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Myxomatous mitral valve disease in Miniature Schnauzers and Yorkshire Terriers | VETgirl Veterinary Continuing Education Podcasts

In today’s VETgirl online veterinary continuing education podcast, we discuss the high prevalence of myxomatous mitral valve disease (MMVD) in dogs, particularly in smaller breeds. We’ll also review the wide variation in outcomes that occur as a result of MMVD and how it has stimulated discussion as what factors may impact these variable outcomes. Some dogs with MMVD will never develop clinical signs referable to their disease during their lifetime, while others will progress to outcomes such as congestive heart failure, pulmonary hypertension, or cardiac arrhythmias. Whether significant differences in clinical progression occur among breeds has not been extensively investigated outside of Cavalier King Charles Spaniels, Dachshunds, and Miniature Poodles.

So, DeProspero et al out of North Carolina State University wanted to evaluate this in a study entitled Myxomatous mitral valve disease in Miniature Schnauzers and Yorkshire Terriers: 134 cases (2007–2016). The purpose of this retrospective study was to investigate if any differences in baseline characteristics of MMVD were present between Miniature Schnauzers and Yorkshire Terriers or in comparison to the data that describes MMVD in the overall dog population. While both popular, small breed dogs, Miniature Schnauzers have a closer genetic relation to Standard Schnauzers while Yorkshire Terriers are more closely related to other small breed terriers. So we don’t want to automatically judge these books by their covers!

This study was retrospectively designed and performed at a single university teaching hospital. Medical records over a ten-year period were reviewed for diagnosis of MMVD in dogs of either breed. Data from the initial examination at which the diagnosis of MMVD was made were recorded, only. In addition, total numbers of dogs of each breed that were at least three years of age and examined at the facility during the ten-year span were tabulated. Baseline demographics, clinical history, physical examination findings, blood pressure assessment, and the results of electrocardiography, Holter monitor analysis, thoracic radiographs, and echocardiography were recorded. Exclusion criteria included lack of purebred status, congenital heart disease, cardiomyopathy, neoplasia, vegetative endocarditis, and systemic hypertension. For thoracic radiographs, a vertebral heart scale (VHS) of < 9.7 was considered normal for Miniature Schnauzers, and < 9.9 for Yorkshire Terriers based on previously published data. Echocardiographic data included assessment of cardiac dimensions, systolic function, mitral regurgitation severity, degree of mitral valve prolapse, presence of pulmonary hypertension based on peak tricuspid regurgitation velocity (with velocity > 3 m/s considered pulmonary hypertension), and stage of MMVD according to the previously published consensus guidelines from 2019.

So, what’d they find in this study? Overall, 12.9% of Miniature Schnauzers had MMVD according to the inclusion/exclusion criteria, compared with 5.7% of Yorkshire Terriers. Mean age of Miniature Schnauzers was 10 years (+/- 2) compared with 11 years (+/- 3) for Yorkshire Terriers. Both of these differences proved to be statistically significant, which may suggest that MMVD is more common and occurs at an earlier age in Miniature Schnauzers compared with Yorkshire Terriers…more on that later. Syncope was the most common clinical sign in Miniature Schnauzers presenting with MMVD, occurring in 43.5% of dogs. This was in contrast with coughing as the most common reported clinical sign in Yorkshire Terriers. Importantly, these differences may be readily explainable by other factors. A significant number (14.6%) of Miniature Schnauzers in the study had findings consistent with sick sinus syndrome (SSS), a condition to which the breed has a known predisposition and which commonly leads to syncope. Similarly, the greater occurrence in coughing in Yorkshire Terriers in this study may be attributable to a contribution from a noncardiac etiology, specifically tracheal collapse, as the breed is commonly afflicted with this disease and there was no specific evidence in the current study to suggest that a majority of the coughing observed in Yorkshire Terriers was definitively cardiogenic in origin.

Stage of MMVD at time of diagnosis (e.g., B1, B2, C, or D) did not differ between groups. 28% and 18.9% of Miniature Schnauzers and Yorkshire Terriers, respectively, presented with radiographic signs of congestive heart failure, which indicates that in dogs with MMVD of either breed, the majority do not present with congestive heart failure at the time of diagnosis with MMVD, which is consistent with what is known about this disease in dogs, as a whole.

There was no significant difference between the breeds with regards to echocardiographic parameters at time of diagnosis with the exception of presence of mitral valve prolapse (to any degree), which was more commonly reported in Miniature Schnauzers. Unfortunately, this parameter carries relatively weak strength in my mind, as it was reported in an entirely subjective manner and therefore highly prone to bias. The existing use of cardiac medications at the time of diagnosis of MMVD was more common in the Yorkshire Terrier group (75% compared with 43.5%). The most common medications in use at the time of diagnosis in either group were furosemide, angiotensin-converting enzyme (ACE) inhibitors, and pimobendan. Given that the distribution of dogs among MMVD stage did not differ among groups, the authors postulated that the higher incidence in cardiac medication use in Yorkshire Terriers may have been empirical treatment of the coughing that occurred more commonly in Yorkshire Terriers, even if this coughing was not confirmed to be cardiogenic in origin.

Pulmonary hypertension was present in 23.2% and 20.0% of Miniature Schnauzers and Yorkshire Terriers, respectively, which was not significantly different between the groups. Median peak tricuspid regurgitation velocity, the primary method by which pulmonary hypertension was diagnosed, was not reported to differ between the breeds either, but I do think it is noteworthy that for Miniature Schnauzers this velocity was reported to be 3.3 m/s compared with 4.2 m/s for Yorkshire Terriers. Using the modified Bernoulli equation of 4v2 , which is the method by which measured echocardiographic velocity is converted into a pressure gradient, this would suggest that the median peak estimated systolic pulmonary artery pressure in Miniature Schnauzers was 43.6 mm Hg, compared with 70.6 mm Hg in Yorkshire Terriers. Clinically, this is a notable difference in my mind. As a cardiologist, I do not have much specific clinical concern for a dog with an estimated systolic pulmonary artery pressure of 43.6 mm Hg, as this is quite mild pulmonary hypertension as > 35 mm Hg is considered abnormal in most circles. My degree of concern would be significantly greater, by comparison, with a pressure of 70.6 mm Hg, which is in the upper end of the moderate range, which is generally accepted to be between 50-75 mm Hg. Again, from a clinical standpoint, I would not likely be considering a pulmonary vasodilator, such as sildenafil, for the dog with the pressure gradient of 43.6 mm Hg, but I very well might be considering it for the dog with the pressure gradient of 70.6 mmHg.

It is possible that the small numbers of dogs in each group led to this, which though not statistically significant, still seems clinically relevant to me. Perhaps with a larger sample size, the median tricuspid regurgitant velocities would have been virtually equal. But for fun, let’s consider for a moment that with larger sample size the difference would not only persist, but achieve statistical significance. There would be two potential physiologic explanations for this finding in my opinion. The first is that Yorkshire Terries are more likely to have additional factors, such as chronic respiratory disease, playing a role in the development of pulmonary hypertension. This seems most likely given that Yorkshire Terriers commonly develop chronic lower airway disease. The second is that Yorkshire Terriers are simply more sensitive or reactive to passive pulmonary venous congestion from MMVD than are Miniature Schnauzers, which while possible, seems less likely and very difficult to prove or disprove.

So, what can we take from this VETgirl podcast? The relevance of some of the differences between the two breeds in this study is not entirely clear. It is true that Miniature Schnauzers had a higher incidence of MMVD, and at an earlier age than Yorkshire Terriers in this population, but the authors point out that almost twice the total number of Yorkshire Terriers than Miniature Schnauzers were tabulated in the study population overall, which may be a reflection of the referral base and could have skewed the results. With regards to the absolute age difference, I think this would best be interpreted in the context of average lifespans of the two breeds, which was not discussed or specified in this study. If Yorkshire Terriers have a longer lifespan, on average, than Miniature Schnauzers, the genetic, or telomeric “age” of dogs in each breed at time of onset of MMVD may not actually be that different.

As with all retrospective studies, this study openly admits to suffering from the risks and biases that come with incomplete data, inconsistencies in diagnostic operators or process, and lack of long-term follow-up in many dogs, among others. The exclusion criteria may also have impacted data as presumably many dogs with the conditions listed as exclusionary may have had concurrent MMVD.

References:
DeProspero DJ, O’Donnell KA, DeFrancesco TC et al. Myxomatous mitral valve disease in Miniature Schnauzers and Yorkshire Terriers: 134 cases (2007–2016). J Am Vet Med Assoc 2021;259 (12): 1428-1432.

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