Glaucoma surgery is aimed at decreasing the intraocular (eye) pressure by creating an alternate pathway for drainage of fluid inside the eye which is called as aqueous humour.
Glaucoma surgery can be combined with cataract surgery or can be done in isolation. 
Most commonly done glaucoma procedures include:
1.Conventional surgery called Trabeculectomy
2.Valve Surgery

Trabeculectomy
In this procedure a window is made in the sclera through which aqueous humour flows out of the eye bypassing the clogged drainage canal.The procedure is usually augmented with the use of antimetabolite called Mitomycin-C or an implant called as Ologen implant.Use of this antimetabolite and implant increases the chances of success of surgery by reducing the chances of future scarring around the scleral window. 

When is Trabeculectomy needed:
1.Uncontrolled intraocular pressure on medications and eye drops
2.As primary procedure in cases of congenital glaucoma where drops doesn’t work
3.Poor compliance of patient where patient doesn’t follow the medication properly
4.Poor follow up of patient and it becomes difficult for the glaucoma surgeon to assess the effectiveness of the eye drops
5.If patient finds drops are costly and unable to buy them regularly as glaucoma treatment is for life in most cases

Information given to the patient regarding glaucoma surgery:
1.Patient requires frequent follow up visits after trabeculectomy surgery
2.Vision after Trabeculectomy may fluctuate for around a month
3.Swimming is a contraindication after the surgery for at least a month
4.Cataract progresses faster after the glaucoma surgery
5.Slight drooping of the eyelid can happen after the glaucoma surgery
6.There is a risk of complete loss of vision in cases of severe advanced glaucoma where central vision is badly involved before the glaucoma surgery only.The condition is called as wipe out.

Ahmed Glaucoma Valve Implant

When is valve surgery for glaucoma advised: 
1.Failed Trabeculectomy
2.As a primary procedure in cases of secondary glaucoma most commonly neovascular glaucoma, uveitic glaucomaas in these cases risk of failure of trabeculectomy is high
3.Cases of ocular surface disorders like Steven Johnson Syndrome, ocular pemphigoid where the chances of failure of conventional trabeculectomy surgery is high 
4.In cases of congenital glaucoma where sclera is very thin

Valve surgery for glaucoma also called as tube shunt surgery. In this procedure a flexible glaucoma drainage device is implanted under the conjunctiva to divert the aqueous humour from inside to outside the eye. It has 2 parts; the tube part goes inside the eye (in the front part of eye called as anterior chamber) and the external portion is like a plate under the conjunctiva.

Risks and possible complications after the shunt surgery
Any tube shunt procedure (glaucoma drainage devices) which to begin with is done in high risk cases can fail in the long run due to natural healing mechanisms of the eye. The reaction of the eye to the plate can cause scarring around it and then can prevent the effective drainage of fluid. In those cases, eye medication may need to be hiked up to control the eye pressure. On the flip side sometimes after the valve surgery for glaucoma, eye pressure may become too low. If it persists then in some cases valve may need to be removed. Cataract progression can happen after this surgery as well. And just like any other surgery there is a risk of other complications like infections, reaction inside the eye, bleeding inside the eye etc. These can be managed with timely treatments.

Both the types of glaucoma surgery are done only in those cases where risk of future damage due to glaucoma is high. The damage to the field of vision that is caused by glaucoma is permanent and cannot be reversed. Consequently, glaucoma surgeries cannot bring about any improvement in vision. Therefore, the reason for advising the procedure is preventive because without it, vision is very likely to worsen or, in rare cases, be totally lost. In most cases, glaucoma eye doctor will not recommend a valve surgery unless they believe nothing else will work. Therefore, for most patients, the benefits of the surgery tend to outweigh the risks. However, there is no generalization of recommendations and it must be individualised based on the eye parameters, stage of glaucoma and control with traditional eye medications.