HCM! Is it only a concern for pedigree cats?

What is hypertrophic cardiomyopathy?

Hypertrophic cardiomyopathy (HCM) is the most common heart disease of all cats, whether they are random bred or pedigreed. It is also a disease that affects humans, dogs, skunks, camels and many more animals. It is a heart muscle disease causes abnormal thickening of the left ventricle muscle walls. Unlike thickening of the walls in response to a physical stress (exercising) where the thickening occurs to deal with the extra workload placed on the heart, the thickening with HCM is not normal. How much the muscle walls thicken ultimately determines the severity of the disease – some cats (and people) have mild disease, others have severe disease. Additionally, at least in people, the disease can also result in arrhythmias and sudden death, although this appears to be less common in cats. HCM is often a progressive disease, and some affected cats develop heart failure if the muscle hypertrophy and subsequent scarring of the heart muscle significantly affects heart function. It is important to rule out other causes of thickening of the heart muscle, like chronic hypertension, hyperthyroidism and outflow obstructions.

What causes HCM in cats? Is HCM genetic?

This is currently unknown in most cats although inheriting HCM has been observed as an autosomal dominant inherited trait in humans as well as in several breeds, such as the Maine Coon and American Shorthair. It is not proven that HCM is inherited in many other breeds. It is important to note that the most commonly affected cat breed is the regular “domestic shorthair” cat, suggesting that many cases have no clear inheritance pattern. Heart muscle hypertrophy in cats can be caused by other diseases, like high blood pressure and hyperthyroidism. To date, no viral or nutritional causes of HCM have been identified in humans or cats. The disease has variable expression; meaning some cats are severely affected, others are only mildly to moderately affected, and some cats may not have evidence of the disease yet produce affected offspring.

How is HCM diagnosed?

HCM is diagnosed using ultrasound of the heart - an echocardiogram. Echocardiography is a good way to detect moderate to severely affected cats. However, it may not always detect the mildly affected cats where changes in the heart can be minimal.

In addition to an echocardiogram, other tests may also be useful in assessing cats with HCM. For example, a chest x-ray is necessary to detect heart failure in cats with severe HCM. An electrocardiogram is useful in cats that have an abnormal heart rhythm. Blood pressure measurement and blood testing for hyperthyroidism is indicated to rule out other diseases that mimic HCM, especially mild to moderate HCM.

Ideally, an echocardiogram to test cats for HCM should be performed by a board-certified radiologist or cardiologist.

Should my cats be tested for HCM and how often should they be tested?

In clinical practice, the most common patients tested for HCM with echocardiography are cats with suggestive clinical signs of heart disease, such as a heart murmur.

Testing cats used in a pedigreed breeding program is a more difficult endeavor. Echocardiography is not a perfect tool for diagnosis of HCM - some affected individuals will escape detection and access to good quality ultrasound services may be difficult and expensive for some breeders. At the very least, breeding cats should be auscultated (examined by a vet with a stethoscope) for heart murmurs or arrhythmias once yearly. Any cat with an abnormality should have an echocardiogram. A significant percentage of cats with HCM will not have a heart murmur, however.

At what age should a cat be tested for HCM?

HCM can affect cats at any age. It has been seen in kittens only a few months of age and in cats over the age of 10. In Maine Coons, most affected male cats have evidence of disease by 2 years of age, and most affected females have evidence of disease by 3 years of age although instances have been documented where the disease has not shown up until 7 years of age. Ragdolls with severe disease seem to develop it earlier in life, often at under 1 year of age. Guidelines for other breeds have not yet been developed. It is therefore hard to recommend a specific age to start testing. It may make sense to test most breeding cats for the first time by the age of 2 years.

Will we ever eliminate HCM from my breed?

The tools we currently have to diagnose HCM (i.e., echocardiography and necropsy) are not perfect and will not allow us to totally eliminate this disease. However, echocardiographic screening will be able to reduce the incidence of HCM within a breed if enough breeders are involved.

Breeders should use all the information they can gather about HCM in family lines, including pedigree analysis based on accurate identification of affected cats.

Any cat that dies suddenly or dies from HCM should have a necropsy (i.e., post mortem examination). Most cats with HCM will have a heart that weighs more than 20 grams and most cats with severe HCM will have a heart that weighs more than 30 grams. Myocardial fiber disarray, the hallmark microscopic heart muscle abnormality seen in humans with familial HCM is seen in all Maine Coon cats with HCM. Unfortunately, most veterinary pathologists are not trained to recognize this lesion.

In the long term, we will need a genetic test for HCM in each breed. A genetic test would allow us to identify affected cats before they were bred and do so accurately. Once a mutation is identified, if all breeders cooperate by testing their breeding cats for the mutation the disease could be eliminated from the breed within several generations. However, the money and resources necessary to identify the gene or genes and to develop a genetic test for each breed are scarce in veterinary medicine.

Can two normal parents produce a kitten with HCM?

Since HCM is known to be an autosomal dominant trait in the breeds where the inheritance is known, each affected cat must have one affected parent. However, there are possible situations in which an affected cat may come from two apparently normal parents.

The first possibility is that one of the parents has been misdiagnosed. This can happen due to inexperience of the ultrasonographer or poor quality equipment. It can also happen if a cat's status is decided on the basis of only one or two ultrasounds early in life. Since HCM can develop at any age, a cat that is normal on ultrasound one year could still have HCM and show signs later in life.

Since the trait has variable expression, not every affected cat will have echocardiographic evidence of HCM. It is therefore possible for a cat to test negative for HCM on ultrasound, and yet still carry the defective gene and pass it to offspring.

Finally, it is possible for spontaneous mutations to occur in cats from normal parents. These cats may then pass on their mutation to offspring. We do not know how often spontaneous mutations causing HCM occur in cats. Statistically, spontaneous mutations are more likely to occur in random bred cats than in pedigreed cats.

What does “HCM free cattery” mean?

There is no universally agreed upon definition of an HCM free cattery. The terminology is currently unclear, as different breeders mean different things when they use this term. Ideally, each breed should develop a specific definition and guidelines for use of this designation for catteries.