Glaucoma is due to a build up of pressure inside the eye that eventually damages the optic nerve. The pressure builds up because normal drainage channels are blocked.
Various types of glaucoma surgery use the same principle. The eye surgeon creates a new drainage route for the fluids inside the eye. By allowing the fluid to get out of the eye, the pressure is relieved.
The procedures usually require anesthetizing the eye. Then a new opening is created into the space between the iris and the cornea. This allows fluid to drain out.
In some cases, a tube is placed to keep the hole open. This tube may be connected to an artificial chamber that controls the pressure. Usually, however, the tube is reserved for a more difficult situation where previous drainage techniques have failed.