What is Hypertrophic Cardiomyopathy?

Hypertrophic Cardiomyopathy (HCM) is a condition where the muscular walls of the heart become abnormally thick (hypertrophied), reducing the heart's ability to fill with and pump blood effectively. It is the most common heart disease in cats and also affects certain dog breeds. HCM can lead to congestive heart failure, arterial thromboembolism (blood clots), and sudden death.

The disease is insidious because many affected animals show no outward signs until a sudden, life-threatening event occurs.

Causes

In Cats

HCM is primarily a genetic disease in cats. Specific gene mutations have been identified in:

  • Maine Coon: MyBPC3 mutation (A31P)
  • Ragdoll: MyBPC3 mutation (R820W)

However, HCM occurs in virtually all cat breeds and in mixed-breed cats. Many other genetic mutations likely exist but have not yet been identified.

In Dogs

HCM is less common in dogs than in cats but does occur, particularly in:

  • Cavalier King Charles Spaniel
  • Boxer
  • German Shepherd
  • Rottweiler

Symptoms

Many cats with HCM are asymptomatic for years. When symptoms do appear, they may include:

  • Rapid or labored breathing
  • Open-mouth breathing (a sign of respiratory distress in cats)
  • Lethargy and decreased activity
  • Loss of appetite
  • Acute hindlimb paralysis (a sign of saddle thrombus — a blood clot blocking blood flow to the hind legs, which is a medical emergency)
  • Sudden collapse or death

In dogs, symptoms include exercise intolerance, coughing, and fainting episodes.

Saddle Thrombus

One of the most feared complications of HCM in cats is aortic thromboembolism (ATE), commonly called "saddle thrombus." A blood clot forms in the enlarged heart and travels to the aortic bifurcation, blocking blood flow to the hind legs. Signs include:

  • Sudden, severe pain
  • Paralysis of one or both hind legs
  • Cold, pale paw pads
  • Absent pulses in affected limbs

This is a veterinary emergency. Prognosis depends on severity, but approximately 33–40% of cats do not survive the initial event.

Screening

Echocardiography (Heart Ultrasound)

The gold standard for diagnosing HCM. A board-certified veterinary cardiologist measures the thickness of the heart walls (normal <6mm in cats). Screening should be performed:

  • Before breeding in predisposed breeds
  • Annually in breeds at high risk
  • When a heart murmur is detected
  • Before anesthesia in at-risk breeds

Genetic Testing

DNA tests are available for the known mutations in Maine Coons and Ragdolls. However:

  • A negative genetic test does not guarantee a cat is free of HCM (other unknown mutations may be present)
  • A positive genetic test means the cat carries the mutation but may not develop clinical disease
  • Both genetic testing and echocardiography are recommended for the most complete assessment

NT-proBNP Blood Test

A blood test that measures a cardiac biomarker. Elevated levels suggest heart disease and can be used as a screening tool, though echocardiography is needed for definitive diagnosis.

Why Screening Matters for Breeders

  • All breeding cats of predisposed breeds should have annual echocardiograms by a board-certified cardiologist
  • Cats with HCM or that carry known HCM mutations should be removed from breeding programs
  • Offspring of HCM-positive cats should be monitored closely
  • Responsible screening has helped reduce HCM prevalence in well-managed catteries

Treatment

There is no cure for HCM. Treatment aims to manage symptoms and prevent complications:

  • Beta-blockers (atenolol) to slow heart rate and reduce thickening
  • Calcium channel blockers (diltiazem) as an alternative
  • Diuretics (furosemide) if congestive heart failure develops
  • Anticoagulants (clopidogrel) to reduce clot risk
  • ACE inhibitors in some cases
  • Strict monitoring of respiratory rate at home

Prognosis

Prognosis varies widely:

  • Some cats with mild HCM live normal lifespans
  • Cats that develop congestive heart failure have a median survival of 12–18 months
  • Cats surviving a saddle thrombus have a guarded prognosis
  • Early detection and treatment improve outcomes significantly
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