Hypertrophic cardiomyopathy is the most common heart disease of cats, affecting as many as one in seven cats in the UK. Often hypertrophic cardiomyopathy goes undetected by owners, until things go wrong quite suddenly and dramatically. It’s important to know whether your cat may be at risk.

What is hypertrophic cardiomyopathy in cats?

The heart is a muscle, made up of four chambers called the left atrium, right atrium, left ventricle and right ventricle. It functions as a pump to squeeze blood around the body. 

In hypertrophic cardiomyopathy, the heart muscle becomes abnormally thick, and the heart’s ability to pump blood is impaired. The thickening of the heart muscle is known as ‘hypertrophy’. This hypertrophy can lead to the development of an abnormal heart rhythm, cause blood clots to form in the left atrium, but most commonly, causes the left ventricle to stop working properly. The left ventricle is the chamber of the heart responsible for pumping oxygenated blood around the body. When hypertrophic cardiomyopathy causes the wall of the left ventricle to become thickened, it also becomes less flexible. The left ventricle can’t stretch or relax well enough to properly fill with the blood that the body needs. 

Blood can back up, sitting around for too long in the pulmonary system, with fluid from the blood vessels leaking into the lungs (‘pulmonary oedema’) and chest (‘pleural effusion’). Pulmonary oedema and pleural effusion are both types of what is known as ‘congestive heart failure’. 

What causes hypertrophic cardiomyopathy and is my cat a susceptible breed?

In humans, it is known that this condition is caused by a defect affecting the genes that code for heart muscle proteins. Several different genetic defects have been identified in cats that appear to be responsible for this disease in different breeds. However, it’s not fully understood how this genetic defect is passed on. Domestic shorthairs (aka, the moggie) are most commonly affected, but pedigree breeds at particularly high risk of hypertrophic cardiomyopathy include the Maine Coon, Ragdoll, Persian, Bengal, British Shorthair, Norwegian Forest Cat, Sphynx and Birman. The disease most commonly affects middle-aged cats, but all ages may be affected.

What are the symptoms of hypertrophic cardiomyopathy in cats?

In around 5% of cases, an abnormal heart rhythm develops that leads to sudden death. A small number of cats might develop a blood clot within the left atrium that can then travel out through the aorta and cause a blockage of blood flow, usually affecting the back legs. In these cases, the cat suffers sudden paralysis, severe pain, and often, death. You can read more about this condition and how it is treated here.

Most of the time however, the disease is silent, and signs may not be apparent until quite far along in the disease process. Some cases can be mild, and others more severe. Cats are masters of hiding and masking signs of illness, so often even cats with severe hypertrophic cardiomyopathy may appear completely normal to their owner. Cats don’t tend to cough when they have heart problems, the way that humans and dogs do. Some of the signs can be very subtle and non-specific, such as sleeping more or having less of an appetite.

Sometimes the first clue is when a heart murmur or abnormal heart rhythm is detected by a vet during a check-up. 

The following would be considered classic signs of a deteriorating heart issue in cats, including hypertrophic cardiomyopathy:

  • Difficulty breathing
  • Faster or more laboured breathing
  • Loss of appetite
  • Weight loss
  • Lethargy
  • Sudden loss of use of one or more legs (caused by a blood clot)
  • Vomiting
  • Sudden death

How is hypertrophic cardiomyopathy diagnosed?

If your vet suspects a heart problem, the next step might be an echocardiogram. This is a non-invasive ultrasound examination of the heart, where the heart can be measured and assessed for how well it is beating.  

Further tests may be required in some instances, including x-rays of the chest. These will be helpful to identify whether there is any fluid in the lungs or chest space. 

Blood tests may be advised to screen for any other underlying issues. Conditions like high blood pressure and hyperthyroidism can also be responsible for changes in the heart, and often need to be ruled out. Your vet will likely want to check your cat’s blood pressure. There are also specific blood tests of the heart, which look for proteins produced by the heart (NTproBNP). Elevations in this protein can be seen in cats with hypertrophic cardiomyopathy, and it can be a useful test if your vet is uncertain as to whether it is your cat’s heart that is causing their problems or not.

How is hypertrophic cardiomyopathy treated in cats?

Unfortunately, there is no cure for hypertrophic cardiomyopathy in cats. The changes occurring in the heart muscle are irreversible. 

One of the treatment aims is to help the left ventricle relax and to better fill with blood. There are a few drugs that can be tried to achieve this, but none have been proven to be truly effective. At the moment, it’s quite acceptable to not treat cats with hypertrophic cardiomyopathy prior to the onset of congestive heart failure. This may change over time as new treatments are developed.

Mild or asymptomatic cases 

These cats may receive no therapy at this point.

Cats with evidence of congestive heart failure 

These cats may be treated with diuretics to help reduce fluid build-up, as well as an angiotensin-converting enzyme (ACE) inhibitor such as benazepril, which helps widen blood vessels and lower blood pressure, making things easier for the heart to pump against. With severe fluid build-up, the vet may need to physically remove the fluid with a catheter to help the cat breathe.

Cats that are at risk of clot formation, or clot reoccurrence 

They may be treated with anti-clotting drugs like clopidogrel and rivaroxaban. 

If your cat has been diagnosed with hypertrophic cardiomyopathy, you’ll need to be very attuned to any changes in your cat’s health. Your vet might be able to show you how to count their breaths, and an increased respiratory rate might be one of the first signs that your cat’s health is deteriorating. Open mouth breathing or panting is a bad sign when seen in a cat. Cats with hypertrophic cardiomyopathy will need to access veterinary care as quickly as possible if you have any concerns.

What is the prognosis for my cat?

The rate at which a cat can go downhill varies, depending on the severity of the disease. Cats with severe hypertrophic cardiomyopathy have a poor prognosis. Some cats may die suddenly, without obvious signs of illness.

Some cats may live with milder disease for a number of years. However, once the heart begins to fail and congestive heart failure starts, survival times generally range between 12-18 months after diagnosis. 

Pedigree cats and hypertrophic cardiomyopathy

A genetic test is available for the Ragdoll and Maine Coon breeds, but this test won’t work on other breeds of cat. It’s also worth noting that not all cats that test positive go on to develop hypertrophic cardiomyopathy and not all cats with hypertrophic cardiomyopathy test positive. Instead, it’s recommended that those breeds susceptible ought to have an annual ultrasound examination of the heart to screen for the disease.

Before breeding your cat, be sure you understand all the possibilities of what could happen. It’s generally recommended not to breed from a cat that has a positive genetic test for hypertrophic cardiomyopathy, to prevent the possibility of the condition being passed on. Talk to your vet to find out more.

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