Acquired Heart Disease in Cats: Hypertrophic Cardiomyopathy

Hypertrophic Cardiomyopathy (HCM) is a big term that describes the changes seen in the heartAcquired Heart Disease in Cats: Hypertrophic Cardiomyopathy muscle in a cat afflicted with this disease.  Click here to learn about normal functioning of cat hearts. It is the most commonly diagnosed heart conditions in the cat.  It is extremely rare in dogs.

There are a number of different causes.  Primary gene mutations in the components of muscle cells have been shown to cause HCM.  In addition to basic muscle cell defects, it has also been proven that there is an inheritable gene defect in Maine Coon cats that will cause HCM.  Unfortunately, this trait is dominant, which means only one parent needs to be affected for the offspring to be afflicted with this disease.  Cats may either have one or two copies of this gene, depending on their parents.  Cats with one copy are more likely to die of heart failure, while cats with two copies are more likely suddenly (similar to a heart attack).

HCM in cats can also be caused by other diseases affecting your pet’s health.  For example, over active thyroid (hyperthyroid) is a very common cause of HCM.  Interestingly, HCM secondary to other diseases will typically have less increase in wall thickness than primary gene defects.  Rarely, the heart may become thickened due to an abnormal infiltration of cells, such as cancer cells as in lymphoma.

The actual changes in the heart are thickening of the walls of the ventricles, most often the left ventricle.

With this increase in thickness, the heart does not function as well.  First, there is not as much space in the ventricle to fill with blood, thereby decreasing the amount that is being circulated to the rest of the body.  In addition, these thick walls do not relax quickly to help with normal filling of blood.  So not only do the walls get closer together in the chamber, but they also do not adapt to accommodate more blood.  Thus the heart needs to work harder because different organs are not receiving enough oxygen or other nutrients, and they, in turn, send signals to the heart to send more blood.  This increased work causes further thickening of the muscle and scar tissue formation.  The scar tissue forms because the blood supply to the cardiac muscle cells decreases due to the increased thickness.  Once the blood supply decreases, these cells lack the proper nutrients to survive.  As one can imagine, this is a perpetuating cycle.

The other significant abnormality is systolic anterior motion of the mitral valve (called SAM).  This complicated sounding term describes an abnormal movement of the mitral valve in cats with this disease.  Due to abnormal alignment of the heart muscles and where they attach to the valves, a piece of the mitral valve gets caught in the outflow tract into the first part of the aorta.

This presents two issues.  The first is the decrease in blood flowing out of the heart.  Closing off part of this outflow tract will also increase the pressure of blood flowing out into the aorta.  Think of a putting your thumb over a hose and the increase in velocity of the water that occurs.  This will cause damage to the lining of the blood vessels and lead to further issues.  The second, and likely more significant, is that blood flows back into the left atrium.  This abnormal flow will eventually lead to left sided heart failure.

This is an image of an echocardiogram of a cat with SAM.  Note the arrow on the left labeled SAMAcquired Heart Disease in Cats and mitral valve septal contact pointing to a bright white oval shape.  This is part of the mitral valve that is blocking blood moving out into the aorta (a black area between the arrows labeled AOV).  The aorta will normally appear as a uniform black rectangle.  In HCM and this image, it is obstructed.  For comparison, note the normal heart diagram on the bottom rotated to match the echocardiogram image.  Acquired Heart Disease in Cats and Rotate Heart

 

Diagnosis of Hypertrophic Cardiomyopathy (HCM) in the Cat

Your veterinarian will listen to your cat’s heart on a normal physical exam.  Instead of sharp sounds (lub-dub lub-dub, like this), the sounds will not be distinct (schhh-dub schhh-dub, like this).  These abnormal sounds are called murmurs.  Depending on your cat’s symptoms, the next steps are xrays or an echocardiogram (an ultrasound of the heart).

A new way of diagnosing heart disease in the cat is pro-BNP (B-type Natriuretic Peptide, for those interested in what the abbreviation stands for).  This is a hormone that has been shown to increase in cats with cardiomyopathies, even those not showing any symptoms of cardiac disease.  A simple blood test will show if this is elevated.

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Dr Roger Johnson and Dr Kyle Marano

Roger K. Johnson, DVM, Diplomate ACVIM (internal medicine) is a board-certified veterinary internal medicine specialist. His professional interests include cardiology as well as using advanced diagnostics to help his patients. His particular favorites include echocardiography, abdominal ultrasonography, and endoscopy. Kyle Marano, DVM is a small animal veterinarian practicing out of Northern Colorado. He has written pieces ranging from sports commentary and analysis to quips on the every day life of veterinary medicine. His furry family includes a chocolate lab mix and an overly nosy cat.

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