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Too Much Heart: A Problem for Middle-Aged Cats


Pet Column for the week of February 28, 2005


Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Kim Marie Labak
Information Specialist

Hypertrophic cardiomyopathy, which is a thickening or overgrowth of heart muscle, is a common heart condition seen in cats, and also happens to be one of the leading causes of sudden heart failure in athletic young men.

When we lift weights, our muscles get stronger and bigger. Likewise, heart muscle also grows with exercise or other increased workload. Under normal conditions, strengthening of heart muscle is usually beneficial. However, Dr. David Sisson, a veterinary cardiologist formerly with the University of Illinois Veterinary Teaching Hospital in Urbana, explains that when heart muscle growth is uncontrolled due to genetic or hormonal abnormalities, the ventricular walls can become thickened, leaving less room in the ventricles for blood.

Thickened ventricle walls also lose their ability to expand, further impairing the heart's ability to fill with blood. If the heart cannot fill with sufficient amounts of blood, heart failure results.
Often, the cause for hypertrophic cardiomyopathy in a cat is idiopathic, or unknown. Dr. Sisson says there is evidence that many cases may be due to an inherited genetic defect that does not become apparent until a cat is middle-aged.

Other cases of hypertrophic cardiomyopathy are caused by hyperthyroidism, a common disease in older cats, in which benign thyroid tumors produce excess T4, or thyroid hormone. T4 increases the heart rate, and the heart muscle thickens in response to this increased workload. T4 can also directly cause the heart muscle to thicken by signaling the heart cells to enlarge. Fortunately, hyperthyroidism is very treatable, and once this is treated, the resulting cardiomyopathy is also resolved.

Dr. Mark Oyama, also a cardiologist at the Veterinary Teaching Hospital, explains that the idiopathic form is treatable, but is more difficult to catch early, before it causes heart failure or blood clots. The prognosis for cats with heart failure or a clot are poor, but if a veterinarian diagnoses a cardiomyopathy before these happen, drug therapy can add several quality years to a cat's life.

The question is: how do veterinarians catch this condition early? Heart murmurs accompany cardiomyopathies in only about half of the cases, so a veterinarian will not always "hear" a problem through a stethoscope. Often thickened ventricular walls can be seen on ultrasound images, but as in human medicine, ultrasound is too expensive and impractical to use as a screening test for all middle-aged cats.

Currently, an efficient screening test in human medicine is used to measure levels of brain naturetic peptide, or BNP, a hormone produced by heart muscle. This simple blood test can tell doctors when BNP levels are abnormal, indicating that an ultrasound is in order to look for heart abnormalities.

Since feline BNP is very different from human BNP, this test does not work for cats, but Dr. Sisson and colleagues, through their research at the University of Illinois, have developed a biochemical test for feline BNP that may be commercially available to veterinarians within the next few years. With a test like this, veterinarians would be able to screen middle-aged cats for heart conditions before the cats display any signs of disease.

For more information about hypertrophic cardiomyopathy or other feline heart conditions, contact you local veterinarian or visit the Web site of the American College of Veterinary Internal Medicine at www.acvim.org and click on "Pet Owners & Public."