Hypertrophic cardiomyopathy (HCM)

What the thickening actually does

A healthy heart muscle relaxes between beats to let the chamber fill. In HCM the left ventricle wall grows thicker and stiffer, so it cannot relax properly. The chamber holds less blood, pressure backs up into the lungs, and blood moves more slowly through the upper chambers.

That slow-moving blood is the part owners underestimate. It can form a clot, and a clot that leaves the heart usually lodges where the aorta splits toward the hind legs. The cat loses use of one or both back legs suddenly and is in severe pain. This is arterial thromboembolism, and it is frequently how an undiagnosed cat presents for the first time.

What you can and cannot see at home

The honest answer is that early HCM is invisible. Cats do not cough with heart disease the way dogs do, and they hide exercise intolerance by simply doing less. There is no reliable at-home screen.

• Faster or harder breathing at rest, especially while asleep, is the single most useful thing to watch. Count breaths over 30 seconds and double it, consistently above 30 breaths per minute at rest warrants a call.
• Open-mouth breathing in a cat is an emergency, not a quirk.
• Sudden hind-limb weakness or crying out and dragging the back legs is arterial thromboembolism until proven otherwise. Same-day emergency.
• A heart murmur found on a routine exam is worth following up, though many HCM cats never develop one and many murmurs are innocent.

Breed risk and screening

HCM occurs in any cat, including domestic shorthairs, but specific mutations in the cardiac myosin-binding protein C gene have been identified in Maine Coons and Ragdolls. Those two have a DNA test. A negative DNA test does not mean a cat will never develop HCM, because other unmapped causes exist, which is why breeders screen with echocardiography as well.

• Buying a Maine Coon, Ragdoll, British Shorthair, Sphynx, Persian or Norwegian Forest kitten: ask for the parents' DNA results and their most recent echocardiogram, not a general statement that the line is healthy.
• Echocardiogram by a cardiologist is the diagnostic standard. Radiographs and stethoscope exams miss subclinical disease.
• Hyperthyroidism and high blood pressure cause a similar-looking thickening that is treatable, so a workup checks those too.

Why it matters

HCM is the reason buying a pedigree cat is a health decision rather than an aesthetic one. The disease is common, largely silent, and concentrated in exactly the breeds people choose for temperament and looks. You cannot detect it by watching your cat, so the leverage sits entirely in where the kitten comes from and whether you establish a resting respiratory rate baseline while the cat is well. Counting sleeping breaths once a month costs nothing and is the only home measurement that reliably catches decompensation early.