Cardiology · Cardiomyopathies
Hypertrophic Cardiomyopathy
Asymmetric Hypertrophy · LVOTO · Mavacamten · SCD Prevention
Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiomyopathy, found in approximately 1 in 500 adults by echocardiographic criteria — yet it remains underdiagnosed, underappreciated by generalists, and undertreated. The disease is defined by left ventricular hypertrophy in the absence of an abnormal loading condition sufficient to explain the degree of hypertrophy, most commonly manifesting as asymmetric thickening of the interventricular septum with a small, hyperdynamic left ventricular cavity.[1] In approximately 70% of patients, the hypertrophied septum causes dynamic obstruction of the left ventricular outflow tract (LVOTO) — a process driven by systolic anterior motion of the anterior mitral leaflet and magnified by anything that reduces ventricular filling: dehydration, vasodilators, Valsalva, standing. Downstream consequences include mitral regurgitation, elevated filling pressures, dyspnea, chest pain, and presyncope. In its most dangerous expression, HCM is the leading cause of sudden cardiac death in young competitive athletes in the United States, a consequence of arrhythmias arising in the fibrotic, hypertrophied myocardium.[2] For decades, treatment was limited to negative inotropes and, in severe obstruction, surgical septal myectomy. The FDA approval of mavacamten in April 2022 — a cardiac myosin inhibitor that targets the hypercontractile sarcomere directly — transformed the therapeutic landscape for obstructive HCM, offering the first disease-mechanism-targeted medical therapy for a condition that had been managed symptomatically for sixty years.
In 1957, a British cardiac surgeon named Sir Russell Brock described a clinical puzzle he called "functional obstruction of the left ventricle" — patients with outflow tract gradients attributable not to fixed anatomical obstruction but to something dynamic in the muscular septum itself. A year later, in 1958, Donald Teare — a forensic pathologist who happened to encounter eight young adults who had died suddenly — published a description of "asymmetrical hypertrophy of the heart" in the British Heart Journal, noting the peculiar disorganization of myocardial fiber bundles he observed on histology: a whorling, chaotic arrangement of hypertrophied myocytes that he recognized as a distinct and previously unreported pathological entity. These two papers together defined the disease — one from the catheterization laboratory, one from the autopsy table — and established that what we now call HCM could be recognized in life by its hemodynamic obstruction and at death by its myocyte disarray.[1]
In the 1960s, Eugene Braunwald and colleagues at the National Institutes of Health performed systematic cardiac catheterization studies that clarified the fundamental hemodynamics of obstructive HCM. They showed that the outflow gradient was dynamic — worsening with agents that reduced preload or afterload, and improving with maneuvers that did the opposite — and they recognized that this dynamic behavior was entirely unlike fixed valvular aortic stenosis. The disease was called by many names: idiopathic hypertrophic subaortic stenosis (IHSS), hypertrophic obstructive cardiomyopathy (HOCM), muscular subaortic stenosis. The current term "hypertrophic cardiomyopathy" was eventually adopted because obstruction is neither universal nor the sole clinically important feature. For the next two decades, management rested almost entirely on negative inotropes — propranolol and verapamil — with surgical septal myectomy reserved for those most severely limited. In 1995, Ulrich Sigwart introduced alcohol septal ablation as a catheter-based alternative to surgery, injecting ethanol into the first septal perforator artery to create a controlled septal infarct.[9]
The genetic era began in 1989, when the first HCM mutation was mapped to chromosome 14 and then traced to the beta-myosin heavy chain protein, a major constituent of the cardiac sarcomere. The subsequent decades uncovered hundreds of pathogenic variants — predominantly in genes encoding sarcomere protein filaments — with MYH7 and MYBPC3 together accounting for 75–80% of genetically characterized cases. The clinical implications were profound. HCM was revealed to be not one disease but a phenotype shared by a range of sarcomere protein mutations, each carrying somewhat different penetrance, expressivity, and long-term risk profiles. Equally important was the recognition in the 1990s and 2000s that sudden cardiac death — previously attributed to obstruction itself — was in fact primarily arrhythmic, arising in the fibrotic substrate created by years of myocyte disarray and compensatory scarring. The implantable cardioverter-defibrillator became the only reliable prevention and the central tool of SCD risk stratification.[6]
The mavacamten era began with the molecular insight that HCM sarcomere mutations cause not weakness but hypercontractility — an excess of myosin heads in an actin-engaged "on" state, creating too much force rather than too little. This observation suggested a specific pharmacological target: a small-molecule inhibitor of cardiac myosin ATPase that could reduce the number of force-generating crossbridges without impairing systolic function. Mavacamten was developed on this hypothesis and tested in the EXPLORER-HCM phase 3 randomized controlled trial, published in the Lancet in 2020. Patients with symptomatic obstructive HCM achieved a composite endpoint of meaningful improvement in peak VO₂ and NYHA functional class at a rate of 37% versus 17% in placebo — with dramatic reductions in LVOT gradient, improvements in NYHA class, and near-complete resolution of SAM in many patients.[4] FDA approval under the brand name Camzyos followed in April 2022. The significance of this approval was not merely pharmacological — it was conceptual: for the first time, a drug targeting the actual mechanism of HCM was available, offering an alternative to surgical intervention for a disease that had been managed symptomatically for sixty years.
In HCM attributable to sarcomere protein mutations, the causal chain from gene to phenotype runs through two broad mechanisms. Missense variants — most characteristic of MYH7 — produce a dominant-negative "poison peptide" effect: the mutant protein integrates into the sarcomere and impairs normal contractile function from within. Frameshift and nonsense variants — most characteristic of MYBPC3 — produce haploinsufficiency through nonsense-mediated decay or proteolysis of a truncated protein, leaving the sarcomere with a deficient complement of its binding protein. Despite these different mechanisms, both converge on the same downstream result: dysregulation of the myosin powerstroke, with an excess of force-generating crossbridges and impaired calcium handling that produce the paradoxical combination of hypercontractility and impaired diastolic relaxation. Myocyte hypertrophy follows as an abnormal growth response to these altered mechanical signals, and the chaotic disorganization of myocardial fiber bundles — myocyte disarray — reflects the failure of the myocardium to organize its growth normally.[1]
The clinical consequences of this molecular disarray cluster around two dominant mechanisms: diastolic dysfunction and LVOT obstruction. Myocyte hypertrophy and interstitial fibrosis increase wall stiffness and impair early diastolic relaxation, raising filling pressures and causing exertional dyspnea even in patients without obstruction. In the approximately 70% of patients who have obstruction at rest or with provocation, a second mechanism amplifies symptoms: dynamic narrowing of the LVOT caused by systolic anterior motion (SAM) of the anterior mitral leaflet.[8] As the hypertrophied septum encroaches on the outflow tract during systole, a Venturi effect — created by the high-velocity jet through the narrowed orifice — draws the anterior mitral leaflet and its subvalvular apparatus forward into the LVOT. This simultaneously reduces effective LVOT area and displaces the mitral leaflet away from the posterior leaflet, creating the posteriorly directed mitral regurgitant jet that is virtually diagnostic of obstructive HCM on Doppler examination.
The dynamic nature of LVOTO has direct and actionable clinical consequences. Obstruction worsens with anything that reduces LV cavity size — dehydration, preload reduction by vasodilators, the Valsalva maneuver, or standing — and it improves with anything that increases preload or afterload: volume loading, squatting, passive leg raise, or hand-grip. A patient who is asymptomatic and gradient-free in a supine clinic examination may have severe hemodynamic obstruction during exercise, after a meal in a hot environment, or during acute volume depletion from a febrile illness. This explains why a single resting echo without provocative maneuvers will underdiagnose provocable obstruction in a substantial fraction of HCM patients, and why exercise echocardiography — or at minimum Valsalva provocation during the resting echo — is essential for complete hemodynamic assessment. The threshold gradient for intervention consideration is 50 mmHg (at rest or with provocation), because this magnitude correlates with symptom burden and with durability of outcomes after septal reduction.[2]
Fibrosis is an independent and underappreciated driver of HCM morbidity. Late gadolinium enhancement (LGE) on cardiac MRI detects myocardial fibrosis and is present in the majority of HCM patients; the extent of LGE correlates with ventricular arrhythmia burden, the risk of sudden cardiac death, and the development of the "burnt-out" phase of HCM in which the hypertrophied, fibrotic ventricle transitions from preserved to reduced ejection fraction — a phenotypic transformation that resembles dilated cardiomyopathy and carries a particularly poor prognosis. Atrial fibrillation develops in approximately 20–25% of HCM patients over their lifetime, driven by progressive left atrial dilation from elevated filling pressures; its presence approximately triples the risk of stroke, requiring anticoagulation regardless of CHA₂DS₂-VASc score in most cases.[1]
HCM is a phenotype, not a single disease, and the clinical subtype determines both the hemodynamic burden and the management strategy. The 2020 AHA/ACC guideline emphasizes the distinction between obstructive and non-obstructive disease as the primary clinical branch point.[2]
| Phenotype | Key Feature | Echo Finding | Management Implication |
|---|---|---|---|
| Obstructive HCM | Resting LVOT gradient ≥30 mmHg; ≥50 mmHg = threshold for intervention | ASH + SAM + turbulent LVOT flow on color Doppler; late-peaking "dagger" CW profile | BB/CCB first-line; mavacamten for NYHA II–III; septal reduction (myectomy or ASA) if ≥50 mmHg + NYHA III–IV despite medications |
| Labile/Provocable Obstruction | Gradient absent at rest; ≥50 mmHg with Valsalva or exercise | SAM may appear only with provocation; gradient induced by Valsalva or exercise echo | Treat as obstructive if exercise gradient ≥50 mmHg with symptoms; eligible for mavacamten and septal reduction |
| Non-obstructive HCM | No significant gradient at rest or with maximal provocation | ASH without SAM; small LV cavity; possible concentric or apical distribution | Symptom management with BB/CCBs; no role for septal reduction; focus on ICD for SCD risk and AF management |
| Apical HCM | Hypertrophy confined to LV apex; common in East Asian populations | "Ace of spades" LV contour; giant TWIs in V3–V5; less ASH; diagnosis may require MRI or contrast echo | Lower LVOTO risk; SCD risk present via arrhythmia; apical aneurysm formation a specific complication requiring anticoagulation |
| Midcavitary Obstruction | Obstruction at midventricular level, not LVOT; papillary muscle hypertrophy common | Hourglass-shaped LV on echo; gradient below the mitral valve level; apical aneurysm in some | Less responsive to standard septal reduction; alcohol ablation directed at midventricular muscle; high SCD risk; ICD consideration |
HCM patients with any of the following features carry high sudden cardiac death risk and should be urgently referred for ICD evaluation: unexplained syncope (especially exertional), sustained VT or resuscitated cardiac arrest, maximal LV wall thickness ≥30 mm, family history of HCM-related SCD in a first-degree relative, LGE ≥15% of LV mass on cardiac MRI, or LV systolic dysfunction (EF <50%). The ESC HCM Risk-SCD model calculates 5-year SCD risk from a validated set of clinical predictors and guides ICD implantation recommendations (Class IIa if 5-year risk ≥6%). The AHA/ACC approach uses the presence of major risk factors individually.
Separate from ICD decisions: all HCM patients should avoid intense competitive sports (the murmur that increases with Valsalva is the bedside reminder of what physical exertion can do to outflow gradients and arrhythmia risk). Avoid dehydration, vasodilators (ACE inhibitors, ARBs, dihydropyridine calcium channel blockers), digoxin, and positive inotropes — each worsens obstruction or increases arrhythmia vulnerability. NSVT detected on ambulatory monitor warrants formal SCD risk assessment regardless of symptom status.
The bedside findings of obstructive HCM follow a coherent physiological logic once the dynamic nature of the obstruction is understood. The dynamic maneuver response — murmur increasing with Valsalva and standing, decreasing with squatting and leg raise — is the single most diagnostically useful bedside finding and distinguishes HCM from every other cardiac murmur.[1]
| Finding | Characteristic | Clinical Pearl |
|---|---|---|
| Dynamic LVOT murmur | Harsh crescendo-decrescendo at LLSB and apex; ↑ Valsalva/standing; ↓ squat/leg raise/handgrip | The dynamic response is pathognomonic — aortic stenosis murmur uniformly decreases with Valsalva; mitral regurgitation murmur from other causes does not change with squatting. If the murmur increases with Valsalva, HCM is the diagnosis until proven otherwise. |
| Bisferiens (bifid) carotid pulse | Two palpable peaks per cardiac cycle — spike-and-dome pattern | The initial rapid ejection spike occurs before obstruction fully develops; the second dome reflects continued ejection after partial LVOT clearance. Best felt in the carotid artery with the neck slightly extended. Found in severe obstructive HCM; pathognomonic when present. |
| S4 gallop | Presystolic gallop at cardiac apex; heard in most HCM patients | The stiff, non-compliant LV with elevated filling pressures relies heavily on atrial contraction for ventricular filling — the abrupt deceleration of this forceful atrial systole against the stiff LV generates the S4. Its loss in AF signals the loss of atrial kick and can precipitate acute decompensation. |
| Deep Q waves on ECG | Narrow, deep Q waves in lateral (I, aVL, V5–V6) and inferior leads | "Septal Q waves" in HCM arise from exaggerated septal depolarization in an abnormally hypertrophied septum. Narrow QRS is the key differentiator from pathological Q waves of MI (which are broad and associated with T-wave changes). Present in 25–50% of HCM patients; absent in valvular AS. |
| LVH on ECG | Voltage criteria for LVH; often with repolarization abnormalities | Massive LVH voltages on ECG (Sokolow-Lyon ≥35 mm) in a young athlete without hypertension or aortic stenosis demands echo evaluation for HCM. Giant negative T waves in V1–V4 in a Japanese patient should prompt apical HCM assessment. Normal ECG makes HCM less likely but does not exclude it. |
POCUS does not diagnose the genetic basis of HCM, but it directly identifies the hemodynamic features that determine clinical urgency, direct management, and guide invasive referral. In the hands of a trained clinician, bedside ultrasound can identify asymmetric septal hypertrophy, systolic anterior motion of the mitral valve, LVOT obstruction, and dynamic gradient provocation — each with management consequences. The challenge is not just seeing ASH (which any competent sonographer can do) but understanding what it means: that the gradient you measure today may be a fraction of what the patient experiences during exercise, that the murmur that increases with Valsalva at bedside reflects a hemodynamic mechanism visible in real time on echo, and that the decisions about mavacamten, septal reduction, and ICD depend in part on what you measure here.[7]
Pearl 1 — Asymmetric Septal Hypertrophy (ASH) on Parasternal Long Axis. The fundamental POCUS finding of HCM is a hypertrophied interventricular septum that is disproportionately thick relative to the posterior wall. Measure IVS thickness at end-diastole on the parasternal long axis (PLAX) view, at the level of the mitral valve leaflet tips, perpendicular to the septum. The diagnostic threshold is ≥15 mm in an adult (or ≥13 mm with a clear family history of HCM). The IVS:LVPW ratio exceeding 1.3 defines asymmetric hypertrophy in the septal distribution. Critically: the LV in HCM is typically small and hyperdynamic with near-complete or complete cavity obliteration at end-systole — this is the opposite of what you see in dilated cardiomyopathy and helps immediately distinguish HCM from secondary causes of hypertrophy.[7]
Pearl 2 — Systolic Anterior Motion (SAM) on M-Mode. SAM is the echo signature of obstructive HCM. On M-mode through the mitral valve on PLAX, the anterior leaflet produces a characteristic anterior deflection during systole — moving toward the septum rather than staying in the posterior position. In severe obstruction, the anterior leaflet contacts the septum, creating a brief flattening or contact artifact on M-mode. The duration of leaflet-septal contact is proportional to gradient severity. On 2D color Doppler in PLAX or apical 4-chamber views, turbulent mosaic flow fills the LVOT during systole — this color aliasing is an immediate qualitative marker of significant obstruction and should trigger CW Doppler gradient measurement.
Pearl 3 — LVOT Gradient by Continuous Wave Doppler. Obtain the LVOT gradient via continuous wave (CW) Doppler from the apical 5-chamber or apical 3-chamber view, with the cursor aligned parallel to the direction of LVOT flow. The velocity profile in obstructive HCM has a characteristic late-peaking "dagger" shape — the velocity continues to rise through systole as SAM-septal contact progressively narrows the outflow tract — which contrasts with the early-peaking symmetric profile of valvular aortic stenosis. Apply the modified Bernoulli equation (gradient = 4v²). A resting gradient ≥30 mmHg = clinically significant obstruction; ≥50 mmHg = threshold for consideration of septal reduction in the context of maximal symptoms.[7]
Pearl 4 — Provocative Testing at Bedside. If the resting gradient is normal or borderline in a patient with symptoms suggestive of HCM, provocation is essential before concluding that obstruction is absent. The Valsalva maneuver — sustained expiratory straining for ≥15 seconds while maintaining CW Doppler alignment in the LVOT — reduces preload and unmasks latent obstruction in patients with labile disease. A gradient increase to ≥50 mmHg with Valsalva constitutes "provocable obstruction" and carries the same therapeutic implications as a resting gradient ≥50 mmHg. At dedicated echo labs, exercise echocardiography is the preferred provocative test, with upright bicycle or immediate post-treadmill echo being more physiologically relevant than the supine Valsalva maneuver.[7]
The diagnostic workup in HCM serves three purposes simultaneously: confirming the diagnosis, identifying the hemodynamic phenotype, and stratifying sudden cardiac death risk. No single test accomplishes all three, and the imaging workup particularly benefits from the complementary strengths of echocardiography (hemodynamics, real-time gradient measurement, provocation) and cardiac MRI (fibrosis detection, complete morphological delineation, tissue characterization).[1]
| Test | Clinical Rationale |
|---|---|
| Transthoracic echocardiogram | Gold standard for diagnosis: wall thickness, LVOT gradient (resting and with provocation), SAM, LA size, LV function, mitral valve morphology, papillary muscle position. Essential at initial evaluation and annually or with symptom change. Exercise echo for provocable obstruction assessment in symptomatic patients with non-diagnostic resting echo. |
| 12-Lead ECG | Abnormal in 90–95% of HCM patients: LVH voltage, deep septal Q waves, repolarization abnormalities. Normal ECG makes HCM substantially less likely. Giant TWIs in V1–V4 are a key clue to apical HCM. ECG also provides baseline QTc (relevant before disopyramide or other antiarrhythmics); serial ECGs detect AF development. |
| Cardiac MRI with gadolinium | Superior to echo for: (1) detecting and quantifying fibrosis via late gadolinium enhancement (LGE) — present in ~65% of HCM patients; LGE ≥15% of LV mass = high SCD risk (ICD Class IIa); (2) delineating apical hypertrophy, midcavitary obstruction, and morphological variants; (3) diagnosis when echo is non-diagnostic (poor acoustic windows, apical disease). Recommended for all HCM patients at initial evaluation or when SCD risk is being formally assessed. |
| Ambulatory cardiac monitoring (Holter/event) | 48–72 hour Holter at initial evaluation for all HCM patients; repeated every 1–2 years or with symptoms. NSVT (≥3 beats at ≥120 bpm) is a major SCD risk factor. AF detection triggers anticoagulation decision. Extended monitoring (2–4 weeks) increases NSVT and AF detection yield significantly. |
| Genetic testing (multi-gene panel) | Recommended for all HCM patients to identify causative sarcomere mutation; positive result enables cascade family screening (first-degree relatives) at low cost. Gene-positive patients have earlier disease onset, more prominent hypertrophy, and worse long-term prognosis compared to gene-negative patients. Panel typically includes MYH7, MYBPC3, and 5–8 additional sarcomere genes. |
| Exercise testing (treadmill or cardiopulmonary) | Exercise-induced hypotension (systolic BP drop ≥20 mmHg during exercise) is a major SCD risk factor. Cardiopulmonary exercise testing (CPET) quantifies functional capacity, guides prognostication, and provides objective endpoint for treatment response (peak VO₂ improvement with mavacamten is the EXPLORER-HCM primary endpoint metric). Useful before and after treatment changes. |
| Metabolic and screening labs | Comprehensive metabolic panel, CBC, renal function before treatment initiation; renal function guides drug dosing. Screen for HCM phenocopies in atypical presentations: alpha-galactosidase A activity and lyso-Gb3 for Anderson-Fabry disease (X-linked, MR imaging shows basal inferolateral LGE); NT-proBNP tracks filling pressure response to therapy. |
Treatment in HCM divides along two independent axes: symptom management (which drives the obstructive vs non-obstructive distinction and the medical-to-invasive stepwise approach) and sudden cardiac death prevention (which depends entirely on ICD candidacy, independent of symptom burden). A patient can be in NYHA class I with no symptoms and still have life-threatening arrhythmia risk requiring ICD; a patient can be severely symptomatic without fitting any SCD risk criteria. These two management tracks must be addressed in parallel at every clinical encounter.[2]
| Intervention | When & How | Evidence & Key Points |
|---|---|---|
| Beta-Blockers | First-line for all symptomatic HCM (obstructive and non-obstructive). Metoprolol succinate 25–200 mg/day; atenolol 50–200 mg/day; propranolol 40–320 mg/day. Titrate to resting HR 60–70 bpm. Non-vasodilatory agents preferred. | Reduce heart rate and prolong diastolic filling time; blunt adrenergic provocation of dynamic obstruction; reduce frequency of exertional angina-like symptoms. Not curative. Non-vasodilating BB essential — avoid carvedilol (vasodilatory; worsens obstruction). Do not abruptly discontinue in obstructive HCM.[2] |
| Non-DHP CCBs (Verapamil/Diltiazem) | Second-line when BB contraindicated or not tolerated. Verapamil 40–80 mg TID up to 480 mg/day; diltiazem 60–120 mg TID. Use in isolation or combined with low-dose BB (cautiously). | Negative chronotropy + inotropy reduce LVOTO and improve diastolic filling. Use with great caution in severe resting obstruction with elevated pulmonary pressures — verapamil can precipitate acute hemodynamic decompensation in this context by reducing afterload without adequately reducing obstruction. Avoid in decompensated HF. Contraindicated with disopyramide.[2] |
| Disopyramide | Add-on negative inotrope for persistent obstructive symptoms despite BB/CCB. 100–150 mg TID–QID; must be combined with BB (prevents disopyramide-induced tachycardia via anticholinergic effect). Monitor QTc and bundle branch block at initiation. | Potent negative inotrope; reduces LVOT gradient up to 50% in many patients and substantially reduces symptoms. Requires ECG monitoring at initiation: stop if QTc >480 ms or LBBB develops. Drug shortages have been a significant practical limitation. Do not combine with verapamil (risk of advanced AV block and hemodynamic collapse).[2] |
| Mavacamten (Camzyos) | For symptomatic obstructive HCM (resting LVOT gradient ≥30 mmHg or ≥50 mmHg with provocation; NYHA II–III) despite BB/CCB, or as first-line obstructive therapy. Start 2.5 mg once daily; titrate by echo gradient at 8 weeks to target resting LVOT <30 mmHg (max 15 mg/day). Hold if EF <50%. | EXPLORER-HCM (Lancet 2020, n=251): 37% vs 17% placebo achieved primary composite endpoint; mean LVOT gradient reduced by 47 mmHg; NYHA class improved in 65% vs 31%; SAM resolved in majority. FDA approved April 2022 (Camzyos). REMS program required (EF monitoring quarterly). Strong CYP2C19 and CYP3A4 interactions.[4] |
| Surgical Septal Myectomy | Gold standard invasive treatment for obstructive HCM: resting or provoked LVOT gradient ≥50 mmHg + NYHA III–IV symptoms despite maximal medical therapy. Transaortic resection of 3–5 g of basal septal muscle under cardiopulmonary bypass. | Provides complete, durable LVOT gradient relief in >95% at high-volume centers. Operative mortality <1% at experienced centers; long-term survival normalized to age-matched general population. Requires expertise — recommend referral to dedicated HCM centers. Concomitant mitral valve repair for intrinsic MV abnormalities where indicated. Class I recommendation (AHA/ACC).[2] |
| Alcohol Septal Ablation (ASA) | Catheter-based alternative to myectomy: injection of 1–3 mL of 98% ethanol into the first (rarely second) septal perforator artery under echocardiographic myocardial contrast guidance, creating a controlled septal infarct. Requires suitable septal anatomy on contrast echo. | Reduces LVOT gradient comparably to myectomy in suitable candidates. Key risks: complete heart block in ~10% (requiring permanent pacemaker in ~5%), RBBB universally, late VT from heterogeneous scar (slightly higher than myectomy). Avoid in patients <21 years (uncertain long-term scar implications). Long-term data less mature than myectomy. Class IIa recommendation (AHA/ACC).[9] |
| ICD — Primary SCD Prevention | For high SCD risk patients with ≥1 year anticipated survival. Major risk factors (AHA/ACC): prior cardiac arrest/VF; sustained VT; maximal LV wall thickness ≥30 mm; unexplained syncope; family history of HCM-related SCD; EF <50%; LGE ≥15% of LV mass. ESC: use HCM Risk-SCD calculator; recommend ICD if 5-year risk ≥6%. | ICD is the only proven therapy for prevention of SCD in HCM. Risk stratification is imperfect — both models (AHA/ACC risk-factor approach and ESC Risk-SCD score) have significant false-positive and false-negative rates. In younger patients with decades of risk ahead, even a 5-year SCD risk of 4–6% may justify ICD consideration. Subcutaneous ICD (S-ICD) is an option when pacing is not needed; transvenous ICD preferred when pacing is anticipated (e.g., post-ASA AV block risk).[6] |
| Sport Restriction & Lifestyle | All HCM patients: avoid intense competitive sports. Moderate recreational exercise (walking, swimming, cycling at moderate intensity) is encouraged and beneficial. Avoid dehydration, alcohol excess, vasodilators, diuretics without fluid maintenance. | HCM is the leading cause of SCD in young competitive athletes in the USA.[5] Exercise restriction substantially reduces this risk. The 2020 AHA/ACC guideline and expert consensus support recreational exercise; the prior absolute restriction from all exercise has been revised. Individualized exercise prescription from HCM specialist is appropriate for athletes who wish to continue moderate activity. Therapeutic lifestyle changes include weight loss in obese patients (reduces filling pressures and obstruction).[2] |
| Recommendation — 2020 AHA/ACC HCM Guideline (Ommen SR et al.)[2] | Class | Level of Evidence |
|---|---|---|
| Echocardiogram at initial evaluation of all patients with suspected HCM, including resting and Valsalva-provoked LVOT gradient assessment | I | B-NR |
| Genetic testing for all patients with HCM to identify causative sarcomere mutation and enable family cascade screening | I | B-NR |
| Beta-blocker or non-DHP CCB for symptomatic patients with LVOTO or non-obstructive HCM | I | B-NR |
| Surgical septal myectomy at experienced centers for NYHA III–IV obstructive HCM refractory to maximal medical therapy | I | B-NR |
| ICD implantation for HCM patients with prior cardiac arrest, sustained VT, or high-risk features with anticipated ≥1 year meaningful survival | I | B-NR |
| Cardiac MRI with gadolinium for all patients with HCM for morphological delineation and SCD risk assessment via LGE quantification | I | B-NR |
| Mavacamten for symptomatic obstructive HCM unresponsive to BB/CCB (Class I per 2024 AHA/ACC Focused Update) | I | A |
| Disopyramide added to BB for persistent obstructive symptoms; alcohol septal ablation as alternative to myectomy in appropriate candidates | IIa | B-NR |
| ICD when 5-year ESC HCM Risk-SCD model estimate ≥6% (ESC guideline) or presence of ≥1 major risk factor (AHA/ACC approach) | IIa | B-NR |
| Vasodilators (ACE inhibitors, ARBs, DHP-CCBs, nitrates), positive inotropes, and digoxin in patients with obstructive HCM (worsen outflow gradient) | III: Harm | B-NR |
The approval of mavacamten was not primarily a triumph of clinical trial design — it was a triumph of basic science translating directly into a therapeutic target. The story begins with a realization that contradicted the intuitive sense of what a "hypertrophic" cardiomyopathy should mean at the molecular level. In a diseased heart with hypertrophied, failing muscle, one might expect weakness — insufficient contractile force. But the sarcomere mutations that cause HCM produce the opposite: hypercontractility, an excess of force-generating actomyosin crossbridges. The MYH7 and MYBPC3 mutations that account for the majority of genetically diagnosed HCM appear to converge on a single functional outcome — an abnormal excess of myosin heads in the "on" state, primed to engage actin — that produces too much contractile force, impairs diastolic relaxation, and drives the downstream myocyte stress signaling that causes hypertrophy, fibrosis, and disarray.[1]
This molecular insight suggested a specific pharmacological intervention: rather than blunting the downstream consequences of hypercontractility (which is what beta-blockers and verapamil do), inhibit the myosin ATPase enzymatic step that powers the crossbridge cycle. Mavacamten does exactly this — it is an allosteric inhibitor of beta-cardiac myosin that stabilizes myosin in a sequestered, inactive state (the "super-relaxed" state), reducing the proportion of force-generating crossbridges available for each contraction. The reduction in contractile force directly reduces LVOT obstruction by decreasing SAM formation, reduces mitral regurgitation, and improves diastolic compliance by allowing the sarcomere to relax more completely. None of this requires surgical myectomy, septal infarction, or chronic negative inotrope exposure.
The EXPLORER-HCM trial demonstrated these theoretical benefits in practice: 251 patients with symptomatic obstructive HCM (resting gradient ≥50 mmHg or provoked ≥50 mmHg and resting ≥30 mmHg) randomized to mavacamten versus placebo for 30 weeks. The primary endpoint — a composite functional improvement in peak VO₂ and NYHA class — was achieved in 37% of mavacamten-treated patients versus 17% of placebo patients (p<0.0001). LVOT gradient fell by a mean of 47 mmHg with mavacamten versus 7 mmHg with placebo. Sixty-five percent of mavacamten patients improved at least one NYHA class, compared to 31% with placebo. The effect on the echocardiogram was striking: SAM resolved completely in many patients, and LVOT gradients dropped below the threshold for septal reduction consideration in the majority of treated patients.[4]
What should fellows take from this mechanistic story? Several things. First, the mechanism of a drug matters — mavacamten works in a different substrate than beta-blockers, and understanding why tells you when to expect a response and what side effects to anticipate (predominantly a reversible reduction in EF in a subset, because the same mechanism that reduces hypercontractility can, at excess doses, impair normal systolic function). Second, the drug requires REMS program enrollment and quarterly echocardiographic monitoring — EF ≤50% triggers dose reduction or discontinuation. Third, CYP2C19 poor metabolizers reach much higher drug concentrations and need lower doses; check genotype if dose-related toxicity is suspected. And fourth — the most important takeaway — HCM is now a disease with a mechanistically specific, Phase 3-proven, FDA-approved medical therapy. The days of beta-blockers and "refer for myectomy" as the only effective options are over. But myectomy, performed at expert centers, remains the most durable treatment for severe obstructive disease, and the existence of mavacamten does not eliminate the surgical option for patients who fail or cannot tolerate it.
251 patients with symptomatic obstructive HCM randomized to mavacamten vs placebo for 30 weeks. 37% vs 17% achieved primary composite endpoint; LVOT gradient reduced by mean 47 mmHg. Led directly to FDA approval of mavacamten (Camzyos) in April 2022 — the first disease-mechanism-targeted therapy for HCM.
Comprehensive evidence-based recommendations covering diagnosis, risk stratification, medical therapy, septal reduction, ICD implantation, and sport restriction. The foundational management framework for HCM; superseded the 2011 ACCF/AHA guideline and incorporated evolving data on genetic testing, cardiac MRI, and risk stratification.
Derivation and validation of a novel multivariable risk prediction model for sudden cardiac death in HCM: a continuous 5-year SCD risk estimate from age, family history, unexplained syncope, NSVT, LV wall thickness, LA diameter, and LVOT gradient. Forms the basis of ESC ICD implantation recommendations (ICD if 5-year risk ≥6%).
Echocardiographic screening of 4,111 young adults in the CARDIA cohort yielding a prevalence estimate of 1 in 500 adults — the foundational epidemiological figure still cited in every HCM guideline and review. Demonstrated that HCM is far more common than previously recognized from clinical presentations alone.
[1] Lorenzini M, Elliott PM. Chapter 42: Hypertrophic Cardiomyopathy — Updated June 2023. In: Fuster V, et al., eds. Fuster and Hurst's The Heart, 15th ed. McGraw-Hill, 2023.
[2] Ommen SR, Mital S, Burke MA, et al. 2020 AHA/ACC Guideline for the Diagnosis and Treatment of Patients With Hypertrophic Cardiomyopathy: A Report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines. J Am Coll Cardiol. 2020;76(25):e159–e240. PMID: 33229116. [PubMed]
[3] Maron BJ, Gardin JM, Flack JM, et al. Prevalence of hypertrophic cardiomyopathy in a general population of young adults: echocardiographic analysis of 4111 subjects in the CARDIA Study. Circulation. 1995;92(4):785–789. PMID: 7641357. [PubMed]
[4] Olivotto I, Oreziak A, Barriales-Villa R, et al. (EXPLORER-HCM study investigators). Mavacamten for treatment of symptomatic obstructive hypertrophic cardiomyopathy (EXPLORER-HCM): a randomised, double-blind, placebo-controlled, phase 3 trial. Lancet. 2020;396(10253):759–769. PMID: 32871100. [PubMed]
[5] Maron BJ, Shirani J, Poliac LC, et al. Sudden death in young competitive athletes: clinical, demographic, and pathological profiles. JAMA. 1996;276(3):199–204. PMID: 8667563. [PubMed]
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