This is where an artificial drainage pathway is created to allow fluid to drain from the inside of the eye to the outside of the eye and thus lower the eye pressure.
The traditional and most common procedure of this type is a trabeculectomy which involves creating an opening in the wall of the eye and partially sealing it off with a 'trapdoor' created from the wall of the eye to provide resistance. The skin of the eye (conjunctiva) is then stitched over the trapdoor allowing the fluid to collect as a bubble ('bleb').
Whilst it is generally considered the gold standard for glaucoma surgery and remains, in many cases, the most reliable way of achieving a sustained and significant drop in eye pressure it requires intensive aftercare with drops for many months and a number of visits as stitches often need adjusting or removing to fine tune the pressure.
Glaucoma Drainage Devices work in a similar way but involve placing an artificial tube inside the eye that drains fluid. These have advantages in certain types of glaucoma where a trabeculectomy would most likely fail due to scarring or in cases where a trabeculectomy has previously failed.