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Editorial

Trabeculectomy—Not Only Filtration

Linnér, Erik MD

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Filtering surgery, with the purpose of lowering intraocular pressure associated with glaucoma, has been used since the 19th century. A thin conjunctival tissue covering the bleb constituted a considerable risk. Attempts were made to improve the results by modifying the technique. Sugar 18 described a procedure, trabeculectomy, in which a sclerotomy opening was covered with a scleral flap of partial thickness. Coryllos 4 reported a new glaucoma operation, trabeculectomy, in which the trephine opening was protected with a scleral flap. The purpose of these attempts was not to abandon the idea of creating an aqueous outflow through scleral filtration, but to increase the safety of the surgical procedure.

At the 2nd symposium of The Ophthalmological Microsurgical Group, “Microsurgery in Glaucoma,” held in Bürgenstock, Switzerland, June 13–15, 1968, Cairns 2 and I 12 presented independent reports based on new concepts. The intention was to create more physiological outflow pathways within the globe, other than scleral filtration.

The purpose of my 12 report was:

“The trabecular meshwork which appeared at the bottom of this opening was then excised, i.e., trabeculectomy was performed... At this time it is impossible to evaluate whether it is a removal of the trabecular meshwork or the outer wall of Schlemm's canal which is the most important. It cannot be decided if there is any flow of aqueous through the opening in the trabecular meshwork into Schlemm's canal and through the normal collector channels.”

I reported again on trabeculectomy the following year. 13 This new surgical procedure was based on the finding that the main aqueous outflow resistance in open angle glaucoma was located in the trabecular meshwork, as reported independently by Goldmann 6–8 and Linnér. 10,11

The report by Cairns 2 described a similar technique:

“The intention is to excise a short length of the canal of Schlemm, with its trabecular adnexa, thus leaving two cut ends opening directly into aqueous humor, with no trabecular tissue remaining as a barrier at this point, and to restore the integrity of the corneo-scleral coat over the area of the excision.”

Cairns referred to a study by Grant that showed that the obstruction to aqueous outflow in glaucoma occurred mainly in the trabecular region on the inner aspect of the Schlemm canal. 9 Later that same year, Cairns published another report on trabeculectomy. 3

For obvious reasons, it was not possible to obtain clinical material for a morphological study of the reaction in the remaining tissue close to the site of the trabeculectomy operation. Only the excised piece of tissue could be examined.

In one patient, the tissue reaction following another surgical procedure, trabeculotomy, became available for morphological study after a subsequent trabeculectomy. Most meshwork was found to adhere directly to the outer wall of the Schlemm canal, the cellular activity was inactive, and no proliferative changes were observed. 5

The effect of trabeculectomy was studied experimentally in 18 eyes of 11 monkeys, which were followed and examined over several months. 1 In five of those eyes, the outflow facility was increased, very little scar tissue developed, and an open connection between the anterior chamber and the lumen of the Schlemm canal was present. In the remaining eyes, the cut ends of the canal were closed by scar tissue and there was no lasting effect on the outflow facility. A moderate fluoresceine filtration was observed in only four eyes. The scar tissue seemed to grow extremely slowly and was not vascularized. The fact that vessels were never found seemed to indicate that the scar tissue was formed by the meshwork proper rather than scleral outgrowth. A comparison between young, healthy monkeys and mostly elderly glaucoma patients must, of course, be made with great caution.

In 1921, Seidel suggested that a transconjunctival escape could be more easily observed by placing a drop of fluoresceine over the bleb area. An increased outflow after a slight pressure on the globe could then be observed. 17

A more sensitive detection of possible filtration could be obtained by staining the aqueous humor with fluoresceine by transferring the dye into the anterior chamber by means of iontophoreses. 14,15 The degree of fluoresceine accumulation over theoperated area was photographed and then classified into three stages in a semiquantitative way.

  • Stage 0: No visible accumulation
  • Stage 1: Moderately, mostly diffuse accumulation
  • Stage 2: A clear, concentrated spot of accumulation

Using this technique, 52 glaucomatous eyes treated with trabeculectomy showed these results: stage 0 appeared in 23 eyes, with no visible filtration, indicating outflow routes within the globe such as through open cut ends of the Schlemm canal; stage 2 was found in 14 eyes, indicating a filtering outflow; and the intermediate stage 1 appeared in 15 eyes, indicating both filtration and outflow routes within the globe. The results of the fluoresceine test found in 23 of 52 operated eyes classified as stage 0 supported the concept that aqueous could escape through pathways within the globe that were created by the trabeculectomy procedure 14,15.

Although more than 30 years have passed since our trabeculectomy procedure was reported 2,12, essential clinical problems remain unsolved. It is an important goal to be able to estimate the target pressure accurately, according to the need of each patient, and then to adjust the surgical procedure accordingly. In some cases, the intraocular pressure should be lowered to the low teens, but in other cases, a higher level may be safe. However, the magnitude of the long-term pressure-lowering effect of trabeculectomy cannot be accurately anticipated and planned.

In the trabeculectomy operation, both vascularized conjunctival and scleral tissue and nonvascularized tissue of trabecular meshwork were removed. In the monkey eyes, scars formed, probably from nonvascularized meshwork proper rather than scleral outgrowth, closing the cut ends of the Schlemm canal. The lumen of the Schlemm canal remained open in only five eyes 1. A similar reaction in the meshwork tissue was found in one human eye after trabeculotomy 5. The mechanism leading to this type of scar tissue closing the free openings to the Schlemm canal in some rather than all of the eyes is not clearly understood, and no satisfactory and safe means of preventing such a development is available.

Many attempts to improve aqueous outflow after trabeculectomy by the use of different compounds have been reported, but only a few examples will be discussed briefly. Different compounds, such as an implant of cross-linked hyaluronic acid, were found to lower pressure in a satisfactory way, but a possible effect on the development of scar tissue in the nonvascularized meshwork was not investigated. The use of antimetabolites such as 5-fluorouracil and mitomycin C, which could limit fibroblast proliferation, have a significant pressure-lowering effect by producing an increase in filtration and bleb formation. The effect on trabecular meshwork has not been studied. Further basic studies are needed, such as on the mechanism of scar formation both in the vascularized and in the nonvascularized tissue.

The study on trabeculectomy in monkeys included speculations about the possibility to improve the results and secure permanent outflow routes by creating several small holes in the trabecular meshwork. 1 In a recent pilot study of 14 human eyes, several points of opening in the inner wall of the Schlemm canal were created by use of an excimer laser technique ab interno, and the patients were followed for a period of 6 months with good results. 16 Long-term follow-up is needed.

References

1. Bárány EH, Linnér E, Lütjen-Drecoll E, et al. Structural and functional effects of trabeculectomy in cynomolgus monkeys. A.v. Graefes Arch Klin Exp Ophthalmol 1972; 184:1–28.
2. Cairns JE. Trabeculectomy. The 2nd Internat Symp on Microsurgery in Glaucoma at Bürgenstock, June 1968. Advanc Ophthalmol 1970; 22:143–53.
3. Cairns JE. Trabeculectomy. Am J Ophthalmol 1968; 66:673–79.
4. Coryllos C. Trabeculectomy: A new glaucoma operation. Bull Hellen Ophthalmol 1967; 35:147–55.
5. Futa R, Marouka S. Furuyoshi N, et al. Fine structural changes of the trabecular meshwork after trabeculotomy. Proc Internat Soc Eye Research 1990; 6:208.
6. Goldmann H. Die Kammerswasservenen und das Poiseuille'sche Gesetz. Ophthalmologica 1949; 118:496.
7. Goldmann H. Der Druck im Schlemm'schen Kanal bei Normalen und bei Glaucoma simplex. Experientia 1950; 6:10.
8. Goldmann H. Abflussdruck, Minutenvolumen und Widerstand der Kammerwasserströmung des Menschen. Doc Ophthalmol 1951; 5–6:278.
9. Grant WM. Further studies on facility of flow through the trabecular meshwork. Arch Ophthalmol 1958; 60:523.
10. Linnér E. Measurement of the pressure in Schlemm's canal and in the anterior chamber of the human eye. Experientia 1949; 5:451–53.
11. Linnér E. The outflow pressure in normal and glaucomatous eyes. Acta Ophthalmol 1955; 23:101–16.
12. Linnér E. Some experiences of microsurgical trabeculotomy and trabeculectomy ab externo in chronic simple glaucoma. The 2nd Internat Symp on Microsurger in Glaucoma at Bürgenstock, June 1968. Advanc Ophthalmol 1970; 22:132–35.
13. Linnér E. Microsurgical trabeculectomy ab externo in glaucoma. Trans Ophthalmol Soc U.K. 1969; 89:475–79.
14. Linnér E. Kammarwasserabflusswege nach trabekulektomie bei glaukom-patienten. Klin Mbl Augenheilk 1989; 195:291–93.
15. Linnér E. Aqueous outflow routes following trabeculectomy. New Trends Ophthalmol 1992; 7:173–76.
16. Neuhann T, Scharrer A, Haeflinger E. Excimer laser trabecular ablation ab interno to treat chronic open-angle glaucoma in a pilot study. Ocular Surgery News 2001;November 10.
17. Seidel E. A.v. Graefes Arch Klin Exp Ophthalmol 1921;104:403.
18. Sugar HS. Experimental trabeculectomy in glaucoma. Am J Ophthalmol 1961; 51:623.
© 2002 Lippincott Williams & Wilkins, Inc.