Congenital valvular dysplasia in the horse

In the case report described in this issue, Bell et al. (2024) describe a previously unreported occurrence of aortic, mitral and tricuspid valve dysplasia (TVD) in a 7-month-old, Warmblood-cross colt. The prevalence of congenital heart disease (CHD) in the horse is limited to historical studies that suggest a prevalence of 0.03%–0.2% (Buergelt, 2003; Michaëlsson & Ho, 2000). Valvular dysplasia appears to make up an even smaller group of CHD in horses, and was identified in only 8/18 foals in one report (Hall et al., 2010). The reported numbers likely underestimate total occurrence, as the true numbers include foals with severe cardiac malformations who die during or shortly after birth, as well as clinically silent defects that go undetected. Despite the suspectedly low occurrence of congenital valvular disease, the equine practitioner must still be aware of these conditions and be familiar with the clinical presentation and available diagnostics for detection and management of congenital valvular dysplasia.

The horse in this report had changes consistent with congenital dysplasia of the aortic, mitral and tricuspid valves. Individual case reports of dysplasia of the mitral valve (Marr & Bowen, 2010; McGurrin et al., 2003; Schober et al., 2000) and mitral and tricuspid valves (Duz et al., 2013) in horses have been published. Anecdotal observation of TVD as an isolated congenital defect has also been previously reported by the authors. Most congenital abnormalities of the tricuspid valve in horses are forms of atresia (Hall et al., 2010; Krüger et al., 2016; Meurs et al., 1997), with some investigators suggesting it is among the most common form of equine CHD (Hall et al., 2010). Atrioventricular valvular dysplasia can also be a component of complex CHD (Bayly et al., 1982; Kohnken et al., 2018; van der Luer & van der Linde-Sipman, 1978). Dysplasia of the aortic valve has been described as an isolated abnormality (Clark et al., 1987; Taylor et al., 2007), as well as in combination with other CHD (Michlik et al., 2014).

The clinical signs of valvular dysplasia tend to manifest early in life, particularly when disease is severe. The clinical presentation of horses with CHD varies greatly based on disease severity. Most foals diagnosed with one or more forms of valvular dysplasia present with lethargy, exercise intolerance, failure to thrive, dyspnoea, cyanosis or syncope within the first days to weeks of life (Scansen, 2019). Clinical signs are often nonspecific, which can complicate diagnosis and delay management. Recognition of disease requires that the practitioner consider CHD as a differential. Abnormal cardiac auscultation is often the first diagnostic that triggers suspicion of CHD, as most forms of valvular dysplasia are associated with a loud heart murmur. The heart sounds in foals are usually readily auscultatable, as their chest walls are thin and the heart abuts the thoracic wall. Physiological murmurs are commonly heard from turbulent blood flow through a patent ductus arteriosus in the neonate up to 4 days of age. These murmurs may be systolic, though continuous murmurs are also auscultated. Continuous murmurs will typically cease to be heard within 24 h after birth and are considered abnormal if detected beyond 4 days of age (Reef, 1985).

Tachycardia can also be suggestive of CHD, as many affected foals present with inappropriate tachyarrhythmias. The heart rate in normal Thoroughbred foals during the first 5 min after birth ranges from 30 to 90 beats/min, with a gradual change of the rate to 72–133 beats/min by 10 h of age (Rossdale, 1967). While tachycardia is a nonspecific finding, it should elicit consideration of CHD as a differential.

Cases of mitral valve dysplasia (MVD) may result in mitral regurgitation (MR) and/or stenosis; however, mitral stenosis (MS) is not commonly reported. The murmur of mitral valve regurgitation is often loud (grade IV/VI or greater), heard best over the left hemithorax towards the apex of the heart and should occur during the systolic phase of the cardiac cycle. In cases of MS, there may also be a soft, diastolic murmur heard in the same location. Patients with severe MVD are often presented with signs of left-sided congestive heart failure (CHF). These signs include tachypnoea, weakness and frank pulmonary oedema. Physical examination findings on presentation often include tachycardia with weak pulses, and auscultation may also reveal coarse bronchovesicular sounds and crackles bilaterally if the horse is in active CHF (Schober et al., 2000).

Tricuspid valve dysplasia will also result in a systolic murmur secondary to tricuspid regurgitation, though this murmur is often quieter than that of MR and is heard loudest over the right hemithorax. This murmur is not to be confused with the loud, right-sided systolic heart murmur associated with a ventricular septal defect (VSD), which is the most commonly reported congenital heart defect in foals (Hall et al., 2010). Other clinical findings such as oedema, hydrothorax, ascites, hydropericardium and enlargement of the liver may be observed if the horse is in right-sided CHF.

Aortic dysplasia may be accompanied by systolic or diastolic murmurs depending on the presence or absence of left ventricular outflow tract obstruction or aortic insufficiency (AI), respectively. Not all patients with aortic valve dysplasia will have an audible murmur (Taylor et al., 2007). This case report from Bell et al. (2024) describes a grade IV/VI, bilateral, continuous murmur. Together with the findings of the echocardiogram and post-mortem examination, it can be inferred that the diastolic component of the murmur was likely due to AI. Systolic and diastolic murmurs may also be described as a ‘to-and-fro’ murmur, rather than a true continuous murmur. The murmur of AI may have a musical decrescendo quality, and is sometimes described as having a ‘creaking’ sound. Although murmur grade has a strong positive correlation to AI severity, arterial pulse quality serves as the best clinical guide. Diastolic runoff caused by AI results in low diastolic pressure, with severely affected horses having diastolic arterial pressures below 50 mmHg. This diastolic runoff leads to a pulse pressure greater than 60 mmHg, which confers to the peripheral pulse palpable hyperkinesis.

The eventual result of severe AI is left ventricular volume overload and subsequent left-sided CHF. Horses with severe AI may present with the same signs of left-sided heart disease as a horse with MR. Clinical signs in animals with CHD may be difficult to discern from other, acquired noncardiac disease, such as pneumonia, dysmaturity, sepsis, parasitic disease, toxaemia, uraemia, drugs, allergic reactions or nutritional deficiencies such as white muscle disease (Reef, 1985).

Differentiation of congenital valvular disease from other differentials can be difficult in the ante-mortem period. Thoracic radiographs can identify generalised cardiomegaly and cardiogenic pulmonary oedema, particularly in foals. As radiography cannot reliably determine chamber size, intracardiac shunts or cardiac function, patients with suspected cardiac disease will benefit from an echocardiogram. Even with an echocardiogram, however, determining the aetiology of changes to the valve leaflets (i.e. congenital malformation vs acquired disease) may prove difficult. Echocardiography may demonstrate specific chamber enlargement depending on which valves are affected. Colour-flow Doppler may reveal a jet of turbulent regurgitant blood flow at the atrioventricular valves. There are other considerations for complete assessment of cardiac function; however, these are beyond the scope of this commentary.

Ultimately, the long-term prognosis for equine patients with congenital valvular dysplasia depends on the severity of the underlying pathology and resultant regurgitation. In mild cases, some level of athletic performance may be achievable. In severe cases that result in cardiac decompensation and CHF, however, the prognosis is generally poor to grave. While treatment of CHF may not be considered in all cases, medical management can be attempted with diuretics, positive inotropes and anti-arrhythmic medications as indicated.

Definitive diagnosis of valvular dysplasia in equine patients often occurs in the post-mortem period. Necropsy findings in affected foals may include marked enlargement of the affected chambers of the heart, and valves often display fused and/or malformed leaflets with shortened chordae tendineae (Marr & Bowen, 2010). If available, histopathological evaluation of valve leaflets is recommended for confirmation of suspected congenital valvular dysplasia.

Clinically significant congenital valvular dysplasia, while rare in foals, is an important differential for the equine practitioner and should be considered when evaluating young horses presented with vague clinical signs.

Todd Sumerfield: Writing – original draft. Carl Toborowsky: Writing – review and editing.

There are no funders to report for this commentary.

No conflicts of interest have been declared.

Not required for this commentary.