Quantitative and qualitative histopathological analysis of myocardial tissue in canine myxomatous mitral valve disease and dilated cardiomyopathy

Quantitative and qualitative histopathological assessments may yield complementary and novel insights into myocardial tissue alterations in dogs with myxomatous mitral valve disease (MMVD) and dilated cardiomyopathy (DCM). This study aimed to compare myocardial histopathological features in multiple, region-matched cardiac specimens from dogs with MMVD, DCM, and cardiac-healthy controls using quantitative and qualitative histopathological methods. Dogs with MMVD (n = 27), DCM (n = 16), and cardiac healthy controls (n = 32) were enrolled. Tissue proportions of cardiomyocytes, fibrosis, and fat, as well as the arterial lumen-to-area ratio (LAR), were quantitatively evaluated in each dog using a digital image analysis software, whereas the presence of endocardial thickening, fibrosis, fat, and attenuated wavy fibers was qualitatively assessed. Clinical MMVD dogs had higher proportions of fibrosis in the left ventricular (LV) lateral wall, the LV posterior papillary muscle, and left atrium compared to controls, and more MMVD dogs had thickened endocardium in the LA compared to controls (all P < 0.05). Clinical DCM dogs had higher proportions of fibrosis in both atria compared to controls (P < 0.05), and higher fibrosis in the interventricular septum compared to clinical MMVD dogs (P < 0.05). Dogs with DCM and atrial fibrillation had higher proportions of atrial fibrosis compared to DCM dogs without atrial fibrillation (P < 0.05). Neither the LAR nor the presence of attenuated wavy fibers differed between the groups in any region. In conclusion, myocardial histopathological alterations were predominantly left-sided in MMVD dogs and bilateral in DCM dogs. Histopathological characteristics in certain cardiac regions varied with disease type and severity, underscoring their potential role in disease pathogenesis.