Congenital Subclavian Vein Aneurysm in an Infant.

Congenital thoracic lymphovenous aneurysms are exceedingly rare. They often present with compressive symptoms or thrombosis, necessitating surgical excision for symptom relief and cosmetic reasons. We report a symptomatic subclavian vein aneurysm in a 4-month-old infant who presented with a rapidly enlarging axillary mass. Multimodal imaging with ultrasound, computed tomography, and magnetic resonance imaging confirmed the diagnosis of subclavian vein aneurysm. The patient underwent surgical resection, and histology confirmed a lymphovenous malformation. Postoperative recovery remained uneventful aside from a chylothorax. On early follow-up at 15 months, the child remained well with no recurrence or residual symptoms.