Mutations in genes responsible for storage diseases also cause a phenotype resembling HCM (genocopy or phenocopy). In approximately 40% of HCM patients the causal genes remain to be identified. MYH7 and MYBPC3, encoding β-myosin heavy chain and myosin binding protein C, respectively, are the two most common genes involved, together accounting for about 50% of the HCM families. Mutations in over a dozen genes encoding sarcomere-associated proteins cause HCM. Atrial fibrillation is also a common complication and is not well tolerated. This complication can usually be averted by implantation of a cardioverter-defibrillator in appropriate high-risk patients. Nonsustained ventricular tachycardia, syncope, a family history of sudden cardiac death, and severe cardiac hypertrophy are major risk factors for sudden cardiac death. However, HCM is also an important cause of sudden cardiac death, particularly in adolescents and young adults. In the majority of patients, HCM has a relatively benign course. The hypertrophy is also frequently associated with left ventricular diastolic dysfunction. The histologic features of HCM include myocyte hypertrophy and disarray, as well as interstitial fibrosis. Left ventricular outflow tract obstruction is present at rest in about one third of the patients, and can be provoked in another third. It is commonly asymmetric with the most severe hypertrophy involving the basal interventricular septum. Hypertrophic cardiomyopathy (HCM) is a genetic disorder that is characterized by left ventricular hypertrophy unexplained by secondary causes, and a non-dilated left ventricle with preserved or increased ejection fraction.
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