Literally, the term "cardiomyopathy" means disease of the heart muscle. More specifically, cardiomyopathy (CM) is a disease of the heart muscle in which either the heart walls thicken greatly (hypertrophic and restrictive forms) or stretch greatly (dilated form). Hypertrophic cardiomyopathy is the most common form. In either form, the heart's function is greatly compromised, and heart failure results.
The left ventricle is primarily affected. This is the largest most muscular chamber of the heart that is responsible for pumping blood to the most distant organs and tissues. All of the forms of CM compromise the left ventricle's ability to pump oxygen-carrying blood to the tissues. The left atrium is the smaller chamber above the left ventricle. In advanced disease, it becomes dilated and poorly functional.
Several causes of feline cardiomyopathy have been identified. A deficiency of taurine, an essential amino acid, will cause dilated CM in the cat. Taurine was deficient in many commercial cat foods until its deficiency was identified; most cat foods are now properly supplemented. Hyperthyroidism, a non-cancerous growth of the thyroid gland, can cause a variation of the hypertrophic form of CM. The restrictive form is associated with an unidentified inflammatory process within the heart muscle. Genetics is suspected but has not been proved. However, most cases of CM are not caused by any of these processes, and we do not understand their origin.
Cardiomyopathy is a disease that usually takes several weeks to months to progress to a serious stage. During the early weeks of the disease, the cat will probably look normal. Cats have the ability to hide serious illness until it reaches a crisis stage. Therefore, most cats that develop clinical signs of cardiomyopathy will appear to have been ill for only a few days. A few days of inactivity and poor appetite occur first. Just prior to the state of heart failure and death, the cat may become very inactive and exhibit labored breathing. Both may be due to insufficient oxygen transport to the body's tissues; the latter may also be due to a collection of fluid in or around the lungs.
As explained below, blood clots may be released from the heart and obstruct major blood vessels. The most common occurrence is obstruction of the large arteries going to the rear legs, resulting in paralysis and pain. Strokes are also possible due to a clot going to the brain. These cats appear incoherent and dazed and may walk in staggering movements, often in a circle, or have one pupil larger than the other. The head will tilt markedly to the side in some of these cats. If a blood clot goes to a front leg, there will be lameness or paralysis to that leg.
Diagnosis is generally made with a chest radiograph (x-ray). The heart will have an abnormal shape and fluid in or around the lungs may be detected. If a large amount of fluid is present around the lungs, it may be necessary to remove it and take more x-rays because the presence of this fluid interferes with evaluation of the heart. Many cases also require better visualization of the heart with an echocardiogram, or sonogram. This is a non-invasive method for looking at the heart while it is pumping. Sound waves are used to make this dynamic study of the heart. The x-ray can tell us about the size and shape of the heart but nothing about heart function. The echocardiogram can provide this information. It will also allow measurement of the heart muscle to determine if it is too thick (hypertrophic CM) or too thin (dilated CM). Finally, an electrocardiogram (EKG) is useful to evaluate the rate and rhythm of the heart.
Determination of the level of thyroid hormone in the blood (T4) is often indicated in evaluating cats with hypertrophic CM. This simple blood test can help identify an overactive thyroid gland as the underlying cause of heart disease.
Treatment is based on the type of CM present. Different drugs are used for the two different forms to support heart function. Therefore, if at all possible, tests necessary to define the specific form of CM are performed before treatment begins. Fortunately, most of these cats can be stabilized with the correct drug; however, continual medication may be necessary since the disease often cannot be cured. The exception to this is the cat with hyperthyroidism. If hyperthyroidism is identified in a cat with hypertrophic CM, the heart disease is potentially reversible if the cat receives appropriate and timely treatment for the thyroid disease.
Aspirin is another component of treatment. It is used to reduce the formation of blood clots, although it will not dissolve any that have already formed. Aspirin is not routinely given to cats due to it very slow breakdown resulting in toxic levels in just a few days. However, it can be given safely if dosed properly.
Most cats with cardiomyopathy develop signs of heart failure, as previously described. However, cats with CM are prone to producing blood clots within their hearts. When these clots escape the heart, they travel through various arteries leading from the heart. They eventually lodge in a narrow part of the artery. The most common site for clots to lodge is the point at which the aorta splits before going into the rear legs. Thus, these cats often become paralyzed in the rear legs very suddenly and are in significant pain. In many cases, it is paralysis and pain that first becomes noticeable and is the reason that medical treatment is sought. Some owners mistake this event for an uncomplicated lameness, or even a broken leg. When these cats are examined, there are no pulses to one or both rear legs, the legs are cold, and the footpads appear blue (cyanotic) due to the lack oxygen.
Treatment of the paralyzed cat concentrates on drugs to relieve pain and to hasten the return of circulation to the legs. Since these cats also have severe heart disease, they make poor surgical candidates. Therefore, surgery to remove the clot is not advisable due to the high incidence of death during surgery.
The prognosis for the paralyzed cat is variable but is improved if the cat receives immediate attention. Within 3-10 days, circulation is restored, and leg function returns in most cats. However, the heart disease must be controlled quickly or they will die.
The prognosis for CM is variable depending on the form of the disease and the severity at the time of diagnosis. If congestive heart failure is present, the first few days are critical, and the outcome could be good or disappointing. If congestive heart failure is not present or is resolved, the treatment outcome is very likely to be good. Until not long ago, the average survival time was about 6 months. However, the use of newer drugs shows promise of giving quality life to these cats for many years.
When cardiomyopathy is caused by hyperthyroidism, the expectation is even better. If hyperthyroidism is successfully treated, the heart function will generally return to normal, and the cat will no longer require treatment.