Glaucoma surgery and aqueous outflow: how does nonpenetrating glaucoma surgery work?

B LEBS. SHALLOW CHAMBERS. Bleb leaks. Flat chambers. Dellen. Blebitis. No bleb. Late failure. Hypotony. Choroidal effusions. Maculopathy. Is there a better way? If the problem is in the meshwork, why do we cut a hole in the eye? Viscocanalostomy and deep sclerectomy are surgical procedures for glaucoma that have been designed to avoid some of the complications of conventional glaucoma surgery. The concept of nonpenetrating glaucoma surgery was first introduced in 1962, and variations have been described since then. Viscocanalostomy is reported to lower intraocular pressure (IOP) without creating a filtering bleb. Deep sclerectomy, a nonperforating filtration procedure, is reported to avoid shallow anterior chambers and the other early postoperative problems of conventional filtering surgery and also produce lower, more diffuse blebs. Both procedures involve fashioning a partial-thickness scleral flap and then removing a second layer of sclera deep to the initial flap. This unroofs Schlemm’s canal and exposes Descemet’s membrane. The resulting trabeculo-Descemet membrane acts as a semipermeable layer of tissue, allowing aqueous to percolate through it. In viscocanalostomy, after removal of the deep scleral layer, Schlemm’s canal is cannulated and expanded with a viscoelastic material. Viscoelastic material is also injected into the region of excised sclera, or “scleral lake,” to prevent healing. By never entering the anterior chamber or removing the trabecular meshwork, hypotony, hyphema, and other complications are said to be avoided. Variations of these procedures include (1) removing the inner wall of Schlemm’s canal and adjacent meshwork but leaving the inner meshwork intact and (2) placing a collagen implant in the filtration bed to prevent episcleral fibrosis. Do these procedures relieve the specific pathological problem of primary open-angle glaucoma (POAG)? Or do they function as simply another way to make a hole in the eye?