Internal thoracic veins: Anatomy, plasticity and clinico-imaging relevance in small animal practice.

The internal thoracic veins (ITVs) are small paired vessels located on the ventral surface of the thoracic cavity that drain the ventro-cranial abdominal wall, the ventro-lateral thoracic wall, the diaphragm and part of the mediastinum, conveying blood from these regions into the cranial vena cava. These vessels demonstrate a high level of anatomic plasticity and haemodynamic adaptability in both humans and small animals with blood flow impairment of the main abdominal and thoracic venous trunks. The ITVs may act as a natural bypass between the cranial and caudal venous system and between the portal vein and the cranial vena cava, depending on the level of the obstruction, giving rise to a wide spectrum of collateral pathways: intrathoracic cavo-caval, thoraco-abdominal cavo-caval, abdomino-thoracic cavo-caval, porto-cranial caval and lateral thoracic-azygos ITV collaterals. This paper provides a brief overview of the normal and pathologic anatomy of the ITVs described in dogs with cranial and caudal vena cava obstruction and portal hypertension as shown by CT angiography. Collateral ITV pathways need to be distinguished from other vascular anomalies in dogs, and their identification during routine CT studies could help radiologists to reach a more accurate diagnosis of caval or portal flow disturbance.