Abstract
OBJECTIVE To compare biomechanical and histologic features of heart valves and echocardiographic findings between Quarter Horses with and without heritable equine regional dermal asthenia (HERDA).
DESIGN Prospective case-control study.
ANIMALS 41 Quarter Horses.
PROCEDURES Ultimate tensile strength (UTS) of aortic and mitral valve leaflets was assessed by biomechanical testing in 5 horses with HERDA and 5 horses without HERDA (controls). Histologic evaluation of aortic and mitral valves was performed for 6 HERDA-affected and 3 control horses. Echocardiography was performed in 14 HERDA-affected and 11 control horses. Biomechanical data and echocardiographic variables of interest were compared between groups by statistical analyses,
RESULTS Mean values for mean and maximum UTS of heart valves were significantly lower in HERDA-affected horses than in controls. Blood vessels were identified in aortic valve leaflets of HERDA-affected but not control horses. Most echocardiographic data did not differ between groups. When the statistical model for echocardiographic measures was controlled for body weight, mean and maximum height and width of the aorta at the valve annulus in short-axis images were significantly associated with HERDA status and were smaller for affected horses.
CONCLUSIONS AND CLINICAL RELEVANCE Lower UTS of heart valves in HERDA-affected horses, compared with those of control horses, supported that tissues other than skin with high fibrillar collagen content are abnormal in horses with HERDA. Lack of significant differences in most echocardiographic variables between affected and control horses suggested that echocardiography may not be useful to detect a substantial loss of heart valve tensile strength. Further investigation is warranted to confirm these findings. Studies in horses with HERDA may provide insight into cardiac abnormalities in people with collagen disorders.
Heritable equine regional dermal asthenia is an autosomal recessive disorder in horses characterized by loose, fragile skin that tears easily.1–4 Despite the name suggesting that the disease is limited to the horse's skin, recent studies indicate that a number of diverse tissues are involved.5–7 A missense mutation in the peptidyl-prolyl isomerase B (PPIB) gene, which encodes CYPB, causes HERDA.8,9 Cyclophilin B is a member of the peptidyl-prolyl cis-trans isomerase family of proteins that catalyze prolyl-containing bonds in procollagen, a step that is required to form the triple helix.10 Cyclophilin B is a rate-limiting enzyme in fibrillar collagen synthesis, and it functions in procollagen trafficking, processing, and chain association.11,12 The HERDA mutation modifies a region of CYPB that identifies improperly folded proteins in the endoplasmic reticulum, and these interactions are presumed to alter collagen organization.9 It has been shown to rearrange the flexible N-terminal end of the polypeptide chain, making it more rigid and likely to result in altered interactions with other endoplasmic reticulum–resident chaperones and foldases.13
We and other authors4,6,14 have recently shown that biomechanical properties of diverse tissues with high fibrillar collagen content, including skin, tendons, ligament, and great vessels, are altered in horses with HERDA. The superficial digital flexor tendons, deep digital flexor tendons, and suspensory ligaments of the forelimbs in horses with HERDA are biomechanically weaker6 than those of horses without the disease, and some affected horses have a clinically hypermobile phenotype.15 Alterations in corneal thickness and curvature and in tear production have also been found in HERDA-affected horses, resulting in a higher clinical incidence of corneal ulcers, compared with that in unaffected horses.5,7
Heritable equine regional dermal asthenia is most prevalent in American Quarter Horses, especially among horses from cutting and reining bloodlines.2,8,16 Within the cutting-horse industry, 32 of 113 (28.3%) elite performance horses and 14 of the top 100 (14%) cutting-horse sires were identified as carriers.2,16 The condition has also been reported in Quarter Horses in several other countries.17,18 People with similar inherited connective tissue disorders are described as having EDS, which is caused by mutations in genes that encode various collagen types, enzymes that modify collagens, and other extracellular matrix proteins.19–24 The clinical manifestations of EDS vary depending on the mutation and the resultant type of collagen defect.19–22,24 A detailed classification scheme has been developed for the condition, but clinical features of the different types overlap.24 Regardless, subtypes of EDS have several common features including joint and skin hypermobility and increased skin fragility.19 Cardiovascular lesions such as mitral and tricuspid valve prolapse, valvular regurgitation, dilation of the aortic root, rupture of the aortic sinus, dissecting aneurysms, varicose veins, capillary fragility, problematic bruising, abnormal retinal blood vessels, and rupture of medium-sized arteries have also been reported in human patients with various subtypes of EDS.20,23–33
The HERDA phenotype in Quarter Horses is similar to that of EDS type VI (the kyphoscoliotic form) found in humans.4 This subtype is characterized by a deficiency in collagen lysyl hydroxylase I and an increased deoxypyridinoline-to-pyridinoline ratio in urine.4,21,24 This ratio is also abnormal in HERDA-affected horses.a,b Symptoms of the kyphoscoliosis subtype in humans include progressive kyphoscoliosis, joint laxity, fragile, hyperextensible skin, ocular rupture, corneal degeneration, blue sclera, muscular hypotonia, cardiac and valvular disease, and arterial rupture.19,21,23,24 Clinical manifestations of HERDA and EDS are not completely the same, but each includes ocular changes, joint laxity, and hyperextensibility and fragility of the skin.1,4–7,15 To our knowledge, cardiac abnormalities have not previously been reported in horses with HERDA. Therefore, the purpose of the study reported here was to compare the tensile strength and histologic features of heart valves and echocardiographic findings for HERDA-affected Quarter Horses with those of Quarter Horses that tested negative for the HERDA-associated mutation of the peptidyl-prolyl isomerase B gene. We hypothesized that biomechanical testing of heart valves of horses with HERDA would reveal decreased tensile strength, compared with results for unaffected horses. We also hypothesized that histologic characteristics of heart valves would differ between horses with and without the disease, and that HERDA-affected horses would have echocardiographic abnormalities similar to those found in people with EDS, such as dilation of the aortic root and increased prevalence of valvular regurgitation and other valvular anomalies, compared with findings for unaffected subjects.
Materials and Methods
Horses
A total of 41 Quarter Horses (22 with and 19 without HERDA) were included in the study. Thirty-seven were from university teaching herds (32 from Mississippi State University and 5 from Michigan State University), and 4 were client-owned horses. For all HERDA-affected horses in the study, HERDA-positive status was confirmed through genetic evaluation performed by the Department of Molecular Medicine at Cornell University; each of these horses was confirmed to be homozygous for the causative mutation of the peptidyl-prolyl isomerase B gene by testing of a blood sample, hair root sample, or both and had lesions consistent with the disease on physical examination, including soft, pliable, hyperextensible skin; scarring; and sloughing and lacerations of the skin. All control horses were confirmed to test negative for the same mutation (neither affected nor carrier) through genetic evaluation of a blood sample, hair root sample, or both by the Veterinary Genetics Laboratory of the University of California-Davis or the Department of Molecular Medicine at Cornell University; these horses were also phenotypically normal and deemed healthy on general physical examination. Owner consent was obtained prior to study inclusion for privately owned horses. All experimental protocols were approved by the Mississippi State University and Michigan State University institutional animal care and use committees.
Biomechanical evaluation
Aortic and mitral valves were collected from 5 HERDA-affected and 5 control horses and frozen at −20°C (−4°F) within 1 hour after confirmation of death. The thawed tissues were used to determine UTS by means of a previously described procedure.34 The HERDA-affected horses were part of the research herd at Mississippi State University and ranged from 2 to 10 years of age (mean ± SD, 5.8 ± 3.3 years). Control horses were clinical patients at the Mississippi State University Animal Health Center that were euthanized for reasons unrelated to the study or to cardiac disease. Control horses ranged from 2 to 20 years of age (mean ± SD, 7.8 ± 7.2 years). Euthanasia was performed via IV injection of an overdose of pentobarbital. Aortic valve leaflet tissue strips (length approx 4 times the width) were dissected by use of a scalpel blade in both radial and circumferential directions. Tissue strips were similarly dissected from the mitral valve anterior leaflets in the radial and circumferential directions.
A uniaxial testing devicec was used to determine the uniaxial mechanical properties of explanted valvular tissues. For each evaluation, the tissue strip was mounted on the testing device with 2 stainless steel grips cushioned with emery paper. After being preconditioned to a peak load of approximately 10% of the failure load for 10 cycles, the tissue strip was loaded up to failure at a grip ramping speed (strain rate) of 0.1 second−1. All testing was carried out in a PBS solution bath at approximately 80.6°F (27°C).
Histologic evaluation
Aortic and mitral valve leaflets were obtained from 9 horses. Six were HERDA-affected animals from 1 to 10 years of age (mean ± SD, 3.1 ± 3.4 years) and 3 were unaffected animals from 2 to 27 years of age (mean ± SD, 10.3 ± 14.4 years) that were euthanized for reasons not related to the study or to cardiac disease. These horses were euthanized as described for biomechanical experiments. Valve leaflets were collected within 1 hour after confirmation of death, fixed in neutral-buffered 10% formalin, and subsequently dissected in a radial or circumferential direction. The specimens were routinely processed, embedded in paraffin, sectioned at a thickness of 5 μm, and stained with Movat pentachrome stain for examination by light microscopy. Movat pentachrome stain was used to aid differentiation of the distinct layers (lamina fibrosa, lamina spongiosa, and lamina ventricularis; some authors designate the lamina radialis and indicate that the lamina ventricularis is an additional layer between the lamina radialis and ventricular endothelium35) of the valves. The solution stains collagen yellow, glycosaminoglycans blue, and elastin black.
The paraffin-embedded tissues were also examined by immunohistochemistry for factor VIII–related antigen. A standardized technique that has been validated for horses was followed; materials, including the recommended reagents,d,e were used in accordance with the manufacturer's instructions. Briefly, formalin-fixed paraffin-embedded sections were deparaffinized and pretreated with proteinase K. A polyclonal rabbit antibody directed against human factor VIII–related antigen,d known to react with von Willebrand factor, to cross-react with equine tissues, and to stain endothelial cells, was used at a 1:800 dilution in tris-HCl and applied for 30 minutes at 72°F (22.2°C). Slides were rinsed with tris-buffered saline (0.9% NaCl) solution (pH, 7.6) and incubated with a biotinylated goat anti-rabbit IgG secondary antibody for 10 minutes at 72°F (22.2°C), followed by peroxidase-labeled streptavidin for 10 minutes. The detection system used was a commercially available kit with proprietary dilutions.e The positive control tissue was lung tissue (obtained from a healthy, purpose-bred research dog euthanized for an unrelated project). Descriptive data were recorded for results of histologic examinations.
Echocardiographic examinations
Fourteen HERDA-affected horses from 1 to 10 years of age (mean ± SD, 4.1 ± 2.8 years) underwent echocardiographic evaluation. Three horses from this group were subsequently included in the biomechanical portion of the study. Body weight of the 14 horses ranged from 291 to 577 kg (640 to 1,269 lb) with a mean ± SD weight of 439.2 ± 93 kg (966.2 ± 204.6 lb). Affected horses comprised 10 mares, 3 geldings, and 1 stallion. Nine and 5 of these horses belonged to the research herds at Mississippi State University and Michigan State University, respectively. Eleven control horses that ranged in age from 1 to 12 years (mean ± SD, 4.6 ± 3.0 years) underwent the same evaluations. Body weight of control horses ranged from 296 to 563 kg (651 to 1,239 lb) with a mean ± SD weight of 417.9 ± 79.4 kg [919.4 ± 174.7 lb]); there were 9 mares, 1 gelding, and 1 stallion. None of the horses were in athletic training, and none had clinical signs of cardiovascular disease. Four of the control horses were privately owned; the remaining 7 were from the herd maintained by Mississippi State University.
Echocardiographic examinations at Mississippi State University (n = 20) were performed by 1 veterinary radiologist (ELB) using a 1- to 4-MHz phased-array probe.f The remaining 5 examinations were performed at Michigan State University by 1 veterinary cardiologist (BO) using a 4-MHz phased-array probe.g All echocardiograms were obtained according to a standardized method established for use in horses.28 The hair was clipped from the cranioventral aspect of the thorax bilaterally, and alcohol and coupling gel were applied for all examinations. Horses were sedated, when necessary, with butorphanol tartrate,h detomidine hydrochloride,i or xylazine hydrochloride.j Measurements were performed with the leading edge-to-leading edge method. The mean and maximum aortic valve areas of all 25 horses were measured and recorded on the right parasternal short-axis views. For 20 horses (9 HERDA-affected and 11 controls), the mean and maximum width of the aorta and the mean and maximum height of the aorta (aortic diameter) at the valve annulus were also measured on right parasternal short-axis views. For the same 20 horses, measurements obtained from right parasternal long-axis images included the mean and maximum width of the aortic valve annulus, mean and maximum width of the widest part of the sinuses of Valsalva, and mean and maximum width of the proximal aspect of the ascending aorta. Location of valvular regurgitation was recorded when present.
Statistical analysis
Biomechanical data were normally distributed on the basis of results of a Shapiro-Wilk test. An unpaired Student t test was used to determine whether there was a significant difference in UTS between HERDA-affected and control horses. For echocardiographic data, Fisher exact testsk were used to determine whether there were significant associations between the presence of valve regurgitation and the diagnosis of HERDA. Separate analyses were conducted for pulmonic, mitral, aortic, and tricuspid valve regurgitation.
For each echocardiographic measurement, the data were visually assessed for normality by examination of histograms and q-q plots.l Each outcome was found to be approximately normally distributed. An ANOVA was conducted for each outcome.m Rather than include HERDA status, sex, body weight, and age in 1 model for each outcome, 4 simpler models were tested for each outcome (least squares means ± SEM) with HERDA status alone or in combination with sex, age, or weight included as the explanatory variable. This approach was used in consideration of the relatively small sample size. For the horses that underwent echocardiography, an unpaired Student t test was used to determine whether there was a significant difference in the mean body weight of HERDA-affected and control horses. Values of P < 0.05 were considered significant.
Results
Biomechanical evaluation
The UTSs of aortic and mitral valve leaflets of HERDA-affected horses were significantly (P ≤ 0.02 for all comparisons) lower than those of control horses for samples dissected in both circumferential and radial directions (Table 1). The magnitude of UTS of affected (n = 5) aortic valve leaflets was 45% lower in the circumferential direction and 29% lower in the radial direction, compared with results for respective control specimens (n = 5). However, the UTS magnitude of mitral valve leaflets of affected horses (n = 5) in the circumferential direction and radial direction were 25% and 27% lower than in control specimens (n = 5), respectively. The valves of 3 HERDA-affected horses examined via echocardiography were also examined in the biomechanical portion of the study, and for each horse, the mean UTS for the aortic valve and mitral valve dissected in each direction was subjectively lower than the corresponding mean values in the control horses.
Results (mean ± SD) of biomechanical testing of aortic and mitral valve leaflets for HERDA-affected (n = 5) and unaffected control (5) Quarter Horses.
| Valve and dissection approach | HERDA-affected | Control | P value |
|---|---|---|---|
| Aortic valve UTS (kPa) | |||
| Radial | 309.24 ± 51.2 | 434.3 ± 31 | < 0.001 |
| Circumferential | 2,515.14 ± 554.3 | 4,593.72 ± 878 | 0.001 |
| Mitral valve UTS (kPa) | |||
| Radial | 955.57 ± 162.5 | 1,310.65 ± 148.3 | 0.009 |
| Circumferential | 1,335.4 ± 210.4 | 1,768.8 ± 288 | 0.02 |
Valve leaflet tissue strips were dissected with a scalpel in radial and circumferential directions. The UTS testing was performed with a uniaxial testing device according to a previously described procedure.34
Histologic evaluations
Sections from the aortic valves of 3 of 6 HERDA-affected horses, but none of the 3 control horses examined, revealed the presence of microvessels in the lamina spongiosa (Figure 1). The endothelium of these microvessels stained strongly positive for factor VIII–related antigen by immunohistochemistry (Figure 2). Other than the presence of vessels, there were no differences noted between aortic valves of HERDA-affected and control horses. Evaluation of mitral valves sectioned in both radial and circumferential directions revealed microvessels in the lamina spongiosa of 6 of 6 HERDA-affected and 1 of 3 control horses (Figure 3). No structural differences in mitral valves were identified between the 2 groups.
Representative photomicrograph of an aortic valve (radially oriented cross section) from a Quarter Horse with HERDA. The lamina fibrosa predominantly contains collagen (yellow staining), the lamina spongiosa predominantly consists of glycosaminoglycan (blue staining), and the lamina ventricularis mainly comprises elastin fibers (black staining). Small blood vessels (arrows) are present in the lamina spongiosa. Movat pentachrome stain; bar = 100 μm. LF= Lamina fibrosa. LS = Lamina spongiosa. LV= Lamina ventricularis.
Citation: Journal of the American Veterinary Medical Association 250, 5; 10.2460/javma.250.5.538
Representative photomicrograph of the aortic valve (radially oriented cross section) from a HERDA-affected horse showing results of immunohistochemical testing for factor VIII–related antigen. Notice strong positive staining of endothelial cells (brown stain uptake; arrow). Immunohistochemical stain with hematoxylin counterstain; bar = 50 μm.
Citation: Journal of the American Veterinary Medical Association 250, 5; 10.2460/javma.250.5.538
Representative photomicrograph of a mitral valve (circumferentially oriented cross section) from a HERDA-affected horse. Small blood vessels (arrows) are prominent in the lamina spongiosa. Movat pentachrome stain; bar = 200 μm. See Figure 1 for remainder of key.
Citation: Journal of the American Veterinary Medical Association 250, 5; 10.2460/javma.250.5.538
Echocardiographic examinations
No association was found between the presence of regurgitation at any valve on echocardiography and HERDA status. The aortic valve was evaluated for the presence of regurgitation in 24 horses. The presence or absence of aortic valve regurgitation could not be determined for 1 horse owing to a suboptimal imaging plane. Fifteen of 24 (63%) horses (7/13 HERDA-affected and 8/11 controls) had mild regurgitation at the aortic valve. The remaining valves were evaluated for the presence of regurgitation in the 20 horses examined at Mississippi State University. According to sonographer preference, these valves were not examined in the horses at Michigan State University. Three of 20 (15%) horses (2/9 HERDA-affected and 1/11 controls) had mild regurgitation at the mitral valve. One of 11 control horses and none of the 9 HERDA-affected horses had regurgitation at the tricuspid valve. One of 9 horses with HERDA had pulmonic valvular regurgitation, and no pulmonic valvular abnormalities were found in the 11 control horses. Mild aortic regurgitation was present on echocardiography of 1 of the 3 HERDA-affected horses that were subsequently included in biomechanical evaluations. No other valvular regurgitation was observed for this subset of horses.
Analysis of data from 9 HERDA-affected and 11 control horses revealed no significant association between HERDA status and any of the aortic measurements obtained on long-axis echocardiographic images, including mean and maximum widths of the aorta at the annulus, sinuses of Valsalva, and proximal aspect of the ascending aorta.
Measurements of the aorta at the valve annulus obtained in short-axis images included aortic valve height and width (n = 20 [9 HERDA-affected and 11 control horses]) and area (25 [14 HERDA-affected and 11 control horses]). One HERDA-affected horse was excluded from statistical analysis of aortic valve height and width as the weight of this horse was not available. Several observations were made. There was no significant association between the mean or maximum width of the aorta, the mean or maximum height of the aorta, or the mean or maximum aortic valve area and HERDA status alone; there was also no significant association found when the statistical model included sex or age as explanatory variables. When the variability of aortic measurements owing to body weight was taken into account in the model, there was no association between mean or maximum aortic valve area and HERDA status, but there were significant associations between HERDA status and the mean (P = 0.033) and maximum (P = 0.036) width of the aorta and mean (P = 0.031) and maximum (P = 0.044) height of the aorta (Table 2). When aortic width and height were plotted against body weight for horses with HERDA (mean ± SD, 455.0 ± 80.1 kg [1,001.0 ± 176.2 lb]; n = 8) and control horses (mean ± SD, 417.9 ± 79.4 kg [919.4 ± 174.7 lb]; 11), it was noted that the control horses generally had larger aortic measurements than those with HERDA, especially at higher weights, although this was not evaluated statistically (Figure 4). There was no significant (P = 0.558) difference between the mean ± SD body weights of HERDA-affected (439 ± 93.1 kg [965.8 ± 204.8 lb]; n = 13) and control (418 ± 79.4 kg [919.6 ± 174.7 lb]; 11) horses used for all of the aortic measurements.
Scatterplots depicting relationships of body weight with mean width (A), mean height (B), maximum width (C), and maximum height (D) of the aorta at the valve annulus on right parasternal short-axis echocardiographic images in HERDA-affected horses (diamonds; n = 8) and control horses (squares; 11).
Citation: Journal of the American Veterinary Medical Association 250, 5; 10.2460/javma.250.5.538
Results of analysis of aortic measurements (least square mean ± SEM) on right parasternal short-axis echocardiographic images of HERDA-affected and control horses with a statistical model accounting for the effect of body weight.
| Valve | HERDA-affected | Control | P value |
|---|---|---|---|
| Aortic valve area (cm2) | |||
| Mean | 31.53 ± 1.278 | 32.04 ± 1.390 | 0.789 |
| Maximum | 33.20 ± 1.277 | 33.33 ± 1.389 | 0.947 |
| Aortic width (mm) | |||
| Mean | 63.21 ± 1.780 | 68.75 ± 1.511 | 0.033 |
| Maximum | 65.98 ± 1.849 | 71.61 ± 1.570 | 0.036 |
| Aortic height (mm) | |||
| Mean | 59.91 ± 1.546 | 64.78 ± 1.312 | 0.031 |
| Maximum | 62.00 ± 1.462 | 66.26 ± 1.241 | 0.044 |
Measurements were obtained at the level of the valve annulus. For HERDA-affected horses, n = 8 for measurements of aortic width and height and 13 for aortic valve area measurements. For control horses, n = 11 for all measurements.
Discussion
Previous reports of HERDA indicate that affected horses can have lesions of the integumentary system, tendinoligamentous structures, large blood vessels, and eyes.2–7 To the authors' knowledge, this study of Quarter Horses provides the first report of cardiac valvular abnormalities associated with HERDA in horses, and there are no reports describing the tensile strength of the cardiac valves of people affected with EDS, although valve fragility was reported in a clinical report of a patient with EDS that underwent cardiac surgery.23 Most reports involving cardiovascular lesions in human patients address the distensibility and strength of the walls of the blood vessels, including the root of the aorta and carotid arteries.36
We found that the UTSs for aortic and mitral valve leaflets were significantly lower for horses with HERDA than for control horses. The effect of age on UTS of the valves tested in the present study was unknown; one of the horses in the control group for the biomechanical evaluation was 20 years old, whereas the oldest horse in the HERDA group was 10 years old. Cardiac valve disease in people mainly affects elderly patients. In human and porcine heart valves, there appears to be an increase in stiffness (tensile modulus) and a higher collagen fiber distribution with increasing age,37,38 but there appears to be a paucity of literature on valvular tensile strength with respect to aging. In 1 study39 of human patients with aneurysms of the ascending portion of the aorta who also had bicuspid aortic valves, the UTS of the valve leaflets decreased with age, and this knowledge reduced the concern that inclusion of older horses in the control population could have influenced the results. In addition, the biomechanical results from the 20-year-old horse did not yield an outlier data point. In a study40 of middle-aged and older horses (median age, 14 years; range, 1 to 45 years), animals with left-sided valvular regurgitation were not at higher risk of death from any cause than were those without the condition. The cause of death in that study was owner reported, and the exact cause of death can be somewhat speculative in such cases; however, most deaths in the study population were attributed to orthopedic problems and gastrointestinal issues.40
Histopathologic examination of an abnormal mitral valve in a human patient affected by EDS revealed subjectively fewer collagen bundles, compared with those of patients without EDS.23 In our study, histologic examination of mitral valve leaflets revealed the presence of microvessels in the lamina spongiosa in HERDA-affected and control horses. This was not an unexpected finding, considering that mitral valves are much thicker than aortic valves, and the presence of vessels is interpreted as normal, providing a mechanism by which the valves obtain additional oxygen and nutrients.41 Interestingly, in the present study, microvessels were observed in the lamina spongiosa of aortic valve leaflets of HERDA-affected but not control horses. The presence of these vessels in aortic valve leaflets of horses with HERDA was potentially a pathological change.42,43 The presence of vessels in either the mitral valve or aortic valve has not previously been reported in horses to the authors' knowledge. It should be considered that this novel finding, particularly of vascularization of the aortic valve, resulted from evaluation of only a small number of horses. A more thorough evaluation of valve histology is needed to reveal the origin and nature of the vasculature in aortic and mitral valve leaflets of horses with and without HERDA.
Cardiovascular lesions have been described in human patients with various subtypes of EDS.20,23–33 The most common lesions include dilation of the aortic root and mitral valve prolapse.20,28,30 Valvular regurgitation has also been reported in patients with various subtypes of EDS.23,28–32 These lesions are typically not clinically relevant and are most frequently identified in patients affected with classic (types I and II) and hypermobile (type III) forms of EDS.20,27–30 In 1 study, echocardiographically detected aortic root dilation was identified in 20 of 71 (28%) patients with the classic and hypermobile forms of EDS.20 However, fatal arterial rupture without prior dilation has occurred in patients affected by the vascular form (type IV) and, rarely, the kyphoscoliosis form (type VI) of EDS.19–21,24,30 Milder cardiovascular lesions, such as mitral valve prolapse and valvular regurgitation, can be found in people with EDS type VI.23,32 Other cardiovascular lesions associated with EDS in humans include varicose veins and unwarranted bruising.20 Valvular regurgitation in EDS patients is thought to occur from defective support of the valve apparatus, laxity of the valve leaflets, or elongation of the chordae tendineae.23 Biomechanical testing of the heart valves of humans with EDS has not been reported but might contribute to the understanding of valvular regurgitation in affected patients.
In the present study, there was no association between HERDA status and the presence of regurgitation at any cardiac valve. However, 15 of 24 horses that underwent echocardiography in the study had mild aortic valve regurgitation, including 7 of 13 horses with HERDA and 8 of 11 control horses. There were also 3 horses with mitral valve regurgitation, 1 with pulmonic valve regurgitation, and 1 with tricuspid valve regurgitation. The number of horses with aortic valve regurgitation was an unexpected finding, given their relatively young ages (all < 12 years old with mean ages of 4.1 and 4.6 years for HERDA-affected and control horses, respectively) and lack of athletic training. Results of previous studies44–46 have shown an association between valvular regurgitation and athletic training in healthy horses. The valvular regurgitation in equine athletes is typically incidental and does not induce clinical signs.47 In a study48 of retired young Standardbred and Thoroughbred racehorses, 12 of 15 horses had regurgitation at the aortic valve. The aortic valve is also the most common site of degenerative valve lesions that are usually visualized in older horses or detected at necropsy.48–50 Mild aortic valve insufficiency is fairly common in horses > 8 to 10 years of age and is generally regarded as an incidental finding.49,51,52 Tricuspid valve regurgitation is also common in horses and is rarely associated with clinical abnormalities.53 Mitral valve regurgitation is a less common finding but is more likely to cause clinical signs than is regurgitation at other valves.48 Compared with results for Standardbred and Thoroughbred racehorses previously studied,48 the control Quarter Horses in our study had a slightly lower prevalence of aortic valve regurgitation (8/11 vs 12/15). However, the numbers of animals in both studies were small, and whether this finding is attributable to differences in training or other factors is unknown.
Even when accounting for potential variation caused by sex, age, and weight, there was no association between HERDA status and any of the echocardiographic aortic measurements obtained from long-axis echocardiographic images. There was no dilation of the root of the aorta (including the annulus, sinus of Valsalva, and proximal ascending aorta) as has been reported in human patients with EDS.20 Two-dimensional echocardiographic measurements of the aorta in people are performed by use of long-axis images to identify the maximum diameter of the root perpendicular to the long axis of the vessel.54–56 Similar measurements were performed in the horses of this study in addition to the measurements obtained from short-axis images. There was also no association between HERDA status alone and the areas of the aortic valve measured on right parasternal short-axis images.
Aortic diameter in horses increases with body weight.57–59 When accounting for the variability introduced by animal weight, a significant association between HERDA status and the mean and maximum diameter of the aorta at the valve annulus measured on right parasternal short-axis echocardiographic images was found. Aortic diameter measurements on short-axis images are subject to variability, compared with images obtained in long axis, for several reasons (eg, inability to determine the maximum diameter of the vessel, changes in cross-sectional area during the cardiac cycle, and refraction at the portion of the aorta that is not perpendicular to the ultrasound beam [width measurement]).56,60,61 However, even with the variability associated with these measurements, there was a significant difference between groups, with the mean and maximum aortic diameter of control horses being greater than that of HERDA-affected horses. To the authors' knowledge, reference ranges of the aortic diameter on 2-D short-axis images in Quarter Horses without athletic training have not been determined. The mean diameter (height) of the aorta for horses in our study (68.3 mm) was lower than that published previously for other breeds (ranging from 72 to 85 mm)52,57,62–65 but similar to that reported in a study66 of 50 athletic Quarter Horses. In the latter study,66 the authors did not specify the imaging plane (short axis or long axis) or mode (brightness mode or motion mode) used to obtain measurements of aortic diameter. It was surprising that the aortic diameters of the control horses in our study were larger than those of the affected horses in the model controlled for weight; however, most of the difference in size appeared to be attributable to 2 horses. The finding of a significant association between HERDA status and this variable may have been a spurious result. Additionally, larger aortic diameters can be related to aortic valve regurgitation and the resultant increase in stroke volume.53 As previously mentioned, aortic diameter can be increased in people with the classic and hypermobile forms of EDS.20 However, 2-D measurement of the diameter of the aorta on short-axis images is not a standard echocardiographic measurement in people.54 Although these short-axis measurements are routinely obtained in horses, they should be evaluated in in a larger number of Quarter Horses with and without HERDA to determine their clinical value.
Arterial aneurysms and fatal arterial rupture have been reported in people with the vascular form (type IV) and the kyphoscoliosis form (type VI) of EDS.19–24,30,31 A defect in type III collagen is responsible for causing premature vessel rupture in people with the vascular form of the disease.22,67 In patients with this form of EDS, the thickness of the intima and media of the carotid arteries is decreased, and the carotid arteries are more distensible than in clinically normal people, likely allowing for dissection and rupture.36 Although defects in collagen type III have not been identified by histology in horses with HERDA,68 the CYPB mutation would be expected to affect all fibrillar collagens, including type III.
The incidence of aortic rupture in horses is currently unknown. Despite results of a previous study6 indicating that the aorta in HERDA-affected horses is considerably weaker than that of healthy horses, to our knowledge, aortic rupture has not previously been reported in this population. Genetic testing of horses with aortic aneurysm or rupture may reveal an additional clinical effect of HERDA or a similar mutation affecting collagen.
Like many horses with HERDA, the horses in this study were not athletically tested. It is unknown whether athletic training in HERDA horses would lead to echocardiographically detectable abnormalities. The affected horses in this study were all ≤ 10 years of age. It is possible that more echocardiographic lesions may have been detected if older horses with HERDA had been examined; however, affected horses are generally humanely euthanized at a young age because of the progressive nature of the disease and are often not available as research subjects. In people with the vascular form of EDS, the median survival time is 48 years.69 Interestingly, these patients do not typically have any detectable dilation of the aorta before catastrophic dissection and aortic rupture occurs, so echocardiography is not often helpful.20
A limitation of this study, in addition to the small number of horses included in the various evaluations, was the use of sedation for echocardiography for many of the examinations. Sedation was necessary to obtain diagnostic images in these horses. Fortunately, diastolic aortic measurements are not affected by sedation with detomidine.70 Although the treatment can affect left ventricular measurements and fractional shortening,70,71 these measurements were not assessed in this study. Although detomidine administration appeared to cause an increase in the incidence of aortic valve regurgitation in horses in 1 study,71 the association was not found to be significant.
Echocardiography of more normal Quarter Horses, with specific attention to aortic size, should be performed to establish reference ranges. An interesting observation in this study was that, although the mean UTSs of valves for the 5 HERDA-affected horses examined were 25% to 45% lower than those of the 5 control horses, only 1 of the 3 HERDA-affected horses that underwent both evaluations had any echocardiographic abnormality detected (mild aortic valvular regurgitation). Biomechanical evaluations in a previous study6 by our group revealed significantly (P = 0.039) lower elastic modulus of the aorta and pulmonary artery in horses with HERDA (n = 6), compared with those for unaffected horses (6). These findings together suggest that significant alterations in the biomechanical properties of heart valves and great vessels can be present in horses without substantial echocardiographic abnormalities, and this should be a consideration when evaluating horses with this diagnostic modality.
Acknowledgments
Dr. Brazile was a PhD student at the time of the study.
Supported in part by the American Quarter Horse Association, the Morris Animal Foundation Veterinary Student Scholars program, and the Department of Large Animal Clinical Sciences, Michigan State University.
The authors declare that there were no conflicts of interest.
ABBREVIATIONS
| CYPB | Cyclophilin B |
| EDS | Ehlers-Danlos Syndrome |
| HERDA | Heritable equine regional dermal asthenia |
| UTS | Ultimate tensile strength |
Footnotes
Swiderski C, Pasquali P, Schwartz L et al. The ratio of urine deoxypyridine to pyridinoline identifies horses with hyperelastosis cutis (AKA hereditary equine regional dermal asthenia) (abstr), in Proceedings. 24th Annu Forum Am Coll Vet Intern Med 2006;756.
Hill A, Schwarz E, Cooke E, et al. Skin from horses with hereditary equine regional dermal asthenia (HERDA) contains collagen crosslinking patterns that cause reduced tensile strength (abstr), in Proceedings. 28th Annu Forum Am Coll Vet Intern Med 2010;688.
Mach-1 V500cs Micromechanical Systems, Biosyntech, Bloomington, Minn.
Rabbit anti-human factor VIII-related antigen, Dako North America Inc, Carpinteria, Calif.
LSAB2 System-HRP, Dako North America Inc, Carpinteria, Calif.
My Lab 50, Esaote North America Inc, Indianapolis, Ind.
Vivid 7, GE Healthcare, Piscataway, NJ.
Torbugesic, Fort Dodge Animal Health, Fort Dodge, Iowa.
Dormosedan, Pfizer Animal Health, New York, NY.
AnaSed, Lloyd Laboratories, Shenandoah, Iowa.
FREQ, SAS, version 9.2, SAS Institute Inc, Cary, NC.
UNIVARIATE, SAS, version 9.2, SAS Institute Inc, Cary, NC.
GLM, SAS, version 9.2, SAS Institute Inc, Cary, NC.
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