Cartilaginous Intrusion of the Atrioventricular Node in a Quarter Horse with a High Burden of Second-Degree AV Block and Collapse: A Case Report.
Authors: Nissen Sarah Dalgas, Saljic Arnela, Kjeldsen Sofie Troest, Jespersen Thomas, Hopster-Iversen Charlotte, Buhl Rikke
Journal: Animals : an open access journal from MDPI
Summary
# Editorial Summary Second-degree atrioventricular block affects a substantial proportion of horses (40–90% depending on breed) and is typically benign, resolving during exercise; however, this Quarter Horse case demonstrates that a high burden of blocks—approximately 300 per hour—combined with recurrent syncope and collapse warrants investigation beyond the standard assumption of elevated vagal tone. Diagnostic workup including 24-hour ECG recording, cardiac echocardiography, and ultimately implantable loop recorder monitoring identified prolonged pauses exceeding 5 seconds, yet conventional imaging revealed only a hyperechoic area at the atrioventricular nodal region until post-mortem examination revealed severe cartilaginous intrusion of the aortic root into the AV node, particularly affecting the His bundle. This pathological finding suggests a structural mechanism for impaired AV conduction that may have contributed to the clinical collapse episodes, though causality remains uncertain given the absence of rhythm recordings during actual syncope events. The case highlights the limitations of current diagnostic modalities in identifying structural AV nodal disease in horses and underscores the need for greater understanding of AV nodal pathology—particularly whether cartilaginous infiltration represents a distinct disease entity or incidental finding. Clinicians evaluating horses with high-burden second-degree AV block and clinical signs should consider that conventional examinations may miss significant structural abnormalities, potentially warranting more aggressive investigation or specialist referral in symptomatic cases.
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Practical Takeaways
- •While second-degree AV blocks are common and usually benign in horses at rest, a high frequency of blocks (~300/hour) with clinical signs of syncope or collapse warrants advanced diagnostic investigation including echocardiography and rhythm monitoring
- •Structural abnormalities such as cartilaginous aortic root intrusion into the AV nodal region may underlie refractory AV conduction problems and should be considered in the differential diagnosis of horses with frequent blocks and collapse
- •Implantable loop recorders can provide valuable long-term arrhythmia documentation in horses with recurrent collapse, though obtaining recordings during clinical events remains challenging
Key Findings
- •An 8-year-old Quarter horse presented with a high burden of second-degree AV blocks (~300 blocks per hour) associated with multiple collapses
- •Cardiac examination revealed a hyperechoic area in the AV nodal region and severe cartilaginous aortic root that appeared to penetrate the AV node
- •Implantable loop recorder detected consecutive second-degree AV blocks with pauses exceeding 5 seconds
- •Cartilaginous intrusion of the aortic root into the His bundle region may explain the hampered AV conduction, though the direct causation of collapses remains uncertain