Hypertrophic Cardiomyopathy (HCM) in Cats

At a glance

  • What is hypertrophic cardiomyopathy? Hypertrophic cardiomyopathy is a condition in which a portion of the
    muscular walls of the heart becomes thickened which makes it hard for the heart to work effectively.
  • Symptoms: Lethargy, exercise intolerance, difficulty breathing, weight loss, loss of appetite, abnormal heart sounds.
  • Treatment: Manage symptoms as well as slowing down the progression of the disease and may include beta-blockers and ACE inhibitors, diuretics low salt diet and restrict exercise.

What is hypertrophic cardiomyopathy?

Feline hypertrophic cardiomyopathy (HCM) is the most common heart disease in cats and one of the most common causes of death. The main feature of HCM is an excessive thickening of the left ventricular wall, papillary muscles, and septum.

Cardio=heart, myopathy=muscle disease and hypertrophic=thickened. The heart is a pump with four chambers, two on the left and two on the right. Each side has two chambers, a ventricle, and atrium, the ventricles serve as pumping chambers and the atria as receiving chambers.

  • The left atrium and left ventricle.
  • The right atrium and right ventricle.

The right atrium receives deoxygenated blood from the body which is pumped into the right ventricle. From here, the blood is pumped into the lungs where it is oxygenated. The left atrium receives the oxygenated blood, pumping it into the left ventricle, from here, the blood is pumped into the rest of the body.

In some cases, systemic diseases can lead to a thickening of the heart. These include high blood pressure (hypertension), hyperthyroidism and acromegaly.


Feline hypertrophic cardiomyopathy

  • Enlargement of the heart wall causes stiffening of the muscle, preventing the heart from expanding to receive blood properly.
  • As the walls thicken, the size of the heart chambers decreases resulting in less blood pumping through the heart. The heart has to work harder, beating faster to maintain blood flow throughout the body.
  • It may also reduce the ability of the valves to work properly, and in some circumstances obstruct the flow of blood out of the heart.
  • Arrhythmias, irregularities of the heartbeat and conduction disturbances are also common complications of hypertrophic cardiomyopathy. The thickened wall sometimes distorts one leaflet of the mitral valve, causing it to leak.
  • As the left ventricle shrinks and stiffens due to a thickening of the wall, it is less able to accept as much blood, this results in a build-up in the left atrium and the lungs and the lining of the chest, leading to congestive heart failure and pulmonary edema, making breathing difficult.
  • Blood clots can form in the left atrium and be carried into the systemic arterial system, most often lodging in the terminal artery, causing paralysis of the hind legs.


The cause of HCM hasn’t been established, however, it is known that HCM in Maine Coons (MYBPC3-A31P) and Ragdolls (MYBPC3-R820W) is inherited as an autosomal dominant trait with incomplete penetrance. Cats have two sets of genes, one from the mother, one from the father. If the cat inherits one mutant allele (an allele is an alternative form of a gene), he is at greater risk of developing HCM, and an even higher chance if he inherits two mutant alleles (one from each parent). Around 30% of Maine Coons have this genetic mutation.

There is anecdotal evidence that HCM may be inherited in Norwegian Forest Cats, Scottish Folds, and American Shorthairs, but this is not the same gene mutation as the ones the Maine Coon or Ragdoll have, but possibly similar. However, there is a higher incidence in domestic cats.


A cat with hypertrophic cardiomyopathy may display no symptoms at all but die suddenly and unexpectedly. Symptoms can develop over time and be subtle. Many symptoms are due to fluid build-up in the lungs and pleural space making breathing more difficult.

  • Lethargy
  • Rapid laboured and noisy breathing
  • Decreased activity
  • Irregular heart rhythm
  • Heart murmur
  • Anorexia
  • Weight loss
  • Blue-tinged gums due to a lack of oxygen circulating and/or pulmonary edema and pleural effusion.
  • Lameness or paralysis of the hind legs due to saddle thrombosis
  • Sudden death

Are some cats at increased risk?

As noted, some breeds can inherit a genetic mutation making them more likely to develop HCM.

The disease occurs most often in middle-aged cats and older. Male cats are more prone to developing HCM as are overweight cats.


A physical examination may reveal abnormal heart or lung sounds, irregular or gallop heart rhythm or a heart murmur, this may well be the first indication that your cat has HCM.

  • Echocardiogram: An ultrasound of the heart will reveal a thickened left ventricle wall that can range from mild to pronounced.
  • X-ray: Can show if there is fluid in the chest and heart enlargement.
  • Electrocardiogram: This test records the electrical activity of the cat’s heart to check for irregular rhythm.
  • Blood tests: Complete blood count and chemistry panel can help provide information on the function of other organs. This information is important when determining methods of treatment.
  • T4 (total T4 or thyroxine) test: Measures T4 concentrations in the blood for cats with suspected hyperthyroidism. 
  • Arterial blood pressure: An inflatable cuff is placed on the cat’s front leg or tail and a Doppler or oscillometric device is used to measure the cat’s blood pressure.
  • Genetic testing: HCM usually doesn’t become apparent in Maine Coons or Ragdolls who carry the genetic mutation until they reach adulthood. A genetic test is available to find out if the cat carries one (heterozygous) or two (homozygous) copies of the mutation. Most heterozygous cats will not get sick.


Asymptomatic cats may require no treatment, but your veterinarian will want to monitor the cat closely. It is not possible to cure HCM, treatment is aimed at managing symptoms and slowing down the progression of the disease.

  • Beta-blockers: The thick ventricles of HCM contract and relax abnormally, and to assist the relaxation phase beta-blockers such as atenolol or propranolol can slow down the heart rate to give the heart more time to fill the chambers.
  • ACE inhibitors: Medications to relax and dilate the blood vessels which lowers blood pressure and decreases demand on the heart.
  • Calcium blocking channels: to slow down the progression of the disease.
  • Diuretics: Medications help the kidneys get rid of excess water and salt in the body which can relieve fluid build-up.
  • Aspirin: Controlled doses for cats who have a high risk of blood clot formation.
  • Controlled doses of blood-thinning drugs such as aspirin for cats who have a high risk of clot formation.
  • Thoracocentesis (pleural tap): A medical procedure that involves the placement of a needle through the chest wall to remove fluid build-up from the pleural space.
  • Restrict activity: Reduces the strain on the heart, the veterinarian may prescribe a period of cage rest.


  • Julia Wilson, 'Cat World' Founder

    Julia Wilson is the founder of Cat-World, and has researched and written over 1,000 articles about cats. She is a cat expert with over 20 years of experience writing about a wide range of cat topics, with a special interest in cat health, welfare and preventative care. Julia lives in Sydney with her family, four cats and two dogs. Full author bio