Historical and Contemporary Debates in Schlemm's Canal-Based MIGS

Abstract

Glaucoma is one of the primary causes of blindness worldwide. Canal opening surgery, a type of minimally invasive glaucoma surgery (MIGS) applied in cases of mild to moderate glaucoma, has gained increasing popularity in recent years due to its efficacy in reducing the intraocular pressure, its safety profile, the simplicity of its technique, and the reduced likelihood of compromised vision. Nevertheless, the existing body of histopathological studies remains insufficient for a comprehensive understanding of post-surgical wound healing. Consequently, debates persist among researchers regarding the mechanism through which Schlemm's canal opening surgery reduces the intraocular pressure, as well as the surgical techniques that may impact the outcomes and the factors influencing surgical success. As the history of MIGS is relatively short and lacks sufficient systemic reviews or meta-analyses evaluating the influence of individual factors, this review was conducted to illuminate the disparities in researchers' opinions at the current stage of research.

Keywords: Kahook dual blade; Schlemm’s canal opening surgery; T hook; Tanito micro-hook; canaloplasty; history; minimally invasive glaucoma surgery (MIGS); surgical success; trabectome; trabeculectomy; trabeculotomy.

Figure 1

A schematic drawing depicting the histopathology of human eyes following an unsuccessful trabeculotomy [66]. Surgical opening of the trabecular meshwork resulted in closure in 14 of 31 eyes (A) and it remained open in 10 of 31 eyes (B) to the anterior chamber six years (ranging from 3.4 to 22.6 years) after external trabeculotomy. These specimens were obtained during subsequent trabeculectomy after the initial trabeculotomy ab externo. In 10 eyes, in which Schlemm’s canal (SC) remained open to the anterior chamber, the inner wall of the SC was lined with extended Schlemm’s canal endothelium and/or fibrous proliferation. In 14 eyes with a closed SC, the trabecular meshwork showed sign of fibrous degeneration. Additionally, the intracanalicular space might be filled with fibrous proliferation, leading to the shrinkage of the SC space. In four eyes, the trabecular meshwork was covered by Schlemm’s canal endothelial cells, and, in three eyes, peripheral anterior synechiae covered the trabecular meshwork. Open triangles indicate lumens surrounded by the extended endothelium of Schlemm’s canal. Arrows point to fibrotic tissue surrounding the expanded Schlemm’s canal. (Copyright License #5796400157661 by Exp. Eye Res. Elsevier).

Figure 2

The Kahook dual bade is a device to excise the trabecular meshwork (internal trabeculectomy) between two parallel blades near the tip (product of New World Medical Rancho Cucamonga, CA).

Figure 3

Tanito micro-hook for internal trabeculotomy: A device inserted into Schlemm’s canal to push away trabecular meshwork tissue (product of Inami, Tokyo).

Figure 4

Chihara T-hook for internal trabeculotomy. The T-shaped head allows the bidirectional opening of the trabecular meshwork. The tip of this device is rounded and designed not to injure outer wall of Schlemm’s canal (product of Inami Tokyo; Handaya Tokyo; and AOI: Advanced Ophthalmic Innovations, Singapore).