Mitral valve endocardiosis

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Severe mitral valve regurgiation leading to congestive heart failure in a Cavalier King Charles Spaniel[1]

Mitral valve endocardiosis (Myxomatous mitral valve disease; chronic mitral valve insufficiency) is a common heart disease of dogs accounting for up to 50% of canine heart disease and is characterized by mitral valve degeneration and congestive heart failure[2].

A genetic predisposition has been reported in the Cavalier King Charles Spaniel[3] but many small and toy breeds are affected.

Mitral valve endocardiosis results from slowly increasing left atrioventricular valve regurgitation from the left ventricle into the atrium. This leads to compensatory increased left ventricular hypertrophy and stroke volume and heart rate. Over time, eccentric left ventricular hypertrophy and atrial enlargement occur, causing mitral valve prolapse[4] and pulmonary edema. These degenerative changes have been shown to correlate with biochemical increases in transforming growth factor-β (TGF-β) and matrix metalloproteinases at the molecular level[5].

Clinically affected dogs may be asymptomatic for many years, but progressive deterioration of the valve leads to intermittent coughing, reduced exercise tolerance and generalized weakness.

Advanced congestive heart failure and sudden death due to thromboembolism[6], left atrial rupture, aortic rupture or cardiac tamponade are not uncommon[7].

A left apical systolic heart murmur may be evident on auscultation and ECG abnormalities are commonly observed such as P-wave dispersion[8], bundle branch block and sick sinus syndrome (bradycardia-tachycardia syndrome due to fatty infiltration of the sinoatrial node)[9].

Blood tests are often unrewarding, although elevated levels of creatinine, NT-proANP[10], cardiotropin[11] and C-reactive protein (≥ 6.0 μg/mL) [12] are common.

Radiographs often reveal cardiomegaly, pleural effusion and pulmonary edema.

Echocardiography is usually required for definitive diagnosis, especially use of M-mode Doppler echocardiograms, often showing fractional shortening, but requires specialist interpretation[13]. However, prognostic assessment of eventual congestive heart failure is difficult on echocardiography alone[14].

Histolopathological examination of postmortem hearts usually reveals gross changes in the mitral valve complex with tissue swelling on the edge of valve leaflets, chordae tendineae, and the chordal-papillary muscle junction with myxomatous degeneration to fibrillar tissue[15].

A differential diagnosis would include Bartonella spp endocarditis[16], hyperthyroidism, cardiomyopathy, patent ductus arteriosus, Duchenne muscular dystrophy, endocarditis, ventricular septal defect and infections with Dirofilaria spp.

Treatment usually requires use of preload drugs such as pimobendan, the β-blocker carvedilol[17], or angiotensin-inhibitors such as enalapril or benazepril together with diuretics such as furosemide and in severe cases, digoxin.

Treatment of heart failure dogs with thyroid hormone or thyroid hormone analogues may improve cardiac performance[18].

Although this is an invariably fatal disease, many dogs live a good quality of life for many years prior to heart failure or elective euthanasia[19].

References

  1. Louisiana State Uni
  2. Baumgartner C & Glaus TM (2004) Acquired cardiac diseases in the dog: a retrospective analysis. Schweiz Arch Tierheilkd 146(9):423-430
  3. Chetboul V et al (2004) Epidemiological, clinical, echo-doppler characteristics of mitral valve endocardiosis in Cavalier King Charles in France: a retrospective study of 451 cases (1995 to 2003). Can Vet J 45(12):1012-1015
  4. Guarda F et al (1989) The pathology of mitral valve prolapse in dogs. Dtsch Tierarztl Wochenschr 96(4):172-174
  5. Obayashi K et al (2011) Effects of transforming growth factor-β3 and matrix metalloproteinase-3 on the pathogenesis of chronic mitral valvular disease in dogs. Am J Vet Res 72(2):194-202
  6. Seki Y et al (1998) Transmural myocardial infarction caused by thromboembolism associated with mitral insufficiency in a dog. J Vet Med Sci 60(6):741-743
  7. Kurt S & Kovacevic A (2012) Atrial rupture and pericardial effusion as a complication of chronic mitral valve endocardiosis. Schweiz Arch Tierheilkd 154(9):397-401
  8. Noszczyk-Nowak A et al (2011) Comparison of P-wave dispersion in healthy dogs, dogs with chronic valvular disease and dogs with disturbances of supraventricular conduction. Acta Vet Scand 53:18
  9. Nakao S et al (2012) The anatomical basis of bradycardia-tachycardia syndrome in elderly dogs with chronic degenerative valvular disease. J Comp Pathol 146(2-3):175-182
  10. Häggström J et al (1997) Effects of naturally acquired decompensated mitral valve regurgitation on the renin-angiotensin-aldosterone system and atrial natriuretic peptide concentration in dogs. Am J Vet Res 58(1):77-82
  11. Nam SJ et al (2010) The cardiac biomarker sodium-calcium exchanger (NCX-1) can differentiate between heart failure and renal failure: a comparative study of NCX-1 expression in dogs with chronic mitral valvular insufficiency and azotemia. J Vet Intern Med 24(6):1383-1387
  12. Rush JE et al (2006) C-reactive protein concentration in dogs with chronic valvular disease. J Vet Intern Med 20(3):635-639
  13. Bonagura JD & Schober KE (2009) Can ventricular function be assessed by echocardiography in chronic canine mitral valve disease? J Small Anim Pract 50(1):12-24
  14. Brown DJ et al (2005) Quantitative echocardiographic [corrected] evaluation of mitral endocardiosis in dogs using ratio indices. J Vet Intern Med 19(4):542-552
  15. Corcoran BM et al (2004) Identification of surface morphologic changes in the mitral valve leaflets and chordae tendineae of dogs with myxomatous degeneration. Am J Vet Res 65(2):198-206
  16. Pesavento PA et al (2005) Pathology of bartonella endocarditis in six dogs. Vet Pathol 42(3):370-373
  17. Gordon SG et al (2012) Retrospective review of carvedilol administration in 38 dogs with preclinical chronic valvular heart disease. J Vet Cardiol 14(1):243-252
  18. Shahrara S et al (1999) Upregulation of thyroid hormone receptor beta 1 and beta 2 messenger RNA in the myocardium of dogs with dilated cardiomyopathy or chronic valvular disease. Am J Vet Res 60(7):848-852
  19. Ettinger SJ et al (1998) Effects of enalapril maleate on survival of dogs with naturally acquired heart failure. The Long-Term Investigation of Veterinary Enalapril (LIVE) Study Group. J Am Vet Med Assoc 213(11):1573-1577
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