Hypertrophic Cardiomyopathy
The thickened heart — the most common cause of sudden cardiac death in young athletes, and how to manage it safely.
What Is It
Understanding Hypertrophic Cardiomyopathy
Hypertrophic cardiomyopathy (HCM) is a genetic disease of the heart muscle in which the walls of the left ventricle become abnormally thickened (hypertrophied) — not due to high blood pressure or any other secondary cause, but as a primary disorder of the heart muscle itself.
The thickened muscle is also disorganised at the microscopic level (myocyte disarray) and develops fibrosis (scarring). This combination — thick walls, disorganised fibres, and scarring — creates an electrically unstable heart that can trigger life-threatening ventricular arrhythmias, most dangerously ventricular fibrillation, particularly during intense physical exertion.
The Genetics of HCM
HCM is an autosomal dominant condition — a child of an affected parent has a 50% chance of inheriting the mutation. It is caused by mutations in genes encoding proteins of the cardiac sarcomere (the contractile unit of the heart muscle). The most commonly affected genes are MYH7 (beta-myosin heavy chain) and MYBPC3 (myosin-binding protein C), together accounting for over 50% of genetically identified cases.
A negative genetic test does not exclude HCM — genetic testing identifies a causative mutation in only 50–60% of clinically diagnosed patients. All first-degree relatives of a confirmed HCM patient should be screened with echocardiography regardless of genetic test results.
Obstructive vs. Non-Obstructive HCM
Obstructive HCM (HOCM) — ~70%
The thickened septum (interventricular wall) bulges into the left ventricular outflow tract (LVOT), obstructing blood flow to the aorta. A gradient (>30 mmHg at rest or >50 mmHg with provocation) is present. Causes a characteristic murmur that increases with Valsalva and decreases with squatting. Often more symptomatic.
Non-Obstructive HCM — ~30%
Significant hypertrophy is present but without outflow tract obstruction. May still carry a high risk of sudden death despite the absence of obstruction. The apical variant (Yamaguchi syndrome) — where thickening predominantly involves the apex — is more common in Asian populations.
Symptoms
Many children and adolescents with HCM are entirely asymptomatic — the condition is discovered only on family screening or incidentally during a pre-participation sports examination. When symptoms do occur, they include:
| Symptom | Mechanism | Significance |
|---|---|---|
| Breathlessness on exertion | Diastolic dysfunction — stiff heart filling poorly | Common; often the first symptom |
| Chest pain / tightness | Microvascular ischaemia; LVOT obstruction | Common; usually exertional |
| Palpitations | Atrial fibrillation; ventricular ectopics | Requires Holter evaluation |
| Syncope (fainting) | Obstruction, arrhythmia, or vasovagal | Red flag — always needs urgent cardiology review |
| Sudden cardiac arrest | Ventricular fibrillation, often during exercise | May be the first presentation in undiagnosed cases |
Diagnosis
Echocardiography is the primary diagnostic tool, demonstrating left ventricular wall thickness ≥15 mm in adults (or ≥2 standard deviations above the mean for body size in children), systolic anterior motion (SAM) of the mitral valve in obstructive cases, and the outflow gradient.
Cardiac MRI is invaluable — it accurately quantifies the distribution and extent of hypertrophy, identifies late gadolinium enhancement (fibrosis/scarring), and helps risk-stratify patients. Extensive fibrosis on MRI is an independent risk factor for sudden death.
Holter monitoring detects non-sustained ventricular tachycardia (NSVT), atrial fibrillation, and other arrhythmias — key components of the sudden death risk score.
Exercise stress test assesses the haemodynamic response to exercise and provokes obstruction in borderline cases. An abnormal blood pressure response during exercise is an additional risk factor.
Genetic testing identifies the causative mutation in 50–60% and enables cascade family screening.
Sudden Death Risk Stratification
Not all HCM carries the same risk. Risk stratification is essential to identify patients who need an ICD. The HCM Risk-SCD calculator (ESC guidelines) incorporates: maximum wall thickness, family history of SCD, unexplained syncope, NSVT on Holter, abnormal blood pressure response to exercise, and left atrial diameter. A 5-year SCD risk >6% is the threshold above which an ICD is generally recommended.
Treatment
Activity restriction: Competitive high-intensity sport is generally contraindicated in HCM due to the risk of triggering ventricular fibrillation during exertion. This is one of the hardest conversations in paediatric cardiology — it must be individualised, empathetic, and evidence-based. Recreational low-to-moderate intensity activity is usually permissible after thorough risk assessment.
Beta-blockers and calcium channel blockers (verapamil) reduce the outflow gradient, improve diastolic filling, and alleviate symptoms in obstructive HCM. They do not reduce sudden death risk.
Mavacamten — a first-in-class cardiac myosin inhibitor — was approved for symptomatic obstructive HCM in adults (2022) and is transforming the management of HOCM. It reduces the outflow gradient by directly inhibiting excess myosin cross-bridge formation. Paediatric data is still emerging.
Septal reduction therapy for refractory symptomatic HOCM with a resting or provocable gradient >50 mmHg: Surgical myectomy (Morrow procedure — surgical removal of a portion of the hypertrophied septum) is the gold standard with excellent long-term outcomes. Alcohol septal ablation (injecting alcohol into a septal artery to create a controlled heart attack and thin the septum) is an alternative in adults but is generally avoided in children.
Implantable Cardioverter-Defibrillator (ICD): Recommended for secondary prevention (after survived cardiac arrest) and for primary prevention in high-risk patients based on the risk calculator. The ICD does not treat the underlying condition but terminates ventricular fibrillation before it is fatal.
Heart transplantation is reserved for end-stage HCM with severe heart failure refractory to all other treatments — a minority of patients.
Not necessarily. Left ventricular hypertrophy (LVH) has many causes. In children, the main differential diagnoses include: hypertensive heart disease, athlete’s heart (physiological adaptation to training), storage disorders (Pompe disease, Fabry disease, Danon disease), and syndromic conditions (Noonan syndrome with HCM). The distinction matters enormously because treatment differs. A specialist cardiologist will evaluate the pattern and distribution of hypertrophy, genetics, associated features, and clinical history to reach the correct diagnosis.
This is one of the most emotionally charged aspects of HCM care, and I understand how devastating this restriction feels for a young athlete. The evidence is clear that intense competitive exercise significantly increases the risk of sudden cardiac death in HCM — physical exertion is the trigger in the majority of exercise-related SCD events. The restriction is not arbitrary; it is based on the biological reality of what happens to an unstable heart under extreme stress. This decision should be made jointly with the family after thorough discussion of the evidence, the individual risk profile, and the available alternatives.
Yes — absolutely and urgently. HCM is autosomal dominant, meaning each first-degree relative has a 50% chance of inheriting the mutation. All first-degree relatives (parents, siblings, children of the affected individual) should have an echocardiogram. If a genetic mutation has been identified, targeted genetic testing in relatives is efficient and definitive. Family screening is one of the most important things that comes out of a new HCM diagnosis.
With modern management, the annual mortality from HCM in specialist centres is approximately 1% per year — significantly lower than historical figures. The majority of patients live full, productive lives. The key to good outcomes is: accurate risk stratification, appropriate ICD implantation where indicated, management of symptoms, avoidance of high-risk activities, and regular specialist follow-up. HCM is a condition that requires lifelong cardiology care, but it is entirely compatible with a long and meaningful life.
Mavacamten (brand name Camzyos) is a first-in-class oral medication that works by reducing the number of active myosin-actin cross-bridges in the heart muscle, directly addressing the underlying cause of obstruction in HOCM. It significantly reduces the outflow gradient and improves symptoms. It was approved for adults with symptomatic obstructive HCM in 2022 and is changing how we treat this condition. Paediatric clinical trials are underway. It is not yet standard therapy in children, but selected adolescents may be considered on a case-by-case basis at specialist centres.
Key Takeaways
- HCM is the most common cause of sudden cardiac death in young athletes — present in 1:500 of the general population.
- It is a genetic (autosomal dominant) disease of the cardiac sarcomere — all first-degree relatives must be screened.
- Up to 70% have outflow obstruction (HOCM); the classic murmur increases with Valsalva and decreases with squatting.
- Risk stratification using the ESC HCM Risk-SCD calculator guides ICD recommendations.
- Competitive high-intensity sport is generally contraindicated — a difficult but life-saving restriction.
- Mavacamten is a new drug directly targeting the mechanism of obstruction, transforming adult HOCM care; paediatric data is emerging.
- Annual mortality in specialist centres is ~1% — most patients live full, normal lives with appropriate management.
Sources
Ommen SR et al. 2020 AHA/ACC Guideline for the Diagnosis and Treatment of Patients with Hypertrophic Cardiomyopathy. JACC 2020;76(25):e159–e240.
Elliott PM et al. 2014 ESC Guidelines on Diagnosis and Management of Hypertrophic Cardiomyopathy. Eur Heart J 2014;35(39):2733–2779.
Allen HD et al. Moss and Adams’ Heart Disease in Infants, Children, and Adolescents, 10th Ed. Wolters Kluwer, 2021.
Educational purposes only. Not a substitute for professional medical advice.
