There was data offered at ISNVD that in unilateral (one sided) optic neuritis the patient was invariably found to have unilateral jugular stenosis-on the same side. (mostly left)
Also if there is a problem up north, like in the transverse sinus which could be congenital or acquired, it might diminish the flow to that jugular enough that the vein never develops fully, or if acquired, declines and becomes sticky and closes up. Veins need flow to stay open.
Dental infections can drain into the jugular vein and make a mess of it too.
CCSVI is both congenital and acquired.
I have been MSsooter but need to change my name after libration. Don't use a scooter anymore! I am LaFemmeMSketeer!