Presented in part at the 28th American College of Veterinary Internal Medicine Forum, Anaheim, California, June 2010.
Objectives: To describe the echocardiographic findings and pedigree analysis of golden retrievers with subvalvular aortic stenosis.
Methods: Seventy-three golden retrievers were evaluated by auscultation and echocardiography. A subcostal continuous-wave Doppler aortic velocity ê2·5 m/s and presence of a left basilar systolic ejection murmur were required for diagnosis of subvalvular aortic stenosis. Three echocardiographic characteristics were recorded: evidence of aortic insufficiency, subvalvular ridge or left ventricular hypertrophy. A disease status score was calculated by totalling the number of echocardiographic -characteristics per subject.
Results: Thirty-two of 73 dogs were affected and their aortic velocities were as follows: range 2·5 to 6·8 m/s, median 3·4 m/s and standard deviation 1·2 m/s. Echocardiographic characteristics of 32 affected dogs were distributed as follows: left ventricular hypertrophy 12 of 32, aortic insufficiency 20 of 32 and subvalvular ridge 20 of 32. Disease status score ranged from 0 to 3 with a median of 2. There was a statistically significant correlation between aortic velocity and disease status score (r=0·644, P<0·0001). Subvalvular aortic stenosis was observed in multiple generations of several families and appears familial.
Clinical Significance: Subvalvular aortic stenosis in the golden retriever is familial. Severity of stenosis correlates well with cumulative presence of echocardiographic characteristics (left ventricular hypertrophy, subvalvular ridge and aortic insufficiency).
Subvalvular aortic stenosis (SAS) is one of the most commonly reported canine congenital heart defects (Tidholm 1997, Buchanan 1999). Mildly affected dogs with SAS may have a normal life span but moderate to severely affected dogs are at risk of developing severe complications (sudden death, congestive heart failure and endocarditis) and at least in one study have an average life span of 19 months (Kienle and others 1994). The gold standard for a diagnosis of SAS is the post-mortem identification of a subvalvular aortic ridge. However, the best pre-mortem non-invasive diagnosis of SAS is typically based on an estimation of increased aortic outflow velocity by continuous-wave Doppler echocardiography measurement. Other supportive echocardiographic findings of SAS may be useful in diagnosis and include presence of a visible subvalvular ridge, left ventricular hypertrophy (LVH) and aortic insufficiency (AI) (O’Grady and others 1989).
Physical examination, echocardiographic records and pedigrees (when available) were evaluated from golden retrievers recruited for study at The Ohio State University, College of Veterinary Medicine; Washington State University, College of Veterinary Medicine or from owners of dogs that had been evaluated by one of numerous veterinary cardiologists (19). All dogs had been evaluated with auscultation and echocardiographic examination with subcostal continuous-wave Doppler measurements of aortic velocity by an ACVIM-board certified cardiologist. Dogs were classified as affected, unaffected or equivocal. Criteria for dogs affected with SAS were the presence of a left basilar systolic ejection murmur and a subcostal continuous-wave Doppler aortic velocity ≥2·5 m/s. Dogs with subcostal, continuous-wave Doppler aortic velocities between 1·81 and 2·49 m/s were considered equivocal. Dogs were classified as unaffected when at least 12 months of age and maximal subcostal, continuous-wave Doppler aortic velocity was ≤1·8 m/s. Presence or absence of a heart murmur was recorded for all study participants.
The records from affected dogs were evaluated more completely for additional phenotypic data for SAS in golden retrievers, while the remaining dogs were utilised for the purposes of pedigree analysis only. Three possible echocardiographic characteristics of SAS were recorded in affected dogs as yes or no variables: evidence of AI, a subvalvular ridge or LVH. Only presence or absence of these characteristics was recorded from the cardiac summaries provided by ACVIM-board certified cardiologists because of the variable way in which they were provided in the reports. AI was assessed by colour Doppler evaluation in any two-dimensional (2D) imaging plane and severity of insufficiency was not evaluated. The definition of LVH was determined by the submitting cardiologist and their routine standards of measure based upon body weight, in some cases only a subjective assessment was provided, rather than wall measurements. The presence or absence of LVH was noted in the cardiac evaluation summary page for each dog evaluated; however, quantitative data for these measures were not available. A disease status score of 0 to 3 was calculated by totalling the number of these echocardiographic characteristics per subject.
A three-generation pedigree was obtained for 66 of 73 golden retrievers enrolled. Commercial pedigree analysis software (Pedigree Viewer 6.3, B Kinghorn, University of New England) was utilised to visualise patterns of inheritance by labelling each dog as affected, unaffected or unknown (the dog was unavailable for evaluation or within the equivocal range for aortic velocity). Families of golden retrievers with affected offspring were identified and evaluated for a known pattern of inheritance including autosomal dominant, autosomal recessive, X-linked dominant and X-linked recessive traits (Ortiz-Lopez and others 1996).
Range, median, sd, quartiles and interquartile range of subcostal aortic velocities of affected dogs were calculated. A D’Agostino and Pearson omnibus normality test was performed for each group by use of commercially available software (Prism 5, GraphPad Software Inc). Spearman rank correlation was performed for correlation of aortic velocity of affected dogs to disease status score, and correlation of age to disease status score by use of commercial software (Prism 5, GraphPad Software Inc.). Pearson correlation coefficient was performed for correlation of aortic velocity to age of affected dog by use of commercial software (Prism 5, GraphPad Software Inc.) Logistic regression was performed using commercial software (SYSSTAT 13, SYSTAT Software Inc.) and OR and 95% confidence intervals (CI) were determined for each echocardiographic characteristic evaluated (subvalvular ridge, AI, LVH). Overall fit of logistic regression was tested using a chi-squared test with 1 df. Null hypothesis was rejected at P<0·05. A statistical consultation service was utilised for statistical interpretation (Statistical Consulting Services, Washington State University).
Seventy-three golden retrievers (30 males, 43 females) were evaluated. Thirty-two dogs (10 males, 22 females) were classified as affected; 27 were classified as normal (15 males, 12 females); and 14 were classified as equivocal (5 males, 9 females). Affected dogs ranged from 10 weeks to 9 years of age at time of evaluation (median 2 years) and had aortic velocities that ranged from 2·5 to 6·8 m/s with a median of 3·4 m/s, sd of 1·2 m/s, interquartile range of 1·9 m/s, 25th percentile of 2·7 m/s and 75th percentile of 4·6 m/s. Affected dog aortic velocity data passed D’Agostino and Pearson omnibus normality test. Normal dogs ranged from 1 to 14·7 years of age at time of evaluation (median 5·4 years) and had aortic velocities that ranged from 0·99 to 1·8 m/s with a median of 1·68 m/s, sd of 0·19 m/s, interquartile range of 0·23 m/s, 25th percentile of 1·5 m/s and 75th percentile of 1·7 m/s. Normal dog aortic velocity data did not pass D’Agostino and Pearson omnibus normality test. Equivocal dogs ranged from 8·5 months to 9·2 years of age at time of evaluation (median 2·8 years) and had aortic velocities that ranged from 1·9 to 2·3 m/s with a median of 2·1 m/s, sd of 0·13 m/s, interquartile range of 0·3 m/s, 25th percentile of 2·0 m/s and 75th percentile of 2·2 m/s. Equivocal dog aortic velocity data passed D’Agostino and Pearson omnibus normality test.
A heart murmur was identified in 39 of the 73 dogs of this population (53%). All dogs within the affected category had a left basilar, systolic, ejection murmur noted (32/32; 100%). Eight of the 14 dogs (57%) in the equivocal category had a left basilar systolic ejection murmur. Four of the 27 dogs in the normal category had a left basilar systolic ejection murmur.
The following echocardiographic characteristics were recorded as present or absent in the affected dogs. Twelve of 32 dogs had LVH, 20 of 32 had AI and a subvalvular ridge was observed in 20 of 32 dogs. Disease status score ranged from 0 to 3 with a median of 2. Spearman rank correlation revealed a statistically significant correlation between aortic velocity and disease status score (r=0·644, P<0·0001; Fig 1), but not between age and disease status score (r=0·156; P=0·39). Aortic velocity did not correlate with age of golden retriever (Pearson r=–0·08; P=0·63). Logistic regression results were as follows: for every 1 m/s increase in aortic velocity (>2·5 m/s), a statistically significant OR for demonstrating presence of subvalvular ridge, LVH and AI was identified (OR=2·95, P=0·007, CI=1·2 to 8·6; OR=4·65, P=0·0001, CI=1·8 to 13·2; OR=2·54, P=0·01, CI=1·1 to 6·7, respectively). When aortic velocity was ≥4·0 m/s (range 4·0 to 6·8 m/s), the combination of LVH, AI and subvalvular ridge (disease status score level 3) was reported in 10 of 11 (90%) golden retrievers.
Three-generation pedigrees for 66 golden retrievers were analysed for evidence of the familial nature of this disease. SAS was observed in multiple generations (at least three) of several families of wide geographic distribution (20 states) and appears familial (Fig 2). Evaluation of pedigree data is not consistent with a simple autosomal dominant or X-linked dominant mode of inheritance because two unaffected parents produced affected offspring (Family 1, Fig 2). An X-linked recessive mode of inheritance is ruled out as an affected female produced an unaffected male offspring (Family 2, Fig 2).
SAS in the golden retriever appears familial based on the observation of the trait in multiple generations of several families of wide geographic distribution as well as the known increased breed prevalence and OR (Buchanan 1999). Although some modes of inheritance (simple autosomal dominant, X-linked recessive) appeared to be excluded based on our families, we did not have enough information to conclusively determine a mode of inheritance. Autosomal recessive mode of inheritance and polygenic inheritance cannot be excluded. The effect of penetrance cannot be established within the population studied. The most accurate way to determine pattern of inheritance in the absence of a known mutation is through a prospective breeding experiment, which was beyond the scope of this study. Ideally, identification of a mutation responsible for SAS will be identified and allow complete description of a definitive pattern of inheritance.
As might be expected, severity of stenosis determined by aortic velocity correlates well with the cumulative presence of echocardiographic characteristics (LVH, subvalvular ridge and AI). Although disease status score could have been impacted by the length of time (age), the heart was exposed to the elevated pressure, logistic regression data did not correlate disease status score to age of animal.
This study has some limitations; but nevertheless provides new insight into this disease in golden retrievers. Although golden retrievers were enrolled from a vast regional distribution, participant bias may impact the genetic diversity of the population evaluated. Although samples were submitted from a wide geographic distribution, the collection of samples at dog shows and referral institutions may produce further participant bias. Utilisation of sample submissions from multiple ACVIM board certified cardiologists with varied reporting forms made it difficult to use quantitative measures for LVH and AI. In some cases, only the presence or absence was noted by an ACVIM board certified cardiologist. Additionally blood pressure was not recorded in all subjects with LVH and cannot be excluded as a confounding variable. Affected dogs were evaluated to report phenotypic description and subcostal continuous-wave Doppler aortic velocities of 2·5 m/s were required for assignment of affected status. This number as well as the cut-off for unaffected was based on a modification of the current American College of Veterinary Medicine Registry of Cardiac Health (ARCH) recommendations for congenital SAS screening. The cut-off value for affected status was intentionally higher than that recommended by the ARCH recommendations. In the absence of a gold-standard antemortem test for SAS, the authors acknowledge that some dogs included in the equivocal category could actually represent a mildly affected state of SAS and vice versa. Therefore, the authors extended the equivocal zone of subcostal Doppler aortic velocity measurements to increase confidence in the normal versus diseased categories. The recommended diagnostic cut-off for determining presence of SAS by aortic velocity has changed over time, such that extending pedigree analysis beyond the third generation was not possible.
In conclusion, SAS in the golden retriever appears to be inherited. As disease severity progresses supportive echocardiographic findings (LVH, subvalvular ridge and AI) increase in frequency independent of age.
This work was supported in part by the Morris Animal Foundation – Pfizer Animal Health Fellowship.
Conflict of interest
None of the authors of this article has a financial or personal relationship with other people or organisations that could inappropriately influence or bias the content of the paper.