Maine Orchid

Hypertrophic Cardiomyopathy (HCM)


Hypertrophic cardiomyopathy (HCM) is the most common heart condition in cats. Its not only present in Maine coons, other breeds are affected by it too, more or less.

HCM is a progressive disease, the cat is not born with it. The disease is typically characterised with a thickening of the left ventricular and/or septal wall of the heart. However, other parameters could indicate HCM as well, such as thickening of papillary muscles and/or presens of SAM (systolic anterior motion of the mitral valve) etc. The changes of the heart will reduce elestacity of the organ and make it difficult for the heart to pump blood to the body. The atrium may become enlarged due to the amount of blood trapped there, since it can not reach the ventricle. And as a subsequent effect to that the lungs may fill with fluid, due to plasma from the blood, making the affected individual short of breath.

The changes to the heart often result in turbulence of the blood and/or leakage over the valve. That is why you can hear a murmur through ausculation on some of the HCM affected cats. However, it's important to remember that not all cats with a heart murmur have HCM, there are other reasons for these too (non leathal reasons).

Some cats that develop HCM show no symptoms of the disease at all. One day they just will not wake up any more, or they suddenly fall dead to the floor due too severe heart failure. As mentioned before, some cats may be caught short of breath due to the fluid in their lungs. And other cats may get paralyzed due to clogs that get trapped often in their rear legs. These clogs are a result of the turbulence of the blood in the HCM affected heart. If one of those clogs from the heart is released into the aorta they will jam when the blood vesels get too narrow for them to pass any longer. That's why the affected cat often becomes lame in one or both of its legs. If you touch the paws of one of those legs they are often cold, since the circulation have been shut off completely.

Establish a diagnosis

To establish that the cat do suffer from HCM you need to make a necropsy of the cat. Why the heck then? Well, you need to look at the hearts tissue to determine if the cat do have the changes related to HCM or not. There may be changes to the tissue connected to HCM without presenting the thickening of the heart or enlargement of the papillary muscles... and you can't check for that until the cat is dead. However, the best tool we have to try to detect HCM before the cat is dead is ultrasound. Ultrasound makes it possible to meassure the left ventricular and the septal wall of the heart. You can also detect turbulences in the blood and changes of the valves, papillary muscles etc. It's not a 100% guarantee even if the ultrasound is normal, but it's the best we can do to establish that the cat is healthy.

Since HCM develops over time its important to check the cat continously so you make sure the cat you use (or have used) in breeding is all right. You want to detect the small initial changes of the disease as soon as possible, in order to give you a chance to take the cat out of breeding, should it be needed. You also want to make sure the cat will survive a pregnacy or the stress during the mating, since that will increase the physical load on the cats heart.

To establish a well developed HCM will not require any deap cardiology expertiese. However, for the breeders we are interested in finding those first signs of a developing HCM so we can remove that cat from breeding as soon as possible. For that you need to consult an expert. At PawPeds website you will find a list of veterinarians that are qualified for this examination of your cat. All of them have adequat education and experience in the field, as well as the advanced equipment required.

It's not establised exactly how this disease is inherited, but it is suspected to be dominant with incomplete penetrance. Therefore the disease may skip one or several generations before it "pops up" again. For humans it has been shown that there are several genes that may cause HCM due to mutations. The majority of those genes are involved in or connected with the sarcomere of the heart cell, the functional part that make the heart cell beat. At this point one mutation has been maped to one gene and connect it to the development of HCM in a group of related Maine coons. I've written more about this at the page DNA-testing.

Primary vs secondary HCM?

When we screen the cats for HCM we are looking for the primary form of the disease. A primary HCM will never revert or improve with medical treatment. It's a permanent change of the heart that often lead to the death of the cat. When death occure is hard to say, since the cat may die from other causes before actuall heart failure. The grading mild, moderate and severe do not indicate how soon the cat will die. It is more a grading related to how severe the changes are in the heart. There are cats with mild assessment that have died the next day, and cats with moderate to severe assessment that have lived several years after they were established to suffer from HCM.

However, in some cases other diseases or conditions may cause a change of the heart and result in a secondary HCM. A secondary HCM will revert if the primary condition is dealt with and set right. Biology is both fascinating and frustrating to work with, nothing is black and white in biology... Known causes that may affect the heart to resemble HCM is hypothyroidism, dehydration, infections, kidney disease etc. A speciallist still notice that something is not right in the aspect of a true HCM and will most likely assess the cat as equivocal instead. If the infection is over next time the cat is evaluated it may have a normal heart again and the assessment will change from equivocal to normal. A primary HCM will still be there next time.


It is recommended to examine the status of the cats heart with ultrasound before breeding. Any of the listed veterinarians on the PawPed website have adequat equipment and education for this evaluation. First examination is often done arount the age of one year, when the cat is first mated. Second test at two years, third at three years or in connection to mating at those ages. A forth test is performed around the age of five and a fifth can be performed at the age of eight years. A cat classified with HCM is not used in breeding. A cat classified as equivocal need to be retested before it is ruled out or still included in a breeding programme. More about the breeding recommendations can be read at the PawPeds website.

Last updated: 2014-11-23