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Hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy, or HCM, is a disease of the myocardium (the muscle of the heart) in which a portion of the myocardium is hypertrophied (thickened) without any obvious cause.1 It had frequently been cited as one of the leading causes of sudden cardiac death in young athletes in the United States.2 However, this same work of passive surveillance saw exclusion of 52% of cases because they had structurally normal hearts drawing much skepticism to the previous hype. more...

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A cardiomyopathy is any disease that primarily affects the muscle of the heart. In HCM, the normal alignment of muscle cells is disrupted, a phenomenon known as myocardial disarray. HCM also causes disruptions of the electrical functions of the heart. HCM is believed to be due to a mutation in one of many genes that results in a mutated myosin heavy chain, one of the components of the myocyte (the muscle cell of the heart). Depending on the degree of obstruction of the outflow of blood from the left ventricle of the heart, HCM can be defined as obstructive or non-obstructive.

HCM is also known as idiopathic hypertrophic subaortic stenosis (IHSS) and hypertrophic obstructive cardiomyopathy (HOCM). A non-obstructive variant of HCM is apical hypertrophic cardiomyopathy 3, which is also known as nonobstructive hypertrophic cardiomyopathy and Japanese variant hypertrophic cardiomyopathy (since the first cases described were all in individuals of Japanese descent).

While most literature so far focuses on European, American, and Japanese populations, HCM appears in all racial groups. The incidence of HCM is about 0.2% to 0.5% of the general population.

Genetics

Hypertrophic cardiomyopathy is attributed to mutation in one of a number of genes that encode for one of the sarcomere proteins (usually effecting either the α or β myosin heavy chain on chromosome 14 q11.2-3). While the severity of the disease process is dependent on the particular gene mutation, about 80% of cases are inherited in an autosomal dominant pattern. Other gene mutations that are associated with HCM include mutations in α-tropomyosin (on chromosome 15), troponin T (on chromosome 1), and myosin-binding protein C (on chromosome 11). The prognosis is variable, based on the gene mutation.

The MYH7 gene (encoding the Β-myosin heavy chain) was the first specific gene identified in familial hypertrophic cardiomyopathy. About 50 percent of all familial cases involve mutation in the MYH7 gene. In individuals without a family history of HCM, the most common cause of the disease is also mutations of the gene that produces the β-myosin heavy chain. Many different mutations in this gene have been identified, and the prognosis is dependant on the particular mutation.

An insertion/deletion polymorphism in the gene encoding for angiotensin converting enzyme (ACE) has been associated with some cases of HCM. The D/D (deletion/deletion) genotype of ACE is associated with more marked hypertrophy of the left ventricle and may be associated with higher risk of adverse outcomes.8,9

Anatomic characteristics

Individuals with HCM have some degree of left ventricular hypertrophy. Usually this is an asymmetric hypertrophy, involving the inter-ventricular septum, and is known as asymmetric septal hypertrophy (ASH). This is in contrast to the concentric hypertrophy seen in aortic stenosis or hypertension. About 2/3 of individuals with HCM have asymmetric septal hypertrophy.

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Outcome predictors in hypertrophic cardiomyopathy - Tips from Other Journals
From American Family Physician, 4/15/03 by Bill Zepf

The debate continues over whether the degree of left ventricular outflow obstruction is an important discriminator of cardiac risk in patients with hypertrophic cardiomyopathy. Maron and colleagues conducted a prospective study of outflow obstruction in patients with hypertrophic cardiomyopathy and its association with death or heart failure.

The authors enrolled 1,101 consecutive patients diagnosed with hypertrophic cardiomyopathy at two cardiac referral centers in Italy and one in the United States. Echocardiographic measurement of the peak outflow gradient in the left ventricle was obtained under resting conditions, taking care to avoid any inclusion of the mitral regurgitation jet. Mean duration of follow-up was 6.3 years for risk of sudden death or progression to severe heart failure (New York Heart Association functional class III or IV).

At the time of last follow-up, 12 percent of the patient cohort had died as a result of hypertrophic cardiomyopathy, and 24 percent of the 914 surviving patients (216 patients) had progressed to severe heart failure. A peak outflow gradient of 30 mm Hg was considered the threshold at which the risk for death or heart failure progression increased, especially in patients older than 40 years (see accompanying figure). Outflow gradients higher than 30 mm Hg did not confer additional risk.

The authors concluded that echocardiographic measurement of a left ventricular outflow gradient greater than 30 mm Hg in patients with hypertrophic cardiomyopathy predicted an increased risk of death or severe heart failure, especially in patients older than 40 years.

EDITOR'S NOTE: The diagnosis of hypertrophic cardiomyopathy is relatively straightforward (i.e., hypertrophied left ventricular wall without chamber dilation). However, predicting the long-term outcome is not as clear-cut. The clinical course in the disease varies from incidental findings noted on echocardiography in asymptomatic patients to sudden death at a young age. While some previous studies had suggested that an outflow gradient greater than 50 mm Hg was a relative indication for intervention, other investigations noted that the degree of outflow obstruction did not seem to correlate with adverse outcomes. This larger study confirms a predictive role for quantifying outflow obstruction and sets a lower cutoff value for identifying patients at elevated risk for complications. The high rate of death and progression to severe heart failure over the relatively short time period in this study emphasizes the importance of stratifying risk and planning interventions in patients with hypertrophic cardiomyopathy.--B.Z.

COPYRIGHT 2003 American Academy of Family Physicians
COPYRIGHT 2003 Gale Group

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