Mitral Valve Degeneration (MMVD)

Overview

Myxomatous mitral valve degeneration (MMVD) is a disease of the mitral valve, which is the major heart valve separating the left atrium from the left ventricle. This valve prevents blood from flowing backwards when the heart (left ventricle) pumps blood to the body. When the valve is diseased, it becomes leaky, and blood is pumped backward every time the heart beats. If enough blood leaks backward, it increases the pressure in the left atrium and lungs. When that pressure is high enough, it forces water out of the blood vessels into the lungs. At that stage, it is called congestive heart failure (CHF), and can be fatal unless treated.

MMVD is the most common heart disease in dogs, accounting for approximately 75-80% of all heart disease diagnosed in dogs. As a degenerative disease, it most commonly affects older dogs. It is estimated that >75% of dogs over the age of 13 years have MMVD, but it can affect dogs at much earlier ages (dogs as young as 2 years of age have been reported). It is more common in males than females, and is far more common in toy or small breeds of dogs.

The exact cause of MMVD is unknown, but there appear to be genetic and familial factors involved in the disease. Some breeds (Cavalier King Charles Spaniels, Chihuahuas, poodles, dachshunds) have a higher prevalence of MMVD.

Diagnosis

MMVD causes thickening of the valve leaflets, which pulls the valve apart and causes it to become leaky. This leak of blood is very turbulent, which often creates a murmur that a veterinarian can hear with a stethoscope. This is akin to putting your thumb over the end of a hose; it causes the water to flow faster through the narrow orifice, which causes a hissing noise. The murmur generally increases with increasing leakage of blood, so it is important to note if the murmur is getting louder. This can be a sign of worsening disease, and may require further workup such as chest radiographs or an echocardiogram. Radiographs (X-rays) allow a veterinarian to look at the overall size and shape of the heart, and also allow us to look at the lungs to assess if there is congestion present. An echocardiogram (cardiac ultrasound) allows the cardiologist to look directly at the mitral valve and other cardiac structures. This can confirm the diagnosis, helps with prognosis, and evaluates for any complicating factors/diseases.

Prognosis

It is unclear how long any dog will live with MMVD. Many dogs with MMVD will live their entire lives without problems related to the disease. Some dogs, however, will progress from asymptomatic disease to congestive heart failure. A cardiologist can help with your pet’s likely prognosis; however, there is no exact way to predict progression in each case.

There is no medication or intervention that is currently available to cure or slow progression of the disease. In humans, MMVD is most often treated surgically; unfortunately, this requires cardiopulmonary bypass at this time, and is thus unavailable or cost-prohibitive in a majority of veterinary cases. Additionally, there has not been a study proving that surgery is better than medical management in dogs. Nevertheless, if you are interested in a surgical option, please ask a cardiologist for advice. Some medications may be effective if started prior to the onset of CHF, but this decision is usually made on an individual basis.

Symptoms

There are usually no signs of MMVD until the leakage is severe enough to cause congestion. Typically, the first sign of MMVD will be a cough. The cough is usually dry, non-productive, and worse in the morning or during excitement. Another common initial symptom is gradual “slowing down”, or exercise intolerance; your dog may not walk as often or as far, or may tire easily. Infrequently, MMVD may cause collapse (fainting) or difficulty breathing as the first sign of disease. Both are medical emergencies, and you should take your dog to the veterinarian immediately if these are noted.