Hypertrophic cardiomyopathy is a thickening (hypertrophy) of the muscles of the wall of the left ventricle of the heart causing the heart to be less able to relax between contractions.

In some cases, this can lead to an increase in pressure in the left atrium which then enlarges. The increase in pressure in the left atrium causes back pressure in the blood vessels of the lungs and a fluid build up in the lungs. In severe cases, a blood clot can sometimes form within the enlarged left atrium.

 

Echocardiograms of feline hearts showing normal heart (left) and heart affected by HCM(right); - note thickened wall of left ventricle (LV - Left Ventricle, RV - Right Ventricle, LA - Left Atrium, RA - Right Atrium)

In people, up to 60% of all cases have been found to have a genetic cause, with over 450 mutations currently identified. In cats, so far 2 genetic mutations have been linked with HCM, one mutation in Maine Coons and one in Ragdolls. It is suspected that there are many more mutations in the cat population that are as yet unidentified.

Certain diseases, including high blood pressure (hypertension) and an overactive thyroid gland (hyperthyroidism) can cause the heart muscle of the left ventricle to thicken. Once these conditions are treated, the heart often returns to a more normal appearance.

Which cats are affected with HCM?

There are several breeds of cat thought to be at a higher risk of developing HCM (including Ragdolls and Maine Coons) however it is most commonly diagnosed in domestics/moggies. It often first develops in young to middle aged cats and may be more common in males. In the pilot study for this project, 34% of 199 cats had a heart murmur, of these cats with murmurs only 50% showed thickening of their left ventricle when examined using a heart scan (echocardiography).

How is HCM diagnosed?

A heart murmur can be heard in some cats affected with HCM. Unfortunately, completely healthy cats can also sometimes have a heart murmur and some cats with HCM don’t have heart murmurs at all. Cats with HCM without murmurs are often diagnosed when they start to show clinical signs or when they are screened due to a related cat being previously diagnosed.

Currently, the most reliable method of diagnosis is a heart scan (echocardiogram). This involves a small amount of fur being clipped from the cat’s chest and an ultrasound probe being used to create images of the heart. This is a painless procedure (illustraded left) which only requires the cats to be lightly restrained for a short period of time.

A blood test has recently been developed that can distinguish between cats with heart disease and cats without.

What clinical signs can I expect to see in my cat with HCM?

Clinical signs in HCM range from non-existent all the way through to severe and life threatening:

  • Asymptomatic: Some cats with HCM may never show any clinical signs of their heart disease and go on to have a normal life expectancy.
  • Mild clinical signs: Affected cats can show increased effort with their breathing, they may faint (syncope) or they may be less able or willing to exercise than they used to.
  • Congestive Heart Failure: Fluid accumulating on or around the lungs can cause cats to struggle to breathe and collapse.
  • Aortic Thromboembolism: For some severely affected cats, a blood clot produced in the heart can block off the blood supply to the hind legs causing a very painful paralysis.
  • Sudden death: For a small minority of cats, the first sign of the disease is sudden death.

How is HCM treated?

There is currently no evidence that those cats without any clinical signs benefit from any kind of treatment. Owners of these cats are advised to get their cat examined by a vet on a regular basis and to monitor their cats for the development of clinical signs.

Once a cat develops clinical signs attributable to their heart disease, treatments are prescribed by a vet based on clinical signs and what can be seen on the heart scan (echocardiogram). Not all cats are prescribed the same drugs and not all cats tolerate the same drugs. It is important to consult a vet if you believe your cat to be developing any signs.

How long will my cat with HCM live?

Cats without clinical signs may not show any decrease in survival compared to cats without any disease. Recent studies have shown this group of cats are able to survive in excess of 10yrs once they are diagnosed, often dying from conditions unrelated to their heart.

Cats that develop congestive heart failure have reported survival times varying from 3 months to 2 years. Poor survival times have been reported for those cats whos first sign is a blood clot. These cats require intensive care and may survive less than 6 months.

It’s important to remember that every cat is different and some will do better or worse than the described survival times.

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