Thursday, January 6, 2011

Heart disease (part 3: HCM)

Let me take a moment to briefly point out that heart disease in dogs and cats just plain sucks. We know what the diseases are, how to identify them correctly, and how to manage the end-stage result (congestive heart failure) - but as for preventing them or curing them, nope. We can do nothing. These diseases, once identified, are progressive. What course they will take often depends on breed of the animal, other disease processes occurring at the time, age, and genetics.

Cats being what they are, must have their own weird brand of heart disease. As I said in the last post, they can have DCM, just like dogs. The more common type of heart disease in cats however is hypertrophic cardiomyopathy (HCM). Maine coon cats are in particular prone to developing HCM. It is known to be a genetic trait - autosomal dominant for those hardcore nerds out there.

In HCM, the left ventricle of the heart and the wall that separates the ventricles start to thicken. As those walls thicken, the chamber becomes smaller and smaller, with less and less room for blood to fill the heart. Blood flow becomes turbulent, and a murmur can be heard.

Concurrently with this heart thickening, there is often something called SAM of the MV. This acronym stands for systolic anterior motion of the mitral valve. What it means in normal person speak is that one of the little leaflets of the mitral valve gets sucked into the outflow tract of the heart. It causes worsening turbulence and may contribute to blood clot formation in the heart.

There are some notable differences in cat heart disease that are important for all veterinarians to know. First and foremost, murmurs in cats are often much more difficult to hear. They are usually very focal - meaning that they can only be heard in one place on the chest (dog murmurs are often heard easily - unless they are very soft). Thus, very careful cardiac auscultation is absolutely necessary to make sure that cat murmurs are not missed. On top of that, cats often develop an unusual arrhythmia. It is called a gallop rhythm. It is so named because of the sound it makes. A traditional heart beat sounds like a lub-dub, lub-dub. A gallop rhythm sounds like a horse galloping - a tallot, tallot, tallot, tallot...

If a gallop rhythm is ausculted, this IS ABSOLUTELY A SIGN OF DISEASE. A gallop rhythm in a cat is ALWAYS pathogenic, bar none. If your veterinarian hears an abnormal rhythm in your cat, further investigation is warranted.

Like heart disease in dogs, there is no prevention, and there is no cure. Some cardiologists use ACE inhibitors like enalapril or benazepril presumptively, but there has been no proof that this helps with the heart disease.

Diagnosis is the same - careful auscultation of heart and lungs, xrays of the chest, and echocardiogram. Progression of the disease can follow a similar course as dogs (to congestive heart failure), but there are notable differences in cats. Cats are often asymptomatic up until they develop heart failure. Whereas dogs will often have a steadily worsening cough, possible exercise intolerance, and syncopal episodes, cats may be totally normal until they develop a severe problem (heart failure or a blood clot or both).

Cats tend to develop blood clots in the heart as a result of the turbulent blood flow. These clots will eventually break free from their moorings and enter into the bloodstream. These clots choose to lodge in one of two places - either at the blood supply to the front leg (usually one) or at the place where the aorta splits to the rear limbs. Classic signs of this include paralysis in the rear limbs. The legs will be cold, the muscles hard/rigid, and the condition is extremely painful. Often cats will present screaming in pain. There is no satisfactory or effective treatment for this, and often veterinarians will recommend euthanasia at this point.

The clots can break down with time, so sometimes supportive care with pain medications and monitoring can give the patient some time. The downside is that the clots will always recur - sometimes in a matter of days, sometimes not for months; but it will happen again. Thus, when cats throw clots, it is a very, very grim prognosis. It is ultimately fatal. Less commonly, the clots can lodge in the blood supply to one of the front legs, leading to limping and pain, as well as death of that leg. Like the back legs, the clots can break up with time, and the prognosis is slightly better, although still grim long-term.

These clots form in the absence OR presence of congestive heart failure. If heart failure is also present, the prognosis is absolutely bleak.

Cats also like to be unusual in that they develop pleural effusion with their congestive heart failure. Dogs develop pulmonary edema (fluid IN the lungs). Cats develop that AND fluid AROUND the lungs. Thus, managing them is a much more challenging situation. No one knows exactly why cats do this and dogs do not. There are several theories, but at this time, the reason is not known.

HCM in cats can be primary/idiopathic, related to breed (Maine coon cats), or secondary to metabolic disease such as hyperthyroidism and hypertension. Elucidating a cause is important. If the HCM is secondary to hyperthyroidism, it cannot be cured or fixed, but the thyroid can be managed, hopefully stopping the progression of heart disease.

Other than HCM, cats also develop unclassified cardiomyopathies, arrhythmogenic cardiomyopathies, and restrictive cardiomyopathies, but these are less common.

1 comment:

Nicki said...

Thanks for the good review. Once I had a saddle thrombus cat live a few years. Always had one leg that was never quite right but really beat the odds!