Dilated and arrhythmogenic cardiomyopathy in dogs (Proceedings)
In this lecture we will discuss canine dilated (DCM) and arrhythmogenic cardiomyopathy (ARVC). We will pay particular attention to breed specific findings.
Canine Dilated Cardiomyopathy - DCM
Strictly speaking, dilated cardiomyopathy (DCM) is defined as an idiopathic functional abnormality of the myocardium causing systolic dysfunction and/or arrhythmias. Definitive breed predispositions exist. Even though the disease is referred to as DCM in each breed, there are important clinical and pathological differences between the breeds mentioned. Although Boxer dogs can get dilated cardiomyopathy, most Boxers develop Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC), a very different disease.Dilated cardiomyopathy is an adult onset disease, with the exception of the Portuguese Water Dog in which it is diagnosed between two and thirty-two weeks.
There appear to be two stages of DCM, an asymptomatic stage, referred to as occult, which may be detected by careful screening, and a stage at which symptoms appear, referred to as overt. There is some preliminary evidence that early recognition of the occult stage may slow progression of the disease therefore, being aware of early signs may be beneficial to patient management.
Clinical signs may include coughing, dyspnea, tachypnea, syncope and occasionally, ascites.
A soft systolic murmur consistent with mitral valve regurgitation and/or a gallop rhythm (S3) may be ausculted at the left apex.
A tachyarrhythmia of sinus, supraventricular or ventricular origin may be noted. In some cases, a murmur or an arrhythmia may be the first signs of the occult form of the disease and should not be overlooked. Since primary valvular disease is relatively uncommon in large breed dogs, and the detection of DCM before the development of congestive heart failure (CHF) may be beneficial in the long-term management of the case, identification of a new murmur, gallop or tachyarrhythmia in suspect breeds should be considered a "red flag" and may warrant a thorough cardiac work-up. Although canine DCM is predominantly a left ventricular disease, biventricular involvement and heart failure with jugular venous distension and ascites is frequently noted, particularly in the giant breeds.
Many dogs with DCM have normal electrocardiograms but atrial and/or ventricular enlargement patterns (R > 3.5 mV Lead II for the left ventricle) may be noted. Sinus tachycardia, atrial fibrillation or ventricular arrhythmias are common. In some cases, ventricular tachyarrhythmias can develop before any ventricular dilation or systolic dysfunction. Routine Holter monitoring may help detect these.
Dilated cardiomyopathy is a progressive myocardial disease. If the disease is diagnosed in the early stages, radiographic findings may be subtle. Therefore, depending on the stage of the disease, thoracic radiographs may be within normal limits or may indicate atrial and ventricular enlargement (typically left) with or without pulmonary venous distension and pulmonary edema. In some cases, biatrial and biventricular enlargement may be noted.
Echocardiography is the diagnostic test of choice for diagnosing canine DCM and is also an important test for occult disease. Echocardiographic findings in the patient with overt disease should include left and sometimes right atrial and ventricular dilation and decreased contractility as characterized by shortening fraction (FS%). Typically the decrease in contractility is quite severe with decreased fractional shortening % (< 20%).
A differential diagnosis for DCM is severe atrioventricular (AV) valve disease since severe ventricular dilation and systolic dysfunction may be occasionally observed in these cases. Consideration of the breed of dog may be helpful in differentiating between DCM and AV valve disease since it is uncommon for many of the large breed dogs to develop significant primary valve disease. An exception to this may be the cocker spaniel, a breed that has a high incidence of primary valve disease, and also is at increased risk of DCM.
Prohormone BNP is released when the ventricles are dilated, hypertrophic or subjected to increased wall tension. Levels of BNP (Idexx) have been shown be increased in dogs with congestive heart failure and can be used to help diagnose, or exclude a diagnosis of heart failure in dogs that presented for cough or dyspnea. At this point it has not yet been shown to be sensitive enough for detection of early disease.
It is clear that several breeds appear to be over represented and some breeds seem to have unique characteristics of the disease that may suggest that this is unique disease for their breed. A familial form of DCM has now been identified in several breeds and is suspected in others. Occasionally, atypical breeds of dogs develop DCM. The etiology of the disease in these cases is unknown and external factors that can insult the myocardium including infectious organisms or nutritional imbalances should be considered.