Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is an inherited heart muscle disease in which the left ventricle thickens without another cause such as hypertension or athletic training.

It is a classic cause of sudden cardiac death (SCD) in young athletes, although UK series show that sudden arrhythmic death syndrome, arrhythmogenic cardiomyopathy and coronary anomalies are also important, so it is not the commonest cause in every cohort. Most people with HCM live normal lives, and exercise advice is now risk-stratified rather than based on blanket avoidance.

The sport and exercise medicine (SEM) clinician meets HCM through screening, an athlete with exertional symptoms or an abnormal electrocardiogram, or a family history of sudden death. The task is to recognise it, tell it apart from the athlete's heart and refer.

HCM is most often caused by variants in genes encoding the cardiac sarcomere and is usually autosomal dominant, so first-degree relatives are at risk. Penetrance is incomplete and expression variable, and some people have no identifiable variant or have a phenocopy such as a metabolic or storage disorder. The defining feature is left ventricular hypertrophy (LVH) not explained by loading conditions, most often asymmetrical and septal. The thick, stiff ventricle fills poorly, limiting the rise in cardiac output with exercise.

In some people the septum, with systolic anterior motion of the mitral valve, obstructs the left ventricular outflow tract (LVOT). Obstruction can worsen with exertion, dehydration or a sudden change in posture.

Disorganised muscle architecture and patchy fibrosis create an electrical substrate for ventricular arrhythmias, the mechanism behind sudden death. Thickness and symptoms vary widely, even within one family.