Malignant pheochromocytoma presenting as a bradyarrhythmia in a dog

Objective: To describe a case of atrioventricular block and syncope secondary to systemic hypertension in a dog with malignant pheochromocytoma. Case Summary: A 13‐year‐old spayed female mixed‐breed dog presented following an acute onset of collapse. The dog was hypertensive and had paradoxical high‐grade second‐degree atrioventricular block with concurrent syncope. Initial emergent therapy included a nitroprusside infusion for the systemic hypertension and vagolytic therapy for the bradyarrhythmia. A left adrenal mass was seen on abdominal ultrasound and was further characterized by MRI. Following medical stabilization with phenoxybenzamine and propantheline, a left adrenalectomy was performed. Histological diagnosis was consistent with a malignant pheochromocytoma. Following tumor removal there has been no further hypertension, atrioventricular block or syncope. Unique information provided: This is the first documented case of a malignant pheochromocytoma presenting as syncope secondary to hypertension induced atrioventricular block.