Trabeculectomy

Introduction

Trabeculectomy is the most common form of modern filtration surgery used to lower intraocular pressure (IOP) in a patient with glaucoma. In trabeculectomy, a fistula for aqueous fluid is created between the anterior chamber and the subconjunctival space, beneath a partial-thickness scleral flap, to create an area of “filtration” called the bleb . Trabeculectomy techniques have evolved from full-thickness sclerectomy forms of filtration surgery to minimize postoperative complications while achieving the goal of successful lowering of IOP. With all forms of filtration surgery, the most sight-threatening complications, such as suprachoroidal hemorrhage, occur in the setting of hypotony. Surgeons embraced trabeculectomy as a way to avoid hypotony, yet historically, the move to trabeculectomy from full-thickness filtration surgery was gradual, as techniques were developed to achieve ultimate success and adequate IOP lowering for the long term.

Trabeculectomy was originally described by Cairns in 1968 and has continued to be modified as surgical instrumentation and suture have improved in recent decades. Modalities to alter wound-healing fibrosis in the subconjunctival space, such as use of antimetabolites (5-fluorouracil and mitomycin C) and laser suture lysis, have continued to offer improvements in postoperative success. Trabeculectomy is still considered the gold standard surgery for primary glaucoma filtration because it is effective, relatively low risk, and technically within the skills of the well-trained ophthalmic surgeon. Variations of technique among surgeons and variability in wound healing among patients are the norm, and it is said that astute and careful postoperative management by the surgeon is key to the successful outcome of trabeculectomy for a patient.

It is helpful to understand the anatomy of a trabeculectomy to recognize the optimal conditions for successful surgery. In many ways, the trabeculectomy is a complicated series of “incisions,” most of which must remain free from healing or scarring to establish a functional filtration bleb. In fact, the only incision that requires complete healing is the conjunctival incision, either at the limbus in a fornix-based flap or at the fornix in a limbal-based flap. Otherwise the iridectomy, sclerectomy, underside of the partial-thickness scleral flap, the sides of the scleral flap, and the extremely important surface area between the episclera and conjunctiva must remain free of obstruction to allow aqueous fluid to migrate through the fistula ( Fig. 10.27.1 ).

Although the goal in trabeculectomy is to establish a subconjunctival filtration bleb that lowers the IOP sufficiently, the bleb itself is the cause of many of the late-onset complications that may occur. Blebs can leak, become infected, and induce endophthalmitis, or cause chronic discomfort ( Fig. 10.27.2 ). Antimetabolite use increases the risk of leaking and infection because of the permanent changes in the conjunctiva that promote a thin-walled bleb ( Fig. 10.27.3 ). Yet antimetabolites offer a more successful long-term decline of IOP in patients who are at high risk for filtration failure and in primary trabeculectomies. It is the risks of the established bleb that have led to the development of non–bleb-forming filtration surgeries. The main disadvantage of non–bleb-forming procedures, where aqueous is shunted to alternative outflow pathways, such as the episcleral venous system or suprachoroidal space, is lowering of IOP to a less significant level that may not be adequate in some patients and may still allow glaucoma progression to occur ( Table 10.27.1 ). Newer devices that allow access to the subconjunctival space with minimal or no (via ab interno insertion through the angle) incisions in the conjunctiva have also been instituted in an attempt to improve the morphology of the bleb and minimize complications. In recent years, after publication of the findings of the randomized Tube Versus Trabeculectomy Study for eyes with previous surgery, as well as the Primary Tube Versus Trabeculectomy Study, surgeons are recognizing that glaucoma drainage implant surgery may rival or improve on the long-term results of trabeculectomy, although controversy remains about the complication profile for each procedure and the ideal surgical procedure for advanced glaucoma.

Indications

The goal of glaucoma management is to preserve visual function while maintaining or improving the patient's quality of life. When surgery is necessary, trabeculectomy is the procedure of choice for most cases of severe primary glaucomas. Success in glaucoma surgical studies is usually reported as a final IOP <21 mm Hg and/or 30% decrease in IOP, with or without additional medications, even though for many eyes that undergo trabeculectomy the goal is for postoperative IOP control in the low teens. Success with the typical definition in forms of open-angle glaucoma and primary angle-closure glaucomas has been reported in approximately 80% of eyes over 1–2 years, with lower IOP achieved with antimetabolite use. Relatively few studies have evaluated the long-term preservation of visual field and optic nerve function as an outcome measure, yet it seems that glaucoma surgery is successful in this regard for the majority of eyes that have resulted in successful control of IOP.

The decision to offer trabeculectomy is based on the patient's current stage of their glaucoma and the risks for ongoing progression of the disease without further intervention. The rate of progression differs among patients, as does the goal for IOP control. Each individual must be carefully evaluated as to the best treatment modality. Some patients are likely to need trabeculectomy soon after diagnosis of their glaucoma, yet there are many patients who will never need trabeculectomy even after many years of the disease ( Box 10.27.1 ).

Surgical Planning

Surgical planning begins in the office with a careful history, ocular examination, and review of the patient's risk factors for failure ( Table 10.27.2 ). On the basis of this assessment the surgeon will plan the approach to surgery, including fornix-based or limbal-based conjunctival flap dissection, placement of the scleral flap and sclerostomy, need for and type of antimetabolite, tightness of the scleral flap sutures to titrate the amount of aqueous fluid egress in the early postoperative period, need for releasable scleral flap sutures, and any preoperative treatments.

Preoperative Factors to Consider

Preoperative factors that need to be considered are given in Table 10.27.3 .