CVT is cerebral venous thrombosis. It is due to clotting of blood and blockage of the veins of Brain. That causes raised pressure in the brain and sometimes brain damage. The blocked veins take a very long time to re-open. Sometimes they don't re-open. There is formation of alternative channels of blood flow - called as collaterals. If the collaterals are good, then even if the previously blocked vessels are not opened, it is still ok.