Hypertrophic Cardiomyopathy is characterized by an inappropriate increase in heart muscle thickness. In these cases, not only do the walls expand outward into the chest cavity but they also grow inward, shrinking the interior space of the heart and subsequent volume and pumping capacity. The heart does not relax normally, and cannot fill with blood normally. This results in elevated pressures in the heart and lungs, which causes heart failure and congestion. Sudden death can also occur due to poor blood flow to the abnormally thick heart muscle. Hypertrophic cardiomyopathy is the most common form of heart disease in cats, and most frequent cause of spontaneous death in indoor adult cats (Eldredge, et. al., 2008).
Hypertrophic cardiomyopathy is a genetically inherited disease. It has been found to have familial inheritance in Maine Coons, Ragdolls, British Shorthairs, American Shorthairs, and Devon Rexes (Eldredge, et. al., 2008). It tends to affect cats at 5 – 7 years of age, although it may be diagnosed at cats of an age (3 months to 17 years).
The cause of HCM in most cats is unknown. In people, most forms of HCM are due to genetic mutations; this is likely true in cats, as well. Two mutations resulting in HCM have been identified in cats thus far. Maine Coons and Ragdolls have slightly different mutations, both resulting in HCM. Genetic tests have been recently developed for these mutations; however, there are many different mutations in humans that result in HCM (over 500 and counting), and the same is expected in cats. Alternatively, a thickened left ventricle may be caused by high blood pressure, hyperthyroidism, or some endocrine disorders – these are usually tested for, as they are treated differently.
Genetic testing (most commonly via cheek swab) may determine if your cat is carrying one of the 2 identified genetic mutations resulting in HCM, but it will not determine if your cat will have HCM. In other words, your cat may have a positive result (carrier) but will not have HCM; or, they may have a negative result, and still have HCM. Thus, the best test at this time remains an echocardiogram. This is an ultrasound of the heart, and will show if your cat’s heart is abnormally thickened or enlarged. There are blood tests being developed to help screen for HCM, but are still investigational at this time.
There is wide variation in disease severity. A cat with HCM may live a normal life and never experience any problems related to heart disease. On the other hand, a cat with HCM may have trouble breathing, stop eating, become weak/paralyzed, or even die suddenly. Diagnostic testing, including an echocardiogram, chest X-rays, and bloodwork can help the cardiologist determine the severity of disease, prognosis, and likely signs that may develop. Again, the prognosis is variable and related to the severity of disease. Diagnostic testing is designed to help determine the most likely progression and prognosis for the individual cat; unfortunately, no test is 100% accurate at this time. Generally speaking, the average cat with HCM and heart failure will live 6 months to one year. This is an average, so some cats live much longer, and some cats do not do as well.
It cannot be stressed enough that cats are absolute masters at self-regulation and masking pain, thereby disguising any weakness or discomfort until it is too late. “Early signs of hypertrophic cardiomyopathy are vague and indefinite” (Eldredge, et. al., 2008). The following signs listed may present themselves, but it is likely that the owner would not notice any symptoms until the cat has entered a state of congestive heart failure. Tragically, the first and only sign of hypertrophic cardiomyopathy may be unexpected sudden death. Still, here is the list of symptoms. If you notice them, visit your veterinarian immediately.
At this point, there is no cure for HCM in people or cats. In people, treatment includes drugs, open-heart surgery, device implantation and cardiac transplantation. In veterinary medicine, many of these therapies are unavailable or cost-prohibitive. Atenolol is a drug frequently used in cats with HCM. It slows the heart rate, which decreases the amount of work the heart must do. Other drugs are withheld until complications of HCM arise, such as congestive heart failure, arrhythmias or blood clot formation. The cardiologist will determine the best therapy for your cat based on test results.