By far the most common form of myocardial disease is known as dilated cardiomyopathy (DCM). It affects the muscular walls of the heart and prevents them from expanding and contracting correctly. In most cases, the cause of the disease is unknown, although it has been associated with nutritional deficiencies, viral infections and certain medications. There is also thought to be a genetic link, since it affects some breeds more than others. Males are more commonly affected.

This illustration shows part of the muscular wall of the heart that can be affected by myocardial disease.

Failure of the heart muscle fundamentally affects the heart's ability to circulate blood around the body.

Although dilated cardiomyopathy can affect any dog, it is more commonly seen in certain large breeds, such as Dobermanns, Irish Wolfhounds, Great Danes, St Bernards and Newfoundlands.