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Summary
This degenerative disease of the heart's valves is the most common cardiac condition in dogs. While it comprises 75% of all heart diseases in dogs, it's considered uncommon in cats.
This disease goes by the name of "endocardiosis," "chronic valvular disease" or "chronic valvular fibrosis." Because it afflicts two valves in particular, it's more commonly referred to as either "mitral valve disease" (affecting the left atrio-venticular valve) or "tricuspid valve disease" (at the right atrio-ventricular valve).
Mitral valve disease is far more common than its tricuspid varietal. Approximately 60% of all endocardiosis cases suffer mitral disease while 30% experience both mitral and tricuspid forms concurrently. Only 10% of endocardiosis cases fall into the tricuspid-only category. As such, this article will only treat the mitral version of the disease.
When the valve between the two left chambers of the heart (the "mitral" valve, between the left atrium and left ventricle) fails to form a tight seal, the blood pumped from the atrium into the ventricle tends to be "regurgitated" back into the atrium.
The heart muscle is a pump that moves blood through the four chambers with involuntary contractions that promote the one-directional flow of the blood. The valves between the chambers prevent the backflow of blood into the preceding chamber, thus keeping the blood flowing in the direction it should.
When the valves degenerate over time they become thickened and deformed, thereby losing the tight seal and causing some blood to move backwards in the heart. This means the heart has to work harder to pump the volume of blood the body needs for normal functions.
In mitral valve disease, "insufficiency" of the valve comes from a progressively degenerative process of unknown--though presumably genetic--origin. Older, small breed dogs are predisposed, with males accounting for slightly more patients than females.
Symptoms and Identification
Dogs with early or mild mitral valve disease will be identified by a heart murmur of low-grade severity heard loudest over the left side of the chest. In these cases, no other symptoms are typically seen. As this progressive disease advances, exercise intolerance, increased respiratory rate and/or effort, and a cough may develop. Syncope (fainting) may also occur after dogs experience abnormal heart rhythms. Sudden death is rare, and usually occurs only when a catastrophic degree of severity causes the left atrium to rupture.
Dogs with advanced or severe mitral valve disease may also experience symptoms consistent with heart failure. Heart failure occurs when the heart can no longer pump the requisite amount of blood for normal body functions. What happens then is that the blood backs up in the lungs on its way to the diseased, left side of the heart. This is what we call "pulmonary edema."
To identify this disease among dogs who suffer characteristic murmurs or other telltale symptoms, diagnosis is best achieved through simple chest X-rays (which may demonstrate an enlargement in the left side of the heart), electrocardiograms (EKG) and echocardiography (ultrasound of the heart) with doppler (demonstrating the flow of blood) to elucidate the abnormal change in the dimensions of the heart and the blood flow through the region of the mitral valve.
Seeing a cardiologist is always the best option when it comes to severely affected dogs.
Affected Breeds
Cavalier King Charles spaniels are notoriously predisposed to mitral valve disease. The following breeds are also predisposed:
Treatment
Treatment for mitral valve disease is relegated purely to the pharmacologic. In other words, drugs alone are the go-to therapies for all degenerative valve diseases in pets.
Luckily, a great many of our mitral valve disease patients don't require any treatment at all until they show symptoms, and most that do tend to live well with drug therapy alone. Only the severely diseased tend to succumb, in spite of treatment.
Though "open-heart" surgeries have been attempted to replace these valves, the risk of highly experimental surgery is not considered worth seeking at the present time. Drugs, low-sodium diets and exercise restriction comprise all available treatments for the present.
Drugs employed are designed to treat the symptoms of the disease rather than the problem itself. Diuretics for fluid removal and beta-blockers for central blood pressure reduction, along with others designed to relieve the symptoms of heart failure, are typically prescribed.
Veterinary Cost
Because most dogs "grow out of it," treatment with medications lasts only a few months. The expense of this kind of treatment is not insignificant (usually a few hundred dollars) but consider that the diagnosis may require sedation X-rays and/or a specialist's opinion (a few hundred more).
Prevention
Preventing mitral valve disease requires removing affected dogs from the breeding pool. Otherwise, there is no known preventative approach. This is especially critical for breeds like the Cavalier King Charles, whose extreme predisposition recommends that they receive cardiac ultrasounds (echocardiograms) before they enter any breeding program.
References
Haggstrom J, Pedersen H, Kvart C. New insights into degenerative mitral valve disease in dogs. Vet Clin North Am Small Anim Pract 2004;34:1.
Swenson L, Haggstrom J, Kvart J, et al. Relationship between parental cardiac status in Cavalier King Charles Spaniels and prevalence and severity of chronic valvular disease in offspring. J Am Vet Med Assoc 1996;208:2009-2012.
Olsen L, Fredholm M, Pedersen H. Epidemiology and inheritance of mitral valve prolapse in Dachshunds. J Vet Intern Med 1999;13:448-456.
Kvart C, Haggstrom J, Pedersen HD, et al. Efficacy of enalapril for prevention of congestive heart failure in dogs with myxomatous valve disease and asymptomatic mitral regurgitation. J Vet Intern Med 2002;16(1):80-8.
Olsen LH, Martinussen T, Pedersen HD. Early echocardiographic predictors of myxomatous mitral valve disease in Dachshunds. Vet Rec 2003;152:293-297.