Cats are experts at hiding illness, and nowhere is that truer than with heart disease. Hypertrophic cardiomyopathy, or HCM, is the most common heart condition in pet cats, affecting an estimated 10 to 15 percent of them. The muscle walls of the heart thicken abnormally, the pumping chambers shrink, and the heart works harder to move less blood. Many cats show no symptoms at all until the disease is advanced, which is exactly why every owner should know how it is detected.
What Actually Goes Wrong
In HCM the left ventricle’s walls stiffen and thicken, reducing how well the heart fills and pumps. Over time this can lead to irregular heart rhythms, congestive heart failure with fluid in or around the lungs, and the complication owners fear most: a blood clot that forms in the heart and lodges in the hind legs, causing sudden, painful paralysis known as saddle thrombus. Some cats with severe HCM die suddenly with no warning at all.
Who Is Most at Risk
Any cat can develop HCM, but genetics play a real role. Specific mutations have been identified in two breeds, and several others show elevated rates:
| Breed | Notes |
|---|---|
| Maine Coon | MYBPC3 gene mutation; screening recommended before breeding |
| Ragdoll | Same gene family mutation; can appear in young adults |
| British Shorthair, Sphynx, Bengal | Higher reported risk; no single mutation explains all cases |
| Mixed-breed cats | Most HCM patients are not purebred; male and middle-aged cats are overrepresented |
Secondary causes exist too: hyperthyroidism and high blood pressure can thicken the heart muscle and are always ruled out during diagnosis.
How HCM Is Found
Early HCM is silent. The first clue is often a heart murmur or abnormal rhythm heard during a routine exam, though not every cat with HCM murmurs and not every murmur means HCM. The definitive test is an echocardiogram, an ultrasound of the heart performed by someone experienced in cardiac imaging; X-rays, stethoscopes, and even blood biomarkers like pro-BNP can raise suspicion but cannot confirm it. For at-risk breeds, annual or twice-yearly screening is the standard advice, and cats diagnosed early but asymptomatic are typically rechecked every six to twelve months.
Treatment and the One Check Worth Learning
There is no cure for HCM, but medication can extend and improve life. Depending on severity, vets use beta-blockers to slow the heart, diuretics such as furosemide to clear lung fluid, ACE inhibitors, pimobendan to support pumping, and the anti-clotting drug clopidogrel to reduce stroke and saddle thrombus risk. At home, the single most useful skill is counting your sleeping cat’s resting respiratory rate: under 35 breaths per minute is reassuring, while a steady climb toward that number is an early warning that the heart is struggling and deserves a same-week call to your vet.
Bottom line. HCM hides precisely because cats compensate well, so waiting for symptoms is a losing strategy. Ask for the heart to be listened to at every checkup, screen at-risk breeds with echocardiography, learn your cat’s resting breathing rate, and treat open-mouth breathing or sudden hind leg paralysis as a drop-everything emergency.