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Hypertrophic Cardiomyopathy (HCM) in Cats

Posted January 19, 2015 in Cat Diseases & Conditions A-Z

Maine Coon Cat

Hypertrophic Cardiomyopathy (HCM), a type of abnormal thickening of the heart, is the most common cause of heart disease in cats. It is often an “occult” silent killer, and it can result in sudden death in cats. HCM is often diagnosed in young to middle-aged male cats. Certain breeds, such as the Maine Coon or Ragdoll, may be predisposed, suggesting a possible genetic component. However, most of the time, the cause of HCM in cats remains unknown.

With HCM, the normal heart muscle is replaced by fibrous connective tissue (scar tissue). This causes the heart to become too muscular (i.e., thicker or “hypertrophied”), and the main heart chamber that pumps blood to the body (left ventricle) becomes too small, making less room for blood. This can result in:

  • An abnormally dilated left atrium
  • Abnormal blood flow to develop within this part of the heart

With HCM, the fibrous connective tissue doesn’t function like normal heart muscle so the heart does not contract normally; again, this results in less blood being pumped into circulation. Because HCM results in a severely thickened and poorly functioning heart muscle, congestive heart failure, an irregular heart beat (e.g., arrhythmia) and/or life threatening blood clots (commonly called a “saddle thrombus”) can occur, making this disease the most common cause of sudden death in indoor cats.

How HCM hides
Unfortunately, HCM can often go undiagnosed at home. A heart murmur or arrhythmia may be heard on an annual physical exam by your veterinarian and can be the first clinical sign. For this reason, any cat with a heart murmur or arrhythmia should ideally be worked up to rule out underlying heart disease (see “Diagnosis” below).

Symptoms of HCM
Note that not all cats with HCM have heart murmurs that veterinarians can “hear” with the stethoscope. This makes diagnosing the disease very difficult in the early stages. For many cats with HCM, signs consistent with congestive heart failure will be the first indication something is wrong. These sings include:

Other symptoms include: 

  • A heart murmur
  • Increased respiratory rate (especially after exertion)
  • Open-mouth breathing
  • Difficulty breathing
  • Blue-tinged gums
  • Decreased activity
  • Decreased appetite
  • Weight loss
  • Sudden onset of paralysis in hind legs (from a saddle thrombus)
  • Weakness or collapse
  • Sudden death

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Justine has more than 18 years of experience in the veterinary industry and is a board-certified emergency critical care veterinary specialist and toxicologist as well as the CEO and founder of Vetgirl. She is also a founding member of IDEXX’s Pet Health Network team.