Ab externo Schlemm's canal surgery: viscocanalostomy and canaloplasty

Ab externo Schlemm's canal (SC) surgery (e.g. viscocanalostomy and canaloplasty) is a valuable alternative to glaucoma filtration surgery. It targets the abnormally high resistance to outflow in the trabecular meshwork and reestablishes the physiologic outflow system. In viscocanalostomy, viscoelastic substance is injected to dilate SC which in turn leads to microdisruptions of the inner wall. In canaloplasty, the additional intracanalicular suture stent keeps the canal patent and enhances the circumferential flow. A prerequisite for these procedures to work is the integrity of the distal outflow system, which can be evaluated by two clinical tests before surgery: provocative gonioscopy with blood reflux and fluorescein channelography. Ab externo SC surgery is suitable for open-angle glaucoma, but also for angle closure glaucoma in combination with cataract extraction. IOP reduction to the mid-teens for viscocanalostomy, and to the lower teens for canaloplasty can be expected. The majority of complications seen in filtering surgery are largely eliminated by the nonpenetrating and bleb-independent approach. Postoperative care is minimal as no bleb management like needling is required, and hypotony-related complications are largely avoided by the intrinsic resistance of the physiologic outflow system. For its efficacy and high safety profile, ab externo SC surgery will continue to play an increasing role and will change the current concept of glaucoma surgery towards earlier intervention. Surgeons will be well advised to implement these antimetabolite-free procedures into their armamentarium to meet the expectations of the demanding glaucoma patient.