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Findings: Injection into the left distal subclavian vein. There is complete occlusion of left innominate vein. Collateral flow is (a) through the jugular venous arch and inferior thyroid veins to the right innominate vein, (b) through the highest intercostal vein which drains into the accessory hemiazygous vein which anastomoses with the hemiazygous and azygous veins, and (c) through the internal thoracic vein with anastomoses with the accessory hemiazygous vein.

Diagnosis: Thrombosis of left innominate vein with collateral circulation.

Discussion: The SVC drains two major venous systems via the innominate vein, the jugular venous system and the subclavian veins. In unilateral obstruction of the left innominate vein, drainage is through extensive interconnecting pathways via direct crossing anastomosis and indirect anastomosis with the azygous system. The collateral pathway to the SVC in left innominate venous occlusion include the following:

A. Anastomoses between innominate tributaries and azygous tributaries:

1. Highest intercostal vein: drains into the accessory hemiazygous vein which drains into the the azygous vein via (a) the hemiazygous venous anastomosis or (b) the posterior intercostal vein through the external vertebral venous plexus, internal venous plexus, vertebral venous ring and contralateral posterior intercostal vein.
   
2. Internal thoracic vein: drains into the accessory hemiazygous vein via the anterior intercostal vein anastomosis with the posterior intercostal vein (to azygous as above).
   
3. Lateral thoracic vein: (tributary of axillary vein) anastomoses with the internal thoracic vein via posterior intercostals (to azygous as above).

1. Internal thoracic vein: drains into the contralateral internal thoracic vein via crossing veins.
   
2. Internal jugular vein: drains into the contralateral internal jugular via the middle and superior thyroid veins.
   
3. Inferior thyroid vein: drains into the contralateral innominate.
   
4. Anterior jugular veins: communicate through the jugular venous arch which anastomose with the inferior thyroid tributaries of the internal jugular veins and external jugular veins.
   
5. Cephalic vein: (axillary vein tributary) anastomoses with the internal jugular vein.

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