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Hypertrophic Caridomyopathy
Introduction
Hypertrophic cardiomyopathy (HCM) is the most common heart disease in cats. It develops and progresses insidiously over time and its early signs may be subtle or nonexistent. Certain breeds, including Maine Coon, Ragdoll and British Shorthaired appear to have an increased risk for development of HCM. A cat that seems healthy may appear to become very ill quickly, or even die suddenly. At Shahnoor we have our breeding queens and studs examined via echocardiogram and auscultation regularly. We do not have HCM identified in our cattery and have made a firm decision to never breed with nor purchase from HCM identified cats or catteries.
What is HCM?
Hypertrophic cardiomyopathy is a heart muscle disease in which the muscular walls of the left ventricle become abnormally thickened. Internal cardiac structures, such as the papillary muscles, those that anchor and support the heart valves, also thicken. HCM is diagnosed when it is determined that the muscle thickening is not caused by another illness such as hyperthyroidism, systemic hypertension or any of several other more rare diseases. HCM is seen in cats at almost any age. Familial occurences have been observed in several breeds, and in these instances cats are most commonly diagnosed when they are less than five years of age. While specific feline genetic mutations have not been isolated yet, the pattern of inheritance in families of cats strongly indicates an autosomal dominat mode of inheritance, as is the case with humans. To date, no viral or dietary causes of HCM have been identified in humans or animals.
 
The feline heart
As HCM develops and progresses, it alters the structure of the heart and impairs its function in several ways. While the predominant indication of feline HCM is thickening of the left ventricular walls, early HCM is usually also characterized by enlarged papillary muscles and an effect called systolic anterior motion (SAM) of the mitral valve. This abnormal motion of the mitral valve may partially obstruct the outflow of blood from the left ventricle to the aorta. The abnormal mitral valve position also allows blood to flow backward, or regurgitate, into the left atrium. This distortion in the mitral valve's position causes a systolic heart murmur that often becomes lauder with excitement or softens as a cat relaxes.
With HCM, the thickened left ventricular walls become stiffened, which prevents the ventricle from relaxing sufficiently to fill with blood at a normal pressure from the left atrium. The increased pressure in the relaxed left ventricle increases pressure in the left atrium, which, in turn, enlarges. This increased pressure also backs up into the capillaries into the lung tissue, causing pulmonary oedema, and into the chest cavity, causing pleural effusion. These abnormalities are called 'congestive heart failure and make it difficult for the cat to breathe.
Blood flow in the enlarged left atrium slows, which can cause blood clots to form in this heart chamber. A blood clot can dislodge and be forced by blood flow into the body's circulatory system, where it eventually becomes lodged and blocks the flow of blood to tissue beyond that point. A classic example of this is rear leg paralysis, which is caused when a clot lodges in the descending aorta at the point where it branches to go to each of the back legs. This situation is commonly reffered to as a saddle thromboembolus.
Clinical signs
Outward signs of HCM may include a barely noticeable increase in breathing rate to marked signs of respiratory difficulty, rear leg paralysis, or the sudden death of a cat that seemed healthy only months earlier.
Diagnosis
Early signs can be so insignificant that even an observant owner may not notice them. Many cats with HCM develop a heart murmur, a gallop rhythm, and/or a rapid heart rate, which the cat's veterinarian may detect during a routine examination. An echocardiogram (an ultrasound of the heart) with color Doppler imaging is the most conclusive means of diagnosing HCM. While electrocardiograms and X-rays may provide the veterinarian with additional useful information, they cannot be used to arrive at an unequivocal diagnosis. HCM may also be diagnosed at veterinary autopsy. The clinician must consider several factors, such as the thickness of the left ventricular walls, the weight of the heart, and the size of the left atrium. The heart often contracts after death, causing it to thicken. Consequently, an increase in thickness alone cannot be used to diagnose HCM. A normal cat heart should weigh less than 20 grams.
Treatment
At present there is NO cure for HCM. The veterinarian may, however, prescribe one or more medications in an effort to reduce the signs of heart failure and to help the heart function as efficiently as possible. Treatment options and the specific drugs prescribed will vary depending on the cat's clinical signs and the heart's functional impairments. Calcium channel blockers and beta blockers are drugs that are commonly used in cats with HCM, either when they are in heart failure or before heart failure occurs. Beta blockers are particularly effective at reducing SAM. Cats in heart failure need a diuretic and an angiotensin converting enzyme (ACE) inhibitor. Fluid withdrawal from the chest is required if fluid is present in the chest cavity. Aspirin or other agents may be given in an attempt to prevent clot formation. A cat that is suffering from respiratory distress, acute heart failure, or limb paralysis requires veterinary care as quickly as possible. There is nothing you can do at home to relieve your cat's distress or pain. Continued veterinary care is essential to the cat's survival.
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