Exertional syncope caused by acquired obstruction of the pulmonary artery due to an organized thrombus in a dog

An 8.5-year-old Boxer was referred for evaluation of frequently occurring exercise-induced seizure-like episodes for 2.5 months. Cardiac auscultation revealed a left basilar systolic murmur with an intensity of IV/VI. Echocardiography showed severe pulmonary arterial obstruction with a pressure gradient of 90 mmHg due to a suspected intraluminal mass. Cardiac catheterization was performed, where a right ventriculogram revealed a large rounded filling defect in the pulmonary artery trunk. No balloon angioplasty, biopsy or stenting were attempted. After euthanasia, post-mortem examination revealed an organized thrombus of unknown etiology attached to the wall of the pulmonary artery trunk. The adjacent arterial wall showed a markedly thickened intimal layer. The etiology remains unclear, primarily due to the chronic nature of the condition. Antemortem blood tests and postmortem examination failed to reveal any prothrombotic conditions. Acquired causes of pulmonary arterial obstruction are extremely rare in dogs, and an organized thrombus has not been reported yet.