Membrane formation in the chamber angle after failure of argon laser trabeculoplasty: analysis of risk factors

Aim—Membrane formation in the chamber angle induced by argon laser trabeculoplasty (ALT) can be a cause of treatment failure. Identification of risk factors for membrane formation was the primary aim of this retrospective study. Methods—Semithin sections of trabeculectomy specimens obtained in a 2 year period were examined by light microscopy. 122 eyes which were treated with one or more ALTs before trabeculectomy were identified. In 46 eyes, a suYcient amount of trabecular meshwork was obtained to permit morphological analysis. Results—Eyes treated with ALT had a significantly higher incidence of membrane formation (p=0.001). In 23/46 specimens a cellular and collagenous membrane was observed covering the entire trabecular meshwork. In 14/23 specimens (61%), this membrane was readily visible at low power magnification (×40). Comparison of these eyes with those without membrane formation revealed a significant diVerence in the number of ALTs (mean 2.07 (SD 0.73) v 1.48 (0.59); p=0.026) and in preoperative IOP (32.0 (9.7) v 26.2 (8.4) mm Hg; p=0.04). Conclusions—Membrane formation in the chamber angle is a frequent cause of ALT failure. The major risk factor is the number of ALTs performed. (Br J Ophthalmol 2000;84:48–53) Since the introduction of argon laser trabeculoplasty (ALT) in 1979 by Wise and Witter it has become one of the standard treatments for glaucoma. Recent reports, however, reveal a failure rate of 15–25% in the first year and annual failure rates of 5–10% thereafter. Within 10 years most patients (68–95%) fail and require further intervention. Since 1973, we have gathered trabeculectomy specimens in order to determine if intraocular pressure (IOP) regulation is dependent on the anatomical site of excision, specifically comparing excisions that were fashioned in proximity to the cornea with those more peripherally based to include uveal tissue and scleral spur. More recently, we have seen repeat ALT treated patients with acutely elevated IOP in the 50s, despite intensive medical therapy, who required emergency trabeculectomy. We investigated a possible cause of such a pressure rise by more closely examining trabeculectomy specimens using light and electron microscopy. In many specimens we found a significant membrane covering the trabecular meshwork. The aim of this retrospective study was to find out if there is any relation between membrane formation, pressure elevation, and previous ALT. Material and methods Over a period of 2 years, 388 eyes of 290 patients underwent filtering surgery at our department. A standard trabeculectomy with a fornix based conjunctival flap was performed. The inner block of tissue at the trabeculectomy site was excised using a diamond knife and straight corneal scissors. Immediately after excision, the tissues were immersed in fixative consisting of 2.5% glutaraldehyde in 0.1 M cacodylate buVer, pH 7.3. The tissues remained in this fixative for 12 hours at 4°C. They were then postfixed in osmium tetroxide for 1 hour, gently dehydrated and embedded in Epon 812. Sections of 0.5 μm were cut, stained with methylene blue, and observed with Zeiss Axiophot. Thin sections were cut, stained with uranyl acetate and lead citrate, and observed with a Hitachi 7000 transmission electron microscope. Tissues were oriented so that sagittal sections through the trabecular region were cut (Fig 1). Of these 388 eyes 122 had one or more previous argon laser trabeculoplasties. We selected only those specimens in which all important structures (that is, Schwalbe’s line, canal of Schlemm, trabecular meshwork) were visible and only one eye per patient was evaluated. This resulted in 54 specimens assigned to the Figure 1 Low power light micrograph depicting a normal chamber angle region. Schlemm’s canal (S), the trabecular meshwork (T), parts of the ciliary muscle (C), and the iris (I). Original magnification ×10. Br J Ophthalmol 2000;84:48–53 48 Augenklinik, UniversitätsSpital, Zürich, Switzerland