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Hypertrophic cardiomyopathy is often asymmetrical, meaning one part of the heart is thicker than the other parts. The condition is usually passed down through families (inherited). It is believed to be a result of several problems (defects) with the genes that control heart muscle growth.

Younger people are likely to have a more severe form of hypertrophic cardiomyopathy. However, the condition is seen in people of all ages.

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 dependant 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.

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. Symptoms of HCM include shortness of breath, chest pain (sometimes known as angina), uncomfortable awareness of the heart beat (palpitation), light-headedness, dizziness, blackouts and sudden cardiac death. Risk factors for sudden death in individuals with HCM include a young age at first diagnosis (age < 30 years), an episode of aborted sudden death, a family history of HCM with sudden death of relatives, specific mutations in the genes encoding for troponin T and myosin, sustained supraventricular or ventricular tachycardia, recurrent syncope, and bradyarrhythmias (slow rhythms of the heart).

The physical findings of HCM are associated with the dynamic outflow obstruction that is often present with this disease. Upon auscultation, the cardiac murmur will sound similar to the murmur of aortic stenosis. However, this murmur will increase in intensity with any maneuver that decreases the volume of blood in the left ventricle (such as standing or the strain phase of a Valsalva maneuver). Upon cardiac catheterization, catheters can be placed in the left ventricle and the ascending aorta, to measure the pressure difference between these structures. In normal individuals, during ventricular systole, the pressure in the ascending aorta and the left ventricle will equalize, and the aortic valve is open. In individuals with aortic stenosis or with HCM with an outflow tract gradient, there will be a pressure gradient (difference) between the left ventricle and the aorta, with the left ventricular pressure higher than the aortic pressure. This gradient represents the degree of obstruction that has to be overcome in order to eject blood from the left ventricle.

The Brockenbrough-Braunwald-Morrow sign is observed in individuals with HCM with outflow tract gradient. This sign can be used to differentiate HCM from aortic stenosis. In individuals with aortic stenosis, after a premature ventricular contraction (PVC), the following ventricular contraction will be more forceful, and the pressure generated in the left ventricle will be higher. Because of the fixed obstruction that the stenotic aortic valve represents, the post-PVC ascending aortic pressure will increase as well. In individuals with HCM, however, the degree of obstruction will increase more than the force of contraction will increase in the post-PVC beat. The result of this is that the left ventricular pressure increases and the ascending aortic pressure decreases, with an increase in the LVOT gradient. While the Brockenbrough-Braunwald-Morrow sign is most dramatically demonstrated using simultaneous intra-cardiac and intra-aortic catheters, it can be seen on routine physical examination as a decrease in the pulse pressure in the post-PVC beat in individuals with HCM.

Hypertrophic cardiomyopathy is usually caused by gene mutations. It’s thought these mutations cause the heart muscle to grow abnormally thick. People with hypertrophic cardiomyopathy also have an abnormal arrangement of heart muscle fibers. The heart muscle cells become jumbled, known as myofiber disarray. This disarray can contribute to an irregular heartbeat (arrhythmia) in some people.

The severity of hypertrophic cardiomyopathy varies widely. Most people with hypertrophic cardiomyopathy have a form of the disease in which the wall (septum) between the two bottom chambers of the heart (the ventricles) becomes enlarged and obstructs blood flow. This is sometimes referred to as hypertrophic cardiomyopathy with obstruction or hypertrophic obstructive cardiomyopathy. About 70 percent of those with hypertrophic cardiomyopathy have some form of obstruction.

Sometimes hypertrophic cardiomyopathy occurs without significant obstruction of blood flow. However, the heart’s main pumping chamber (the left ventricle) may become stiff, which reduces how much blood the ventricle can hold and how much blood gets pumped out to the body with each contraction. Doctors sometimes refer to this as hypertrophic cardiomyopathy without obstruction or nonobstructive hypertrophic cardiomyopathy.

Research has shown that about 50 percent of patients with hypertrophic cardiomyopathy have a close relative who also has the condition. Other nongenetic influences may play a part in determining who will develop hypertrophic cardiomyopathy. The condition appears to occur equally in men and women.