The devices that are used to reduce the eye pressure in glaucoma by draining the aqueous humour (natural fluid of the eye) from inside the eye to a small blister or bleb behind the eyelid are called Aqueous shunts.
Draining the aqueous humour, using a shunt, reduces the pressure on the optic nerve that causes loss of vision in glaucoma. The purpose of lowering the eye pressure is to prevent further loss of vision. Control of the eye pressure with an aqueous shunt will not restore vision already lost from glaucoma.
Aqueous shunts have various other names such as tube implants, glaucoma tube shunts, glaucoma drainage devices and glaucoma drainage implants. These all refer to the same thing. Although there are many types of shunts available, two main types are in use at Moorfields Eye Hospital Abu Dhabi and they function in a similar fashion.
These are called the Ahmed Glaucoma Valve and The Baerveldt Glaucoma Implant.
The Ahmed Glaucoma Valve contains a type of valve that helps to prevent very low eye pressure during the first few weeks after surgery.
The Baerveldt implants do not contain valves but do have other advantages. they must be blocked with a stitch that is either tied around the outside of the silicone tube (external ligature), or threaded through the inside of the tube (occluding suture) at the time of surgery. The purpose of the stitches is to prevent the shunt from draining excessively in the first few weeks after surgery and causing the eye pressure to be too low.