Hypertrophic cardiomyopathy (HCM) is a disease of the heart muscle, or myocardium, that impairs the heart’s ability to receive and pump blood throughout the body. It primarily affects cats and there is some evidence of a genetic cause for this disease. A gene test is available for the diagnosis. However, it should be stressed that HCM is a familiar disease, and the same clinical picture could be induced by different gene mutations. Therefore, a negative gene test does not necessarily mean that a cat does not necessarily exclude the presence of the disease. Affected cats can be as young as one year old, though it is most common in cats aged four to seven years. There is no cure for HCM, but its symptoms can be managed with medication and many cats never progress to heart failure.
To better understand how the changes that occur with HCM affect the normal function of your pet’s heart, please see Normal Heart.
In HCM, an unknown process at the cellular level causes the muscular wall of the left ventricle to thicken (figure). The thickened muscular wall effectively expands inward and fills the space that is normally available for blood during diastole. Over time, the volume of blood in the left ventricle during diastole is decreased, and so is the volume of blood ejected during systole. Blood from the lungs returns to the left atrium and then is pushed into the left ventricle. The decreased diastolic function (i.e., filling) of the left ventricle causes the pressure within the chamber to rise, and this increased pressure can be transmitted back to the left atrium (leading to left atrial enlargement) and to the lungs causing fluid retention within the lungs and pleural space (i.e., pulmonary edema and pleural effusion). This condition is known as congestive heart failure.
Because these changes to the heart muscle occur over a long period of time, an animal with HCM can compensate for a very long time and appear completely normal until suddenly developing signs of heart failure, such as difficulty breathing. An echocardiogram is necessary for a definitive diagnosis of HCM.
A common and serious complication of HCM in cats is represented by arterial thromboembolism. Left atrial enlargement favors blood stasis and thereby formation of clots in the heart (called thrombi). These thrombi can leave the heart and travel along the aorta, finally lodging in and obstructing, partially or totally, the vessel. It usually occurs at the level the aorta divides to originate the two vessels heading to the rear limbs. As a consequence, the blood supply to the rear part of the body is blocked and affected cats can present with sudden onset of pain, paresis and weakness. This is an emergency situation and you should see your veterinarian or an emergency service as soon as possible.
Treatment and Prognosis
Cats with HCM are commonly prescribed medications that maximize the heart’s relaxation/filling ability during diastole, thus allowing the heart to eject as much blood as possible during systole such as beta-blockers or calcium antagonists. Although there is no cure for HCM, managing signs of heart failure. if and when they occur can maintain a good quality of life for as long as possible. Patients can survive months or even years with this disease. Many cats actually never progress to heart failure. Cats with thromboembolic disease have a more guarded prognosis. Sudden death is also possible with this disease.
Early recognition of heart failure is key to preserving your pet’s quality of life. Utilizing the Heart Failure Diary and following the recommendations in your personalized treatment/monitoring plan ensures the highest level of care for your cat.
Short axis cardiac ultrasound image of a cat with HCM (left). The lumen (black area in the center) is reduced. The right picture represents the same ultrasound view but in a normal cat.