The commonest heart condition in cats is called hypertrophic cardiomyopathy, or HCM for short. Although commonly described as a “swollen heart”, it’s actually more like a thickened heart, with massively enlarged muscle walls. While this may sound great (after all, who doesn’t want a super-strong heart?), it’s actually harmful and, eventually, fatal.
What actually is it?
The best description of HCM is “concentric thickening of the heart muscle”. The increased thickness of the muscle makes the heart much stronger – however, it also reduces the space inside the heart (the lumen) for the blood. As a result, eventually the heart is unable to pump enough blood, and the cat goes into heart failure.
What causes it?
There are two major causes in cats. The first is genetics – some cats have gene mutations that cause the muscle in their heart to lay down additional protein fibres inside each cell. Over time, this results in thickening of the heart muscle and HCM developing. These genes may be found in any cat, but are particularly common in breeds such as the Maine Coon and the Ragdoll (with up to 40% of cats either affected or carriers).
The second cause is due to overwork – most commonly due to hyperthyroidism (overactive thyroid gland) but also potentially high blood pressure caused by kidney disease. Like any other muscle, the harder the heart works, the bigger it gets, and in these diseases, it continues to grow until HCM occurs.
What are the symptoms?
HCM can cause two different syndromes.
The first is congestive heart failure. As the heart’s pumping function becomes progressively weaker, fluid is retained in the body (to try and maintain blood pressure). This results in fluid on the abdomen (ascites) and chest (pulmonary oedema), and it’s this that causes the problems. Initially, affected cats may just have a slightly faster breathing rate, but then they struggle to catch their breath; their gums may go blue as they cannot get enough oxygen in; and eventually they suffer from internal drowning. Very occasionally, their heart will just fail and they will die of cardiac arrest, but this is sadly uncommon (sadly because it seems to be a much nicer way to go than internal drowning).
The second possible syndrome is thromboembolic disease, better known as blood clots. The abnormal flow of blood in the heart causes blood clots to form (typically in the atria, the top chambers in the heart). Sometimes, a little bit of blood clot will break off and travel around the body. If it comes to rest in the brain, it causes a stroke; in the lungs it causes a pulmonary embolism; and in the hindlegs a saddle thrombus. When this occurs, the tissues supplied by that blood vessel are starved of oxygen and die; this may be merely unpleasant, or potentially fatal, depending on where the clot comes to rest.
How is it diagnosed?
There are blood tests that can tell if a cat’s heart isn’t working quite right; however, for a definitive diagnosis your cat would need to have a heart scan. This involves clipping the sides of their chest and using an ultrasound machine to look inside the heart as it beats. The thickness of the walls can be measured, and so can the speed of blood moving through the chambers. If clots are building up in the atria, they are usually visible as “smoke” on the screen. Most cats can be scanned conscious, or with a very mild sedative, but it’s often helpful if you can be there to reassure them!
Is there any treatment?
If the condition is genetic, there is no cure. However, the cat is suffering from HCM secondary to hyperthyroidism, then effective treatment of the hyperthyroidism will often result in resolution of the HCM as the heart gradually returns to its normal size and shape. Likewise, control of abnormally high blood pressure reduces the load on the heart and it will tend to revert to a healthier condition.
If the underlying condition cannot be treated, the best treatments are drugs to relax the heart so it can fill more effectively (diltiazem is licensed for this purpose). If congestive heart failure develops, there are medications to control it (diuretics or “water tablets”, and ACE inhibitors to reduce fluid build-up).
In all cases of uncontrolled HCM, however, blood thinners to prevent the build-up of blood clots can and should be used.
If the HCM can be reversed, the prognosis is good. If not, sadly, the condition will almost always progress and eventually become fatal – however, early diagnosis can delay that, and medication can keep your cat happy and comfortable for a long time.
If you think your cat may have a heart condition, make an appointment to get them checked out by one of our vets as soon as possible.