Spontaneous Closure of a Ventricular Septal Defect in a Horse

Abstract

A 2-month-old Quarter Horse Paint colt was referred to the University of Pennsylvania, Widener Hospital for Large Animals for evaluation of bilateral cardiac murmurs. The murmurs had been detected during a routine neonatal physical examination when the colt was 3 days old. There had been no changes in the cardiac murmurs and no other signs of cardiac disease during the first 2 months of life. Physical examination at presentation to the hospital identified a grade 5/6 pansystolic coarse band-shaped murmur with point of maximal intensity over the tricuspid valve region and a grade 4/6 holosystolic coarse band-shaped murmur with point of maximal intensity over the pulmonic to aortic valve regions. Physical examination was otherwise unremarkable. A ventricular septal defect (VSD) with relative pulmonic stenosis was considered the most likely cause of the cardiac murmurs. A complete echocardiographic examination (2D, M-mode, and Doppler [color flow, continuous wave, pulsed wave] echocardiography) was performed using a variable frequency 2.5–3.5 mHz cardiac transducer.a A small defect in the caudoapical portion of the interventricular muscular septum was detected. The diameter of the defect was 1.23 × 0.95 cm on long- and short-axis views, respectively. Color flow Doppler echocardiography revealed blood flow across the interventricular septum from left to right during systole at the site of the defect. Color flow Doppler also indicated that the shunted blood flow from the VSD coursed dorsocranially within the right ventricle, along both the right ventricular free wall and the right side of the interventricular septum. Continuous wave Doppler echocardiography from the right cardiac window revealed high-velocity blood flow from left to right across the defect with a peak velocity of 4.27 m/s. The apex of the right ventricle appeared mildly rounded and the tricuspid, mitral, and aortic valves appeared mildly thickened. All 4 cardiac valves were thoroughly examined by Doppler echocardiography and no evidence of regurgitation was detected. The echocardiographic findings were consistent with an isolated restrictive caudoapical muscular VSD. Prognosis for a normal life expectancy and use as a western pleasure horse were considered to be good. Annual echocardiographic reevaluation was recommended. It was recommended that the foal not be bred as an adult because there is evidence for a hereditary component with VSDs in a few animal species and in some human families.1–5 Approximately 8 months later, the referring veterinarian evaluated the colt for mucopurulent, bilateral nasal discharge and an occasional cough. At this time, the colt's cardiac murmurs had decreased in intensity. The colt was treated with trimethoprim sulfamethoxazole and its respiratory signs improved. However, because of the decrease in intensity of the cardiac murmurs the colt was readmitted to the Widener Hospital for Large Animals for cardiac reexamination. On physical examination, the colt was in good body condition and had grown appropriately, weighing 348 kg. An occasional cough was present. Pulmonary auscultation disclosed harsh bronchovesicular sounds cranioventrally. An occasional wheeze was auscultated over the right midthorax with a rebreathing examination. Cardiac auscultation revealed that both cardiac murmurs had decreased in intensity by 1 grade and the right-sided murmur was holosystolic rather than pansystolic. Physical examination findings were otherwise unremarkable. The small caudoapical muscular VSD was still discernable on echocardiographic examination. The defect measured 1.32 × 1.04 cm on long- and short-axis views, respectively. Color flow Doppler echocardiography identified a very narrow jet of blood flow through the interventricular septum, which continued dorsocranially along the right side of the interventricular septum (Fig 1). A velocity of 3.48 m/s was obtained with continuous wave Doppler echocardiography, but the measurement was thought to be an underestimate of the peak shunt velocity because of poor alignment with shunt flow. Increased right ventricular pressure because of increased pulmonary vascular resistance secondary to the colt's respiratory tract disease also could explain the decreased shunt velocity across the VSD and decreased intensity of the cardiac murmurs. Right-sided cardiac catheterization was recommended because the lack of tricuspid regurgitation precluded noninvasive estimation of pulmonary arterial pressure. Thoracic radiographs and thorough evaluation of the colt's respiratory disease also was recommended. The owner declined these diagnostic procedures and elected to continue monitoring the colt at home. Reevaluation was recommended if the respiratory signs were to persist or progress, the cardiac murmurs to change, or the foal to develop clinical signs consistent with cardiac compromise. Color flow Doppler, short-axis echocardiogram of the left ventricle and interventricular septum obtained from the right cardiac window during the second examination when the horse was 10 months old. Color flow mapping revealed a narrow jet of blood flow from left to right consistent with a ventricular septal defect. Follow-up communication a few months later indicated that the colt's respiratory signs had resolved shortly after the cardiac reexamination and that it was apparently healthy. Follow-up communication with the referring veterinarian 1 year later indicated that the cardiac murmurs had continued to decrease in intensity over time and had become inaudible. The colt had continued to grow normally and had no signs of cardiopulmonary disease. Echocardiographic reevaluation was recommended to determine if closure of the VSD had occurred. The gelding was presented for reevaluation at 25 months. Physical examination at the time revealed no abnormalities. The loud bilateral systolic murmurs that were present previously were no longer audible. Echocardiographic reexamination confirmed closure of the VSD. A dimple-like irregularity was present in the myocardium in the caudoapical portion of the septum where the defect had been detected previously. Color flow, continuous wave, and pulsed-wave Doppler echocardiography failed to identify blood flow shunting across the ventricular septum, consistent with spontaneous closure of the VSD. Physical examination on follow-up examination when the gelding was 6 years old, in work as a show horse, in good body condition and weighing 622 kg, also disclosed no abnormalities. The dimple-like irregularity in the myocardium in the caudoapical portion of the interventricular septum where the VSD had been located previously still was discernable on a long-axis left ventricular view from the left paracostal window and measured 0.64 cm. In addition, a thin band of echoic tissue, presumably fibrous tissue, was observed overlying the right side of the interventricular septum at the site of the dimple (Fig 2). The right ventricular free wall periodically was in direct apposition with this portion of the interventricular septum during systole (Fig 2). Although color flow Doppler echocardiography indicated blood flow into the dimple on the left side of the interventricular septum, color flow, continuous wave, and pulsed-wave Doppler echocardiography failed to identify blood flow shunting across the interventricular septum (Fig 3). These findings were consistent with persistent closure of the VSD. Cardiac reevaluation was recommended if the gelding's murmurs returned. 2D long-axis echocardiogram of the left and right ventricles and interventricular septum obtained from the left, 5th intercostal space on the fourth examination when the horse was 6 years old. Notice the thin band of echoic tissue (arrow), presumably fibrous tissue, overlying the right side of the interventricular septum at the site of the dimple where the ventricular septal defect had been located previously. Note the close apposition of the right ventricular free wall (arrow head) to the dimple in the interventricular septum during systole. Color flow Doppler, long-axis echocardiogram of the interventricular septum obtained from the left 5th intercostal space on the fourth examination when the horse was 6 years old. Color flow mapping identified turbulent blood flow into and out of the dimple but not across the interventricular septum. The most common form of congenital heart disease in humans and horses is a VSD.5–7 The prognosis for humans with isolated small VSDs often is good. Many patients are asymptomatic and many of the VSDs spontaneously close early in life.5,6,8–10 Contrary to what is reported for horses, many isolated VSDs in humans occur in the muscular septum.5–7,11 Some studies in humans suggest that spontaneous closure of isolated muscular VSDs is more frequent than with isolated VSDs in other locations.5,6,11 Spontaneous closure rate has been reported to be as high as 88.9% for small, isolated muscular VSDs in human neonates evaluated by echocardiography from birth.11 Although spontaneous closure of VSDs has been reported in several animal species, it appears to be rare compared with what is observed in humans. Spontaneous closure has been reported in 2 dogs with isolated perimembranous VSDs and in 2 dogs with complex congenital heart disease after surgical closure of a patent ductus arteriosus.12–14 Surgically induced VSDs in dogs also have been reported to close spontaneously.15 Other species in which spontaneous closure of a VSD has been reported include Yucatan miniature pigs and pika.16,17 Mechanisms of spontaneous closure depend upon VSD location.5,18,19 Muscular VSDs in humans close by apposition of the muscular borders of the VSD. This occurs by muscular in-growth and fibrous proliferation from the septum resulting in formation of a plug.5,18 Alternatively, apposition may be achieved by hypertrophy of the right ventricular myocardium.5 Perimembranous VSDs in humans usually close with a thin layer of fibrous tissue formed by progressive adhesion of tricuspid valve tissue or chordae tendinae to the borders of the VSD.5,18,19 Closure via growth of fibrous tissue from the periphery of the perimembranous VSD also has been reported but is believed to be less common.5,18,19 Physical examination findings in humans and animals with spontaneously closing VSDs include decreasing intensity and eventual disappearance of cardiac murmurs.5,9,12–14 2D and Doppler echocardiographic findings reported in humans with spontaneously closing VSDs include resolution of volume overload, decrease in defect size, inability to identify the defect, membranous ventricular septal aneurysm, and narrowing of the band-width of color flow Doppler signals.5,18–21 2D and Doppler echocardiographic signs of spontaneous closure reported in the veterinary medical literature include presence of redundant tissue or a membranous ventricular septal aneurysm at the site of the VSD and lack of Doppler signals consistent with transseptal flow.12,13 Physical examination findings for the horse of this report were in accordance with those reported for spontaneous closure in humans and other animals. The echocardiographic signs of very mild volume overload detected during initial examination of this horse resolved by the time of the second examination. The thickened appearance of the tricuspid, mitral, and aortic valves at the initial echocardiographic examination may have been associated with the VSD blood flow. If so, it is difficult to understand why the pulmonic valve was not also affected. Transient vavlulitis secondary to a viral infection was considered possible. The thickened appearance of the valves had resolved by the second echocardiographic examination 8 months later. Consequently, it did not seem likely that the valves were dysplastic. The dimple-like irregularity in the myocardium was smaller than the original VSD, possibly consistent with growth of muscular tissue. In contrast to humans with spontaneous closure of muscular VSDs, however, a gradual decrease in the size of the VSD was not detected. The location of the defect in this horse made it difficult to image and obtain reproducible measurements, but more frequent examinations may have been useful in documenting measureable decreases in defect size. Furthermore, the borders of the VSD were not apposed with a muscular-fibrous plug or hypertrophied muscle of the right ventricle. Instead, the defect was closed by a very thin band of echoic tissue, presumably fibrous tissue on the right side of the septum. Consequently, closure of the muscular septal defect appeared to be more similar to closure described for perimembranous defects in both humans and dogs.5,12,13,19 Thus, the manner of spontaneous closure of the muscular VSD in this horse does not appear to be typical of that reported for muscular VSDs in humans. The conditions leading to this type of closure in this patient are unknown. Small defect size, small shunt volume, and lack of shunting during diastole may have contributed to the manner of VSD closure in this horse. The close proximity and periodic apposition of the right ventricular free wall and interventricular septum may have partially or intermittently closed the defect, decreasing the already small shunt, and thus facilitating growth of fibrous tissue and spontaneous closure. Membranous VSDs are the only VSDs commonly reported in horses.7,22 Relatively little information regarding VSDs in other locations in horses is available. Four cases of muscular VSDs in horses were documented in which progression and outcomes were evaluated.22 One of these horses was a successful racing Standardbred. The other 3 horses had additional severe cardiac abnormalities including large or multiple VSDs, valvular dysplasia or prolapse, severe regurgitation of 1 or more valves, substantial volume overload, main pulmonary artery dilatation, arrhythmias, and myocardial dysfunction which necessitated euthanasia. Muscular VSDs in horses are documented far less commonly than in humans, and the few that have been documented were associated with more severe cardiac disease and had a poorer prognosis than what has been reported in human beings. However, the true prevalence and clinical outcomes of muscular VSDs are not known because studies screening horses for congenital heart disease have not been conducted. The incidence of VSDs in human neonates and rates of positive outcomes such as spontaneous closure dramatically increased once echocardiography became more routinely used in neonates.6,8,10 Similar studies performed in horses may result in similar epidemiologic findings. In conclusion, spontaneous closure of a VSD in a horse has not been reported previously. Spontaneous closure should be considered a potential outcome in horses with VSDs if cardiac murmurs are noted to decrease in intensity or disappear. However, decreasing murmur intensity also occurs with progression of cardiac disease and increased pulmonary vascular resistance. Thus, when spontaneous closure is suspected, an echocardiographic reexamination should be performed to eliminate progression of cardiac disease and to confirm spontaneous closure. Additional clinical and epidemiologic studies are needed to describe congenital heart disease in horses. aVivid Five; GE Vingmed Ultrasound, Horten, Norway