In traditional glaucoma surgery, a small guarded hole (fistula) is made to drain out fluid from inside the eye, thereby reducing the IOP. This is the most common surgery performed for glaucoma, and is known as trabeculectomy.
In this technique, a tiny guarded hole in the wall of the eyeball at the level of the junction of the cornea and the sclera is performed to allow the egress of the aqueous humor through a new alternative outflow way. The aqueous humor is directed through the tiny hole to a reservoir at the sub-Tenon/sub-conjunctival space that is called filtering bleb. The sutures aim to control the amount of flow, and from bleb, the aqueous is then absorbed through the capillaries to the blood circulation. Surgery is usually done under the superior eyelid.
After surgery, a small white coloured blister / scar can be seen in the upper part of the eye and is called a “bleb”. In most cases, the bleb remains covered by the superior eyelid and cannot be seen unless the eyelid is retracted.