—specifically a left ventricular wall thickness of 15 mm or greater—that occurs independently of abnormal loading conditions, accompanied by a dynamic left ventricular outflow tract (LVOT) gradient of 30 mm Hg or more1,2. As the most common monogenic heart disease, it arises primarily from autosomal dominant mutations affecting genes encoding sarcomeric proteins2,4. The disease process is functionally characterized by hyperdynamic myocardial contraction, marked asymmetric septal hypertrophy, and systolic anterior motion (SAM) of the mitral valve apparatus, which together precipitate dynamic mechanical obstruction during systole1,4. Affected individuals present across a broad clinical spectrum ranging from complete asymptomatic status to disabling dyspnoea, angina pectoris, exercise-induced presyncope, life-threatening arrhythmias, and sudden cardiac death, particularly in young competitive athletes1,2.

Etiology & Risk Factors

  • Genetics and Inheritance Pattern: Inherited predominantly as an autosomal dominant trait with high penetrance and variable expressivity, caused by more than 1,500 distinct point mutations across at least 11 thick and thin myofilament sarcomeric genes2.
  • Primary Pathogenic Genes: Mutations in the beta-myosin heavy chain gene (MYH7) and the myosin-binding protein C gene (MYBPC3) account for the vast majority of genetically resolved cases, alongside rarer mutations in troponin T (TNNT2) and troponin I (TNNI3)2.
  • Non-Sarcomeric Phenocopies: Metabolic storage disorders and infiltrative cardiomyopathies that mimic HOCM, including Anderson-Fabry disease, Pompe disease, PRKAG2 cardiac glycogenosis, and amyloidosis.
  • Demographic and Environmental Risk Factors: Family history of premature sudden cardiac death, adolescent or young adult growth spurts triggering phenotypic expression, and intense physical exertion which exacerbates dynamic gradients.

Pathophysiology

The structural foundation of HOCM is disarray of myocardial fibers accompanied by interstitial fibrosis and microvascular remodeling. Genetic mutations within the cardiac sarcomere induce hyperdynamic cellular contraction and compensatory cellular hypertrophy. As the interventricular septum thickens asymmetrically relative to the posterior left ventricular wall, the geometry of the left ventricular outflow tract is severely narrowed. During ventricular systole, the high-velocity blood ejection rushing through the narrowed subaortic region creates a Venturi effect, pulling the anterior leaflet of the mitral valve into contact with the ventricular septum—a phenomenon known as systolic anterior motion (SAM). This mechanical interaction physically obstructs the outflow tract, sharply elevating intraventricular systolic pressure. Simultaneously, the combination of myocardial hypertrophy, altered calcium handling, and subendocardial ischemia impairs diastolic relaxation and ventricular compliance (stiffening of the ventricle), leading to elevated left ventricular end-diastolic pressures, pulmonary venous congestion, and secondary left atrial enlargement1,2.

Hypertrophic obstructive cardiomyopathy (HOCM) — Pathophysiological Cascade & Clinical Flowchart 1 Genetic Mutation Mutations in sarcomeric protein genes (e.g., MYH7, MYBPC3) alter contractile apparatus function. 2 Hypertrophic Response Compensatory myocardial hypercontractility and localized asymmetric hypertrophy of the interventricular septum. 3 Outflow Obstruction Systolic anterior motion of the mitral valve creates dynamic left ventricular outflow tract obstruction. 4 Haemodynamic Impairment Elevated intraventricular pressures, impaired diastolic filling, and reduced stroke volume. 5 Therapeutic Intervention Beta-blockade, surgical myectomy, or alcohol septal ablation to relieve obstruction and prevent sudden cardiac death.

Clinical Presentation & Classic Clinical Signs

  • History & Cardinal Symptoms: Patients frequently report exertional dyspnoea resulting from elevated filling pressures, typical or atypical angina pectoris driven by increased myocardial oxygen demand and microvascular ischaemia, and stress-dependent presyncope or syncope secondary to transient outflow tract obstruction and impaired cardiac output1,2. Palpitations are common and often herald the onset of paroxysmal supraventricular or ventricular arrhythmias.
  • Physical Examination & Named Clinical Signs: Auscultation reveals a harsh, medium-pitched crescendo-decrescendo systolic ejection murmur heard best at the left lower sternal border and apex, which paradoxically increases in intensity during maneuvers that decrease ventricular volume or preload (e.g., Valsalva manoeuvre, standing from a squatting position, or administration of nitroglycerin) and diminishes with maneuvers that increase preload or afterload (e.g., squatting, passive leg raise, handgrip). Carotid upstroke is characteristically brisk and bisferiens (double-peaked pulse). The apical impulse may be sustained and bifid due to a prominent presurging atrial contraction (fourth heart sound, S4).

Diagnostic Workup & Interpretation (Laboratory, Imaging & ECG/Pathology)

Diagnostic Modality / Test Expected Clinical & Radiological / Laboratory Finding
Gold-Standard InvestigationTransthoracic echocardiography (TTE) demonstrating basal interventricular septal thickness $\ge$15 mm without other loading causes, resting or provocable LVOT gradient $\ge$30 mm Hg, and classic systolic anterior motion (SAM) of the mitral valve1.
Laboratory PanelElevated serum cardiac biomarkers (troponin T or I, NT-proBNP) reflecting ongoing myocardial wall stress, mechanical stretch, and microvascular ischaemia. Genetic testing panel for sarcomeric mutations.
Imaging / Radiology (X-ray, US, CT, MRI)Cardiac Magnetic Resonance (CMR) imaging with late gadolinium enhancement (LGE) to accurately quantify left ventricular mass, map regional wall hypertrophy patterns, and visualize extent of myocardial fibrosis/scarring. Chest radiography may reveal mild-to-moderate cardiomegaly with left atrial enlargement.
ECG / Histopathology / ProceduralElectrocardiogram shows prominent voltage criteria for left ventricular hypertrophy, deep narrow ("dagger-like") Q waves in inferior and lateral leads, and repolarization abnormalities (T-wave inversions). Histopathology reveals haphazard arrangement of hypertrophied myocytes (myofibrillar disarray) interspersed with interstitial fibrosis.

Staging, Classification & Clinical Scoring Systems

Criterion / Stage / Score Component Clinical Threshold, Points & Management Implication
HCM Risk-SCD CalculatorEstimates 5-year risk of sudden cardiac death using age, maximal wall thickness, left atrial diameter, max LVOT gradient, family history of SCD, non-sustained VT, and unexplained syncope. Score $\ge$6% strongly supports prophylactic ICD placement.
NYHA Functional ClassClass I (no limitation) to Class IV (symptoms at rest). Guides escalation from medical management to septal reduction therapies.
LVOT Gradient SeverityMild (30–49 mm Hg), Moderate (50–79 mm Hg), and Severe ($\ge80 mm Hg or provocable gradient \ge$50 mm Hg). Direct indicator for invasive intervention in drug-refractory symptomatic patients1.

Differential Diagnosis

Differential Condition Distinguishing Clinical, Biochemical & Imaging Features
Hypertensive Heart DiseaseConcentric rather than asymmetric left ventricular hypertrophy, history of long-standing systemic hypertension, and wall thickness rarely exceeding 15 mm unless severely uncontrolled.
Aortic StenosisCalcified, immobile aortic valve leaflets seen on echocardiography; pressure gradient is valvular rather than subvalvular, and the murmur radiates to the carotids without accentuation on Valsalva.
Cardiac AmyloidosisInfiltrative cardiomyopathy featuring "speckled" myocardial texture on echocardiography, low-voltage ECG despite increased wall thickness, and systemic features of extracellular protein deposition.
Athlete's HeartPhysiological hypertrophy (sympathetic conditioning response) with symmetrical LV wall thickness rarely exceeding 15 mm, normal diastolic function, and complete regression of hypertrophy upon deconditioning.

Management & Treatment Protocol

  • Initial Stabilization & Resuscitation: Avoid volume depletion, dehydration, and administration of positive inotropic agents or pure vasodilators (such as dihydropyridine calcium channel blockers, nitrates, and ACE inhibitors), which worsen the LVOT gradient. Administer intravenous normal saline cautiously if volume depletion occurs. Manage acute tachyarrhythmias aggressively.
  • First-Line Medical / Pharmacological Therapy (with exact adult dosing):
    • Beta-blockers: Metoprolol succinate 50–200 mg orally once daily (or metoprolol tartrate 50–100 mg orally twice daily) titrated to heart rate 60–70 bpm to prolong diastolic filling time and decrease myocardial contractility2.
    • Non-Dihydropyridine Calcium Channel Blockers: Verapamil (extended-release) 120–240 mg orally once daily (up to 480 mg daily) used as an alternative or adjunct for patients intolerant to beta-blockers (avoid in patients with severe resting hypotension or advanced heart failure).
    • Myosin Inhibitors: Mavacamten 5 mg orally once daily, adjusting dose based on echocardiographic ejection fraction and gradient response to directly target hypercontractility.
  • Surgical / Interventional Indications: Reserved for patients with severe drug-refractory symptoms (NYHA Class III–IV) and resting or provocable LVOT gradients $\ge$50 mm Hg.
    • Surgical Septal Myectomy (Morrow Procedure): Gold-standard surgical excision of the basal septum.
    • Percutaneous Alcohol Septal Ablation: Subselective catheter-based injection of absolute ethanol into the first septal perforator branch of the left anterior descending artery to induce controlled localized septal infarction and subsequent shrinkage.

Complications & Prognosis

  • Key Acute and Chronic Complications: Sudden cardiac death secondary to ventricular tachyarrhythmias, heart failure with preserved or reduced ejection fraction, atrial fibrillation predisposing to systemic thromboembolism and stroke, infective endocarditis on the abnormal mitral valve or septum, and progressive syncope1.
  • Prognostic Factors: Annual mortality in specialized cohorts is low (1%), but risk is markedly elevated by severe baseline symptoms, a family history of sudden cardiac death, extreme left ventricular wall thickness ($\ge$30 mm), unexplained syncope, nonsustained ventricular tachycardia on Holter monitoring, and extensive myocardial fibrosis identified via cardiac magnetic resonance imaging1,2.