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Spontaneously occurring restrictive nonhypertrophied cardiomyopathy in domestic cats: a new animal model of human disease
Cardiovascular Pathology 23 (2014) 28–34
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Cardiovascular Pathology
Original Article
Spontaneously occurring restrictive nonhypertrophied cardiomyopathy in domestic cats: a new animal model of human disease Philip R. Fox a, Cristina Basso b, Gaetano Thiene b, Barry J. Maron c,⁎ a b c
Caspary Research Institute, The Animal Medical Center, New York, NY Department of Cardiac, Thoracic and Vascular Sciences, University of Padua Medical School, Italy Hypertrophic Cardiomyopathy Center, Minneapolis Heart Institute Foundation, Minneapolis, MN 55407
a r t i c l e
i n f o
Article history: Received 25 June 2013 Received in revised form 1 August 2013 Accepted 2 August 2013 Keywords: Cardiomyopathy Echocardiography Heart failure Feline
a b s t r a c t Background: Spontaneously occurring small animal models of myocardial disease, closely resembling the human condition, have been reported for hypertrophic cardiomyopathy (in cats) and arrhythmogenic right ventricular cardiomyopathy (in cats and boxer dogs). Nonhypertrophied restrictive cardiomyopathy (RCM) is a well-recognized but relatively uncommon primary heart muscle disease causing substantial morbidity in humans. We describe RCM occurring in felines here as a potential model of human disease. Methods: We used two-dimensional and Doppler echocardiography to define morphologic and functional features of RCM in 35 domestic cats (25 male; 10±4 years old) presenting to a subspecialty veterinary clinic. Ten underwent complete necropsy examination. Echocardiographic parameters of diastolic filling were compared to those in 41 normal controls. Results: The 35 cats presented with congestive heart failure (n=32), lethargy (n=2), or syncope (n=1), associated with thromboembolism in 5 and supraventricular tachyarrhythmias in 8. During an average 4.4-year follow-up period, 18 died or were euthanized due to profound heart failure, and 3 died suddenly; survival from clinical presentation to death was 0.1 to 52 months. Echocardiographic and necropsy examination showed biatrial enlargement, nondilated ventricular chambers, and normal wall thicknesses and atrioventricular valves. Histopathology demonstrated disorganized myocyte architecture and patchy replacement myocardial fibrosis. Pulsed Doppler demonstrated restrictive physiology with increased early (E) mitral filling velocity (1.1±0.3 m/s) and peak E to peak late (A) flow ratios (4.3±1.2), reduced A filling velocity (0.3±0.1 m/s), and shortened mitral deceleration time (40.7±9.3 ms; all Pb.001 vs. controls), with preserved left ventricular systolic function. Conclusions: A primary myocardial disease occurring spontaneously in domestic cats is remarkably similar to restrictive nondilated and nonhypertrophied cardiomyopathy in man and represents another potential animal model for human disease. © 2014 Elsevier Inc. All rights reserved.
1. Introduction Recent reports have described distinctive clinical and pathologic features of spontaneously occurring cardiomyopathies in small animals, notably hypertrophic cardiomyopathy (HCM) and arrhythmogenic right ventricular cardiomyopathy (ARVC) [1–6]. Clinical and morphologic profiles in these cats and dogs resemble (if not replicate) their respective conditions in humans and potentially represent spontaneously occurring animal models of human disease. Another myocardial disease occurring in man is characterized by impaired ventricular filling and restrictive physiology, associated with
the phenotype of enlarged atria in the presence of normal ventricular thickness and chamber size, and normal atrioventricular valves [7]. The natural history of this condition (known as primary restrictive cardiomyopathy [RCM]) in patients is usually adverse due to congestive heart failure and sudden death [8–16], but without definitive treatment strategies other than heart transplant [12], and with an incompletely understood pathogenesis. In the present study, we report the first comprehensive description of the clinical, echocardiographic, and pathologic features of RCM occurring spontaneously in domestic cats and closely resembling the same disease in humans. 2. Methods
Funding sources: This study was supported by a grant from the Caspary Research Institute of the Animal Medical Center. Registry for Cardio-Cerebro-Vascular Pathology, Veneto Region, Venice, Italy. Disclosures: none. ⁎ Corresponding author. Minneapolis Heart Institute Foundation, Minneapolis, MN 55407. Tel.: +1 612 853 3996; fax: +1 612 863 3875. E-mail address: [email protected] (B.J. Maron). 1054-8807/$ – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.carpath.2013.08.001
2.1. Study population This protocol was approved by investigator Institutional Animal Use and Care Committee, with owner consent to participate. Between June 2002 and January 2006, clinical and echocardiographic records of
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The Animal Medical Center (Caspary Research Institute and Bobst Hospital) in New York City were reviewed for feline patients presenting to the Cardiology Specialty Clinic for evaluation of acute dyspnea and/or weakness, or congestive heart failure with radiographic evidence of pulmonary congestion or pleural effusion (n= 1125). Cats with severe anemia, systemic hypertension, or thyrotoxicosis were excluded. Over this period of time, 54 cases (4.8%) met our criteria for RCM on the basis of a distinctive phenotype by echocardiography or at autopsy, and with signs of heart failure: (a) marked biatrial enlargement, (b) normal left ventricular (LV) wall thickness and ventricular chamber size, (c) normal LV systolic function, and (d) restrictive LV filling pattern with pulsed Doppler echocardiography. Nineteen animals with technically suboptimal mitral inflow velocities were excluded, and the remaining 35 cats constitute the study group. RCM cats were 1.5 to 17.1 years old (mean 10±4); 25 (71%) were males. Body weight was 2.4 to 8.8 kg (mean, 4.9±1.5) (Table 1). Breeds included domestic short hair (n=25), Siamese (n=2), and one each American short hair, domestic long hair, Manx, Birman, Singapura, Burmese, Himalayan, and Maine Coon. None of these cats were known to be related. At initial diagnosis, 32 of the 35 RCM cats had clinical signs consistent with congestive heart failure including tachypnea, dyspnea, ascites, or jugular venous distention; 2 presented with lethargy, and 1 presented with syncope and transient paresis. Of 34 animals that died, 10 were available for gross and histologic postmortem examination.
2.2. Controls Forty-one healthy cats without clinical evidence of cardiovascular disease and with normal echocardiographic and Doppler studies were selected during the same study period for comparison with RCM cats: 1.0 to 17.7 years old (mean, 10.1±4.6) and 2.5 to 10.6 kg in weight (mean, 4.8±1.8); 23 (56%) were neutered males (Table 1). Breeds included domestic short hair (n=24), domestic long hair (n=3), Maine coon cat (n=2), Himalayan (n=2), Persian (n=2), Siamese (n=2), and one each of Scottish fold, Angora, Siamese, Burmese, Abyssinian, and American short hair breeds. Control cats did not differ from RCM cats with regard to age (P=.9), gender (P=.2), or body weight (P=.6).
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In addition, 6 other normal cats that died from trauma without evidence of cardiovascular disease were selected as controls for comparison with the 10 RCM cats that underwent autopsy examination. Controls were 2 to 11 years old (mean, 6.5±3.4 years); weight was 2.9 to 5.9 kg (mean, 4.5±1.1); four were male. None of the 41 control cats studied with echocardiography was assessed postmortem. 2.3. Echocardiographic methods Standard M-mode, two-dimensional, and Doppler echocardiographic examinations were performed in conscious, unsedated, manually restrained cats as previously described [1,4,17] using commercially available cardiovascular ultrasound systems equipped with 7 and 7.5-mHz, phased-array transducers. M-mode echocardiograms were derived from two-dimensional images under direct anatomic visualization with simultaneous electrocardiographic (ECG) recording [1] to achieve as closely as possible the standard imaging planes described in man [18]. Measurements were reported after averaging three to five consecutive cycles. Left ventricular enddiastolic (LVIDd) and end-systolic (LVIDs) diameters were measured, and LV short axis shortening fraction (% FS) was calculated by the following formula: LVIDd−LVIDs/LVIDd×100. Ventricular septum and LV posterior free wall thicknesses were measured at end-diastole at the right parasternal short-axis view at the level of chordae tendineae. Left atrial (LA) dimension at endsystole and aortic root (Ao) dimensions at end-diastole were measured from the left basal short-axis view. Right atrium was judged to be dilated if comparable in size to the enlarged left atrium in two echocardiographic views. Pulsed-wave Doppler echocardiography measured transmitral early (E) and atrial (A) filling velocities, ratio of peak early to peak late flow (E/A), and E-wave deceleration time [19,20] from the left apical four-chamber view with the sample volume in the LV inflow tract at the tips of the mitral leaflets. 2.4. Radiographic and ECG methods Thoracic radiographs were obtained in each RCM cat. Pulmonary edema was judged to be present by LA enlargement with alveolar or interstitial lung patterns [21]. ECG was recorded from a right lateral recumbent position [1,4,22] in 20 RCM cats, and simultaneous lead II ECG was recorded during echocardiographic examination in all RCM cats.
Table 1 Demographic, echocardiographic, and morphological comparison of cats with RCM and normal controlsa Parameter
RCM
Controls
No. of animals Age, years Male, n (%) Body weight, kg. Max. LV wall thickness, mm VS LVPW LV cavity, mm End-diastole End-systole Fractional shortening, % LA dimension, mm LA:Ao ratio E-wave, m/s A-wave, m/s E/A ratio E-wave deceleration (ms)
35 10±4.0 (1.5–17.1) 25 (71) 4.9±1.5 (2.4–8.8)
41 10.1.±4.6 (1–17.7) 23 (56) 4.8±1.8 (2.5–10.6)
P value .88 .23 .6
4.4±0.8 (3.0–5.8) 4.5±0.8 (3.0–5.8)
4.1±0.6 (3.1–5.0) 3.9±0.5 (3.0–5.0)
.03 b.001
15.6±2.0 (9.5–18.9) 8.4±1.3 (5.3–10.2) 45.2±8.6 (35.2–60.2) 20.5±2.4 (16.3–27.6) 2.3±0.3 (1.8–3.4) 1.1±0.27 (0.7–1.8) 0.27±0.1 (0.12–0.47) 4.3±1.2 (2.8–7.6) 40.7±9.3 (21.9–62.2)
14.9±2.1 (10–18.2) 7.1±1.4 (4.5–10.2) 52.2±5.8 (38.1–60.1) 11.9±1.0 (10–13.5) 1.2±0.1 (1.1–1.3) 0.69±0.12 (0.5– 0.9) 0.68±0.12 (0.5–1.0) 1.0±0.3 (0.6–1.5) 86.5±20.6 (53.5–191.6)
.12 b.001 b.001 b.001 b.001 b.001 b.001 b.001 b.001
Abbreviations: A-wave=transmitral atrial (A) filling velocity; E-wave=transmitral early (E) filling velocity; LVPW=left ventricular posterior free wall; m/s=meters/second; ms= millisecond; max=maximum; VS=ventricular septum. a Expressed as mean±S.D. (range).
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2.5. Pathological methods Necropsy examinations were performed in 10 RCM cats. Hearts were fixed in 10% phosphate-buffered formalin. A semiquantitative scoring system described cardiac chamber enlargement: 0=absent, 1=mild, 2=moderate, 3=severe [1,23]. LV wall thickness was measured at three sites, and average thickness was calculated. Transmural tissue blocks were taken perpendicular to the long-axis of the right ventricular (RV) wall, left ventricular (LV) free wall, and ventricular septum. Tissue sections were 5 μm thick, embedded in paraffin, and stained with hematoxylin/eosin, Heidenhain (azan) trichrome, and Congo Red stains. Myocytes were assessed for hypertrophy (transverse diameter N12 μm) and spatial arrangement of cardiac muscle cells (i.e., disarray) [1,24–26]. Fibrosis was categorized as either pericellular or replacement. Extent of myocardial disarray or replacement fibrosis was graded 0 to 3+ (0=absent/minimal, 1=mild, 2=moderate, 3= severe) [25,26]. 2.6. Follow-up Morbidity and mortality data for the 35 RCM cats were obtained by reviewing patient medical records. For cases in which the clinical outcome could not be accessed from the patient database, owners of the cats were contacted to determine the clinical status of their animals. 2.7. Statistical analysis Categorical data are presented as proportions, and continuous data are expressed as range and mean±S.D. Differences between continuous variables were analyzed with unpaired Student’s t test or Mann– Whitney rank sum test. Differences between proportions were assessed with Fisher’s Exact Test. Measure of statistical dependence between two variables was tested by Spearman rank order correlation. Survival curves were estimated by Kaplan–Meier procedure. Median survival was reported. A P value b.05 was regarded as statistically significant.
tachycardia, gallop rhythm, jugular vein distention, palpable abdominal effusion, or hepatosplenomegaly. The three other cats presented with lethargy (n=2) or syncope (n=1). 3.2. Echocardiography 3.2.1. Cardiac dimensions RCM cats had substantial biatrial enlargement (Fig. 1) (20.5±2.4 mm). LA was substantially larger compared to controls (11.9±1.0 mm) (Pb.001); LA:Ao ratio in RCM (2.3±0.3) also significantly exceeded that in controls (1.2±0.1) (Pb.001). Right atrium was judged enlarged from echocardiograms (and thoracic radiographs) in each RCM cat (Fig. 1). Ventricular septal thicknesses were slightly greater in RCM cats (4.4±0.8 mm) than in controls (4.1±0.6 mm; P=.034), as were LV posterior free wall thicknesses (4.5±0.8 mm vs. 3.9±0.5 mm; Pb.001) (Table 1). However, both septal and free wall thicknesses were b6 mm in each animal, i.e., did not meet the conventional cutoff value used for diagnosis of feline HCM [1,19,27,28]. End-diastolic LV cavity dimension did not differ between RCM and control cats (P=.125). Systolic anterior motion of the mitral valve was absent. LV shortening fraction was lower in RCM (45.2%±8.6%) vs. controls (52.2%±5.8%; Pb.001), but in each cat was within the normal reference range [1,19,20,27,28]. In RCM cats, there was no relation between age and LA dimension (P=.06), LV end-diastolic dimension (P=.214), or ventricular septal (P=.29) and LV free wall thickness (P=.28). 3.2.2. Doppler LV filling patterns Pulsed Doppler mitral inflow examination demonstrated restrictive filling patterns in each of the 35 RCM cats (Table 1; Fig. 2). Mitral E-wave deceleration time was significantly shorter in RCM cats (40.7±9.3 ms) compared to controls (86.5±20.6 ms; Pb.001). RCM cats also showed higher E-wave velocity (1.1±0.3 m/s), lower A-wave velocity (0.27±0.1m/s), and higher E/A ratio (4.3±1.2) compared to controls (0.69±0.1m/s, 0.68±0.1m/s, and 1.0±0.3, respectively) (all Pb.001). In each RCM cat, the E:A ratio exceeded the highest E/A recorded in control cats (i.e., 1.5).
3. Results 3.3. Radiography 3.1. Clinical presentation Of the 35 RCM cats, 32 presented with ≥1 physical finding characteristic of heart failure: acute tachypnea/dyspnea, ausculatory findings of inspiratory rales, muffled lung and heart sounds,
Thirty-two of 35 cats had evidence of congestive heart failure radiographically, including 13 with changes compatible with leftsided congestive heart failure (e.g., patchy or diffuse alveolar or interstitial infiltrates). Six of these 32 cats had evidence of right-sided
Fig. 1. Two-dimensional echocardiographic features characteristic of RCM in two domestic cats. Apical four-chamber view in end-diastole shows marked biatrial enlargement with normal LV wall thickness and normal-sized ventricular cavities. (A) A 12-year-old, domestic Maine Coon cat. (B) A 15-year-old domestic short-hair cat (also shown in Figs. 2A and 3A). RA=right atrium.
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Fig. 2. Distinctive restrictive LV inflow filling patterns by pulsed Doppler echocardiography (at mitral valve leaflet tips in apical four-chamber view) from two domestic short-hair cats with RCM. (A) Age 15. (B) Age 9. Both show characteristically tall E-wave, small A-wave, and high E/A ratio (3.9 and 3.5, respectively), as well as shortened E-wave deceleration time (41 and 52 ms, respectively). Recording at 100 mm/s and 66 mm/s, respectively. Tracing in panel (A) is from the same cat shown in Fig. 1B and also Fig. 3A. (C) Normal transmitral filling pattern recorded in a normal, healthy domestic short-hair cat. E/A ratio is 1.3, and E-wave deceleration time is 83 ms. Recorded at 100 mm/s.
heart failure, including pleural effusion, ascites or hepatomegaly, pericardial effusion, and inferior vena caval dilatation; 13 other cats had changes consistent with biventricular heart failure. 3.4. Electrocardiography On presentation, a variety of arrhythmias were evident in the RCM cats, including nonsustained supraventricular tachycardia (n=8) or ventricular premature complexes or tachycardia (n=10). Atrial fibrillation developed in one cat 10 months after diagnosis. 3.5. Gross pathology Heart weights were 24.3±4.8 g and significantly exceeded those previously reported in normal cats (18.4±3.6 g; P=.009) [29].
Compared to controls, all 10 RCM cats showed moderate-to-severe left and right atrial dilatation (Figs. 3 and 4). A mural thrombus was observed in left atrium or appendage in two (Fig. 4). Left and right ventricular chambers appeared grossly normal in size in all 10 RCM cats. Ventricular septal thicknesses did not differ between RCM cats (5.7±1.6 mm) and controls (6.5±0.3; P=.37).
3.6. Histopathology Histological features identified in ventricular septum and/or LV free wall included (Fig. 3) the following: (a) myocyte disarray (Fig. 3C) in each cat (judged moderate in nine; mild in one) associated with increased interstitial fibrosis in each; score: 1.9±0.3 vs. 0.7±0.5 in controls; (b) myocardial replacement fibrosis in 9/10 (moderate in 3, mild in 6; score: 1.2±0.6 vs. 0 in controls); and (c) abnormal
Fig. 3. (A) Heart from a 15-year-old domestic short-hair cat with RCM opened along the two-dimensional echocardiographic four-chamber tomographic plane showing nondilated ventricular chambers, normal LV wall thickness, and normal atrioventricular valves, but associated with biatrial enlargement. Same cat as shown in Figs. 1B and 2A. (B) Intramural coronary arteriole with modest thickening of the wall (media), adjacent to area of replacement fibrosis. Stained with trichrome Heidenhain; magnification ×100. (C) Disorganized myocardial architecture in which adjacent myocytes are arranged at perpendicular and oblique angles, with interstitial fibrosis evident in areas of disarray. Stained with trichrome Heidenhain; magnification ×100. FW=left ventricular free wall.
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Fig. 4. Gross heart specimen from a 3-year-old American short-hair domestic cat with RCM, sectioned to correspond with the two-dimensional echocardiographic four-chamber view. The two portions of the transected heart are shown in panels (A; posterior) and (B; anterior). LV wall thickness and atrioventricular valves are normal, ventricular cavities are nondilated, and the right and left atria are enlarged. In panel (B), a huge thrombus occupies the entire LA chamber and appendage.
intramural coronary arterioles with thick walls and narrowed lumen in 8/10 vs. 0 in controls (Fig. 3B). Amyloidosis and hemochromatosis were absent, and there was no evidence of endomyocardial disease in the cat hearts.
The remaining RCM cat (with heart failure) was lost to follow-up after 2.8 years. Cardiac and noncardiac mortalities are compared in Fig. 5.
3.7. Clinical outcomes
Spontaneously occurring canine and feline cardiac diseases, remarkably similar clinically and morphologically to those conditions occurring in humans, have been reported. We and others have described in detail HCM in domestic [1] and Maine coon cats [2,30], as well as ARVC in both cats [4] and boxer dogs [5,6]. The present report extends our interspecies observations (and recognition of potential spontaneous models of human disease) to RCM, another form of cardiac disease well documented in man with restrictive physiology, and usually identified by transmitral Doppler echocardiography [7–15]. Distinctive from HCM, the primary RCM phenotype is characterized by absence of LV wall thickening in the presence of normal-sized left and right ventricular chambers, substantial biatrial enlargement, structurally and functionally normal atrioventricular valves, and preserved LV systolic function [7–13]. Indeed, all these features in humans with RCM were consistently identified in the cats reported here, and in the absence of evidence for systemic or other cardiac diseases known to be associated with restrictive filling and cardiomyopathy in patients [31,32]. Clinical profiles of our RCM cats shares important similarities with human RCM patients. Notably, these include severe heart failure with exertional dyspnea (sometimes associated with pulmonary edema) [14,15,33,34], as well as thromboembolism [35,36], supraventricular tachyarrhythmias [37,38], and pulsed Doppler waveform patterns consistent with restrictive LV filling [37–41]. Feline cardiomyopathy described here had a profound impact on the clinical course and survival of the animals. Ninety percent of the RCM cats presented with congestive heart failure and despite maximum medical management, their survival thereafter was limited—i.e., 60% died of heart failure, suddenly, or of associated thromboembolic events, with median survival after diagnosis slightly N3 months. Although data are relatively sparse in man, and available literature suggests that children and adults with RCM (similar to the feline model) almost uniformly experience adverse prognosis with substantial morbidity and mortality due to heart failure and sudden death [14,34,37,41].
In order to control heart failure, cats were treated alone or in combination with angiotensin-converting enzyme inhibitors (n=29), diuretics (n=4), sotalol (n=2), and digitalis (n=2), as well as anticoagulation with heparin (n=2). Of the 35 RCM cats, 18 died or were euthanized due to progressive and drug refractory heart failure; 5 of these 18 also had thromboembolism to the distal aorta. Three others experienced sudden death: including a witnessed event in which the cat was startled out of sleep, staggered, and collapsed; and two indoor cats that were asymptomatic in the morning and found dead 7 h later without evidence of trauma. For these 21 cats, survival from presentation to death was 0.1 to 52 months (median, 3.4). Thirteen other cats died from noncardiac causes (chronic renal failure, n=6; lymphoma, n=2; unknown chronic diseases, n=2; and 1 each with hepatic carcinoma, meningitis, intestinal foreign body).
Fig. 5. Kaplan–Meier estimate of the death-free survival of 35 cats with RCM. Curves depict cardiac mortality and noncardiac mortality separately (Pb.05 by log-rank test).
4. Discussion
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However, one area of distinction between human and animal forms for RCM relates to its relative prevalence in these respective populations. For example, identification of 35 RCM cats over only about 3 years at the Animal Medical Center (i.e., about 5% of those cats with echocardiograms for evaluation of congestive heart failure or pulmonary congestion) suggests that the frequency of this disease in felines greatly exceeds that in human patients. Noneosinophilic RCM is a primary myocardial disease for which pathogenesis is incompletely resolved. In man, pathogenic mutations in several genes encoding proteins of the cardiac sarcomere have been linked to primary RCM (e.g., troponin I and T, α-cardiac actin, beta-myosin heavy chain, myosin light chain, myopalladin, αtropomyosin) [9,38,40–47]. Families with relatives having the dissimilar phenotypes of RCM or HCM have been associated with troponin I and T, beta-myosin heavy chain, and α-actin mutations [37,40– 42,44,47]. The molecular basis of RCM (and HCM) in the cat is, at present, unknown. A relationship between primary RCM and HCM, suggested by occurrence of restrictive physiology in both conditions [7,41,44,45,47,48], is further evident by our histopathologic findings in RCM cats studied at autopsy. Histopathologic features of HCM were evident in most of these cats showing areas of disorganized cellular architecture (myocyte disarray) and structurally abnormal intramural coronary arterioles in the LV, and replacement scarring likely the consequence of ischemia and cell death [24,26]. These findings suggest that, in cats, the RCM phenotype could be part of the spectrum of contractile protein disease (e.g., HCM) due to sarcomere mutations [41,44,47,49]. While, in man, RCM is generally considered a primary form of heart disease, our observations in the spontaneous feline model raise the possibility that RCM and HCM phenotypes could be diverse expressions of the same disease state. While some echocardiographic parameters in our RCM cats differed statistically from the control animals without cardiovascular disease, these values nevertheless fell within published normal reference limits for cats [1,17,27,28]. For example, the ventricular septal and LV free wall thickness in RCM cats, while slightly thicker than in controls, in each case were less than the cutoff value regarded as diagnostic for feline HCM. Translating the findings in this report to the development of an animal model suitable for investigational purposes can be challenging. To date, there have been a small number of successful efforts to create colonies of small animals with naturally occurring, familial cardiac disease. Notably, a breeding colony of Maine Coon cats has been established, and phenotypic, pathologic, and genetic evidence of HCM has been reported from this spontaneous model [2,30]. Successful breeding of other heritable cardiovascular diseases have been documented in the canine as well, with familial transmission of discrete subaortic stenosis in the Newfoundland dog [50,51] and conotruncal malformations in the Keeshond dog [52,53]. These reports demonstrate the feasibility of using animal models with familial heart disease in breeding experiments to study phenotypes and genetic mutations. Nevertheless, there are a number of obstacles to establishing and maintaining successful breeding colonies using nonrodent mammals, particularly the feline or canine. Important among these considerations is the fact that identified probands invariably originate from the private sector (as family pets) and that these animals are frequently neutered or otherwise are not candidates for breeding. Moreover, breeding colonies represent an ambitious enterprise requiring substantial resources and devoted facilities and equipment, technical support, and reliable funding over long periods of time; such have not generally been forthcoming in veterinary medicine. Furthermore, experiments with felines may be more difficult than those involving other species with regard to justification of animal involvement, particularly when terminal experiments are contemplated. Nonetheless, as reported here, the opportunity to
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develop and study naturally occurring feline models of human heart disease is germane and relevant. In conclusion, this novel report of naturally occurring feline myocardial disease, i.e., nonhypertrophied, nondilated RCM, is an important addition to the other cardiovascular diseases previously reported to occur spontaneously in dogs and cats, such as HCM and ARVC. Taken together, the observation that complex disease processes (e.g., HCM, ARVC, and now RCM) are similar across species in small animals and man is striking. These are relevant clinical observations, providing potential animal models that could serve to enhance our understanding of cardiac disease in man. References [1] Fox PR, Liu SK, Maron BJ. Echocardiographic assessment of spontaneously occurring feline hypertrophic cardiomyopathy: an animal model of human disease. Circulation 1995;92:2645–51. [2] Meurs KM, Sanchez X, David RM, Bowles NE, Towbin JA, Reiser PJ, et al. A cardiac myosin binding protein C mutation in the Maine Coon cat with familial hypertrophic cardiomyopathy. Hum Mol Genet 2005;14:3587–93. [3] Meurs KM, Norgard MM, Ederer MM, Hendrix KP, Kittleson MD. A substitution mutation in the myosin binding protein C gene in ragdoll hypertrophic cardiomyopathy. Genomics 2007;90:261–4. [4] Fox PR, Maron BJ, Basso C, Liu SK, Thiene G. Spontaneously occurring arrhythmogenic right ventricular cardiomyopathy in the domestic cat: a new animal model similar to the human disease. Circulation 2000;102:1863–70. [5] Basso C, Fox PR, Meurs KM, Towbin JA, Spier AW, Calabrese F, et al. Arrhythmogenic right ventricular cardiomyopathy causing sudden cardiac death in boxer dogs: a new animal model of human disease. Circulation 2004;109: 1180–5. [6] Meurs KM, Mauceli E, Lahmers S, Acland GM, White SN, Lindblad-Toh K. 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Contents lists available at ScienceDirect
Cardiovascular Pathology
Original Article
Spontaneously occurring restrictive nonhypertrophied cardiomyopathy in domestic cats: a new animal model of human disease Philip R. Fox a, Cristina Basso b, Gaetano Thiene b, Barry J. Maron c,⁎ a b c
Caspary Research Institute, The Animal Medical Center, New York, NY Department of Cardiac, Thoracic and Vascular Sciences, University of Padua Medical School, Italy Hypertrophic Cardiomyopathy Center, Minneapolis Heart Institute Foundation, Minneapolis, MN 55407
a r t i c l e
i n f o
Article history: Received 25 June 2013 Received in revised form 1 August 2013 Accepted 2 August 2013 Keywords: Cardiomyopathy Echocardiography Heart failure Feline
a b s t r a c t Background: Spontaneously occurring small animal models of myocardial disease, closely resembling the human condition, have been reported for hypertrophic cardiomyopathy (in cats) and arrhythmogenic right ventricular cardiomyopathy (in cats and boxer dogs). Nonhypertrophied restrictive cardiomyopathy (RCM) is a well-recognized but relatively uncommon primary heart muscle disease causing substantial morbidity in humans. We describe RCM occurring in felines here as a potential model of human disease. Methods: We used two-dimensional and Doppler echocardiography to define morphologic and functional features of RCM in 35 domestic cats (25 male; 10±4 years old) presenting to a subspecialty veterinary clinic. Ten underwent complete necropsy examination. Echocardiographic parameters of diastolic filling were compared to those in 41 normal controls. Results: The 35 cats presented with congestive heart failure (n=32), lethargy (n=2), or syncope (n=1), associated with thromboembolism in 5 and supraventricular tachyarrhythmias in 8. During an average 4.4-year follow-up period, 18 died or were euthanized due to profound heart failure, and 3 died suddenly; survival from clinical presentation to death was 0.1 to 52 months. Echocardiographic and necropsy examination showed biatrial enlargement, nondilated ventricular chambers, and normal wall thicknesses and atrioventricular valves. Histopathology demonstrated disorganized myocyte architecture and patchy replacement myocardial fibrosis. Pulsed Doppler demonstrated restrictive physiology with increased early (E) mitral filling velocity (1.1±0.3 m/s) and peak E to peak late (A) flow ratios (4.3±1.2), reduced A filling velocity (0.3±0.1 m/s), and shortened mitral deceleration time (40.7±9.3 ms; all Pb.001 vs. controls), with preserved left ventricular systolic function. Conclusions: A primary myocardial disease occurring spontaneously in domestic cats is remarkably similar to restrictive nondilated and nonhypertrophied cardiomyopathy in man and represents another potential animal model for human disease. © 2014 Elsevier Inc. All rights reserved.
1. Introduction Recent reports have described distinctive clinical and pathologic features of spontaneously occurring cardiomyopathies in small animals, notably hypertrophic cardiomyopathy (HCM) and arrhythmogenic right ventricular cardiomyopathy (ARVC) [1–6]. Clinical and morphologic profiles in these cats and dogs resemble (if not replicate) their respective conditions in humans and potentially represent spontaneously occurring animal models of human disease. Another myocardial disease occurring in man is characterized by impaired ventricular filling and restrictive physiology, associated with
the phenotype of enlarged atria in the presence of normal ventricular thickness and chamber size, and normal atrioventricular valves [7]. The natural history of this condition (known as primary restrictive cardiomyopathy [RCM]) in patients is usually adverse due to congestive heart failure and sudden death [8–16], but without definitive treatment strategies other than heart transplant [12], and with an incompletely understood pathogenesis. In the present study, we report the first comprehensive description of the clinical, echocardiographic, and pathologic features of RCM occurring spontaneously in domestic cats and closely resembling the same disease in humans. 2. Methods
Funding sources: This study was supported by a grant from the Caspary Research Institute of the Animal Medical Center. Registry for Cardio-Cerebro-Vascular Pathology, Veneto Region, Venice, Italy. Disclosures: none. ⁎ Corresponding author. Minneapolis Heart Institute Foundation, Minneapolis, MN 55407. Tel.: +1 612 853 3996; fax: +1 612 863 3875. E-mail address: [email protected] (B.J. Maron). 1054-8807/$ – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.carpath.2013.08.001
2.1. Study population This protocol was approved by investigator Institutional Animal Use and Care Committee, with owner consent to participate. Between June 2002 and January 2006, clinical and echocardiographic records of
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The Animal Medical Center (Caspary Research Institute and Bobst Hospital) in New York City were reviewed for feline patients presenting to the Cardiology Specialty Clinic for evaluation of acute dyspnea and/or weakness, or congestive heart failure with radiographic evidence of pulmonary congestion or pleural effusion (n= 1125). Cats with severe anemia, systemic hypertension, or thyrotoxicosis were excluded. Over this period of time, 54 cases (4.8%) met our criteria for RCM on the basis of a distinctive phenotype by echocardiography or at autopsy, and with signs of heart failure: (a) marked biatrial enlargement, (b) normal left ventricular (LV) wall thickness and ventricular chamber size, (c) normal LV systolic function, and (d) restrictive LV filling pattern with pulsed Doppler echocardiography. Nineteen animals with technically suboptimal mitral inflow velocities were excluded, and the remaining 35 cats constitute the study group. RCM cats were 1.5 to 17.1 years old (mean 10±4); 25 (71%) were males. Body weight was 2.4 to 8.8 kg (mean, 4.9±1.5) (Table 1). Breeds included domestic short hair (n=25), Siamese (n=2), and one each American short hair, domestic long hair, Manx, Birman, Singapura, Burmese, Himalayan, and Maine Coon. None of these cats were known to be related. At initial diagnosis, 32 of the 35 RCM cats had clinical signs consistent with congestive heart failure including tachypnea, dyspnea, ascites, or jugular venous distention; 2 presented with lethargy, and 1 presented with syncope and transient paresis. Of 34 animals that died, 10 were available for gross and histologic postmortem examination.
2.2. Controls Forty-one healthy cats without clinical evidence of cardiovascular disease and with normal echocardiographic and Doppler studies were selected during the same study period for comparison with RCM cats: 1.0 to 17.7 years old (mean, 10.1±4.6) and 2.5 to 10.6 kg in weight (mean, 4.8±1.8); 23 (56%) were neutered males (Table 1). Breeds included domestic short hair (n=24), domestic long hair (n=3), Maine coon cat (n=2), Himalayan (n=2), Persian (n=2), Siamese (n=2), and one each of Scottish fold, Angora, Siamese, Burmese, Abyssinian, and American short hair breeds. Control cats did not differ from RCM cats with regard to age (P=.9), gender (P=.2), or body weight (P=.6).
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In addition, 6 other normal cats that died from trauma without evidence of cardiovascular disease were selected as controls for comparison with the 10 RCM cats that underwent autopsy examination. Controls were 2 to 11 years old (mean, 6.5±3.4 years); weight was 2.9 to 5.9 kg (mean, 4.5±1.1); four were male. None of the 41 control cats studied with echocardiography was assessed postmortem. 2.3. Echocardiographic methods Standard M-mode, two-dimensional, and Doppler echocardiographic examinations were performed in conscious, unsedated, manually restrained cats as previously described [1,4,17] using commercially available cardiovascular ultrasound systems equipped with 7 and 7.5-mHz, phased-array transducers. M-mode echocardiograms were derived from two-dimensional images under direct anatomic visualization with simultaneous electrocardiographic (ECG) recording [1] to achieve as closely as possible the standard imaging planes described in man [18]. Measurements were reported after averaging three to five consecutive cycles. Left ventricular enddiastolic (LVIDd) and end-systolic (LVIDs) diameters were measured, and LV short axis shortening fraction (% FS) was calculated by the following formula: LVIDd−LVIDs/LVIDd×100. Ventricular septum and LV posterior free wall thicknesses were measured at end-diastole at the right parasternal short-axis view at the level of chordae tendineae. Left atrial (LA) dimension at endsystole and aortic root (Ao) dimensions at end-diastole were measured from the left basal short-axis view. Right atrium was judged to be dilated if comparable in size to the enlarged left atrium in two echocardiographic views. Pulsed-wave Doppler echocardiography measured transmitral early (E) and atrial (A) filling velocities, ratio of peak early to peak late flow (E/A), and E-wave deceleration time [19,20] from the left apical four-chamber view with the sample volume in the LV inflow tract at the tips of the mitral leaflets. 2.4. Radiographic and ECG methods Thoracic radiographs were obtained in each RCM cat. Pulmonary edema was judged to be present by LA enlargement with alveolar or interstitial lung patterns [21]. ECG was recorded from a right lateral recumbent position [1,4,22] in 20 RCM cats, and simultaneous lead II ECG was recorded during echocardiographic examination in all RCM cats.
Table 1 Demographic, echocardiographic, and morphological comparison of cats with RCM and normal controlsa Parameter
RCM
Controls
No. of animals Age, years Male, n (%) Body weight, kg. Max. LV wall thickness, mm VS LVPW LV cavity, mm End-diastole End-systole Fractional shortening, % LA dimension, mm LA:Ao ratio E-wave, m/s A-wave, m/s E/A ratio E-wave deceleration (ms)
35 10±4.0 (1.5–17.1) 25 (71) 4.9±1.5 (2.4–8.8)
41 10.1.±4.6 (1–17.7) 23 (56) 4.8±1.8 (2.5–10.6)
P value .88 .23 .6
4.4±0.8 (3.0–5.8) 4.5±0.8 (3.0–5.8)
4.1±0.6 (3.1–5.0) 3.9±0.5 (3.0–5.0)
.03 b.001
15.6±2.0 (9.5–18.9) 8.4±1.3 (5.3–10.2) 45.2±8.6 (35.2–60.2) 20.5±2.4 (16.3–27.6) 2.3±0.3 (1.8–3.4) 1.1±0.27 (0.7–1.8) 0.27±0.1 (0.12–0.47) 4.3±1.2 (2.8–7.6) 40.7±9.3 (21.9–62.2)
14.9±2.1 (10–18.2) 7.1±1.4 (4.5–10.2) 52.2±5.8 (38.1–60.1) 11.9±1.0 (10–13.5) 1.2±0.1 (1.1–1.3) 0.69±0.12 (0.5– 0.9) 0.68±0.12 (0.5–1.0) 1.0±0.3 (0.6–1.5) 86.5±20.6 (53.5–191.6)
.12 b.001 b.001 b.001 b.001 b.001 b.001 b.001 b.001
Abbreviations: A-wave=transmitral atrial (A) filling velocity; E-wave=transmitral early (E) filling velocity; LVPW=left ventricular posterior free wall; m/s=meters/second; ms= millisecond; max=maximum; VS=ventricular septum. a Expressed as mean±S.D. (range).
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2.5. Pathological methods Necropsy examinations were performed in 10 RCM cats. Hearts were fixed in 10% phosphate-buffered formalin. A semiquantitative scoring system described cardiac chamber enlargement: 0=absent, 1=mild, 2=moderate, 3=severe [1,23]. LV wall thickness was measured at three sites, and average thickness was calculated. Transmural tissue blocks were taken perpendicular to the long-axis of the right ventricular (RV) wall, left ventricular (LV) free wall, and ventricular septum. Tissue sections were 5 μm thick, embedded in paraffin, and stained with hematoxylin/eosin, Heidenhain (azan) trichrome, and Congo Red stains. Myocytes were assessed for hypertrophy (transverse diameter N12 μm) and spatial arrangement of cardiac muscle cells (i.e., disarray) [1,24–26]. Fibrosis was categorized as either pericellular or replacement. Extent of myocardial disarray or replacement fibrosis was graded 0 to 3+ (0=absent/minimal, 1=mild, 2=moderate, 3= severe) [25,26]. 2.6. Follow-up Morbidity and mortality data for the 35 RCM cats were obtained by reviewing patient medical records. For cases in which the clinical outcome could not be accessed from the patient database, owners of the cats were contacted to determine the clinical status of their animals. 2.7. Statistical analysis Categorical data are presented as proportions, and continuous data are expressed as range and mean±S.D. Differences between continuous variables were analyzed with unpaired Student’s t test or Mann– Whitney rank sum test. Differences between proportions were assessed with Fisher’s Exact Test. Measure of statistical dependence between two variables was tested by Spearman rank order correlation. Survival curves were estimated by Kaplan–Meier procedure. Median survival was reported. A P value b.05 was regarded as statistically significant.
tachycardia, gallop rhythm, jugular vein distention, palpable abdominal effusion, or hepatosplenomegaly. The three other cats presented with lethargy (n=2) or syncope (n=1). 3.2. Echocardiography 3.2.1. Cardiac dimensions RCM cats had substantial biatrial enlargement (Fig. 1) (20.5±2.4 mm). LA was substantially larger compared to controls (11.9±1.0 mm) (Pb.001); LA:Ao ratio in RCM (2.3±0.3) also significantly exceeded that in controls (1.2±0.1) (Pb.001). Right atrium was judged enlarged from echocardiograms (and thoracic radiographs) in each RCM cat (Fig. 1). Ventricular septal thicknesses were slightly greater in RCM cats (4.4±0.8 mm) than in controls (4.1±0.6 mm; P=.034), as were LV posterior free wall thicknesses (4.5±0.8 mm vs. 3.9±0.5 mm; Pb.001) (Table 1). However, both septal and free wall thicknesses were b6 mm in each animal, i.e., did not meet the conventional cutoff value used for diagnosis of feline HCM [1,19,27,28]. End-diastolic LV cavity dimension did not differ between RCM and control cats (P=.125). Systolic anterior motion of the mitral valve was absent. LV shortening fraction was lower in RCM (45.2%±8.6%) vs. controls (52.2%±5.8%; Pb.001), but in each cat was within the normal reference range [1,19,20,27,28]. In RCM cats, there was no relation between age and LA dimension (P=.06), LV end-diastolic dimension (P=.214), or ventricular septal (P=.29) and LV free wall thickness (P=.28). 3.2.2. Doppler LV filling patterns Pulsed Doppler mitral inflow examination demonstrated restrictive filling patterns in each of the 35 RCM cats (Table 1; Fig. 2). Mitral E-wave deceleration time was significantly shorter in RCM cats (40.7±9.3 ms) compared to controls (86.5±20.6 ms; Pb.001). RCM cats also showed higher E-wave velocity (1.1±0.3 m/s), lower A-wave velocity (0.27±0.1m/s), and higher E/A ratio (4.3±1.2) compared to controls (0.69±0.1m/s, 0.68±0.1m/s, and 1.0±0.3, respectively) (all Pb.001). In each RCM cat, the E:A ratio exceeded the highest E/A recorded in control cats (i.e., 1.5).
3. Results 3.3. Radiography 3.1. Clinical presentation Of the 35 RCM cats, 32 presented with ≥1 physical finding characteristic of heart failure: acute tachypnea/dyspnea, ausculatory findings of inspiratory rales, muffled lung and heart sounds,
Thirty-two of 35 cats had evidence of congestive heart failure radiographically, including 13 with changes compatible with leftsided congestive heart failure (e.g., patchy or diffuse alveolar or interstitial infiltrates). Six of these 32 cats had evidence of right-sided
Fig. 1. Two-dimensional echocardiographic features characteristic of RCM in two domestic cats. Apical four-chamber view in end-diastole shows marked biatrial enlargement with normal LV wall thickness and normal-sized ventricular cavities. (A) A 12-year-old, domestic Maine Coon cat. (B) A 15-year-old domestic short-hair cat (also shown in Figs. 2A and 3A). RA=right atrium.
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Fig. 2. Distinctive restrictive LV inflow filling patterns by pulsed Doppler echocardiography (at mitral valve leaflet tips in apical four-chamber view) from two domestic short-hair cats with RCM. (A) Age 15. (B) Age 9. Both show characteristically tall E-wave, small A-wave, and high E/A ratio (3.9 and 3.5, respectively), as well as shortened E-wave deceleration time (41 and 52 ms, respectively). Recording at 100 mm/s and 66 mm/s, respectively. Tracing in panel (A) is from the same cat shown in Fig. 1B and also Fig. 3A. (C) Normal transmitral filling pattern recorded in a normal, healthy domestic short-hair cat. E/A ratio is 1.3, and E-wave deceleration time is 83 ms. Recorded at 100 mm/s.
heart failure, including pleural effusion, ascites or hepatomegaly, pericardial effusion, and inferior vena caval dilatation; 13 other cats had changes consistent with biventricular heart failure. 3.4. Electrocardiography On presentation, a variety of arrhythmias were evident in the RCM cats, including nonsustained supraventricular tachycardia (n=8) or ventricular premature complexes or tachycardia (n=10). Atrial fibrillation developed in one cat 10 months after diagnosis. 3.5. Gross pathology Heart weights were 24.3±4.8 g and significantly exceeded those previously reported in normal cats (18.4±3.6 g; P=.009) [29].
Compared to controls, all 10 RCM cats showed moderate-to-severe left and right atrial dilatation (Figs. 3 and 4). A mural thrombus was observed in left atrium or appendage in two (Fig. 4). Left and right ventricular chambers appeared grossly normal in size in all 10 RCM cats. Ventricular septal thicknesses did not differ between RCM cats (5.7±1.6 mm) and controls (6.5±0.3; P=.37).
3.6. Histopathology Histological features identified in ventricular septum and/or LV free wall included (Fig. 3) the following: (a) myocyte disarray (Fig. 3C) in each cat (judged moderate in nine; mild in one) associated with increased interstitial fibrosis in each; score: 1.9±0.3 vs. 0.7±0.5 in controls; (b) myocardial replacement fibrosis in 9/10 (moderate in 3, mild in 6; score: 1.2±0.6 vs. 0 in controls); and (c) abnormal
Fig. 3. (A) Heart from a 15-year-old domestic short-hair cat with RCM opened along the two-dimensional echocardiographic four-chamber tomographic plane showing nondilated ventricular chambers, normal LV wall thickness, and normal atrioventricular valves, but associated with biatrial enlargement. Same cat as shown in Figs. 1B and 2A. (B) Intramural coronary arteriole with modest thickening of the wall (media), adjacent to area of replacement fibrosis. Stained with trichrome Heidenhain; magnification ×100. (C) Disorganized myocardial architecture in which adjacent myocytes are arranged at perpendicular and oblique angles, with interstitial fibrosis evident in areas of disarray. Stained with trichrome Heidenhain; magnification ×100. FW=left ventricular free wall.
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Fig. 4. Gross heart specimen from a 3-year-old American short-hair domestic cat with RCM, sectioned to correspond with the two-dimensional echocardiographic four-chamber view. The two portions of the transected heart are shown in panels (A; posterior) and (B; anterior). LV wall thickness and atrioventricular valves are normal, ventricular cavities are nondilated, and the right and left atria are enlarged. In panel (B), a huge thrombus occupies the entire LA chamber and appendage.
intramural coronary arterioles with thick walls and narrowed lumen in 8/10 vs. 0 in controls (Fig. 3B). Amyloidosis and hemochromatosis were absent, and there was no evidence of endomyocardial disease in the cat hearts.
The remaining RCM cat (with heart failure) was lost to follow-up after 2.8 years. Cardiac and noncardiac mortalities are compared in Fig. 5.
3.7. Clinical outcomes
Spontaneously occurring canine and feline cardiac diseases, remarkably similar clinically and morphologically to those conditions occurring in humans, have been reported. We and others have described in detail HCM in domestic [1] and Maine coon cats [2,30], as well as ARVC in both cats [4] and boxer dogs [5,6]. The present report extends our interspecies observations (and recognition of potential spontaneous models of human disease) to RCM, another form of cardiac disease well documented in man with restrictive physiology, and usually identified by transmitral Doppler echocardiography [7–15]. Distinctive from HCM, the primary RCM phenotype is characterized by absence of LV wall thickening in the presence of normal-sized left and right ventricular chambers, substantial biatrial enlargement, structurally and functionally normal atrioventricular valves, and preserved LV systolic function [7–13]. Indeed, all these features in humans with RCM were consistently identified in the cats reported here, and in the absence of evidence for systemic or other cardiac diseases known to be associated with restrictive filling and cardiomyopathy in patients [31,32]. Clinical profiles of our RCM cats shares important similarities with human RCM patients. Notably, these include severe heart failure with exertional dyspnea (sometimes associated with pulmonary edema) [14,15,33,34], as well as thromboembolism [35,36], supraventricular tachyarrhythmias [37,38], and pulsed Doppler waveform patterns consistent with restrictive LV filling [37–41]. Feline cardiomyopathy described here had a profound impact on the clinical course and survival of the animals. Ninety percent of the RCM cats presented with congestive heart failure and despite maximum medical management, their survival thereafter was limited—i.e., 60% died of heart failure, suddenly, or of associated thromboembolic events, with median survival after diagnosis slightly N3 months. Although data are relatively sparse in man, and available literature suggests that children and adults with RCM (similar to the feline model) almost uniformly experience adverse prognosis with substantial morbidity and mortality due to heart failure and sudden death [14,34,37,41].
In order to control heart failure, cats were treated alone or in combination with angiotensin-converting enzyme inhibitors (n=29), diuretics (n=4), sotalol (n=2), and digitalis (n=2), as well as anticoagulation with heparin (n=2). Of the 35 RCM cats, 18 died or were euthanized due to progressive and drug refractory heart failure; 5 of these 18 also had thromboembolism to the distal aorta. Three others experienced sudden death: including a witnessed event in which the cat was startled out of sleep, staggered, and collapsed; and two indoor cats that were asymptomatic in the morning and found dead 7 h later without evidence of trauma. For these 21 cats, survival from presentation to death was 0.1 to 52 months (median, 3.4). Thirteen other cats died from noncardiac causes (chronic renal failure, n=6; lymphoma, n=2; unknown chronic diseases, n=2; and 1 each with hepatic carcinoma, meningitis, intestinal foreign body).
Fig. 5. Kaplan–Meier estimate of the death-free survival of 35 cats with RCM. Curves depict cardiac mortality and noncardiac mortality separately (Pb.05 by log-rank test).
4. Discussion
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However, one area of distinction between human and animal forms for RCM relates to its relative prevalence in these respective populations. For example, identification of 35 RCM cats over only about 3 years at the Animal Medical Center (i.e., about 5% of those cats with echocardiograms for evaluation of congestive heart failure or pulmonary congestion) suggests that the frequency of this disease in felines greatly exceeds that in human patients. Noneosinophilic RCM is a primary myocardial disease for which pathogenesis is incompletely resolved. In man, pathogenic mutations in several genes encoding proteins of the cardiac sarcomere have been linked to primary RCM (e.g., troponin I and T, α-cardiac actin, beta-myosin heavy chain, myosin light chain, myopalladin, αtropomyosin) [9,38,40–47]. Families with relatives having the dissimilar phenotypes of RCM or HCM have been associated with troponin I and T, beta-myosin heavy chain, and α-actin mutations [37,40– 42,44,47]. The molecular basis of RCM (and HCM) in the cat is, at present, unknown. A relationship between primary RCM and HCM, suggested by occurrence of restrictive physiology in both conditions [7,41,44,45,47,48], is further evident by our histopathologic findings in RCM cats studied at autopsy. Histopathologic features of HCM were evident in most of these cats showing areas of disorganized cellular architecture (myocyte disarray) and structurally abnormal intramural coronary arterioles in the LV, and replacement scarring likely the consequence of ischemia and cell death [24,26]. These findings suggest that, in cats, the RCM phenotype could be part of the spectrum of contractile protein disease (e.g., HCM) due to sarcomere mutations [41,44,47,49]. While, in man, RCM is generally considered a primary form of heart disease, our observations in the spontaneous feline model raise the possibility that RCM and HCM phenotypes could be diverse expressions of the same disease state. While some echocardiographic parameters in our RCM cats differed statistically from the control animals without cardiovascular disease, these values nevertheless fell within published normal reference limits for cats [1,17,27,28]. For example, the ventricular septal and LV free wall thickness in RCM cats, while slightly thicker than in controls, in each case were less than the cutoff value regarded as diagnostic for feline HCM. Translating the findings in this report to the development of an animal model suitable for investigational purposes can be challenging. To date, there have been a small number of successful efforts to create colonies of small animals with naturally occurring, familial cardiac disease. Notably, a breeding colony of Maine Coon cats has been established, and phenotypic, pathologic, and genetic evidence of HCM has been reported from this spontaneous model [2,30]. Successful breeding of other heritable cardiovascular diseases have been documented in the canine as well, with familial transmission of discrete subaortic stenosis in the Newfoundland dog [50,51] and conotruncal malformations in the Keeshond dog [52,53]. These reports demonstrate the feasibility of using animal models with familial heart disease in breeding experiments to study phenotypes and genetic mutations. Nevertheless, there are a number of obstacles to establishing and maintaining successful breeding colonies using nonrodent mammals, particularly the feline or canine. Important among these considerations is the fact that identified probands invariably originate from the private sector (as family pets) and that these animals are frequently neutered or otherwise are not candidates for breeding. Moreover, breeding colonies represent an ambitious enterprise requiring substantial resources and devoted facilities and equipment, technical support, and reliable funding over long periods of time; such have not generally been forthcoming in veterinary medicine. Furthermore, experiments with felines may be more difficult than those involving other species with regard to justification of animal involvement, particularly when terminal experiments are contemplated. Nonetheless, as reported here, the opportunity to
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develop and study naturally occurring feline models of human heart disease is germane and relevant. In conclusion, this novel report of naturally occurring feline myocardial disease, i.e., nonhypertrophied, nondilated RCM, is an important addition to the other cardiovascular diseases previously reported to occur spontaneously in dogs and cats, such as HCM and ARVC. Taken together, the observation that complex disease processes (e.g., HCM, ARVC, and now RCM) are similar across species in small animals and man is striking. These are relevant clinical observations, providing potential animal models that could serve to enhance our understanding of cardiac disease in man. References [1] Fox PR, Liu SK, Maron BJ. Echocardiographic assessment of spontaneously occurring feline hypertrophic cardiomyopathy: an animal model of human disease. Circulation 1995;92:2645–51. [2] Meurs KM, Sanchez X, David RM, Bowles NE, Towbin JA, Reiser PJ, et al. 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