Myxomatous mitral valve disease (MMVD) is considered the most common congenital heart disease in dogs as it accounts for more than 70% of all canine heart disease. The disease is chronic and progressive, early symptoms is usually a heart murmur, developing after the age of six. Approximately 30% of dogs with MMVD progress to mitral regurgitation (MR) and eventually heart failure.

The disease is common in small breeds and the incidence is particularly high in some breeds such as the Cavalier King Charles spaniel (CKCS).

The heart has 4 chambers, 2 upper chambers (atria) and 2 lower chambers (ventricles). Blood passes through a valve before leaving each chamber of the heart. The valves prevent the backward flow of blood (regurgitation). The valve between the left atrium and left ventricle is called the mitral valve. The mitral valve is responsible for making sure that blood flows from your left atrium to your left ventricle. It also ensures that blood doesn't flow backward between those two chambers.

Dogs with MMVD have progressive mitral valve degeneration. over time, the abnormal valve allows blood to leak backward, or regurgitate, into the left atrium. As the left atrium becomes bigger and bigger to accommodate the extra blood from the regurgitation, increased pressure develops in the left atrium. The blood returning from the lungs into the left atrium is backed up, causing fluid to build up in the lungs. This is called congestive heart failure. Congestive heart failure can lead to difficulty breathing but can be managed with diuretic medications for a certain period of time.