Degenerative Mitral Valve Disease

By Dr. Michael Luethy

Degenerative mitral valve disease (MVD), also known as myxomatous valvular degeneration or endocardiosis, is the most common cause of congestive heart failure in the dog.  The disease may affect either the mitral or tricuspid valve, or both, but clinical signs are most frequently associated with the effects on the mitral valve.  Clinical evidence of the disease has been reported in greater than 30% of dogs 13 years or older with classic valvular changes of the disease detected in a substantially higher percentage of post mortem dogs in this age group.   The disease appears to have a genetic basis in most cases with a likely polygenic mode of inheritance.  Any breed can be affected by the disease, but small breeds are affected most frequently with the Cavalier King Charles spaniel, Chihuahua, miniature schnauzer, Maltese, Shih Tzu, and toy poodle among the breeds known to have a strong breed predilection.  There is a slightly higher prevalence among males and clinical signs are most frequently noted in the geriatric population.

The exact etiology of MVD is unknown but collagen abnormalities or abnormalities in the activity of endothelin have been proposed as potential mechanisms.  Affected valve leaflets demonstrate thickening, nodular distortions, and changes of the chordae tendinae which all act to prevent normal coaptation of the valve leaflets.  Histologic changes are dominated by deposition of mucopolysaccharides  in the spongiosa with a lesser amount of fibrosis also noted.  The deterioration and abnormal coaptation of the valve leaflets leads to insufficiency of the mitral valve and regurgitation of blood from the left ventricle into the left atrium during systole.

The majority of dogs presented to the veterinarian for clinical signs associated with heart disease present for coughing.  Coughing may occur because of the presence of pulmonary edema, but more frequently, the cough associated with MVD is related to airway elevation and compression caused by left atrial enlargement.  Decreased mucus clearance also may also be a factor in cough production.  It is important to remember that the same breeds affected by MVD are often affected by collapsing trachea and chronic airway disease and coughing in a dog with a heart murmur may not be related to the heart disease.  Owners may also present heart disease patients to the veterinarian for tachypnea/dyspnea, syncope, decreased stamina, or ascites (when the tricuspid valve is affected).  Many dogs that develop MVD will not have the disease progress to the point of congestive heart failure.

The classic physical exam finding in a dog with MVD is a systolic murmur with the PMI over the left cardiac apex.  Systolic clicks may also be heard over this area and may precede the development of a murmur.  The murmur of mitral regurgitation associated with MVD cannot be distinguished from the murmur of mitral regurgitation associated with dilated cardiomyopathy,  a congenitally dysplastic valve, or mitral endocarditis.   Respiratory rate and character, lung sounds, mucous membrane color and refill, femoral pulse strength and regularity, abdominal palpation, and the presence or absence of jugular distension will all provide additional useful information about the cardiovascular status of the MVD patient.

Thoracic radiography provides the most essential information needed to successfully evaluate and manage most cases of MVD.  Radiographs allow gross evaluation of left heart size and are critical in evaluating pulmonary vasculature and parenchyma to determine if a patient demonstrating clinical signs of coughing or respiratory distress has congestive heart failure (CHF) or primary airway/lung disease.  Echocardiography provides a large amount of additional, non-invasive data that is extremely valuable in providing information for making management decisions and for providing owners with an accurate prognosis.  Echocardiography is a recommended adjunct to the workup of a MVD patient, but it should never supplant radiography as a diagnostic test in this disease.  Electrocardiography may provide useful information regarding the rhythm abnormalities that may affect MVD patients , but it is too insensitive and non-specific to provide useful information about chamber enlargement.

The progressive mitral regurgitation associated with MVD places many affected patients at risk for the development of left side CHF with pulmonary edema.  Right sided CHF with ascites may develop if the tricuspid valve is affected.  Unfortunately, at this time no nutritional supplements or medications have been developed that will successfully prevent the progression of MVD.  Treatment strategies are centered at eliminating pulmonary edema and delaying/controlling subsequent episodes of congestion in patients with advanced mitral regurgitation.  Studies evaluating the use of ACE inhibitors such as enalapril  to delay the onset of CHF in dogs with moderate to severe mitral regurgitation have yielded equivocal results with no clear cut benefit to initiating therapy prior to the onset of CHF.  Current recommendations for the treatment of MVD patients typically involve initiation of “triple therapy” with furosemide, ACE inhibition, and Vetmedin when clinical evidence of CHF develops.  Additional diuretics, vasodilators, or neurohormonal moderating agents may be added in as the disease progresses.  Current treatment guidelines for MVD may be found in the ACVIM Cardiology consensus statement: Guidelines for the Diagnosis and Treatment of Canine Chronic Valvular Heart Disease at www.acvim.org.  Another useful resource providing guidelines, case studies, and other educational tools can be found at the Cardiac Education Group web site at www.cardiaceducationgroup.org.

 

Abbott J:  Acquired Valvular disease.  In Tilley L, Smith F, Oyama M, Sleeper M, eds:  Manual of canine  and feline cardiology,  ed 4,  St Louis, 2008  Saunders.

Atkins CD, Keene BW, Brown WA, et al:  Results of the veterinary enalapril trial to prove reduction in onset of heart failure in dogs chronically treated with enalapril alone for compensated naturally  occurring mitral valve insufficiency.  J Am Vet Med Assoc 231:  1061-1069, 2007.

Buchanan JW:  Chronic valvular disease (endocardiosis) in dogs, Adv Vet Med 21:75, 1979.

Ettinger SJ, Benitz AM, Ericsson GF, et al:  Effects of enalapril maleate on survival of dogs with naturally acquired heart failure.  J Am Vet Med Assoc 213:  1573-1577, 1998.

Haggstrom J, Kvart C, Pedersen HD:  Acquired valvular disease.  In Ettinger SJ, Feldman EC, eds:  Textbook of Veterinary internal medicine, ed 6, Philadelphia, 2005, WB Saunders.

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  16(1):  80-88, 2002.

Sisson D, Kvart C, Darke PG:  Acquired valvular heart disease in dogs and cats.  In Fox PR, Sisson D, Moise NS, eds:  Textbook of canine and feline cardiology:  principles and clinical practice, ed 2,  Philadelphia, 1999, WB Saunders.

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