Hypertrophic cardiomyopathy (HCM) is the most common heart disease in cats, affecting an estimated 10-15% of the general feline population. The condition is characterized by abnormal thickening (hypertrophy) of the left ventricular heart muscle, which reduces the chamber’s ability to fill with blood and can lead to congestive heart failure, blood clot formation, and sudden death. HCM can affect cats of any age but is most commonly diagnosed in middle-aged cats.

Overview

In HCM, the muscular walls of the left ventricle become abnormally thick, stiffening the heart and impairing its ability to relax and fill with blood during diastole. This leads to elevated pressure in the left atrium and pulmonary veins, which can ultimately result in fluid accumulation in or around the lungs (congestive heart failure). The thickened heart muscle may also create turbulent blood flow, and the enlarged left atrium provides an environment where blood clots can form — a potentially devastating complication known as aortic thromboembolism (ATE or “saddle thrombus”).

HCM has a known genetic basis in certain breeds. A mutation in the myosin-binding protein C (MYBPC3) gene has been identified in Maine Coons and Ragdolls. Other commonly affected breeds include British Shorthairs, Persians, Sphynx, and Bengals, though any cat, including domestic shorthairs, can develop HCM.

Causes & Risk Factors

  • Genetic mutation — Autosomal dominant inheritance in Maine Coons and Ragdolls (MYBPC3 mutation); genetic basis suspected in other breeds
  • Breed predisposition — Maine Coon, Ragdoll, British Shorthair, Sphynx, Persian, Bengal, and others
  • Sex — Male cats appear to be affected more frequently and more severely
  • Age — Typically diagnosed between 5-7 years, though some cats develop disease as young as 1 year
  • Secondary causes — Systemic hypertension and hyperthyroidism can cause ventricular thickening that mimics HCM and must be ruled out

Symptoms

Many cats with HCM are asymptomatic for years, and the disease may be first detected incidentally (heart murmur, gallop rhythm, or arrhythmia found during a routine exam). When clinical signs develop, they may include:

  • Increased respiratory rate or effort (breathing faster than 40 breaths per minute at rest)
  • Open-mouth breathing or panting
  • Lethargy and decreased activity
  • Reduced appetite
  • Sudden hind-limb paralysis and severe pain (aortic thromboembolism — this is an emergency)
  • Sudden death (may be the first indication of disease in some cats)
  • Heart murmur or gallop rhythm detected by your veterinarian

Aortic thromboembolism (ATE) is an emergency. If your cat suddenly cannot use one or both hind legs, is crying in pain, or the hind paws feel cold, seek emergency veterinary care immediately.

Diagnosis

  • Echocardiography (cardiac ultrasound) — The gold standard for diagnosing HCM. Measures left ventricular wall thickness (greater than 6 mm is diagnostic), assesses left atrial size, and evaluates heart function. Should be performed by a veterinary cardiologist when possible.
  • Physical examination — Heart murmur, gallop rhythm, or arrhythmia may be detected, but some cats with HCM have no auscultable abnormalities.
  • Cardiac biomarkers — NT-proBNP and cardiac troponin I blood tests can help screen for heart disease, though they are not definitive on their own.
  • Chest radiographs — Evaluate heart size and detect pleural effusion or pulmonary edema in cats with congestive heart failure.
  • Electrocardiogram (ECG) — May identify arrhythmias but is not diagnostic for HCM.
  • Blood pressure and thyroid testing — Essential to rule out secondary causes of cardiac thickening.
  • Genetic testing — Available for the MYBPC3 mutation in Maine Coons and Ragdolls. A positive test indicates risk but does not confirm clinical disease.

Treatment & Medications

There is no cure for HCM. Treatment goals are to manage symptoms, reduce the risk of heart failure and thromboembolism, and slow disease progression:

  • Atenolol — A beta-blocker that reduces heart rate, lowers myocardial oxygen demand, and may improve diastolic filling. Commonly used in cats with significant hypertrophy or outflow tract obstruction.
  • Diltiazem — A calcium channel blocker sometimes used as an alternative to atenolol to improve cardiac relaxation.
  • Furosemide — A loop diuretic used to manage congestive heart failure by removing excess fluid from the lungs or chest cavity. Dose is carefully titrated to the minimum effective amount.
  • Clopidogrel (Plavix) — An antiplatelet medication given to cats with significant left atrial enlargement to reduce the risk of aortic thromboembolism. Typically dosed at 18.75 mg (one quarter of a 75 mg tablet) once daily.
  • Pimobendan — An inodilator increasingly used in cats with HCM and congestive heart failure, particularly when systolic function is impaired. Its use in preclinical HCM is still under investigation.
  • Thoracocentesis — Drainage of pleural effusion (fluid around the lungs) in emergency situations.
  • Oxygen therapy — For cats in acute respiratory distress from heart failure.

Cats with asymptomatic (preclinical) HCM and only mild to moderate thickening may not require immediate medication. Your veterinarian or cardiologist will determine the appropriate treatment plan based on the severity of disease.

Prognosis

Prognosis for HCM varies widely depending on the stage of disease at diagnosis:

  • Cats with mild, preclinical HCM may live normal lifespans with monitoring alone
  • Cats with moderate disease and no heart failure have a median survival of several years with appropriate management
  • Cats that develop congestive heart failure have a more guarded prognosis, with median survival times of approximately 12-18 months with treatment
  • Aortic thromboembolism carries a poor prognosis; approximately 40-50% of affected cats are euthanized at the time of the event, and recurrence is common in survivors
  • Regular echocardiographic monitoring (every 6-12 months) allows early detection of progression

Frequently Asked Questions

Should I screen my cat for HCM? Screening echocardiography is recommended for breeds with known genetic predisposition (Maine Coons, Ragdolls, British Shorthairs, Sphynx) before breeding. It is also advisable for any cat with a detected heart murmur or prior to anesthesia.

Can HCM be prevented? In genetically predisposed breeds, responsible breeding using genetic testing and echocardiographic screening can reduce the incidence. There is no known way to prevent HCM in individual cats, but early detection allows proactive management.

Is HCM painful for my cat? HCM itself does not typically cause pain. However, aortic thromboembolism is extremely painful, and congestive heart failure causes significant respiratory distress. Quality of life monitoring is an important part of long-term management.

How do I monitor my cat’s breathing at home? Count your cat’s resting respiratory rate while they sleep. A normal rate is below 30 breaths per minute. A sustained resting rate above 40 breaths per minute may indicate developing heart failure and warrants prompt veterinary evaluation.

This information is for educational purposes only and does not replace professional veterinary advice. Always consult your veterinarian or a veterinary cardiologist for diagnosis and treatment of your cat’s specific condition.