[Non penetrating filtering surgery, evolution and results]

Trabeculectomy is currently the standard filtration procedure for glaucoma surgical treatment. Despite several advantages over full-thickness procedures, trabeculectomy can be responsible for early postoperative complications related to sudden ocular decompression that can lead to hypotony with its sight-threatening complications. Nonpenetrating glaucoma surgeries, namely viscocanalostomy and deep sclerectomy with external trabeculectomy, have been developed in recent years in order to improve the safety of conventional filtering procedures. The goal of these procedures is to reduce intraocular pressure (IOP) by enhancing the natural aqueous outflow channels, while reducing outflow resistance, attributed for 75% to the trabecular meshwork and for 25% to the outer wall of Schlemm's canal (SC) or tissue surrounding it. In these procedures, the anterior chamber is not opened so that complications related to full thickness procedures are mainly avoided. In the last few years, viscocanalostomy and deep sclerectomy with external trabeculectomy have become the most popular nonpenetrating filtering procedures. Both involve the removal of a deep scleral flap, the external wall of SC, and corneal stroma behind the anterior trabecula and Descemet membrane, thus creating a scleral lake. The aqueous humor leaves the anterior chamber through the intact trabeculodescemetic membrane and reaches the scleral lake, from where it will egress into different pathways. In viscocanalostomy, a high-molecular viscoelastic substance is injected into the ostia of the SC in order to enlarge the SC and its collector channels. In deep sclerectomy with external trabeculectomy, the main goal is to remove the inner wall of the SC and the adjacent trabecular layers involved in aqueous outflow resistance, while leaving the innermost trabecular layers intact so that the anterior chamber does not open during operation. Different surgical adjuvants designed to maintain the scleral lake open are commercially available. Retrospective and prospective published studies have reported similar midterm results with trabeculectomy and nonpenetrating filtering procedures in terms of IOP control, with fewer postoperative complications and better visual acuity recovery with the nonpenetrating procedures. This article reviews the current nonpenetrating surgical procedure techniques, their mechanism of action, and their outcome.