It's uncertain. Possibly the dural sinuses, ascending lumbars, renal vein, iliac vein, vertebral veins, and/or the innominate vein. I think Dr. McGuckin checks out the superior vena cava.

Of all the ones listed, the renal vein seems to be the most important, because a compression of the renal vein causes the high 500 ml/min flow to go into the ascending lumbar and hemiazygous vein, which slows down the cerebrospinal drainage.

Next in importance would be the innominate vein, because that's what the jugulars flow into, and while we rarely hear about a blockage there, there have been a few.

Next most important from the list would be the dural sinuses, in order to check that the vein that appears to be the jugular truly is the jugular (because only the jugular goes all the way to the jugular bulb to the sinuses).

Next would be the iliac vein, because a May thurner compression can be treated, but Dr. Sclafani recently said something about the iliac vein not playing much of a role in CCSVI.

Vertebral veins and ascending lumbars are least on my list, because it would be rare to treat a vertebral vein, and the ascending lumbars are sometimes ill-formed or hyperplastic too-small and not treatable. If it can't be treated, ok, it's good to know about it, but that's all.

Hope that helps! Are you thinking of getting more veins looked at?