Complete atrioventricular septal defect in Thoroughbred foals: echocardiographic and postmortem findings.

Overview

• This study describes the clinical signs, echocardiographic features, and postmortem findings in three Thoroughbred foals diagnosed with complete atrioventricular septal defect (AVSD).
• The research highlights the rarity of complete AVSD in horses, characterizes its presentation and diagnosis, and discusses the poor prognosis associated with this congenital heart defect.

Background and Significance

• Complete atrioventricular septal defect (AVSD) is a congenital heart anomaly characterized by a deficiency in the atrial and ventricular septa and a common atrioventricular (AV) valve rather than separate mitral and tricuspid valves.
• It is rarely reported in horses, making detailed data on clinical features and diagnostic imaging scarce.
• Identifying this defect early in foals is important due to the severe cardiovascular compromise it causes.

Purpose and Study Design

• The study retrospectively examined three Thoroughbred foals presented from 2017 to 2023.
• All foals had complete AVSD confirmed by necropsy (postmortem examination).
• The researchers reviewed clinical signs, echocardiographic (heart ultrasound) findings, and anatomical features observed during necropsy.

Clinical Presentation

• Foals exhibited early signs consistent with congenital cardiac defects:
• Weakness and failure to thrive (poor growth and development)
• Presence of cardiac murmurs detectable by auscultation
• Age at presentation varied among the foals, but all showed significant cardiovascular issues.

Echocardiographic Findings

• Transthoracic echocardiography was used as a non-invasive diagnostic tool before death (antemortem diagnosis).
• Key echocardiographic features included:
• A common atrioventricular junction—there was one AV valve rather than separate valves for atria and ventricles.
• Combined atrial and ventricular septal defects—there were defects in both the walls that separate the heart’s upper chambers (atria) and lower chambers (ventricles).
• All foals’ AVSDs were classified as Rastelli type C, which means the common AV valve bridges both ventricles with features associated with this classification.

Postmortem Examination

• Necropsy confirmed the echocardiographic diagnosis by showing:
• A single common atrioventricular valve structure instead of separate valves.
• Distinct atrial and ventricular septal components of the defect, confirming complete AVSD.
• The anatomical findings verified the accuracy of echocardiographic diagnosis.

Outcomes and Prognosis

• Despite the different ages at which foals were diagnosed and euthanized, all demonstrated significant cardiovascular compromise.
• The prognosis in foals with complete AVSD was poor, emphasizing the severity of the condition.
• Early debilitation signs and heart murmurs in foals should prompt clinicians to consider AVSD as a differential diagnosis.

Clinical Implications

• Transthoracic echocardiography is a reliable and practical tool for diagnosing complete AVSD in foals prior to death.
• Awareness of this rare defect can improve early diagnosis and management decisions, although treatment options are limited and prognosis remains guarded.
• Veterinarians should consider AVSD when encountering foals with failure to thrive and cardiac murmurs early in life.