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 is the leading cause of sudden cardiac death in young athletes.2
A cardiomyopathy is any disease that primarily affects the muscle of the heart. In HCM, the normal alignment of muscle cells is distrupted, 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 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.
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.
Associated symptoms
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).4
Physical examination
| Aortic stenosis | Hypertrophic cardiomyopathy | |
|---|---|---|
| Echocardiography | ||
| Aortic valve calcification | Common | No |
| Dilated ascending aorta | Common | Rare |
| Ventricular hypertrophy | Concentric LVH | Eccentric, often involving the septum |
| Physical examination | ||
| Murmur of AI | Common | No |
| Pulse pressure afer PVC | Increased | Decreased |
| Valsalva maneuver | Decreased intensity of murmur | Increased intensity of murmur |
| Carotid pulsation | Normal or parvus et tardus | Brisk, jerky |
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).
Diagnostic testing
A diagnosis is made using electrocardiogram, echocardiogram, chest X-ray, coronary angiography, or MRI of the heart.
Treatment
Treatment of HCM is directed towards decreasing the left ventricular outflow tract gradient and to abort arrhythmias in individuals at high risk of sudden cardiac death.
The use of a pacemaker has been advocated in a subset of individuals, in order to cause asynchronous contraction of the left ventricle. Since the pacemaker activates the interventricular septum before the left ventricular free wall, the gradient across the left ventricular outflow tract may decrease.
In cases that are refractory to all other forms of treatment, cardiac transplantation is an option.
Related disorders
Feline hypertrophic cardiomyopathy is the most common heart disease in cats; the disease process and genetics are believed to be similar to the disease in humans.5
References
1. Richardson P, McKenna W, Bristow M, Maisch B, Mautner B, O'Connell J, Olsen E, Thiene G, Goodwin J, Gyarfas I, Martin I, Nordet P. Report of the 1995 World Health Organization/International Society and Federation of Cardiology Task Force on the Definition and Classification of cardiomyopathies. Circulation. 1996 Mar 1;93(5):841-2. (Medline abstract) (Full text)
2. Maron BJ, Thompson PD, Puffer JC, McGrew CA, Strong WB, Douglas PS, Clark LT, Mitten MJ, Crawford MH, Atkins DL, Driscoll DJ, Epstein AE. Cardiovascular preparticipation screening of competitive athletes. A statement for health professionals from the Sudden Death Committee (clinical cardiology) and Congenital Cardiac Defects Committee (cardiovascular disease in the young), American Heart Association. Circulation. 1996 Aug 15;94(4):850-6. (Medline abstract) (Full text)
3. Rivera-Diaz J, Moosvi AR. Apical hypertrophic cardiomyopathy. South Med J. 1996 Jul;89(7):711-3. (Medline abstract), (Full article)
4. Maron BJ, Cecchi F, McKenna WJ. Risk factors and stratification for sudden cardiac death in patients with hypertrophic cardiomyopathy. Br Heart J. 1994 Dec;72(6 Suppl):S13-8. (Medline abstract)
5. Kittleson MD, Meurs KM, Munro MJ, Kittleson JA, Liu SK, Pion PD, Towbin JA. Familial hypertrophic cardiomyopathy in maine coon cats: an animal model of human disease. Circulation. 1999 Jun 22;99(24):3172-80. (Medline abstract)