Idiopathic hypertrophic subaortic stenosis

Alternative names
Cardiomyopathy - hypertrophic (HCM); IHSS; Hypertrophic cardiomyopathy; Asymmetric septal hypertrophy (ASH).


Hypertrophic cardiomyopathy (HCM) is a form of cardiomyopathy (disease of the heart muscle) involving thickening of the heart muscle. This interferes with the function of the heart.

Causes, incidence, and risk factors

In this condition, heart muscle becomes too thick to function properly. The thickening is often not symetrical, affecting one part of the heart more than others. It may interfere with the functioning of the heart by reducing the size of the ventricular chamber. It may also reduce the ability of the valves to work properly. The thickening of the heart muscle may, in some circumstances, obstruct the flow of blood out of the heart.

In addition, heart cells become tangled and jumbled up instead of maintaining their normal pattern. One consequence of the disarray of these cells is that the electrical impulses that normally run though the heart muscle cannot proceed normally, which can lead to abnormal heart rhythms (arrhythmias).

HCM is usually an inherited disorder, the consequence of several defects in the genes controlling heart muscle growth. The abnormal muscle growth usually occurs during periods of a person’s rapid general growth, typically during adolescence.

Younger people are likely to have a more severe form of the disease, but HCM may be diagnosed in people of all ages. HCM among people over 60 years of age is often associated with mild hypertension (high blood pressure).


  • chest pain  
  • fainting, especially during exercise  
  • light-headedness, especially after activity or exercise  
  • dizziness  
  • sensation of feeling heart beat (palpitations)  
  • shortness of breath

Additional symptoms that may occur are:

  • fatigue, reduced activity tolerance  
  • shortness of breath when lying down

Some patients with HCM have no symptoms, and may be unaware of their condition until it is detected during routine medical screening.

Unfortunately, the first symptom of HCM among many young patients is sudden death, caused by severe arrhythmias. HCM is a major cause of death in young athletes who seem completely healthy but die during heavy exercise.

Signs and tests

The pulse in the arms and neck may have special characteristics. The doctor may feel an abnormal heart impulse in the chest. Listening with a stethoscope may reveal abnormal heart sounds or a murmur, which may change intensity with different body positions.

Most frequently, however, the physical examination of patients with HCM is relatively normal.

Left ventricular muscle thickness, obstruction of blood flow, or mitral valve regurgitation (malfunctioning of the valve between the left atrium and left ventricle) may appear on:

  • Echocardiography (the most common test) with Doppler ultrasound. For a more accurate evaluation, some patients need a transesophageal echocardiogram (TEE).  
  • Chest x-ray  
  • An ECG may indicate left ventricle muscle thickness, arrhythmias, previous heart attack, and/or ischemic changes. These changes are better captured by a 24-hour Holter monitor.  
  • Cardiac catheterization

Lab tests are not specifically diagnostic for hypertrophic cardiomyopathy, but tests may be used to rule out other suspected diseases.

Your health care provider may recommend that your close blood relatives - even if they have no symptoms - also be tested for HCM.


Treatment is aimed at control of symptoms and prevention of complications. Some patients may require hospitalization until the condition is stabilized.

The thick ventricles of HCM contract and relax abnormally, and to assist the relaxation phase some drugs may be necessary. These include beta-blockers and calcium channel blockers such as verapamil, which improve exercise tolerance and reduce chest pain.

When severe blood outflow blockage exists, an operation called myotomy-myectomy (heart muscle cutting-heart muscle removal) often results in marked improvement. Replacement of the mitral valve during the same surgery is necessary in some HCM patients with mitral valve regurgitation.

Some people with arrhythmias may need anti-arrhythmic medications. If the arrhythmia is atrial fibrillation, the risk of blood clots breaking away from the atrium and clogging arteries in the body may need to be reduced with blood thinner medicine.

Patients with HCM and high risk factors may need to receive an implantable-cardioverter defibrillator (ICD) to prevent sudden cardiac death. These risks include severe heart muscle thickness, potentially lethal heart rhythms, a history of passing out, or a family history of sudden cardiac death.

Expectations (prognosis)

Some affected individuals remain without symptoms for many years and have a normal life span, yet some may deteriorate gradually or rapidly. Progression into dilated cardiomyopathy occurs in some patients.

Patients with hypertrophic cardiomyopathy are at higher risk for sudden death than the normal population, and can be affected at a young age. Hypertrophic cardiomyopathy is a well known cause of sudden death in athletes. People with HCM should follow their doctor’s advice about physical exercise and medical appointments.

Young patients with HCM planning to start a family may benefit from genetic counseling.


  • severe injury or trauma from fainting  
  • cardiac arrhythmias, including lethal arrhythmias  
  • congestive heart failure  
  • dilated cardiomyopathy

Calling your health care provider

Call for an appointment with your health care provider if:

  • You have any symptoms of hypertrophic cardiomyopathy  
  • Chest pain, palpitations, faintness or other new or unexplained symptoms develop

Identify, by family history, which people are at high risk for the genetic form of the disease. Some patients with mild forms of hypertrophic cardiomyopathy are only diagnosed by screening echocardiograms because of their known family history. Treat known high blood pressure.

Johns Hopkins patient information

Last revised: December 4, 2012
by Janet G. Derge, M.D.

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