Diagnosing HCM

Hypertrophic cardiomyopathy may be mild, moderate or severe. Cats with HCM may develop heart failure, die suddenly, or form a clot inside the heart that then breaks loose and travels to different regions of the body (most commonly the hind legs). Only cats with severe HCM develop heart failure. Most, but not all, cats that form a clot have severe HCM. Although the incidence of sudden death appears to be small, it can happen at any time to any cat with HCM.

Cats that are in heart failure accumulate fluid in or around their lungs (pulmonary edema and pleural effusion, respectively). This fluid accumulation results in an increase in breathing (respiratory) rate and, if severe, causes labored breathing (dyspnea, or respiratory distress). Signs such as a mild increase in the respiratory rate are usually so subtle that they go unnoticed. Many cats with HCM develop a fast heart rate, and/or a heart murmur, and/or a gallop rhythm (an extra heart sound) as the disease advances. These signs cannot be relied upon, however, to indicate the presence of disease before it becomes severe, and all these signs occur with other forms of heart disease in cats and so are not predictive of HCM.

An echocardiogram (ultrasound of the heart) with color flow Doppler imaging is the most conclusive means of diagnosing HCM. An echocardiogram reveals both the physical structure and dynamic functioning of the heart. It is non-invasive and poses essentially no risk to the cat. An electrocardiogram and chest X-rays may provide your veterinarian with additional useful information with regard to heart rhythm and presence of fluid accumulation, but they cannot be used by themselves to arrive at a diagnosis.

The findings on an echocardiogram characteristic of HCM include thickening of the left ventricular walls (free wall and interventricular septum), enlarged papillary muscles, and systolic anterior motion (SAM) of the mitral valve. In cats with early or mild disease only the papillary muscles may be thickened. In cats that are destined to develop more severe disease, the wall thickening increases over time (i.e., the disease progressively worsens until the walls reach whatever maximum thickness they will attain).

Cats with severe HCM often have an enlarged left atrium. The left atrium is not enlarged in all cats with HCM but is enlarged in all cats that are in heart failure. Occasionally a clot will be found in the enlarged left atrium of a cat with severe HCM.

In some cats it is impossible to tell if they are normal or have HCM based on an echocardiographic exam. Those cats are generally placed in an equivocal category. In some of these cats the disease will progress so that subsequent exams will be more definitive. Since the early stages of HCM and mild to moderate HCM can be difficult to recognize, it is strongly recommended that a veterinary cardiologist be consulted for diagnosis as well as subsequent disease management.

The veterinarian will also perform other tests to determine if the hypertrophy is secondary to (caused by) another illness such as hyperthyroidism or hypertension. If no other causes are found, the cat is diagnosed with HCM.

A cat whose sibling, offspring, or parent developed HCM may also be at risk for the disease. Periodic echocardiograms might be useful to monitor the status of such a cat. This could be of particular importance if the cat is in a purebred breeding program, since one would not want to breed a cat that has HCM or a genetic mutation that causes HCM. Conversely, if a purebred cat develops HCM, it is very important to check its parents, siblings, and offspring for HCM, so it is important that the owner tell the breeder of that cat about the cat's diagnosis.

Any Maine Coon cat or Ragdoll cat diagnosed with HCM should be tested for their breed’s unique cardiac myosin binding protein C (MYBPC3) mutation. In Ragdolls it appears that this is the definitive diagnostic test. In Maine Coon cats, not every cat with clinical disease tests positive for that breed’s known MYBPC3 mutation, and so it appears that at least one more mutation that can cause HCM in Maine Coon cats remains to be found.

It is also possible to diagnose HCM at necropsy (veterinary autopsy). Since the heart may contract after death, the clinician should consider various factors such as the size and weight of the heart, the appearance and size of the left atrium, etc., in addition to left ventricular wall thickness. More specifically, HCM can be diagnosed when the left ventricular walls are too thick and the heart weighs more than 20 grams. The left atrium is usually, but not always, enlarged. A variety of microscopic changes characteristic of HCM (e.g., myocyte hypertrophy, myocardial fiber disarray, interstitial fibrosis) can also be found but may not be present in all cats with HCM (although they are present in all Maine Coon cats with HCM). Myocardial fiber disarray is the hallmark finding with HCM. Unfortunately, there are very few veterinary pathologists trained to correctly identify this abnormality. 
 

HCM FAQ for breeders

Drs. Kittleson, Gompf, and Little have addressed questions of special interest to cat breeders in their FAQ, Feline Hypertrophic Cardiomyopathy: Advice for Breeders. 
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Clinical case studies

Alice, whose first clinical sign was tachypnea (rapid breathing)

Freckles, whose first clinical sign was a saddle thromboembolus (rear leg paralysis)

Please keep in mind that your own cat's clinical signs, X-rays, and echocardiogram results may differ from those documented in the case studies. The treatment prescribed for your cat may also differ from those described in the studies, since your veterinarian will recommend the treatment most appropriate for your cat's individual needs.