Hypertrophic Cardiomyopathy (HCM) in Cats

Understanding Your Cat's Heart Condition

Hypertrophic cardiomyopathy (HCM) is characterized by unexplained thickening of the muscular walls of the heart. The word "unexplained" implies that other conditions that can cause secondary thickening of the heart walls, such as high blood pressure, hyperthyroidism, and aortic stenosis (another type of heart disease), have been ruled out. When this is the case, a genetic cause is generally suspected. Some breeds of cats, such as Maine Coons, Ragdolls, and Sphyx, have a proven genetic inheritance of this disease.

HCM can lead to congestive heart failure (fluid buildup due to poor cardiac function) due to an inability of the abnormally thick heart walls to relax. That is, the chambers of the heart are unable to adequately open, accept blood from the blood vessels "behind" them, and pass blood along in the forward direction.

HCM is an acquired heart disease (as opposed to congenital diseases, which are present at birth). It is diagnosed most frequently in young adult and middle aged cats, although first detection at either younger or older ages can also occur.

How Will My Veterinarian Diagnose HCM?

Heart disease may first be suspected prior to heart failure during routine physical examination, based on detection of a heart murmur (an abnormal "whooshing" sound associated with the normally crisp heart sounds). However, some cats with HCM do not have a heart murmur. A blood test called NT-pro-BNP can be used to screen for HCM and other heart diseases in asymptomatic cats.

If congestive heart failure is present at the time of diagnosis, other physical exam findings may be present, such as rapid and labored breathing. Breath sounds heard with a stethoscope may be either extremely quiet or unusually loud, depending on where fluid has accumulated.

The Echocardiogram: Confirming the Diagnosis

Definitive diagnosis requires an echocardiogram (ultrasound/sonogram examination of the heart). Echocardiographic findings in cats with HCM are extremely variable. Certain abnormalities, however, are commonly noted. The walls of the heart's strongest chamber, the left ventricle, are abnormally thick. Most cats with HCM, particularly those with concurrent congestive heart failure, also have an enlarged left atrium, the chamber located "behind" the left ventricle. Other abnormalities, such as the presence of valvular insufficiencies (e.g. mitral valvular regurgitation), are sometimes noted.

Additional Tests Your Cat May Need

Several other diagnostic tests may be important during initial and followup evaluation. Chest x-rays provide a "big picture" view of the heart within the chest cavity alongside the lungs. Prior to the onset of congestive heart failure, chest x-rays yield a baseline for future comparison. Once congestive heart failure develops, they comprise the most important imaging test by which medical therapy is monitored.

An electrocardiogram (often called an EKG or ECG) is used to identify and characterize arrhythmias. The relative severity of an arrhythmia dictates whether or not it warrants specific therapy. Followup EKGs are instrumental in assessing progression of previously noted arrhythmias and their response to antiarrhythmic therapy.

As mentioned above, blood pressure measurement is necessary to rule out hypertension (high blood pressure) as a cause of thickened heart walls. A thyroid hormone level is used to screen for the common feline endocrine disease, hyperthyroidism, which is another potential cause of secondary thickening of the heart's walls. Other blood work may be necessary to assess kidney function and electrolyte concentrations. These values can be affected by some of the commonly used medications in the treatment of heart disease and heart failure. It is important to monitor them so that undesirable side effects can be avoided or minimized.

Treatment Options for Your Cat

Until recently, there were no medications that could treat this heart muscle disease directly. However, a newer medication called rapamycin (Felycin) has shown promise in slowing progression of HCM, or even reducing the wall thickening. While cats' hearts on average appeared improved on echocardiograms after starting this medication, it is still unknown if this drug improves survival or delays the onset of symptoms in cats with HCM.

Drugs that slow down the heart rate, such as beta blockers and calcium channel blockers, may be used. This decrease of the heart rate allows the heart chambers more time to fill. However, there is no definitive proof that these drugs improve survival in cats with HCM. ACE inhibitors (e.g. enalapril and benazepril) are medications that lower blood pressure, thereby reducing the workload for the heart. Similarly, ACE inhibitors have not been demonstrated to improve outcome in cats with HCM. If congestive heart failure is present, a diuretic drug called "furosemide" (often referred to by one of its brand names, Lasix) is used to remove excess fluid from the body by causing patients to urinate more. A medication called pimobendan (aka Vetmedin), which improves the contractions of the heart muscle, can also be used to help manage cats with congestive heart failure.

Because cats with heart disease are prone to forming blood clots within the chambers of their hearts, blood thinner medications will often be used to reduce the risk of this happening. These include options such as clopidogrel (Plavix) and rivaroxaban (Xarelto). Aspirin can also be considered, although this appears to provide minimal reduction in blood clot risk for cats.

What to Expect: Prognosis and Quality of Life

Just as the echocardiographic appearance of HCM varies between affected cats, so does the long-term prognosis. The presence of congestive heart failure does worsen overall prognosis, although many cats respond very well to medical therapy for some period of time.

In the absence of heart failure at the time of diagnosis, some cats remain asymptomatic for very long periods, with one study reporting an average survival time of approximately 5 years in such cats. Some cats with mild HCM can live full lives without ever having any symptoms associated with their heart disease. Again, this is variable and, therefore, impossible to accurately predict.

Critical Signs to Watch For at Home

Once the diagnosis of HCM has been made, it is important to watch for signs of heart failure (the buildup of excessive fluid in the body due to heart disease). Symptoms of heart failure include lethargy, weakness, intolerance to activity or exercise, and rapid or labored breathing. Some cats may only show mild and nonspecific behavioral changes such as hiding or loss of appetite.

Finally, a subset of cats with heart disease form blood clots inside their heart, which can then be released and become lodged in an artery downstream. This can result in a sudden onset of difficulty walking, atypical vocalization, and even sudden death.

Observation of even the milder of these symptoms warrants a phone call to your veterinarian. More severe symptoms, such as difficulty breathing or walking, require immediate attention on an emergency basis.

Living with HCM: Moving Forward Together

A diagnosis of hypertrophic cardiomyopathy can feel overwhelming, but you're not alone in this journey. While HCM is a serious condition and the long-term outlook varies from cat to cat, many cats with HCM live comfortably for years with appropriate monitoring and treatment. The key is staying vigilant for changes in your cat's behavior or breathing, keeping up with regular veterinary checkups, and maintaining any prescribed medications. Your veterinary team is your partner in managing this condition, and they're here to answer questions and adjust your cat's care plan as needed. By watching for warning signs and seeking help when needed, you're giving your cat the best chance at a good quality of life.