Heart Disease in Dogs: Dilated Cardiomyopathy

DCM Dilated Cardiomyopathy (DCM) is a common heart disease that affects your pet’s health.  DCM is a blanket term for any dilation noted in the heart chambers (ventricles or atria). To better understand DCM, one must visualize what is actually happening to the heart.  This image shows the changes seen in DCM, most notably the thinning of the walls of the ventricles.  In addition, the atria, most notably the left atrium, has become dilated.  This can have serious side effects on the electrical signals through the heart.

The mechanism of DCM is one of decreased contractility.  The heart muscle itself does not contract with normal strength to help eject blood properly.  This, in turn, creates a situation where the heart has to attempt to pump extra blood.  This is called volume overload.  The heart stretches to accommodate the increased volume of blood.  This stretch changes the way the muscle cells are aligned and, more importantly, changes the structure of the mitral valve that sits between the left atrium and ventricle.

This causes the left side of the heart to backup with blood into the lungs due to the inability to eject blood properly (poor contractility) and improper backflow through the mitral valve (mitral regurgitation).  The lungs will then fill with fluid (pulmonary edema) causing respiratory issues.

The other common side effect of DCM and enlarged left atrium is atrial fibrillation.  The larger tissue of the big left atrium can cause electrical signals to fire abnormally from improper locations and at improper times.  Thus, instead of a nice, controlled and organized contraction, the heart has a very irregular contraction that is even more inefficient thereby compounding the poor contractility of DCM.

Genetic Predilection of Dilated Cardiomyopathy in Dogs

Overall, DCM is seen in less than 0.3% of dogs.  As many of you Doberman pinschers ownersdobermans know, your beloved Dobies are far and away the highest risk breed for this disease.  Boxers make up 15% of DCM cases seen at UC Davis, second to the Doberman’s 33%.  Great Danes, Labrador Retrievers, and Cocker Spaniels rounded out the top 5 highest seen breeds.

The genetic inheritance has been proven in Irish Wolfhounds, Dobermans, and Newfoundlands, but research is still ongoing for most other breeds.  Interestingly, male Dobermans tend to present more frequently (up to 79% of cases were male in one study) and earlier in life with more severe signs than females.  The reason for this is unknown.

Keister DM: A study of Phase IV clinical data on milrinone, a canine cardiotonic.  Ptrocol H., Sterling-winthrop Research Institute, 1988.

Causes of Dilated Cardiomyopathy in Dogs

Like many terms used in both veterinary and human medicine, dilated cardiomyopathy is a blanket term that encompasses many different diseases that incite the same heart changes and signs.  Much of the time, we do not find an inciting cause and are left with idiopathic dilated cardiomyopathy.  “Idiopathic” means that it is a recognized syndrome that we do not know the actual cause for.  This is a long way of saying that when we talk about DCM, we almost always are talking about idiopathic dilated cardiomyopathy.

Other causes of DCM include viral infections (parvo, distemper), tick-borne (Ehrlichia, Lyme disease), parasites (trypanosome), elevated thyroid, and nutritional deficiencies (taurine, carnitine).  Depending on your geographical area, your veterinarian may test for some of these other causes when your pet is suspicious for DMC.  Overall, these make up less than 10% of DCM diagnoses.

Diagnosis of Dilated Cardiomyopathy in Dogs

Diagnosis of DCM starts with listening to your dog’s heart.  Instead of sharp sounds (lub-dub lub-dub, like this), the sounds will not be distinct (schhh-dub schhh-dub, like this).  These abnormal sounds are called murmurs.  After hearing a murmur, your veterinarian may take xrays or recommend an echocardiogram, which is an ultrasound of the heart.  Other signs of heart disease include coughing, weakness, exercise intolerance, or lethargy.  These will guide the diagnosis of heart disease.

A cardiologist will perform an echocardiogram. If heart disease is suspected, chest xrays are taken to evaluate the heart size and for any concurrent fluid in the lungs. The echocardiogram is the most definitive diagnosis of DCM, but it does not show the underlying cause.  As discussed above, 90% of the time we do not find the inciting cause.  Further work up includes testing for deficiencies in different proteins (taurine, carnitine), searching for infectious agents, and careful diet evaluation.

Treatment of Dilated Cardiomyopathy in Dogs

Treatment for DCM varies depending on the severity of the case and the inciting cause.  In nearly all cases of DCM, the primary goal is to increase contractility of the heart.  Pimobendan (vetmedin) is a newer medication that is excellent at increasing contractility.

Treating concurrent rhythm abnormalities such as atrial fibrillation is best done with heart rate limiting drugs.

Treating heart failure, a situation where fluid backs up into the lungs causing difficulty breathing, is done with diuretics such as lasix.  Other long term medications include ACE-inhibitors like enalapril.

Some have advocated dietary supplementation of taurine, carniting, and co-enzyme Q10.  Currently, there does not seem to be much consensus on the effectiveness of these treatments, though the side effects are minimal so they may be worth attempting for a trial period.

In human medicine, a heart transplant is the treatment of choice.  This has been performed in canine patients, though on a very limited basis with only mild success.  Transplants in veterinary medicine face a host of ethical considerations.  Interestingly, there have been surgical attempts at attaching a back muscle around the heart and allowing the normal electrical stimulation of the heart to stimulate the new back muscle.  This essentially acts like a jacket of muscle to pump blood by giving the heart extra contractility.  It has been tried rarely in veterinary medicine with low success.

Prognosis of Dilated Cardiomyopathy in Dogs

The prognosis of DCM is variable depending on the inciting cause.  Cases that are caused by infection or nutritional deficiencies can return to normal function.  Without an inciting cause, as with 90% of cases, the prognosis is an unknown.  Older dogs and males seem to not do as well in the long term.  As with most heart disease, we cannot cure DCM, we can only hope to manage it.

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Dr Roger Johnson and Dr Kyle Marano

Roger K. Johnson, DVM, Diplomate ACVIM (internal medicine) is a board-certified veterinary internal medicine specialist. His professional interests include cardiology as well as using advanced diagnostics to help his patients. His particular favorites include echocardiography, abdominal ultrasonography, and endoscopy. Kyle Marano, DVM is a small animal veterinarian practicing out of Northern Colorado. He has written pieces ranging from sports commentary and analysis to quips on the every day life of veterinary medicine. His furry family includes a chocolate lab mix and an overly nosy cat.

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