The outflow of aqueous via the anterior chamber angle is a constant process. The aqueous is formed by the ciliary processes and then passes through the pupil from the posterior chamber to the anterior chamber (Figure 2.1). About 83%–96% of the aqueous finally exits the eye into the anterior chamber angle via the trabecular meshwork—Schlemm’s canal—venous system (i.e., the conventional or canalicular outflow pathway). The other 5%–15% of aqueous outflow occurs via uveoscleral outflow (i.e., the unconventional or extracanalicular outflow pathway), with aqueous passing through the ciliary muscle and iris, then entering into the supraciliary and suprachoroidal spaces, and then finally exiting the eye through the sclera or along the penetrating nerves and vessels. Glaucoma is usually associated with aqueous outflow problems through a variety of mechanisms. For the developmental glaucomas, the improper development of the outflow structures is the main reason for high eye pressures. In the primary and secondary open-angle glaucomas, the theories to explain the diminished outflow facility are numerous. Possible etiologies are as follows: deposition of foreign material (such as pigment, red blood cells, glycosaminoglycans, extracellular lysosomes, plaque-like material, and proteins) into the trabecular meshwork (TM) and the wall of Schlemm’s canal (SC), loss of trabecular endothelial cells, structural changes of the inner wall of SC, and abnormal phagocytic activity of trabecular endothelial cells. In angle closure glaucoma, the peripheral iris closes the entrance to the TM by the anterior pulling mechanism or the posterior pushing mechanism, resulting in the direct blockage of conventional outflow. The goal of angle and nonpenetrating procedures is to restore aqueous outflow, thereby lowering intraocular pressure (IOP). Angle surgery restores outflow by re-opening the natural channels for aqueous outflow, and nonpenetrating glaucoma surgery creates an artificial external filtration site and partly restores the normal physiologic pathways. In 1936, Otto Barkan was the first to describe a surgical procedure that creates an internal incision into trabecular tissue under direct magnified view of the anterior chamber angle.
Morphological variability of the trabecular meshwork in glaucoma patients: implications for non-perforating glaucoma surgery