Major ReviewThe Changing Conceptual Basis of Trabeculectomy: A Review of Past and Current Surgical Techniques
Introduction
When introduced almost a half-century ago, trabeculectomy gained widespread acceptance in surgical management of glaucoma because low intraocular pressure (IOP) was achieved with fewer complications than those associated with competing procedures. The search for better results continues today as trabeculectomy itself has been modified to improve success rates and reduce complications. Interestingly, the original intention of pioneers in trabeculectomy was to lower IOP without transscleral filtration and the development of a filtering subconjunctival bleb. However, their efforts yielded the guarded filtration procedure that remains the mainstay of modern glaucoma surgery. Today, surgical innovation has returned to the concept of bleb-less glaucoma surgery, with procedures such as deep sclerectomy and viscocanalostomy.
In his seminal paper, Cairns described the goal of trabeculectomy as excising a short length of the Canal of Schlemm, with its trabecular adnexae, thus leaving two cut ends opening directly into aqueous humor, with no trabecular tissue remaining as a barrier at that point, and restoring the integrity of the corneoscleral coat over the area of the excision.34 Although naming this procedure trabeculectomy was appropriate (because trabeculum was removed to open Schlemm’s canal), the procedure might also have been accurately called canalostomy. Importantly, the objective of trabeculectomy was to lower IOP without inducing external filtration, but in Cairns’s initial cases, inadvertent blebs occurred in one-third of patients. Given the complication rates of full-thickness filtering procedures used at that time, Cairns viewed this result as undesirable. Pathology later showed fibrotic closure of the cut ends of Schlemm’s canal. Additionally, the presence of Schlemm’s canal in the trabeculectomy specimen did not correlate with outcomes.226 A filtering bleb was present in the majority of successful cases, and the amount of fluorescein-stained aqueous in the bleb correlated with success rates.226 Therefore, the procedure that started as a “trabeculectomy” actually worked as a guarded filtration procedure, but the nomenclature remained unchanged.
We will not use the word trabeculectomy in the rest of this review unless the procedure being discussed is, in fact, a trabeculectomy. When commenting on the operation typically performed today and commonly called “trabeculectomy” we will use more accurate terminology, such as “sclerokeratectomy,” or whatever actually describes the operative technique.
The aqueous humor leaves the eye through two pathways. The conventional pathway consists of aqueous humor passing through the trabecular meshwork (TM), across the inner wall of Schlemm’s canal into its lumen, and then into draining collector channels, aqueous veins, and finally the episcleral venous system, rejoining the bloodstream from whence it came.10, 89 The nonconventional or uveoscleral pathway may be traced through the uveal meshwork and anterior face of the ciliary muscle, the suprachoroidal space, and out through the sclera. The uveoscleral pathway carries less than 10% of total aqueous outflow.24 Studies on non-glaucomatous eyes have shown that total outflow facility decreases with age. Although Becker19 showed that in primary open-angle glaucoma patients the magnitude of decrease in outflow facility was similar to that reported for non-glaucomatous patients, Larsson et al142a concluded that the absolute value of outflow facility in open-angle glaucoma patients was significantly less than in age-matched controls.
The major site of resistance within the TM has not yet been well characterized; in normal eyes, however, the majority of outflow resistance occurs in the juxtacanalicular trabecular meshwork.5, 71, 116, 155, 183 In normal human eyes 75% of the resistance to the aqueous humor outflow is localized in the TM and 25% occurs beyond Schlemm’s canal.70, 92, 268 Importantly, this is not the case with well-developed glaucoma.66, 98, 99
To this point in this review we have considered the thinking that led those who developed trabeculectomy and the many modifications of trabeculectomy that followed. The goal was to develop a procedure for glaucoma that would be both most effective and least likely to cause complications. Because glaucoma was a condition that was defined by intraocular pressure above a certain level (usually 21 mm Hg), it seemed reasonable to set as a goal for surgery lowering the pressure below that.250 What has since become apparent is that the level of pressure tolerated by different eyes varies widely, some not having progressive optic nerve damage with high intraocular pressures and others rapidly losing vision with low pressures.
Additionally, there are suggestions that fluctuation, as well as the absolute level, of intraocular pressure may play a role in progressive damage to neural tissue.235, 251 It is entirely possible that there are some eyes in which such fluctuation is of importance, and others in which the absolute level is more important.
A changed understanding of glaucoma has led to a changed conceptual framework for glaucoma surgery. With a better knowledge of the precise level of intraocular pressure tolerated by a particular eye, whether or not fluctuation of intraocular pressure is important, and where the site of resistance to aqueous outflow lies for that particular eye, it should become possible to select which of a variety of glaucoma surgical procedures is most appropriate for that particular eye. We detail differences in concept and technique of transscleral or transcanal filtration techniques.
Section snippets
Aqueous Filtration Through Schlemm’s Canal (No Bleb)
The idea of performing trabeculectomy was based on the concept that outflow resistance resides mainly in the TM and inner portion of Schlemm’s canal.92 Sugar248 was the first to perform experimental trabeculectomy with a lamellar scleral flap on eye-bank eyes and then in a living human eye. When performed on a woman with pigmentary glaucoma, the procedure was not satisfactory in controlling IOP, though gonioscopy showed that a portion of the trabeculum had been excised. Sugar248 coined the term
Guarded Filtering Surgery
After it became apparent that the Cairns’s trabeculectomy functioned best in the presence of a subconjunctival filtering bleb, efforts were made to embrace the formation of blebs while minimizing complications. Fronimopoulos et al were the first to introduce the concept of “guarded filtration” by a scleral flap.82 This important modification of full-thickness fistulizing operations, which were the dominant procedures before the introduction of trabeculectomy, significantly reduced the risk and
Summary and Conclusion
Sugar248, 249 and Cairns34 devised a procedure to facilitate egress of aqueous through Schlemm’s canal by excising a fragment of the TM, determined experimentally to be the site of greatest outflow resistance. We now known that, in open-angle glaucoma, pathology may also exist more distal to the trabecular meshwork, especially in chronic forms.98, 99 The major reason for successful IOP reduction in Sugar’s and Cairns’s series was actually aqueous draining to the subconjunctival space. Modern
Method of Literature Search
In order to prepare this review we conducted a Medline and PubMed search of the literature for the period between 1960 and 2010 using the following key words as well as various combinations of them: trabeculectomy, guarded filtering surgery, mitomycin-C, 5-fluorouracil, amniotic membrane, anti-VEGF, beta radiation, non-penetrating glaucoma surgery, viscocanalostomy, deep sclerectomy, Trabectome, iStent, ab interno trabeculectomy, ab externo trabeculectomy, and canaloplasty. Reference lists from
VI. Disclosure
The authors reported no proprietary or commercial interest in any product mentioned or concept discussed in this article.
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