The natural history of
hypertrophic cardiomyopathy (HCM) is extremely heterogeneous. Many patients remain asymptomatic throughout life, some develop severe symptoms of
heart failure, but others die suddenly, often in the absence of previous symptoms and at a young age. Therefore, identification of those patients at high risk of
sudden death represents a major clinical problem and has become an even greater challenge since the
implantable cardioverter-defibrillator (ICD) has proved to be highly effective in preventing
sudden death in HCM. Patients who have survived a
cardiac arrest, or one or more episodes of sustained
ventricular tachycardia, are considered to be at high risk and are candidates for an ICD. However, this patient subset represents a small proportion of the HCM population. The greatest difficulty concerns the identification of high risk patients who are candidates for primary prevention of
sudden death with a prophylactic ICD. Decisions are based on generally accepted
clinical markers which are associated with increased risk, including: family history of
sudden death, extreme left ventricular (LV) wall thickness ( > or =30 mm),
nonsustained ventricular tachycardia on Holter monitoring, unexplained (non-
neurocardiogenic) syncope particularly in young patients, and hypotensive blood pressure response to exercise. Patients with end-stage HCM or a LV apical
aneurysm represent important arrhythmogenic subsets also associated with substantially increased risk. Multiple or single strong risk markers are associated with increased
sudden death risk and justify consideration for a prophylactic ICD.