Pet Health Topics

Hypertrophic Cardiomyopathy (HCM) 

This information is not meant to be a substitute for veterinary care. Always follow the instructions provided by your veterinarian.

Genetic Test for HCM

HCM is a disease that causes thickening of the heart muscle resulting in poor relaxing and filling ability. As the heart’s pumping chamber (ventricle) becomes progressively thicker, less blood can enter the chamber; thus, less blood is ejected out to the body. The cause of HCM is unknown, although certain breeds of cats appear to be predisposed. Middle-aged male cats may be more commonly affected. Sometimes heart muscle thickening similar to HCM can develop secondary to other disorders such as hyperthyroidism (elevated thyroid hormone) and systemic hypertension (elevated blood pressure). Blood pressure measurement and, in cats over five years of age, a blood thyroid hormone test should be done to exclude these secondary causes when cardiac hypertrophy (thickening) is diagnosed.

Some pets show no sign of illness, especially early in the disease. In other cases, signs of left-sided congestive heart failure (fluid accumulation in the lung) may occur. These signs include lethargy, decreased activity level, rapid and/or labored breathing and possibly open mouth breathing with excitement or exercise. Sometimes left and right-sided congestive heart failure develop with fluid accumulation inside the chest or abdominal cavity causing greater respiratory (breathing) effort and abdominal distention. Once fluid accumulations have occurred, clinical heart failure is present and aggressive medical therapy should be sought. Other signs of this disease can include sudden weakness, collapsing episodes, and unfortunately even sudden death due to disturbances in heart rhythm. In some cats with a very large heart chamber (i.e. left atrium) a blood clot may form and if it enters the circulation may cause weakness or paralysis (usually of the rear legs). If this occurs, contact your veterinarian right away to determine if complications related to heart disease (or another disease) are present.

A physical examination performed by your veterinarian may reveal a heart murmur, abnormal heart sounds, abnormal lung sounds, or irregularities in heart rhythm. Chest radiographs (x-rays), an electrocardiogram (ECG..sometimes called an EKG), and an echocardiogram (heart ultrasound) are tests often utilized to confirm a suspected diagnosis and to determine severity. A routine physical exam and one or more of these tests may be recommended every six months to one year to look for any progression of disease in cats without clinical signs.

Asymptomatic pets may not need medical therapy depending on the findings of the tests listed above, but routine reevaluations will often be recommended. Other cats will need medications to slow the heart rate, and promote relaxation of the pumping chambers (ventricles). If arrhythmias or congestive heart failure signs are present, additional medications used may be required. Since this disease can be progressive, the number and the amount of medications used may change with time. Therapy is always tailored to the needs of the individual patient. If congestive heart failure is present, dietary salt reduction is also recommended.

Beta-adrenergic blockers such as atenolol (Tenormin) or propanolol (Inderal). These medications slow the heart rate, which enhances filling and relaxation of the pumping chambers. Beta-blockers also allow more time for blood flow to the heart muscle itself, and reduce the amount of oxygen used by the heart. In some cases, the incidence of arrhythmias is also lessened. Side effects may include bronchospasm (spasm of the airways) (propranolol), fatigue, and in excessive doses,  slow heart rate and low blood pressure.

Calcium-channel blockers such as diltiazem (Cardizem CD, Dilacor XR). This class of drug has similar actions to the beta-blockers. Some differing characteristics of the calcium-blockers include little or no anti-arrhythmic activity in some cases, possible a greater ventricular relaxing effect, and a greater propensity for low blood pressure at higher doses.

Other medications may be prescribed in some patients. Diuretics (furosemide, spironolactone, etc.) may be needed to control edema and effusions (congestive heart failure). In pets that have had or may be prone to blood clot formation, anti-coagulants such as aspirin, warfarin, or heparin may be prescribed.

Medical therapy is always chosen to meet the needs of the individual patient. Frequent recheck examinations and adjustments may be needed especially early in the course of treatment to individualize a medication regimen.

This Pet Health Topic was written by O. L. Nelson, DVM, MS, Diplomate ACVIM (Cardiology & Internal Medicine) Washington State University.

Washington State University assumes no liability for injury to you or your pet incurred by following these descriptions or procedures.