Trabeculectomy

Trabeculectomy is a surgical procedure used in the treatment of glaucoma to relieve intraocular pressure by removing part of the eye's trabecula and adjacent structures. It is the most common glaucoma surgery performed and allows drainage of aqueous humor from within the eye to underneath the conjunctiva where it is absorbed. This outpatient procedure is most commonly performed under monitored anesthesia care in a half awake state using a retrobulbar block or a combination of topical and subtenon (Tenon's capsule) anesthesia.

Procedure
An initial pocket is created under the conjunctiva and Tenon's capsule and the wound bed is treated for several seconds to minutes with mitomycin C (MMC, 0.5-0.2 mg/ml) or 5-fluorouracil (5-FU, 50 mg/ml) soaked sponges. These chemotherapeutics help to prevent failure of the filter bleb from scarring by inhibiting fibroblast proliferation. Some surgeons prefer "fornix-based" conjunctival incisions while others use "limbus-based" construction at the corneoscleral junction which may allow easier access in eyes with deep sulci. A partial thickness flap with its base at the corneoscleral junction is then made in the sclera after careful cauterization of the flap area, and a window opening is created under the flap which a Kelly-punch to remove a portion of the sclera, Schlemm's canal and the trabecular meshwork to enter the anterior chamber. Because of the fluid egress the iris will partially prolapse through the sclerostomy and is usually therefore grasped to perform an excision called iridectomy. This iridectomy will prevent future blockage of the sclerostomy. The scleral flap is then sutured loosely back in place with several sutures. The conjunctiva is closed in a watertight fashion at the end of the procedure.

Mechanism
Intraocular pressure may be lowered by allowing drainage of aqueous humor from within the eye to the following routes: (1) filtration through the sclerostomy around the margins of the scleral flap into the filtering bleb that forms underneath the conjunctiva, (2) filtration through outlet channels in the scleral flap to underneath the conjunctiva, (3) filtration through connective tissue of the scleral flap to underneath the conjunctiva. into cut ends of Schlemm's canal, (4) aqueous flow into cut ends of Schlemm's canal into collector channels and episcleral veins and (5) into a cyclodialysis cleft between the ciliary body and the sclera if tissue is dissected posterior to the scleral spur.

Postoperative Care
Glaucoma medications are usually discontinued to improve aqueous humor flow to the bleb. Topical medications consist typically of antibiotic drops 4 times per day and anti-inflammatory therapy e.g. with prednisolone drops every 2 hours. A shield is applied to cover the eye until anesthesia has worn off (that also anesthetizes the optic nerve) and vision resumes.

Patients are instructed to call immediately for pain that cannot be controlled with over the counter pain medication or if vision decreases, to not rub the eye and to wear the shield at night for several days after surgery.

If 5-FU was used during surgery or if no anti-fibrotic agent was applied, 5 mg 5-FU daily can be injected in the 7 - 14 postoperative days. In the following days to weeks sutures that hold the scleral flap down can be cut by laser suture lysis to titrate the intraocular pressure down by improving outflow. In laser suture lysis a red light laser and a contact lens are used to penetrate noninvasively the overlying conjunctiva and cut the black nylon suture. Some surgeons prefer adjustable flap sutures during the trabeculectomy that can be loosened later on with forceps in a slit lamp office procedure.

Postoperative Challenges
flat bleb - will fail if not formed in first postoperative days; if early scarring is cause, subconjunctival 5-FU injections can prevent conjunctiva from adhering down against wound bed

bleb leak - may cause flat bleb; bandage contact lens for several days followed by leak repair if necessary

flat anterior chamber - reform to prevent corneal decompensation; can often be done in the office at the slit lamp with viscoelastics used in cataract surgery

blebitis - if infectious can progress to devastating endophthalmitis

suprachoroidal hemorrhage - rupture of long posterior ciliary artery from progressive stretching with progressive serous choroidal detachment; usually occurs several days after trabeculectomy with acute pain often while straining

hypotony - wound revision

cataract formation - cataract surgery if visually significant

small encapsulated bleb - injection of subconjunctival MMC and lidocaine can balloon up adjacent conjunctiva followed by needle incision of the bleb side and bleb extension

Conclusion
Trabeculectomy is the most common invasive glaucoma surgery. It is highly effective in the treatment of advanced glaucoma as demonstrated in major glaucoma studies. Even if a prior trabeculectomy has failed a second trabeculectomy can be performed at a different site. If scarring is the main reason, anti-fibrotic and anti-inflammatory therapy has to be intensified in the second procedure. Alternatively, insertion of a glaucoma valve device can be used.

Modifications of Trabeculectomy
Trabeculectomy has undergone numerous modifications, e.g. filtering trepanotrabeculectomy (TTE) is a modification of the operation after J. Fronimopoulos. A triangular scleral flap is created which is approximately one-half as thick as the sclera. Trepanation is performed with a 2 mm trephine. The scleral edge of the trepanation opening is heat-cauterized.