Feline Hypertrophic Cardiomyopathy (HCM) in Cats

Published Jun. 23, 2023

What Is Hypertrophic Cardiomyopathy (HCM) in Cats?

Like the human heart, the feline heart has four chambers—two smaller chambers on the top, the left and right atrium, and two larger chambers on the bottom, the left and right ventricle.

The function of the right atrium is to pump deoxygenated blood from the body into the right ventricle, which pumps the blood to the lungs to pick up more oxygen for use by the body and release carbon dioxide for disposal. The function of the left atrium is to pump oxygen-rich blood from the lungs into the left ventricle, which then pumps the blood back to the body to allow proper cellular and organ function. Because the left ventricle needs to push oxygenated blood to the body, its muscular walls are thicker than those of the right ventricle.

When the walls of the left ventricle become fully enlarged and excessively thickened, the result is hypertrophic cardiomyopathy (HCM). The narrowing of the inner chamber of the left ventricle, due to this muscular thickening, prevents it from filling properly and decreases its efficiency. As a result, less oxygen-rich blood can reach the body to keep it functioning correctly and it causes the heart to beat faster than normal, increasing its need for oxygen and causing the death of heart muscle cells. This can also hamper heart function.

HCM is the most common heart disease in cats. As many as 1 in 7 will develop HCM at some point in their lives, and most show no symptoms.

Causes of Hypertrophic Cardiomyopathy in Cats

One of the most common causes is an inherent disease of the heart muscle itself. This is often related to a genetic predisposition to HCM that is present in several cat breeds, including:

These breeds are most affected by HCM because they are more likely to carry a mutation in a gene called A31P that is involved in maintaining healthy heart muscles. In cats that have two abnormal copies of this gene, death of heart muscle cells and scar tissue in the wall of the left ventricle occurs, which prevents the heart from functioning properly.

In addition to genetics, there can be several medical causes of HCM in cats. Hormonal conditions such as hyperthyroidism or acromegaly (excessive secretion of a growth hormone because of a tumor in the anterior pituitary gland near the brain) can stimulate growth of the muscular walls of the left ventricle. Cancer in the heart can also increase the size and thickness of the walls of the left ventricle.

Other diseases unrelated to the heart, such as kidney disease, can cause high blood pressure in the body, which puts additional strain on the left ventricle as it works harder than normal to pump blood to the body against elevated pressure in the vessels. Over time, this increased effort causes the ventricle walls to become thicker, and the interior chamber narrower.

Symptoms of Hypertrophic Cardiomyopathy in Cats

Loss of appetite
Heart murmurs
Arrhythmias (abnormal heart rhythms)
Blood clots, which can cause pain/vocalization, trouble walking, cold rear limbs, vomiting, and trouble breathing
Heart failure
Fluid in lungs, causing crackles and wheezes when breathing
Difficulty breathing
Fluid in abdomen (ascites), causing visible abdominal swelling
Abnormal gum color: pale, gray, or blue
Blue foot pads and nailbeds
Weak or unstable pulse
Tiring easily during play/exercise
Lethargy/weakness
Fainting/collapse
Sudden death

How Veterinarians Diagnose Hypertrophic Cardiomyopathy in Cats

A veterinarian will start with a thorough physical exam, focusing on listening to the heart and lungs for any abnormal heart rhythms, murmurs, and any excessive crackling and wheezing during breathing. They will also examine the cat’s feet and nail beds for signs of poor circulation, feeling carefully for any weak or unsteady pulses in the limbs, as well as look for any abnormal colors in the gums, such as blue, gray, or pale. The cat’s overall demeanor and ability to walk will also be assessed. HCM can cause lethargy and trouble walking in some cats, particularly if the cat has developed secondary blood clots because of it.

Following the physical exam, blood work, including a thyroid panel, will be performed to determine if there is a medical condition that might cause HCM. X-rays may be helpful in screening for HCM, but it can be missed, depending upon the angle of the image. Electrocardiograms (ECGs) can also be used to pick up any abnormal heart rhythms.

The best way to diagnose HCM in cats is the echocardiogram, an ultrasound of the heart that allows a fuller picture of the thickness of the left ventricle, the flow of blood between heart chambers, and other features of heart function that are best examined as the heart is pumping in real time.

Treatment of Hypertrophic Cardiomyopathy in Cats

A variety of medications can help cats that have HCM:

Clopidogrel: medication that helps prevent blood clots.
Diltiazem: a medication that can prevent some fluid buildup and reduce the thickness of the left ventricular wall in some cats.
Beta blockers: medications such as atenolol can help cats with severe HCM by slowing heart rate in cats with tachycardia (an overly fast heart rate), correcting abnormal heart rhythms, and improving the function of the valves between the chambers of the heart.
- Beta blockers can also decrease blood pressure in cats with hypertension.
ACE inhibitors: medications such as benazepril are often prescribed to cats with HCM who have heart failure. It can reduce blood pressure.
Aspirin: used to thin the blood and prevent clotting in cats with HCM.
Warfarin: another blood-thinning medication that can help prevent blood clots
Furosemide or Lasix®: a common medication prescribed for cats who have congestive heart failure due to HCM.
Spironolactone: another diuretic medication that is used in cats with HCM and heart failure to remove fluid buildup. Spironolactone is often used along with Lasix®.
Nitroglycerin ointment: can be applied inside the ears of cats to treat severe fluid buildup when administered along with Lasix®. It can also improve blood flow to the body by relaxing the ventricles and arteries, so they are able to accommodate more blood.
Pimobendan: can help increase the function of the left atrium and left ventricle during the pumping phase of the heart cycle, when the heart is pushing blood from the left ventricle to the body.

In addition to medications, some cats may need routine pleurocentesis, a procedure that removes excess fluid from the lungs. Cats with severe, acute HCM and heart failure may need oxygen therapy in the hospital. Cats that suddenly develop heart failure may benefit from being put under anesthesia, intubated, and helped to breathe if respiratory failure occurs.

Anxious cats may be given sedatives such as acepromazine to keep them calm, as stress can cause breathing difficulties and increase the risk of heart failure in cats with HCM.

Recovery and Management of Hypertrophic Cardiomyopathy (HCM) in Cats

Unfortunately, HCM has no cure. Cats with no symptoms may live a normal lifespan, but the disease usually worsens. If symptoms develop, cats will generally survive for about two more years. Survival time is generally shorter, perhaps only six months or less, for cats that have blood clots, congestive heart failure, and/or low body temperature. If your cat requires hospitalization, a few things can be done when your cat is home to maintain your cat stable for as long as possible.

Stress can hinder breathing and increase the risk of heart failure in cats. It is important to set up a quiet space for your cat where he can rest away from any noise and activity of people and other pets. Providing easy access to food, water, litter boxes, and soft perches near windows can benefit your cat immensely.

Diet is another factor that can improve the quality of life for cats with HCM. Because HCM, especially with congestive heart failure, can cause loss of appetite, it is essential that cats be fed a high-quality, enticing food that is high in protein. This may require switching from wet food to dry food or vice versa.

Low-sodium broths or food toppers such as Purina® Beyond Mixers Immune Support can encourage cats with lower appetites to eat more. Omega-3 fatty acids, found in fish oils, can decrease inflammation that is often responsible for muscle wasting and low appetite in cats. A liquid supplement such as Nutramax® Welactin is a great addition to your cat’s food.

Cats on medications for congestive heart failure may lose too much vitamin B in their urine and require vitamin B supplements. Work closely with your veterinarian to determine what the best diet and supplements may be for your cat, based on their condition.

Prevention of HCM in Cats

Since HCM can be inherited or may be the result of other medical conditions, the disease is difficult to prevent. Reputable breeders should test susceptible cat breeds for mutations in the A31P gene so that kittens produced from breeding stock do not inherit two mutated copies of the gene.

General care and proper nutrition for all cats can go a long way to supporting heart health and prolonging the asymptomatic stage of HCM. It is crucial for higher-risk cats (senior cats, cats with predisposed conditions, cats with the A31P gene) to be screened annually for any signs of heart disease.

If you are concerned about HCM in your cat, it is best to speak with your veterinarian about her overall risk, which diagnostic tests are most appropriate, and how often those tests are needed. Your veterinarian can also assist with developing a wellness and nutrition plan at home to ensure your cat has the best and longest possible life.

Featured Image: iStock.com/AzmanL