Hypertrophic Cardiomyopathy

Intensive endurance and strength training causes the heart to adapt, and this 'athlete's heart' can include wall thickening that overlaps with mild HCM, a genuine grey zone. Features favouring physiological adaptation include modest, symmetrical thickening, a normal or enlarged left ventricular cavity, normal relaxation and filling, and regression with detraining. Features favouring HCM include marked or asymmetrical thickening, a small cavity, impaired relaxation, a markedly abnormal electrocardiogram, a family history of HCM or sudden death, and symptoms such as exertional chest pain or blackout.

As a practical guide in adults, HCM is generally diagnosed when unexplained maximal wall thickness is at least 15 millimetres, with 13 to 14 millimetres a grey zone that becomes more concerning alongside a family history, a pathogenic variant, an abnormal electrocardiogram, impaired diastolic function, a small cavity, or abnormalities on cardiac magnetic resonance imaging; in children, body-size adjusted measurements and specialist assessment are used. Detraining can support the distinction but should not be the sole test or demanded without specialist input, and modern assessment combines several investigations. Where doubt remains, refer to a specialist in sports cardiology and inherited cardiac conditions rather than clearing or disqualifying on incomplete information.

Many people with HCM have no symptoms and are found through screening or family evaluation. When symptoms occur they include exertional breathlessness, chest tightness, palpitations, light-headedness and, most concerning, blackout during or just after exercise. Some present for the first time with a cardiac arrest, which is why these warning symptoms must be taken seriously in anyone who exercises.

Blackout during exertion is the strongest red flag; collapse just after stopping may be vasovagal but still needs assessment if it is unexplained, recurrent, without a typical warning, or accompanied by palpitations, chest pain, an abnormal electrocardiogram or a family history of sudden death. Examination may be normal, or reveal a murmur that becomes louder on standing or straining. In anyone over 16 with a blackout, give driving advice at the first visit: they should not drive while awaiting specialist assessment, following the Driver and Vehicle Licensing Agency rules.

The electrocardiogram (ECG) is the most useful first-line test and is abnormal in most people with HCM, often before hypertrophy is obvious. Isolated voltage criteria for LVH can be physiological in trained athletes, but lateral or inferolateral T-wave inversion, ST-segment depression, pathological Q waves, ventricular pre-excitation, significant conduction disease or ventricular arrhythmias should prompt specialist assessment. A normal electrocardiogram lowers suspicion but does not clear an athlete who has exertional syncope, exertional chest pain, a concerning family history or a suspicious murmur.

Echocardiography (echo) is the key imaging test, measuring wall thickness, outflow obstruction and diastolic function, and cardiac magnetic resonance imaging (cardiac MRI) adds precise measurement and shows fibrosis for diagnosis and risk assessment. History, ambulatory rhythm monitoring and genetic testing all contribute in specialist hands, and genetic testing enables cascade screening of relatives. Exercise testing should be specialist-led, and when HCM or significant obstruction is suspected, imaging is usually needed first to decide whether and how to test safely.

Management of HCM is specialist and individualised, and the SEM clinician's role is to refer and support rather than to direct it. Symptomatic obstruction is managed by specialists with medicines that slow the heart and improve filling, such as beta-blockers, non-dihydropyridine calcium-channel blockers and sometimes disopyramide; in eligible adults with symptomatic obstructive disease, a cardiac myosin inhibitor (mavacamten) may be added to optimised standard care under current national guidance. Sudden death risk is assessed formally, and an implantable cardioverter defibrillator (ICD) is offered to those at high risk, with septal reduction reserved for selected severe symptoms. Dehydration and significant volume loss should be avoided, as they worsen obstruction.

Decisions about exercise and competition have moved from blanket disqualification towards individualised, shared decision-making between the athlete and a specialist team. Features that usually push advice away from high-intensity or competitive sport include cardiac symptoms, a previous cardiac arrest, unexplained syncope, exercise-induced ventricular arrhythmias, an abnormal blood pressure response to exercise, significant outflow obstruction, and a higher formal sudden death risk estimate; the SEM doctor should recognise these as reasons not to clear an athlete. An ICD reduces arrhythmic death risk in selected patients but is not automatic clearance for high-intensity or collision sport. Athletes who carry a pathogenic variant but have no hypertrophy should still be assessed and followed through inherited cardiac conditions services, where participation may be possible in selected cases with ongoing surveillance. Low-intensity activity is generally encouraged, and emergency planning, including defibrillator access, should be in place wherever the athlete trains.

• •European Society of Cardiology guidance on the cardiomyopathies and on sports cardiology informs UK diagnosis, risk assessment and exercise recommendations.
• •National Institute for Health and Care Excellence (NICE) guidance on transient loss of consciousness supports the assessment of exertional blackout, and national guidance covers the use of a cardiac myosin inhibitor in symptomatic obstructive HCM (TA913).
• •Athlete-specific electrocardiogram interpretation criteria reduce false positives while preserving detection of conditions such as HCM.
• •Family and genetic evaluation is delivered through inherited cardiac conditions services and NHS genomic medicine pathways; testing is most useful when it enables cascade testing, and if no variant is found relatives may still need clinical screening.

• •HCM is usually inherited and autosomal dominant, and is a major, classically tested cause of sudden cardiac death in young athletes.
• •The hallmark is unexplained left ventricular hypertrophy, often asymmetrical and septal, with a stiff, poorly filling ventricle.
• •Athlete's heart versus HCM: a wall thickness of at least 15 millimetres suggests HCM, 13 to 14 millimetres is a grey zone, and regression with detraining favours adaptation.
• •The electrocardiogram is usually abnormal in HCM and is read against athlete-specific criteria; a normal tracing does not clear a symptomatic athlete.
• •Exertional blackout, exertional chest pain and a family history of sudden death under 40 are red flags needing urgent assessment.
• •Exercise and competition decisions are individualised and specialist-led, guided by risk markers rather than automatic disqualification.