Hypertrophic cardiomyopathy (HCM) is a hereditary primary myocardial disease that is most commonly caused by mutations within genes encoding sarcomeric contractile proteins and is characterised by left ventricular hypertrophy in the absence of a cardiac or systemic cause.1,2 The condition is inherited as an autosomal dominant trait and has a prevalence of one in 500.3,4 Marked genetic heterogeneity, diverse clinical phenotypes and a highly variable natural history are well recognised.4–6 Although the overall prognosis is relatively good with an annual mortality rate <1 %, the propensity to potentially fatal ventricular arrhythmias is the most feared complication particularly because the peak incidence of sudden cardiac death (SCD) is during adolescence and early adulthood.6–9 The association with SCD is most frequently highlighted when a young, previously asymptomatic athlete falls victim and HCM is considered the commonest cause of SCD in young athletes worldwide.10–12
The arrhythmogenic substrate comprises left ventricular hypertrophy, myocyte disarray and interstitial fibrosis.13–17 Triggers for arrhythmias may include myocardial ischaemia, excessive sympathetic stimulation, left ventricular outflow tract obstruction (LVOTO) and paroxysmal atrial fibrillation (AF).18–20 The identification of patients at risk of arrhythmogenic SCD is an essential component in disease management given that the implantable cardioverter defibrillator (ICD) is the most effective therapy in preventing SCD.21–25 However, the low risk of adverse events in most patients coupled with the complex and unpredictable relationship between the arrhythmic substrate and triggers for arrhythmias means that risk stratification for arrhythmogenic SCD is a challenging aspect of the disease. A low threshold to implant an ICD into most patients with HCM is not cost-effective and is hampered by the high prevalence of inappropriate shocks and other complications relating to the implantation of the ICD.23–26
Aborted SCD and malignant ventricular arrhythmias are the most powerful risk factors for SCD.27–30 Patients who survive an episode of ventricular tachycardia (VT) or ventricular fibrillation (VF) remain at high risk of recurrent arrhythmogenic events, having an estimated risk of 10.6 % per annum and both the American College of Cardiology Foundation/ American Heart Association (ACCF/AHA) and American College of Cardiology/European Society of Cardiology (ACC/ESC) management guidelines for HCM recommend ICD implantation in such patients.27–30