Hypertrophic Cardiomyopathy (HCM)
HCM is by far the most common form of cardiomyopathy in humans and cats. In HCM, the walls of the left ventricle thicken (hypertrophy). Along with the thickening of the left ventricular walls, the papillary muscles (small muscles within the heart that anchor some of the heart valves) may be enlarged. HCM is a genetic, primary heart muscle disease.
You may have heard about 'secondary HCM'. It is important to rule out other diseases that can mimic cardiomyopathies. Left ventricular walls may also hypertrophy secondary to other diseases. Rule out:
High blood pressure, as it causes secondary thickening of the heart muscle in response to demand.
Hyperthyroidism (overactive thyroid gland), as excess thyroid hormone causes an increase in metabolic rate, which results in an increase in the work the heart has to do and subsequent enlargement of the heart or heart muscle thickening.
Although these diseases can cause the left ventricle to look thick they are not HCM.
Forms of Cardiomyopathy
Restrictive Cardiomyopathy (RCM)
In RCM the wall thickness is normal and the left ventricle can look relatively normal, but in fact it is stiffer than normal so the left ventricle still doesn’t fill very well and generally there will be an enlarged left atrium. This disease can be very difficult to detect in the early stages, as the left ventricle will look normal and the atrium will not be enlarged, therefore it can look like a normal cat!
Dilated Cardiomyopathy (DCM)
DCM is characterized by thinning and weakening of the heart muscle, leading to dilation of the heart chambers because the heart can no longer pump blood effectively. Historically, this disease was associated with a diet deficient in taurine, an essential amino acid. However, since this was discovered in the 1980s, food manufacturers have adjusted taurine levels such that this deficiency is no longer seen in cats eating a commercial diet. DCM is now rare in cats and when seen may represent the ‘end-stage’ or ‘burn-out’ phase of another cardiomyopathy such as HCM. This can happen when the blood flow and oxygenation to the heart muscle itself is compromised and heart muscle cells die off.
Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC)
ARVC has only recently been recognized in cats and is rare, but there is a suggestion that Birmans may be predisposed. In this disease, heart muscle cells of the right ventricle are replaced by fibrous and fatty tissue (‘fibrofatty infiltration’). This causes heart rhythm disturbances and enlargement of the right side of the heart.
Dr. Karen Becker’s video ‘Cardiomyopathy in Cats’