BackgroundThere is controversy about risk of malignant arrhythmias and stroke in patients with apical aneurysms in hypertrophic cardiomyopathy (HCM). ObjectivesThe aim of this study was to estimate the associations of aneurysm size and major HCM risk factors with the incidence of lethal and potentially lethal arrhythmias and to estimate incidence of unexplained stroke. MethodsIn 108 patients (age 57.4 ± 13.5 years, 37% female) from 3 HCM centers, we assessed American Heart Association/American College of Cardiology guidelines risk factors and initial aneurysm size by echocardiography and cardiac magnetic resonance imaging and assessed outcomes after median 5.9 (IQR: 3.7-10.0) years. ResultsImplantable cardioverter defibrillator discharges or sudden cardiac death (SCD) occurred in 21 (19.4%) patients. Of patients with a risk factor, 55% subsequently had ventricular tachycardia (VT), ventricular fibrillation (VF), or SCD at follow-up, compared with 10% in those who did not (P < 0.001). The upper tercile of size had a 5-year cumulative risk of 35%, while the lower tercile had 5-year risk of 6% (P = 0.0046). In those with the smallest aneurysms <2 cm2 and also without risk factors VT, VF, or SCD occurred in only 2.5%. Clinical atrial fibrillation (AF) was prevalent, occurring in 49 (45%). Stroke was commonly associated with AF. Stroke without conventional cause had an incidence of 0.5%/year. Surgery in 19% was effective in reducing symptoms. VT ablation and surgery were moderately effective in preventing recurrent VT. ConclusionsRisk factors and aneurysm size were associated with subsequent VT, VF, or SCD. Patients with aneurysms in the lowest tercile of size have a low cumulative 5-year risk. Clinical AF occurred frequently. Stroke prevalence in absence of known stroke etiologies is uncommon and comparable to risk of severe bleeding.