Small but lethal: cystic tumour of the atrioventricular node-a case report.

Background: Cystic tumours of the AV node (CTAVN) are exceptionally uncommon congenital cardiac tuours, with an incidence of 0.0017-0.03%. Case summary: A 47-year-old woman with recurrent symptomatic tachycardia underwent an ablation. Coronary sinus was noted to be enlarged. Subsequent cardiac CT revelead a well-circumscribed cystic lesioni in the Triangle of Koch, suspicious of a CTAVN. Multimodal imaging, including echocardiography, coronary angiography and cardiac MRI confirmed a non-vascular, encapsulated lesion with imaging characteristics consistent with a benign cystic mass. Given the risk of sudden cardiac death, surgical resection was performed. Histopathology confirmed CTAVN. The postoperative course as complicated by complete heart block requiring pacemaker implantation, with no recurrence at one year. Discussion: CTAVN is a rare but clinically significant benign tumour associated with conduction abnormalities and malignant arrhythmias. This care highlights the importance of multimodal imaging in diagnosis and the role of early surgical intervention to mitigate life-threatening complications.