Prognostic interaction between clinical stage and left atrial remodeling in dogs with myxomatous mitral valve disease

Myxomatous mitral valve disease (MMVD) is leading cause of acquired heart failure in small-breed dogs. Although the American College of Veterinary Internal Medicine (ACVIM) staging is widely used in practice, how each stage and the degree of cardiac remodeling affect survival has not been clearly defined. We aimed to determine whether severe remodeling in dogs with preclinical (stage B2) disease is associated with survival differences compared with dogs with congestive heart failure (CHF; stage C) and less severe remodeling. This retrospective study analyzed 270 dogs with MMVD, stratified by the left atrium-to-aortic root ratio as having mild (<1.9), moderate (1.9–2.2), and severe (≥2.2) left atrial enlargement. Survival for dogs with stage B2 disease with severe LA enlargement did not differ significantly from dogs with stage C disease with mild-to-moderate enlargement. However, within the subgroup with severe LA enlargement, dogs that developed CHF had significantly shorter survival than those that remained in stage B2 (360 vs. 654 days; P = 0.032). Severe LA enlargement in dogs with preclinical disease contributes to a poor prognosis comparable to that of CHF with mild-to-moderate enlargement, indicating that stage alone may be insufficient for determining prognosis and that LA remodeling should also be considered.