Dilated Cardiomyopathy (DCM)

Description:
Dilated Cardiomyopathy (DCM) is a condition characterized by weakness of the heart muscle (poor contractility) that eventually leads to enlargement (dilation) of the heart chambers and complications of congestive heart failure, irregular heart rhythms, and/or sudden death in affected individuals. The condition is infrequently diagnosed in dogs and is rare in cats. It is most common in large or giant breed dogs (i.e. Doberman Pinscher, Newfoundland, Great Dane, Irish Wolfhound, Boxer), with Cocker Spaniels being one notable exception. The prevalence in certain breeds suggests a genetic predisposition for the condition. It affects adult, usually middle-age or elderly patients.

Figure 1A/B: Two dimensional short axis echocardiograms of the left and right ventricles in a normal dog (A) and a dog affected with DCM (B).

Cause:
Genetic factors seem to play a large role in the inheritance and development of DCM in the dog, although a definitive genetic mutation for the condition has not been identified in any breed leading to the term Idiopathic Dilated Cardiomyopathy. Several other non-cardiac conditions can contribute to the development of a dilated, poorly contracting heart muscle including amino acid deficiencies (taurine/carnitine), a low thyroid level, inflammation of the heart muscle (myocarditis), prolonged rapid heart rates, and low blood flow to the heart muscle (ischemia). In cats the condition has for the most part been eliminated after a discovery in the late 1980's of taurine deficiency as the primary cause in this species. Taurine is now supplemented to all commercial cat foods and the condition is rare, either seen in cats fed a home-cooked vegetarian diet or in patients with ischemic disease.

Symptoms:
In many dogs with DCM there is a long preclinical phase where that are no obvious symptoms. Weakness, fainting spells, difficulty breathing, distension of the abdomen with fluid, and coughing are all possible in the later stages of disease. Some animals may die suddenly from an irregular heart rhythm without any previous symptoms.

Diagnosis:
The identification of DCM is initially based on physical exam findings and recognition of characteristic features (dilated, poorly contracting heart) on an echocardiogram. The echocardiogram is interpreted in conjunction with several other tests to rule out other conditions that can cause similar echo findings such as a low thyroid level, amino acid deficiency (taurine, carnitine), inflammation of the heart muscle (myocarditis), or decreased blood flow to the heart muscle (ischemic heart disease secondary to a heart attack, as in people). In some breeds (Boxer, Doberman pinscher), screening with a 24 hour ECG recording (Holter monitor) may be able to identify animals early in the course of the disease process so that therapy can be initiated early and patients can be removed from the breeding population, if they are active breeders. In other affected individuals, a Holter monitor may be recommended to determine the frequency and severity of irregular heart rhythms to help guide treatment. Although research is underway to identify genetic markers for the condition in certain breeds, none have been identified to date. Consequently blood testing to screen for the disease is not yet feasible.

Treatment:
There are no proven strategies for preventing the development of DCM, although many medical protocols and nutritional supplements have been proposed. Some medical therapies have been demonstrated to delay progression of DCM once a diagnosis is reached, and in patients with underlying conditions causing the heart muscle weakness (thyroid deficiency, taurine deficiency), supplementation with the appropriate therapies can halt or reverse the heart muscle changes. If congestive heart failure develops, a combination of diuretics, drugs to support heart muscle contraction, and medications to prevent fluid retention are all indicated. In patients that have irregular heart rhythms, drugs to stabilize the rhythm are also frequently added. There is no cure for DCM, and a heart transplant would be indicated in similarly affected people.

Follow-up:
In patients with DCM that are asymptomatic, recheck echocardiograms, EKGs, radiographs, and Holter monitors are useful to assess therapy and document the rate of progression of disease. In symptomatic patients receiving heart failure therapy, these diagnostics are performed more frequently and combined with intermittent labwork (blood samples) to monitor for side effects of drug therapy and to monitor for other concurrent diseases.

Prognosis:
Unfortunately the overall prognosis for patients with DCM is poor. The individual prognosis varies to some degree with the breed, with Cocker Spaniels having the longest clinical course (>1 year) and Doberman Pinschers having the shortest (3-6 months) once heart failure symptoms are apparent. Any affected breed is at risk for sudden death from the development of lethal arrhythmias.

Summary:
Dilated cardiomyopathy is a disease of predominantly large and giant breed dogs that results in progressive heart muscle dysfunction, dilation, and eventual congestive heart failure or death of affected patients. The exact cause of the condition is unknown but genetic factors are presumed to play a role. There is no known effective preventative strategy for the condition. Treatment for affected individuals may improve their quality of life, delay the onset of heart failure symptoms, and potentially improve life expectancy. Treatment is not curative, however, and most affected individuals eventually die from the disease.

Recommendations:
In breeds that are considered high risk (Doberman Pinscher, Boxer, Great Dane, Irish Wolfhound, Scottish Deerhound, Newfoundland, Cocker Spaniel), yearly screening with an echocardiogram and Holter monitor are recommended for individuals that are in an active breeding program or non-breeding individuals that have a family history of DCM.