To date, raised intraocular pressure (IOP) remains the only modifiable risk factor in glaucoma management. The aim of glaucoma surgery is to lower IOP and thus curb the progression of glaucomatous optic nerve damage, especially in cases refractory to topical therapy.

Non-penetrating glaucoma surgery (NPGS) enhances the natural aqueous outflow channels, while reducing outflow resistance located in the inner wall of Schlemm's canal and the juxtacanalicular trabecular meshwork. Aqueous percolates through the remaining trabeculo-Descemet's membrane (TDM) into the subscleral space.

While full-thickness trabeculectomy penetrates the eye, NPGS does not. With the globe remaining intact, there is a relatively controlled flow of aqueous through the TDM, thus preventing sudden intra- and postoperative hypotony. The superior safety profile of NPGS as compared with full-thickness trabeculectomy is further evidenced by a relatively low risk of blebitis and endophthalmitis.¹

Figure 1: Mitomycin-soaked pledget 'necklace' suture used to avoid losing or 'fishing' for pledgets under the conjunctiva.

In addition, contrary to trabeculectomy, which has been shown to have a high failure rate when combined with cataract extraction, NPGS does not appear to be compromised in its success rate when combined with phacoemulsification and intraocular lens implantation.² We assume that the reason for this is the barrier function of the TDM, which prevents the emergence of post-cataract inflammatory proteins from the anterior chamber into the subscleral space.

Even though there is an extensive literature devoted to fullthickness glaucoma surgery, sufficiently large and comparable longterm studies reporting on the efficacy and reproducibility of NPGS are pending. This is one of the main reasons why, despite its apparent advantages, NPGS is not widely performed.

Variants of NPGS

The idea of NPGS was first explored in the 1960s. Today, the basic NPGS procedure can be described as a deep sclerectomy (DS). In addition, NPGS encompasses viscocanalostomy and canalopasty, both of which aim to increase flow through Schlemm's canal.³

DS can be combined with spacemaintainer implants, the aim of which is to avoid secondary collapse of the superficial flap. The use of antimetabolites such as mitomycin C (MMC) reduces the risk of bleb scarring and, therefore, enhances the rate of success of NPGS, which, although to a lesser extent than trabeculectomy, is bleb dependent.

Operative technique

Figure 2: Evident percolation of aqueous humour through TDM demonstrated with fluorescein.

All our NPGS cases were performed under the operating microscope under topical (proxymetacaine 0.5%) and subconjunctival (lidocaine 2% 0.25–0.5 mL) anaesthesia.