Feline hypertrophic cardiomyopathy

Symptoms an owner can spot at home

• Increased resting or sleeping respiratory rate, consistently above roughly 30 breaths per minute
• Reduced activity, more sleeping, less jumping, usually read as the cat getting older
• Open-mouth breathing or panting after mild exertion, which is never normal in a cat
• Sudden crying out with weakness or dragging of one or both hind legs
• A heart murmur or gallop rhythm found on a routine examination, though many affected cats have neither
• Fainting or collapse episodes

When to see a vet

• Open-mouth breathing, panting, or breathing with visible abdominal effort, go to an emergency hospital now
• Sudden hind-limb weakness, dragging, cold back feet or crying in pain, this is arterial thromboembolism until proven otherwise and is an emergency
• Any resting respiratory rate consistently above 30 breaths per minute, measured while the cat is asleep
• Collapse, fainting, or blue or grey gums
• A newly detected murmur in a cat of an at-risk breed, which warrants a cardiology referral rather than watchful waiting

What it is

Hypertrophic cardiomyopathy is a primary thickening of the left ventricular wall. A normal heart muscle relaxes between beats so the chamber can fill; a thickened, stiff wall cannot, so the chamber holds less blood and pressure backs up into the lungs. It is the dominant feline heart disease by a wide margin, accounting for the large majority of heart disease in cats.

The second consequence matters as much as the first. Blood moves more slowly through the enlarged left atrium, and slow-moving blood can clot. A clot that leaves the heart typically lodges where the aorta divides toward the hind legs, cutting off blood supply to both. The cat loses use of the back legs suddenly and is in severe pain. A distressing number of cats are diagnosed for the first time on the day this happens.

Not all thickening is HCM. Hyperthyroidism and systemic hypertension both produce a similar appearance and are treatable in their own right, so a proper workup rules them out before the diagnosis is settled.

Breed risk and genetics

HCM occurs in cats of any breeding, domestic shorthairs included. Specific mutations in the cardiac myosin-binding protein C gene have been identified in Maine Coons and in Ragdolls, and those two breeds have a DNA test. Several other breeds are over-represented without a mapped mutation, which is why echocardiographic screening remains the standard for breeding cats rather than DNA testing alone.

• A negative DNA test does not rule out HCM. It rules out one known mutation.
• Screening for breeding stock is an echocardiogram by a cardiologist, repeated periodically, because the disease can appear later in life.
• When buying a kitten of an at-risk breed, ask for the parents' most recent echocardiogram reports and any DNA results, in writing.

How vets diagnose it

Echocardiography is the diagnostic standard, because it measures wall thickness directly. Radiographs show the consequences of advanced disease rather than the disease itself, and a stethoscope misses a substantial share of affected cats. Blood tests measuring cardiac biomarkers can flag a cat who warrants an echo but do not replace it.

Blood pressure measurement and a thyroid panel are part of the same workup, since both can cause or worsen the picture. Expect a cardiology referral, and expect the diagnosis to be staged rather than binary, because a subclinical cat and a cat in congestive failure are managed very differently.

Living with a diagnosed cat

Treatment is directed by a veterinary cardiologist and depends on stage. Broad categories include drugs to manage fluid accumulation once heart failure develops, and anticoagulant therapy for cats judged to be at risk of clot formation. There is no treatment that reverses the thickening, and no diet or supplement has been shown to slow it.

The owner's contribution is monitoring and stress reduction. Counting the sleeping respiratory rate once or twice a week gives your vet the single most useful piece of home data, and a rising trend is often the earliest sign of decompensation. Keep a written log rather than trying to remember.

• Count breaths over 30 seconds while the cat is genuinely asleep, and double it. Under 30 is the reassuring range.
• Minimise stressful handling, and tell any vet or groomer about the diagnosis before sedation or anaesthesia.
• Keep the cat lean, since obesity adds cardiac workload, and reduce the need for jumping to reach food, water and resting places.
• Know where the nearest 24-hour emergency hospital is before you need it.