Hypertrophic cardiomyopathy (HCM)

• Hypertrophic cardiomyopathy (HCM) is an inherited disease of the heart muscle in which the left ventricle, usually the septum, becomes thick without a cause such as high blood pressure or a diseased valve.
• It affects roughly 1 in 500 people and often runs in families. Many carriers never develop symptoms.
• The thick septum can narrow the outlet of the left ventricle. The mitral valve leaflet is then pulled into the stream (systolic anterior motion, SAM), which both obstructs flow and leaks.
• The classic murmur becomes louder when the ventricle is emptied (standing, Valsalva) and softer when it is filled or faced with more resistance (squatting, handgrip).
• Most people have a near-normal lifespan. A minority have breathlessness, chest pain, fainting, atrial fibrillation or sudden cardiac death. Risk scores, an implantable defibrillator and drugs that relax the sarcomere have changed what can be offered.

By the end of this page you should be able to:

1. Define HCM, distinguish obstructive from non-obstructive disease, and explain why "HOCM" is only one part of the spectrum.
2. Trace the path from a sarcomere variant to hypertrophy, diastolic dysfunction, ischaemia, obstruction and arrhythmia.
3. Recognise the history and bedside signs, and explain every change in the murmur produced by Valsalva, standing, squatting and handgrip.
4. Choose and interpret the core investigations: ECG, echocardiography, cardiac MRI and genetic testing.
5. Separate HCM from athlete's heart, hypertensive heart disease, aortic stenosis, amyloidosis, Fabry disease, Friedreich ataxia and Noonan syndrome.
6. Estimate sudden cardiac death (SCD) risk with the ESC model and the AHA/ACC risk markers, and state when an ICD is indicated.
7. Treat symptoms with the right drug in the right patient, avoid drugs that worsen obstruction, and know when to refer for septal reduction.
8. Manage atrial fibrillation, pregnancy and family screening, and adapt care when resources are limited.

HCM is a disease of the sarcomere, the molecular motor that makes heart muscle contract. A variant in one of the genes that encodes its proteins makes the muscle work inefficiently. The heart responds by thickening, but the thickening is maladaptive: the ventricle becomes stiff, the septum bulges into the outflow tract, and the muscle's architecture breaks down into disarray and scar.

That single process explains almost everything a patient experiences. A stiff, small ventricle raises filling pressures, so the patient is breathless. A narrow outflow tract produces a murmur and, during exertion, a fall in output that causes fainting. Damaged muscle and scar create the substrate for ventricular arrhythmia, which is why HCM can kill a young person who seemed perfectly well. The clinical skill is to recognise the pattern, measure the obstruction and the risk, and then match treatment to the individual.

The 2023 European Society of Cardiology (ESC) guideline defines HCM broadly, as unexplained left ventricular hypertrophy with or without right ventricular involvement. The 2024 American Heart Association/American College of Cardiology (AHA/ACC) guideline uses a narrower, genetic definition: a primary myocardial disease caused by sarcomere variants, with the same 15 mm threshold.^[6][1] In everyday practice the two definitions meet at the bedside: a thick ventricle with no loading cause, often with a family history, and sometimes with a pathogenic variant.

Several classifications are useful.

• By distribution. Asymmetric septal hypertrophy is a common pattern. Apical, concentric, mid-ventricular, lateral and right ventricular forms occur.^[4] The pattern matters because apical disease can hide from a standard view and mid-ventricular disease can cause an apical aneurysm.
• By obstruction. Obstruction is dynamic. A resting peak gradient of at least 30 mmHg defines resting obstruction; a gradient below 30 mmHg at rest that rises to at least 30 mmHg with provocation is "provocable" or latent obstruction.^[3] A gradient of at least 50 mmHg is the level at which septal reduction is considered.^[12]
• By stage. Most patients are stable. A minority progress to a dilated, "end-stage" phenotype with systolic dysfunction, usually defined as an ejection fraction below 50%.^[1]

The older term "hypertrophic obstructive cardiomyopathy" (HOCM) is still heard. It describes only the obstructive subset, and it can mislead by implying that obstruction is the whole disease. About one third of patients referred to specialist centres have resting obstruction and another third have provocable obstruction; the remainder have no obstruction at rest or on provocation.^[12]

HCM is an important inherited heart muscle disease. Population studies put the prevalence of unexplained left ventricular thickening at 0.16% to 0.23% of adults,^[12] and the familiar figure of 1 in 500 is a reasonable approximation.^[4] Because genotype-positive, phenotype-negative carriers also exist, some estimates reach 0.6%.^[12]

The absolute numbers are large. About 20 million people worldwide are thought to be affected,^[4] and in the United States more than 750,000 people are estimated to have HCM.^[5] Most are never diagnosed: estimates range from about 10% of cases recognised clinically worldwide to roughly 100,000 of an estimated 750,000 cases in the United States, and only about 6% have symptoms.^[4][3] The disease occurs in every ancestry and does not appear to differ in frequency between them, although access to diagnosis does.^[5]

HCM can present at any age, from infancy to old age. Many people are found through family screening or because a murmur is heard during a routine examination. Others present in the third to fifth decade with exertional symptoms.

Roughly 40% to 60% of patients with HCM carry a pathogenic or likely pathogenic variant in a gene encoding a sarcomere protein, and the proportion depends on how strictly the phenotype is defined and on the population studied.^[5][6] The inheritance is autosomal dominant: one altered copy is enough to cause disease, and each child of an affected parent may inherit it.

Two genes account for most identified variants. About 70% of patients with a sarcomere variant have it in MYBPC3 (myosin-binding protein C3) or MYH7 (beta-myosin heavy chain).^[5] The rest cluster in genes such as TNNT2, TNNI3, TPM1, MYL2, MYL3 and ACTC1.^[5] MYBPC3 variants tend to appear later and with lower penetrance.^[2]

Carrying a variant is not the same as having the disease. Penetrance is incomplete and age-dependent: some carriers never develop hypertrophy, while relatives with the same variant can differ widely in wall thickness, obstruction and outcome.^[5] This is why a negative echocardiogram in a young carrier does not end surveillance, and why a negative genetic test in a patient with a typical phenotype does not exclude HCM.

The explanation is not fully known. Modifier genes, epigenetic effects, sex, body mass and environment all probably contribute. Obesity, for example, is associated with higher outflow gradients and worse outcomes, although whether it is causal is unclear.^[5]

Not every thick heart is sarcomeric HCM. Important phenocopies include:

• Fabry disease, an X-linked lysosomal storage disorder caused by alpha-galactosidase A deficiency.
• Cardiac amyloidosis, usually transthyretin (ATTR) or light-chain (AL).
• PRKAG2 and LAMP2 glycogen storage disease, which can produce hypertrophy with pre-excitation.
• Noonan syndrome and other RASopathies, often with pulmonary stenosis and dysmorphic features.
• Friedreich ataxia, a mitochondrial disorder with neurological signs and concentric hypertrophy.
• Danon disease and Pompe disease.

These matter because some have specific treatments (enzyme replacement in Fabry, tafamidis in ATTR amyloidosis) and because they change family counselling.

Genetic testing is recommended for patients who meet diagnostic criteria, because a result can confirm the diagnosis, inform prognosis, guide reproductive choices and allow cascade testing of relatives.^[1] Testing is most useful when the phenotype is typical and there is a family history; it is less useful in isolated, atypical hypertrophy, where a variant of uncertain significance can create more anxiety than clarity.

The sequence is straightforward.

1. Test the proband (the first affected person in the family) with a sarcomere gene panel.
2. If a pathogenic or likely pathogenic variant is found, offer predictive cascade testing to adult first-degree relatives, with pre- and post-test counselling. Testing then cascades outwards to cousins and beyond.^[1]
3. Relatives who carry the variant need cardiac surveillance even if their echocardiogram is normal, because penetrance is age-dependent.
4. Relatives who do not carry the variant can usually be discharged from cardiac screening.
5. If no variant is found in the proband, first-degree relatives still need clinical screening with echocardiography and ECG, because the family may have a variant that current testing cannot detect.

The earliest structural change is myocyte hypertrophy, driven by the mutant sarcomere. The septum is usually affected more than the free wall, producing asymmetric septal hypertrophy. The ratio of septal to posterior wall thickness is often abnormal, although the absolute thickness matters more for diagnosis. The hypertrophied septum bulges into the left ventricular outflow tract (LVOT), the channel between the ventricle and the aorta.

Obstruction in HCM is dynamic, not fixed. As blood accelerates through the narrowed outflow tract, the anterior mitral leaflet is drawn towards the septum by a combination of suction (the Venturi effect) and drag from the flow stream. This is systolic anterior motion (SAM).^[4] Once the leaflet touches the septum, two things happen at once: the outflow tract narrows further, and the mitral valve fails to close properly, producing a posteriorly directed mitral regurgitation jet.

The result is a self-reinforcing loop. Faster flow pulls the leaflet further in, which narrows the tract more, which speeds the flow again. Anything that reduces ventricular volume or increases contractility worsens the loop; anything that fills the ventricle or reduces contractility softens it. That is the physiological basis of the bedside manoeuvres and of drug treatment.

The thickened, fibrotic ventricle is stiff. It relaxes slowly and cannot fill at low pressure, so left atrial pressure rises and the patient becomes breathless, especially on exertion.^[4] A small cavity makes this worse: even a modest increase in volume produces a large rise in pressure. Over time the left atrium dilates, which is the substrate for atrial fibrillation.

The hypertrophied muscle has a higher oxygen demand and a compromised supply. Microvascular dysfunction, reduced capillary density, compression of small vessels during systole and a mismatch between wall thickness and blood supply all contribute. The result is exertional angina even with normal coronary arteries, and patchy subendocardial injury that feeds fibrosis.^[4] This is why chest pain in HCM is not automatically angina from atheroma, and why coronary angiography may be normal.

The arrhythmia substrate is built from three elements: myocyte disarray, replacement fibrosis and ischaemia. Disarray disrupts normal conduction, fibrosis creates slow zones and barriers, and ischaemia makes membranes unstable. Ventricular premature beats and non-sustained ventricular tachycardia (NSVT) are common, and sustained ventricular tachycardia or fibrillation is the mechanism of sudden death. In the atria, stretch and fibrosis produce atrial fibrillation, which is both a symptom and a risk factor for stroke.

A small proportion of patients lose systolic function and develop a dilated ventricle, usually after years of disease. The ejection fraction falls below 50%, and the physiology shifts from obstruction and diastolic dysfunction towards heart failure with reduced ejection fraction. This is often called the dilated or end-stage phase and carries a worse prognosis.^[1]

Gross. The heart is heavy, with a thickened septum and a small left ventricular cavity. The hypertrophy may be focal, asymmetric or concentric, and the outflow tract can be narrowed by a bulging septal bulge. In apical disease the apex is thickened and the cavity is shaped like a spade.

Microscopic. The hallmark is myocyte disarray: bundles of muscle fibres run in different directions instead of in parallel, so that the tissue looks disorganised even at low power. There is also myocyte hypertrophy, nuclear enlargement, interstitial and replacement fibrosis, and abnormal intramural coronary arteries with thickened walls. These changes are not specific to HCM, but their combination in a young patient with unexplained hypertrophy is characteristic.^[4]

Fibrosis can be quantified on cardiac MRI as late gadolinium enhancement (LGE). It is present in up to 65% of patients and is often patchy and mid-wall, favouring the insertion points of the right ventricle and the hypertrophied segments.^[4] Extensive fibrosis is associated with a higher risk of ventricular arrhythmia and adverse outcome.

Many people with HCM have no symptoms. When symptoms occur, they usually reflect obstruction, diastolic dysfunction or arrhythmia.^[3]

• Exertional breathlessness is a common and often dominant symptom. It comes from high left atrial pressure and, when obstruction is present, from a failure of cardiac output to rise with exercise.
• Chest pain is often exertional and can mimic angina. It may be caused by ischaemia, obstruction or diastolic dysfunction.
• Syncope or presyncope, especially during or immediately after exertion, is worrying. It may reflect a fall in output from obstruction, an arrhythmia or abnormal vascular responses.
• Palpitations suggest atrial fibrillation, ventricular ectopy or sustained arrhythmia.
• Fatigue and exercise intolerance are easily dismissed but often the most limiting symptom.

Symptoms can vary from day to day. Breathlessness often worsens after a large meal, alcohol, dehydration or heat, because these change preload and afterload.^[3][12]

HCM can cause sudden cardiac death in young people, including competitive athletes.^[12] The typical victim may have had no symptoms, or only vague warning signs such as exertional dizziness. This is why pre-participation screening, a careful family history and a low threshold for echocardiography matter. It is also why an athlete with a thick ventricle needs a structured assessment rather than an automatic diagnosis of "athlete's heart".

The examination is normal in many patients, especially those without obstruction. When signs are present, they are best elicited in a quiet room with the patient supine, then standing.

Pulse and impulse. The carotid pulse may be brisk and then fall in mid-systole before rising again, giving a jerky or bisferiens quality.^[4] The apical impulse is sustained and displaced outwards. A palpable fourth heart sound (S4) can add a second component, producing a double apical impulse. An S4 is common and reflects a vigorous atrial contraction against a stiff ventricle.^[4]

Murmur. The classic murmur is a harsh, mid-to-late systolic crescendo–decrescendo murmur, loudest at the left lower sternal border or apex, and it does not radiate to the carotids.^[12] When SAM causes mitral regurgitation, a separate holosystolic murmur may be heard at the apex and radiate to the axilla.^[3] The murmur of HCM is dynamic: its intensity changes with manoeuvres that alter ventricular volume, afterload or contractility.

A thick left ventricle has many causes. The history, the ECG, the pattern of hypertrophy and the presence of extracardiac features usually separate them, but occasionally a cardiac MRI, genetic test or biopsy is needed.^[12]

Two practical points follow. First, hypertension and HCM commonly coexist, and hypertension alone does not exclude HCM; the pattern of hypertrophy and the presence of SAM or a sarcomere variant decide.^[12] Second, a patient with "hypertrophy and heart failure" who has low voltage on the ECG should be investigated for amyloidosis before being labelled as HCM, because the treatment is entirely different.

The ECG is abnormal in up to 95% of patients, but there is no single diagnostic pattern.^[4] Common findings are:

• left ventricular hypertrophy with strain;
• deep, narrow Q waves in the inferior or lateral leads, reflecting septal hypertrophy;^[4]
• T-wave inversion, which may be giant and symmetrical in apical HCM;
• left atrial enlargement;
• pre-excitation, which suggests a glycogen storage disease such as PRKAG2 or Fabry disease.^[4]

A normal ECG is reassuring but does not exclude the diagnosis, especially in young carriers and in apical disease.

Figure 1Echocardiography in hypertrophic cardiomyopathy

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Long description

Two transthoracic echocardiogram panels from a patient with hypertrophic cardiomyopathy. Panel A is a two-dimensional apical four-chamber view in which the septum is clearly thicker than the lateral wall and narrows the left ventricular cavity, marked by a yellow arrow. Panel B is the same view with colour Doppler, showing an anteriorly directed mitral regurgitation jet marked by a yellow arrow.

Transthoracic echocardiography (TTE) is the main imaging test and the backbone of diagnosis and follow-up.^[1] It answers five questions.

1. Is the ventricle thick, and where? Measure the maximum end-diastolic wall thickness in all segments. A thickness of at least 15 mm, or at least 13 mm in a relative of an affected patient, supports the diagnosis.^[1]
2. Is there obstruction? Measure the peak LVOT gradient at rest and with provocation (Valsalva, standing, exercise). A resting gradient of at least 30 mmHg defines obstruction; a gradient of at least 50 mmHg is significant enough to consider septal reduction.^[12]
3. Is there SAM and mitral regurgitation? SAM is visible as the anterior leaflet moving towards the septum in systole.^[4]
4. Is the ventricle stiff? Assess diastolic function, left atrial size and filling pressures.
5. Is there an apical aneurysm? Look especially in mid-ventricular and apical phenotypes; an aneurysm with scar is a risk marker for sudden cardiac death.^[1]

Provocation is essential. A resting study alone will miss latent obstruction in a substantial proportion of patients, and exercise echocardiography is used when symptoms are exertional and the resting or provoked gradient does not explain them.

Cardiac MRI (CMR) is useful at initial evaluation in many patients, and it is particularly useful when the echocardiogram is inconclusive, when apical or focal hypertrophy is suspected, or when another diagnosis is possible.^[1] It measures wall thickness accurately, shows the distribution of hypertrophy, identifies apical aneurysms, and characterises tissue.

Late gadolinium enhancement (LGE) marks replacement fibrosis. It is present in up to 65% of patients and is often patchy and mid-wall.^[4] Extensive LGE, usually defined as 15% or more of left ventricular mass, is associated with a higher risk of sudden cardiac death and is used as a risk modifier; some reviews quantify this increase as about twofold, while others describe a continuous association with no fixed multiplier.^[4][12] CMR also helps to distinguish HCM from athlete's heart, amyloidosis and Fabry disease.^[12]

Genetic testing is recommended when it can confirm the diagnosis, inform prognosis or reproductive decisions, or enable cascade testing.^[1] A pathogenic or likely pathogenic variant in a sarcomere gene supports the diagnosis; a variant of uncertain significance should not be used to label a patient or to justify invasive treatment. Genetic testing does not replace clinical screening of relatives, because no test detects every cause.

• Exercise testing with ECG and blood pressure monitoring: assess functional capacity, provocable obstruction, exercise-induced arrhythmia and the blood pressure response, all of which inform the overall assessment.
• Ambulatory ECG monitoring: detect NSVT and paroxysmal atrial fibrillation. The 2024 AHA/ACC guideline gives more weight to longer, faster and more frequent NSVT episodes.^[1]
• Blood tests: natriuretic peptides and troponin are often mildly raised and help with prognosis, but they do not diagnose HCM.
• Coronary assessment: needed before myectomy and when chest pain could be coronary disease.
• Cardiopulmonary exercise testing: useful for assessing functional limitation and for transplant assessment.

The diagnosis rests on demonstrating unexplained left ventricular hypertrophy. In an adult, a maximal wall thickness of at least 15 mm is diagnostic; 13 to 14 mm is borderline and needs supporting features such as a family history, a pathogenic variant or an abnormal ECG.^[1] In a first-degree relative of a patient with definite HCM, a thickness of at least 13 mm is enough.^[1] In children, the threshold is a wall thickness more than two standard deviations above the predicted value for body size.^[6]

Risk stratification is central to management, because an ICD can prevent death but carries a lifelong burden of complications. Two approaches are in use, and they do not always agree.

The ESC HCM Risk-SCD model. This calculator estimates the five-year risk of sudden cardiac death for patients aged 16 years or older. It uses seven variables: age at evaluation, maximum left ventricular wall thickness, left atrial diameter, maximum LVOT gradient, family history of SCD, non-sustained ventricular tachycardia, and unexplained syncope.^[11] The result is a percentage:

• 6% or more: an ICD should be considered.
• 4% to less than 6%: an ICD may be considered.
• Less than 4%: the patient is in the low-risk category; extensive LGE or a low ejection fraction can still enter shared decision-making.^[1]

The AHA/ACC approach. The 2024 AHA/ACC guideline uses a set of major risk factors. An ICD is reasonable when at least one is present:

• sudden death in a first-degree or close relative at age 50 years or younger;
• maximum left ventricular wall thickness of 30 mm or more in any segment;
• one or more recent episodes of syncope that are thought to be arrhythmic;
• a left ventricular apical aneurysm with transmural scar or LGE;
• left ventricular systolic dysfunction with an ejection fraction below 50%.^[1]

Non-sustained VT and extensive LGE are treated as risk modifiers rather than major factors, and the guideline emphasises shared decision-making.^[1]

An ICD is recommended for survivors of cardiac arrest caused by ventricular fibrillation or ventricular tachycardia, and for patients with spontaneous sustained ventricular tachycardia and haemodynamic compromise.^[1] For primary prevention, the decision follows the risk assessment above. An ICD is not a treatment for symptoms, and it should not be implanted solely to allow competitive sport.^[1]

Regular low-to-moderate intensity exercise is recommended for all able patients, because it improves fitness, symptoms and quality of life.^[1] The old advice to avoid all exercise was too broad.

Competitive sport is a more individual decision. The 2024 AHA/ACC guideline states that universal restriction from vigorous activity or competitive sports is not indicated for most patients with HCM.^[1] Athletes with a mild phenotype, no resting or provocable obstruction and no exercise-induced complex arrhythmia may be considered for competitive sport after specialist assessment, whereas those with high-risk features, significant obstruction or exercise-induced arrhythmia should not compete.^[1] Shared decision-making, with a cardiologist experienced in sports cardiology, is essential.

Other lifestyle points:

• Avoid dehydration, heavy alcohol binge drinking and sauna use if they provoke symptoms.
• Treat hypertension, sleep apnoea and obesity, which add to the load on a stiff ventricle.
• Discuss driving and insurance according to local rules.
• Screen and treat atrial fibrillation, because stroke risk is high.

Drug treatment is aimed at symptoms and obstruction, not at curing the disease. The table summarises the main options.

Beta-blockers remain the first-line drug because they slow the heart rate, lengthen diastole and reduce contractility, all of which lessen obstruction and improve filling.^[3] Non-vasodilating beta-blockers are preferred, because vasodilating agents can worsen the gradient.^[6] Verapamil and diltiazem are useful alternatives, but they are negative inotropes and vasodilators, so they must be used carefully in severe obstruction.

Disopyramide is a class IA antiarrhythmic with a negative inotropic effect. It reduces the resting gradient and improves symptoms in some patients, but the evidence comes from small, largely retrospective studies, and it should be started in hospital with QTc monitoring.^[10]

Mavacamten and aficamten are a new class of drugs that partially inhibit cardiac myosin, reducing the number of force-generating cross-bridges and thereby relaxing the sarcomere. In obstructive HCM they lower the LVOT gradient, improve symptoms and functional capacity, and can reduce the need for septal reduction.^[7][6]

They are not harmless. Both reduce systolic function and can cause heart failure, so ejection fraction must be measured before treatment and monitored during titration.^[8][9] Mavacamten is metabolised through CYP2C19, with a terminal half-life of 6 to 9 days in normal metabolisers and 23 days in poor metabolisers, and is available only through a restricted programme; strong CYP2C19 inhibitors are contraindicated.^[8][7] Aficamten has a shorter, more predictable half-life and is metabolised through several pathways, but it still requires echocardiographic monitoring.^[6]

In obstructive HCM, anything that reduces preload or afterload, or increases contractility, can worsen the gradient.

• Vasodilators (including vasodilating beta-blockers, ACE inhibitors, angiotensin receptor blockers and dihydropyridine calcium-channel blockers such as amlodipine) can worsen obstruction and should be avoided when possible.^[12]
• High-dose diuretics can reduce preload excessively and worsen obstruction; low-dose loop diuretics may be used cautiously when there is congestion.^[6]
• Nitrates should be used only when obstruction has been excluded, because they can precipitate syncope in obstructive disease.^[6]
• Digoxin has positive inotropic effects and is best avoided in obstructive HCM; it may be used cautiously for rate control in non-obstructive disease.^[12]

When symptoms persist despite optimal drug treatment and the gradient is high, septal reduction can relieve obstruction. The usual criteria are a resting or maximally provoked LVOT gradient of at least 50 mmHg with NYHA class III–IV symptoms despite maximal tolerated therapy, or exertional syncope with a high gradient.^[1]

• Surgical myectomy removes a wedge of the hypertrophied septum through the aortic valve, often with mitral valve repair if there is SAM-related regurgitation. It is the preferred operation when there is another surgical lesion, such as mitral valve disease, and at experienced centres it relieves obstruction in most patients.^[1] Operative mortality is below 1% at experienced centres, and higher when myectomy is combined with mitral valve repair.^[4][3]
• Alcohol septal ablation injects ethanol into a septal perforator artery, causing a controlled myocardial infarction in the hypertrophied septum. It is an alternative when surgery is contraindicated or the risk is unacceptable, and it requires suitable septal anatomy.^[1]

Both procedures require a multidisciplinary team, a clear gradient, and a patient who understands the trade-offs. Myectomy is generally preferred in younger patients and in those with complex mitral disease; ablation is less invasive but creates a scar and has a higher chance of heart block requiring a pacemaker.

Atrial fibrillation is common in HCM, affecting about one in five patients.^[12] It causes symptoms, worsens heart failure and substantially increases the risk of stroke.

• Anticoagulation is recommended for all patients with HCM and clinical atrial fibrillation, regardless of the CHA2DS2-VASc score. Direct-acting oral anticoagulants are first-line, with vitamin K antagonists as the second-line option.^[1]
• Rate control with a beta-blocker or a non-dihydropyridine calcium-channel blocker is preferred. Digoxin is an alternative only in non-obstructive disease.^[12]
• Rhythm control with cardioversion, antiarrhythmic drugs or catheter ablation can be considered for symptoms, but the decision is individual and should involve a cardiologist.
• Subclinical atrial fibrillation detected by a device or monitor also warrants consideration of anticoagulation.^[1]

Pregnancy is usually tolerated well, but it needs planning.

• Pre-conception counselling should cover inheritance, medication review and the risk of worsening symptoms.^[1]
• Beta-blockers can be continued for symptoms or arrhythmia, with monitoring of fetal growth.^[1]
• Mavacamten is contraindicated in pregnancy because it may cause fetal harm; it should be stopped before conception.^[1][8]
• Anticoagulation for atrial fibrillation should use low-molecular-weight heparin or vitamin K antagonists, depending on the stage of pregnancy.^[1]
• Vaginal delivery is recommended for most women unless there is an obstetric indication, severe heart failure or severe outflow obstruction.^[1]

A pregnancy heart team, including cardiology, obstetrics and anaesthesia, should manage women with significant obstruction or previous arrhythmia.

First-degree relatives should be offered clinical screening with ECG and echocardiography, and cascade genetic testing if a pathogenic variant is known.^[1] Screening usually begins in childhood or adolescence in families with early disease, and continues into adult life. The interval depends on age and findings; many centres repeat echocardiography every few years in genotype-positive, phenotype-negative relatives.

Patients with HCM need lifelong follow-up. The frequency depends on symptoms, obstruction, arrhythmia and risk; regular review with ECG and echocardiography is usual, with more frequent visits when titrating myosin inhibitors or after septal reduction.

• Sudden cardiac death from ventricular arrhythmia, especially in young people and athletes.^[12]
• Heart failure from diastolic dysfunction, obstruction or end-stage systolic dysfunction.
• Atrial fibrillation and stroke, with a high thromboembolic risk.^[12]
• Infective endocarditis, which can occur on a damaged mitral valve, especially after valve surgery; assess and treat according to local endocarditis guidance.
• Apical aneurysm and its complications, including thrombus and arrhythmia.^[1]
• Complications of treatment: heart block after septal ablation, ventricular septal defect or aortic regurgitation after myectomy, and heart failure from myosin inhibitors.^[4]

Most people with HCM have a near-normal life expectancy.^[4] Contemporary cohorts report annual mortality of about 0.9% to 1.5%, falling to around 0.5% per year with modern treatment.^[4] Sudden cardiac death is a feared complication, but its absolute risk in an individual is often low and can be estimated with the risk tools above.

Poor prognostic features include:

• young age at diagnosis and a family history of early sudden death;
• massive hypertrophy (30 mm or more);^[1]
• extensive LGE on MRI;
• left ventricular systolic dysfunction;
• apical aneurysm;
• atrial fibrillation and progressive heart failure.

There is no way to prevent the genetic variant, but the consequences can be reduced.

• Family screening finds affected relatives before symptoms or sudden death.^[1]
• Pre-participation screening of athletes, with history, examination and ECG where the health system supports it, can identify HCM and other causes of sudden death.
• Blood pressure control prevents hypertensive remodelling from being mistaken for, or adding to, HCM.
• Vaccination and general cardiovascular prevention apply as for the general population.

Children and adolescents. HCM in childhood is often genetic, and a RASopathy or metabolic disease should be considered. Growth affects the normal range of wall thickness, so paediatric thresholds use body-surface-area-adjusted values. Competitive sport and school activities need individual advice. Genetic testing of parents is important because a child's diagnosis may reveal an affected parent.

Older adults. Hypertension and aortic stenosis become common, and the combination can mask or mimic HCM. Amyloidosis should be considered when hypertrophy is accompanied by low voltage, carpal tunnel syndrome or heart failure with preserved ejection fraction. Treatment should account for frailty, polypharmacy and the higher risk of drug-induced hypotension.

Pregnancy. See above.

Athletes. See the lifestyle section. The key is not to label a physiological adaptation as disease, and not to clear a patient with definite HCM and high-risk features for competitive sport.^[1]

HCM can be diagnosed and managed with relatively little technology, but access to specialist care determines outcomes.

• Echocardiography is the key test. A transthoracic study can measure wall thickness, detect SAM and estimate the gradient. Where echo is unavailable, a loud murmur that changes with Valsalva, a family history of sudden death and an abnormal ECG should prompt referral.
• ECG is cheap and widely available. It is abnormal in most patients, but a normal ECG does not exclude the disease.
• Beta-blockers are widely available and effective for symptoms and obstruction.^[12] Verapamil is an alternative where beta-blockers are unavailable or contraindicated.
• Genetic testing and cardiac MRI are often unavailable. Do not delay diagnosis or treatment waiting for them; use family history and echo, and refer when possible.
• Septal reduction and ICDs require specialist centres. Refer patients with refractory symptoms, high gradients, syncope or high-risk features. Until then, optimise medical therapy and counsel about exercise and family screening.
• Anticoagulation for atrial fibrillation is essential and can be done with vitamin K antagonists where direct-acting agents are unaffordable.

Read next: cardiac physical examination, ECG interpretation, atrial fibrillation and flutter, aortic stenosis, cardiac amyloidosis, Fabry disease, and genetic counselling. These are covered elsewhere in the library; the principles above are the ones to carry into them.