Equine Congenital Heart Disease (CHD)

Comprehensive Information on Equine Congenital Heart Disease (CHD) for NAVLE Preparation

Definition

• Congenital Heart Disease (CHD): A group of malformations present at birth that result from errors during cardiac development, leading to structural and functional abnormalities in the heart of horses.

Etiology

• Genetic and Environmental Factors:

  • Genetic Predisposition: Familial risks have been observed, particularly in Arabians, Welsh Mountain Ponies, and Standardbred horses.

  • Environmental Factors: Potential teratogens (drugs, toxins, infections) during pregnancy may contribute to CHD development.

  • Unknown Causes: In most cases, the specific cause remains unidentified.

Pathophysiology

• Cardiac Development Errors:

  • Septation Defects: Errors in the septation of the heart chambers and great vessels can lead to various forms of CHD, such as ventricular septal defects (VSD) and atrial septal defects (ASD).

  • Abnormal Cardiac Looping: Affects the right-left orientation of the heart, leading to heterotaxy syndromes (e.g., situs inversus).

  • Persistent Fetal Circulation: Failure of normal closure of fetal shunts (e.g., patent ductus arteriosus [PDA]) can result in postnatal complications.

Clinical Signs

• Foals:

  • Failure to Thrive: Poor growth and development.

  • Respiratory Distress: Dyspnea and cyanosis.

  • Heart Murmurs: Often the first indication of CHD, typically detected on auscultation.

  • Nonspecific Signs: May include syncope, exercise intolerance, or sudden death.

• Adult Horses:

  • Heart Failure Signs: Due to chronic volume overload, including jugular venous distension, subcutaneous edema, and respiratory distress.

  • Arrhythmias: Potential complication in horses with CHD.

  • Syncope: Due to inadequate cardiac output.

Specific Forms of Congenital Heart Disease

1. Ventricular Septal Defect (VSD):

   • Most Common Form of CHD in horses.

   • Types: Perimembranous, muscular, and doubly committed (juxtaarterial) VSDs.

   • Clinical Signs: Left-to-right shunt, heart murmur, left atrial and ventricular enlargement, pulmonary overcirculation.

   • Prognosis: Dependent on defect size and concurrent anomalies. Defects larger than 2.5 cm or involving significant hemodynamic compromise have a worse prognosis.

2. Atrial Septal Defect (ASD):

   • Less Common: Leads to a left-to-right shunt, volume overload of the right atrium and ventricle, and pulmonary overcirculation.

   • Types: Primum, secundum, sinus venosus, and coronary sinus ASDs.

   • Clinical Signs: Typically associated with a heart murmur and may present with signs of right heart failure in severe cases.

3. Patent Ductus Arteriosus (PDA):

   • Failure of Ductus Arteriosus Closure: Leads to continuous left-to-right shunting, pulmonary overcirculation, and heart failure if untreated.

   • Clinical Signs: Continuous "machinery" murmur, detected at the left heart base, and signs of left-sided heart failure.

   • Treatment: Surgical ligation or interventional occlusion is recommended.

4. Tetralogy of Fallot (TOF):

   • Four Components: Right ventricular outflow tract obstruction, VSD, right ventricular hypertrophy, and an overriding aorta.

   • Clinical Signs: Cyanosis, exercise intolerance, and heart failure due to right-to-left shunt.

   • Prognosis: Poor without intervention; affected foals often have a limited lifespan.

5. Atrioventricular Septal Defect (AVSD):

   • Incomplete Septation: Results in a primum ASD and VSD, with a single atrioventricular valve.

   • Clinical Signs: Heart failure due to significant left-to-right shunting and valve regurgitation.

   • Prognosis: Poor, particularly in cases with severe valve involvement.

6. Double Outlet Right Ventricle (DORV):

   • Both Great Vessels Arise from the Right Ventricle: Often associated with VSD, resulting in varying degrees of cyanosis and heart failure.

   • Clinical Signs: Dependent on the degree of pulmonary blood flow and shunt direction.

7. Valvular Dysplasia:

   • Malformed Valves: Can lead to stenosis or regurgitation, particularly affecting the atrioventricular or semilunar valves.

   • Clinical Signs: Heart murmurs, arrhythmias, and heart failure due to volume overload.

   • Prognosis: Variable, dependent on the severity of the malformation.

8. Vascular Ring Anomalies:

   • Abnormal Vascular Structures: Compress the esophagus or trachea, leading to dysphagia or respiratory distress.

   • Common Type: Persistent right aortic arch (PRAA).

   • Treatment: Surgical correction is necessary for resolution.

Diagnostics

• Auscultation: Detection of heart murmurs, often the first clue to CHD.

• Echocardiography: Gold standard for diagnosing specific CHD, assessing shunt direction, chamber sizes, and associated lesions.

• Radiography: May reveal cardiac enlargement and pulmonary changes due to volume overload.

• Electrocardiography (ECG): Useful for identifying arrhythmias.

Treatment

• Medical Management: Primarily supportive, aimed at managing heart failure and arrhythmias.

• Surgical Intervention: Indicated for certain defects, such as PDA, VSD, or vascular ring anomalies, though rarely pursued in equine practice due to the complexity and risk.

Prognosis

• Dependent on Defect: Simple defects with minimal hemodynamic impact may allow for a normal lifespan, while complex or large defects often carry a poor prognosis.

Complications

• Heart Failure: Due to chronic volume or pressure overload.

• Arrhythmias: Resulting from structural abnormalities or myocardial remodeling.

• Sudden Death: Possible in severe cases of CHD, particularly those with right-to-left shunting.

References:

Bonagura, J. D., Reef, V. B., & Schwarzwald, C. C. (2019). Congenital heart disease in foals and adult horses. Veterinary Clinics of North America: Equine Practice, 35(1), 27–56.