Overview

HCM causes abnormal thickening of the left ventricular muscle. Contraction may remain strong, but the heart cannot relax adequately before the next beat. Impaired filling causes blood to back up in the left atrium, which gradually enlarges.

Significant atrial enlargement can lead to two major problems. Increased pulmonary venous pressure can cause pulmonary edema or pleural effusion, presenting as acute open-mouth breathing. Clots can also form in the enlarged atrium and dislodge; a clot blocking the aortic branch to the hindlimbs causes sudden paralysis (ATE). Both are emergencies.

Predisposed breeds

HCM can affect any breed. The ACVIM consensus identifies causal mutations in Maine Coons and Ragdolls and reported predisposition in other breeds. Breed alone does not determine whether disease is present or when testing is needed.

Maine Coon and Ragdoll — established disease-associated mutations
Persian, British Shorthair and Sphynx, among others — reported breed predisposition
Cats with a family history or planned breeding
All breeds — assessment is needed for murmurs, gallops, arrhythmias or breathing changes

Absence from a predisposed-breed list does not guarantee safety. Cats' naturally low visible activity can make exercise-related changes difficult to notice.

What signs should I watch for?

Cats may show little outward evidence of cardiac problems. Any of these signs calls for prompt assessment.

Faster resting breathing or more pronounced flank movement
Open-mouth breathing, almost always an emergency sign in cats
Sudden hindlimb paralysis, cold limbs or severe pain (possible ATE)
Sudden loss of interest in play or hiding in corners
Reduced appetite, weight loss or more frequent vomiting
Pale or bluish gums or paw pads

Cats can hide heart disease; the first sign may be an emergency

HCM may cause no obvious symptoms until breathing difficulty or hindlimb paralysis occurs. Discuss echocardiographic screening and its timing with your veterinarian if there is a murmur, gallop, arrhythmia, family history, breeding plan or breed predisposition.

Diagnostic steps

Echocardiography directly measures left ventricular wall thickness and left atrial size and is central to diagnosis, alongside these steps.

Auscultation + Resting respiratory rate (auscultation may be normal)

Murmurs, gallops or arrhythmias may be audible, but even advanced HCM can have relatively normal auscultation. Predisposed breeds or increased respiratory rate may warrant imaging despite normal auscultation.
Chest X-rays (heart enlargement and pulmonary edema)

Assess silhouette shape, pulmonary venous congestion, edema and pleural effusion. With emergency breathing difficulty, oxygen and minimal restraint take priority. Stabilization determines the order of point-of-care ultrasound and radiographs.
Echocardiography (ventricular thickening, atrial size and relaxation)

Directly measure wall thickness, left atrial size and diastolic indices. This core examination informs diagnosis and ACVIM staging.
NT-proBNP (screening aid)

NT-proBNP helps screen for possible structural heart disease. Staging considers echocardiography together with current or previous clinical signs and clotting-risk findings.
Blood pressure (with thyroid and kidney assessment)

Hypertension and hyperthyroidism can also cause ventricular thickening and are assessed alongside the heart. Kidney function provides baseline information for medication use.

ACVIM staging

ACVIM classifies feline cardiomyopathy into the following stages, which guide treatment and antithrombotic decisions.

Stage	Condition	When to consider treatment
A	Breed or family-history risk, without cardiomyopathy on ultrasound	Reassessment timing based on individual risk and breeding plans
B1	Structural abnormality with normal left atrial size	Reassessment timing based on atrial size, clinical findings and rate of change
B2	Structural abnormality plus left atrial enlargement (increased ATE risk)	Monitor progression and symptoms. Consider antithrombotic treatment for risk findings such as moderate or greater atrial enlargement, impaired atrial function or spontaneous echo contrast
C	Heart failure signs or previous ATE	Medication + Emergency planning + Closer monitoring

This owner-oriented table summarizes general ACVIM guidance. Application depends on individual echocardiographic findings, blood pressure and concurrent conditions.

Treatment and care by stage

HCM progression and complication risk vary between cats. We follow changes in structure and symptoms and select care according to heart failure and clotting risks.

Medication decisions by stage

Evidence does not support routinely giving every asymptomatic cat medication to slow heart rate or improve relaxation. Antithrombotic treatment may be considered for atrial enlargement, blood stasis or spontaneous echo contrast. Stage C congestive heart failure requires indicated treatment such as diuretics. Decisions are individualized using ultrasound, blood pressure, kidney function and clinical status.

Home monitoring — Resting respiratory rate

Count breaths for one minute while your pet sleeps and compare with the usual rate. Contact the veterinarian if faster breathing recurs and agree on individual contact thresholds. For labored or open-mouth breathing, go straight to the nearest 24-hour emergency veterinary hospital regardless of the count.

Emergency signs — Go to a 24-hour emergency veterinary hospital

For open-mouth breathing, sudden hindlimb paralysis, cold limbs, severe pain or reduced consciousness, go directly to the nearest 24-hour emergency veterinary hospital without waiting for our appointment or a social-media reply.

Managing concurrent conditions

Hypertension, hyperthyroidism and chronic kidney disease are common in older cats and may cause ventricular thickening or affect medication choices. Blood pressure, thyroid and kidney checks accompany cardiac follow-up.

Frequently asked questions

Can a cat outside the predisposed breeds develop HCM?

Yes. Any breed, including Korean domestic shorthairs, can be affected. Absence from the breed list is not reassurance; abnormal auscultation or a change in breathing rate warrants assessment.

Does normal auscultation mean no testing is needed?

Normal auscultation cannot fully exclude HCM. Discuss screening based on family history, breeding plans, breed predisposition, planned anesthesia and other clinical findings.

My cat is breathing with its mouth open. Is this an emergency?

Yes. Go straight to the nearest 24-hour emergency veterinary hospital. Minimize excitement and unnecessary restraint during transport and, if possible, notify the emergency hospital before leaving.

My cat suddenly cannot use its hindlimbs. What happened?

In a cat with HCM, sudden hindlimb paralysis raises concern for ATE. Limbs may be cold, pads pale or blue, and pain severe. Go straight to the nearest 24-hour emergency veterinary hospital without waiting for our outpatient appointment.

Can HCM be cured?

The course varies; some cats remain symptom-free for a long time. Not every asymptomatic cat receives the same medication. Regular assessment tracks change and guides treatment according to heart failure and clotting risk.

When should a predisposed breed be screened?

There is no single starting age or fixed interval for all cats. Family history, breeding plans, murmurs, gallops, arrhythmias, systemic disease and previous ultrasound findings determine timing individually.

References

These external resources explain cardiomyopathy classification and risk-based care. They do not define our services or an individual prescription.

Feline hypertrophic cardiomyopathy (HCM)