65 7.1 What Are the Aqueous Veins? The aqueous veins are visible on the surface of the eye and contain aqueous being returned to the general circulation. Aqueous veins are of great importance because aqueous out- flow system models can be judged by their ability to predict and explain properties of directly visible aqueous flow. Aqueous humor circulation through the anterior segment of the eye involves one of the vascular circulatory loops that is driven down a continuous pressure gradient initially set up by the heart. Aqueous exits the eye by passing through the trabe- cular meshwork to Schlemm’s canal. After entering Schlemm’s canal, aqueous enters collector channels that have a lumen in communication with the aqueous veins. The aqueous vein lumen in turn communicates with episcleral veins that return blood to the general circulation. 7.2 Why Are Aqueous Veins Clinically Important Now? We owe so much to the discovery of the aqueous veins. All current ideas about glaucoma have been placed on a solid foundation by the recognition of these unique vessels; it is a remarkable story for a simple reason; their recognition was what established a powerful concept. Aqueous flows! The concept underlies all current approaches to treatment of both open and closed angle glaucoma. Aqueous flows; that remarkably simple premise was the subject of great dispute until Ascher’s 1 and Goldmann’s 2 independent discovery of the aqueous veins in the early 1940s. The concept of a stagnant aqueous held such sway that it was not until the 1970s that the concept was eliminated from discussion. 3 The idea that aqueous flows was little more than specula- tion until such flow was identified in the aqueous veins. Although some laboratory studies suggested aqueous flows, 4 the evidence involved indirect measurements that were not sufficiently definitive to permit development of a consensus opinion. In the absence of a consensus about aqueous flow, the concept of using peripheral iridectomy for treatment of angle closure was on shaky ground. Furthermore, efforts to improve aqueous outflow in open angle glaucoma did not have a satisfactory rationale. Direct observation of flow through the aqueous veins elimi- nated uncertainty, it changed everything. Gonioscopy to char- acterize the status of the aqueous outflow apparatus suddenly became relevant. Advocates of peripheral iridectomy could point to a very clear rationale for their recommendations; the approach was rapidly accepted worldwide. The mechanism of action of glaucoma drugs could be placed within a rational con- text. Specific microsurgical procedures could be envisioned because they could be tied to a clear goal of the restoration of normal flow. Procedures that owe their genesis to the concept of aqueous flow include goniotomy, trabeculotomy, sinuso- tomy, and newer procedures including trabectome, the Glaukos shunt, non-penetrating surgery, and trabecular tightening. 7.3 Why Will the Aqueous Veins be Clinically Important in the Future? Where else can we directly observe and measure return of an extravascular fluid to the vascular system? How remarkable that we can see the behavior involved in normal outflow! How remarkable that we can also directly observe outflow system abnormalities in glaucoma! How remarkable that we can directly observe the effect of drugs that restore aqueous outflow to normal! What a wonderful gift, what a wonderful opportunity to gain new insights. Direct observation is the investigative tool which is the most reliable and the most powerful in the hierarchy of methodologies that establish the validity of evidentiary claims. Because of the aqueous veins, we no longer need to rely entirely on indirect measurements. No longer need our scope of thought be limited by the need to base claims entirely on assumptions made following the examination of fixed tissues. Chapter 7 Aqueous Veins and Open Angle Glaucoma Murray Johnstone, Annisa Jamil, and Elizabeth Martin P.N. Schacknow and J.R. Samples (eds.), The Glaucoma Book: A Practical, Evidence-Based Approach to Patient Care, DOI 10.1007/978-0-387-76700-0_7, © Springer Science+Business Media, LLC 2010 In:The Glaucoma Book Editors: Schacknow - Samples Springer - New York - 2010