Monday, November 20, 2006

Section 8-B: Trabeculectomy: A filtering procedure

It is important to remember that glaucoma surgery is performed to lower intraocular pressure (IOP) to prevent further loss of vision, not to improve vision. Lowering the IOP to the target level helps to slow down or halt the progression of optic nerve damage and prevent further loss of vision. In 2006, it is not possible to recover lost vision or existing optic nerve damage from glaucoma; vision loss from glaucoma is, therefore, irreversible. When medications or laser treatment fail to control the intraocular pressure (IOP) in glaucoma, the next step in the management of glaucoma often involves trabeculectomy, which is referred to as filtering procedure.

Preoperative Considerations
Trabeculectomy is a commonly performed glaucoma surgical procedure when medications fail to adequately control the IOP. It can be performed for most (but not all) types of glaucoma. It is necessary to have intact, non-scarred conjunctiva. The conjunctiva is a thin tissue that coats the surface of the sclera or eye wall. Trabeculectomy can be difficult to perform in an eye that has had previous ocular surgeries with scarred tissue. In this situation, the surgeon may elect to perform a glaucoma drainage tube implant (or seton) procedure (see next section).

Surgical Technique
The basic goal behind trabeculectomy is to create a small hole in the anterior chamber of the eye to allow drainage of the aqueous fluid toward the outside. (Figure 8-6) Trabeculectomy surgery starts with making an incision through the conjunctiva. The surgeon then creates a partial-thickness sclera flap (or trapdoor) on the sclera (eye wall). Underneath the scleral flap, a surgeon cuts a small hole into the anterior chamber, which allows the drainage of aqueous fluid through the scleral flap and into the sub-conjunctival space. An iridectomy (hole in the iris) is performed at this point to allow the scleral opening to stay open without being blocked by the iris tissue. The scleral flap is then tied down with stitches, that are loose enough to allow continuous drainage of the aqueous fluid. Finally, the overlying conjunctival tissue is closed with stitches to allow formation of a bleb or an elevation of conjunctival tissue formed by the aqueous fluid, which is being filtered out of the scleral flap (trapdoor) underneath. The filtering bleb is usually located in the superior aspect of the eye and covered by the upper lid. Consequently, it is not readily noticeable by a casual observer. The aqueous fluid from the filtering bleb is then slowly absorbed by the conjunctival and episcleral (on the surface of the sclera) blood vessels and drain into the orbital venous system.

Trabeculectomy Surgery

Figure 8-6. Illustration of a trabeculectomy surgery.


Starting in the late 1980’s, chemotherapeutic drugs (or anti-metabolite or anti-healing medications commonly used in cancer chemotherapy) have been used during or after trabeculectomy. These anti-metabolite drugs are used to decrease the amount of tissue healing following trabeculectomy. A large multi-center study has shown that 5-fluorouracil (an anti-metabolite medication) is effective in increasing the success rate of trabeculectomy. In 2006, anti-metabolites such as 5-fluorouracil and mitomycin C are widely used during trabeculectomy to increase the overall success of the surgery. On the other hand, the frequent use of the anti-metabolite medications during trabeculectomy can also increase the risk of hypotony (low intraocular pressure below the physiologic level), which is one of the complications of trabeculectomy (see below).

Anesthesia
The surgery uses a local anesthesia to the eye. Traditionally, a retrobulbar (behind the eye) anesthetic injection was used for local anesthesia. This involves injecting a small amount of anesthetic behind the eye under light intravenous sedation. Retrobulbar anesthesia works well not only in trabeculectomy but in many other types of ocular surgery. However, it can be occasionally associated with serious complications such as hemorrhage (bleeding) or even perforation of the eye. In certain cooperative patients, a topical (eye drop) anesthesia can be used to perform trabeculectomy rather than the retrobulbar injection anesthesia. The advantage of the topical anesthesia is quicker visual recovery and decreased risk associated with retrobulbar injection. The surgery usually takes about an hour under local anesthesia and is done as an outpatient basis.

Postoperative Recovery and Follow-Up
Postoperatively, the recovery period is between 6-8 weeks. In trabeculectomy, the post-operative follow-up is particularly important because the success of the surgery depends on the rate and extent of conjunctival healing process. During this period, the surgeon follows the patient closely, usually on a weekly (sometime more frequent) basis initially. During follow-up visits, adjustments can be made to reduce the IOP if is too high. This can be done by cutting (or pulling) stitches from the scleral flap with a laser to allow additional filtration (laser suturelysis, Figure 8-7). Occasionally, the surgeon may elect to needle the bleb post-operatively if there is excessive conjunctival scarring process. A small gauge needle is used to break up the scar tissue to allow more filtration of the aqueous fluid, and usually performed in a minor procedure room under topical anesthesia. If the IOP is too low, the surgeon may reduce the amount of anti-inflammatory (or steroid) medications to allow additional healing process. In short, there are many adjustments that may need be done postoperatively to maximize the chance of surgical success. Thus, it is very important for the patient to have a proper postoperative follow-up under the direction of the treating surgeon.

Post-op Laser Suturelysis

Figure 8-7. Postoperative Laser Suturelysis to increase filtration of trabeculectomy


Trabeculectomy filtering bleb

Figure 8-8. Trabeculectomy filtering bleb.



Success and Complications of Surgery
The success rate of a trabeculectomy is approximately 65-70%. Additional 20% can have a qualified success, meaning the goal IOP is achieved using one or more anti-glaucoma medication(s) postoperatively. Approximately 10-15% of the trabeculectomy may fail in the first few months due to excessive conjunctival scarring. In approximately 1% of the time however, there can be more serious complications such as hemorrhage (bleeding), infection, and other complications. Depending on the complication, it may result in temporary or even permanent reduction in vision. It may also require further surgical procedure to correct the complication. It is important to discuss these potential complications with the treating physician before surgery.

The most common short-term complication of trabeculectomy surgery is that it may fail to adequately lower the IOP. This may occur due to the excessive scarring of the conjunctival tissue with decreased filtration of the aqueous fluid out of the eye. On the other hand, the IOP may be too low (called hypotony) due to the excessive filtering of the aqueous fluid or leaking wound. Low IOP (typically below 5 mmHg) can cause blurry vision and can be associated with shallowing of the anterior chamber, cataract formation, and greater risk of intraocular fluid accumulation (choroidal effusion) or intraocular bleeding (suprachoroidal hemorrhage). The suprachoroidal hemorrhage is a particularly feared complication after trabeculectomy, because it is often associated with pain, elevated IOP, and permanent decrease in vision. It often requires additional surgery to drain the blood.

There are also long-term complications of having a trabeculectomy bleb around the eye. If there is a leak from the bleb, the IOP may become too low. In addition, the bleb leak can increase the risk of infection. An infection in a post-trabeculectomy eye can be serious because of the surgical hole in the sclera may allow a direct access by the offending micro-organism to the inside of the eye. Such intraocular infection can seriously compromise the vision and even integrity of the eye itself. Therefore, any symptoms of infection in a post-trabeculectomy eye such as pain, decreased vision, redness, and purulent discharge, should be reported and examined promptly. This may occur even years after the surgery. For this reason, post-trabeculectomy patients are encouraged to always wear goggles during swimming and are discouraged from wearing contact lenses in order to decrease the possibility of a bleb infection (called blebitis or bleb-associated endophthalmitis).

Even a successful trabeculectomy surgery may not last forever; the surgery is considered successful if it controls the IOP for a period of 7-8 years. If the first trabeculectomy fails, it can be repeated second (rarely third) time to control glaucoma. Subsequent trabeculectomies usually have a higher chance of failure than the primary trabeculectomy surgery. If trabeculectomy fails to control glaucoma adequately, the surgeon may consider a glaucoma drainage tube (or seton) (see next).

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