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V.H. Barocas, E.C. Huang, J.J. Heys; Aqueous Humor and Iris Mechanics in Synechial vs. Appositional Contact in Primary Angle-Closure Glaucoma . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3426.
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Purpose: Primary angle-closure glaucoma (PACG) is characterized by elevated intraocular pressure (IOP) and a narrow anterior chamber angle (ACA). The two major types of PACG are defined by the mechanism of angle closure (i.e., the mechanism of contact between iris and cornea). In synechial contact, the peripheral iris adheres to the corneoscleral shell. In appositional contact, the iris is forced anteriorly by a pressure difference between the posterior and anterior chambers due to relative pupillary block. Methods: We developed a finite-element computer simulation of aqueous humor and iris mechanics in synechial and appositional contact in PACG. Synechial contact is modeled by imposing a Dirichlet boundary condition forcing the peripheral iris near the corneoscleral shell. A possible mechanism for appositional contact was examined with our model by specifying that the iris (i.e., the sphincter iridis) maintains a target pupil diameter. Results: For synechial contact, we found that as the ACA narrowed below 20 degrees, IOP increased dramatically, iris-lens contact became negligible and the central iris exhibited a plateau-like profile. For appositional contact, we found that as pupil diameter decreased, the pressure difference between the anterior and posterior chambers increased significantly. As a result, iris concavity increased, closing the ACA and increasing the IOP. Conclusions:Characteristic features of PACG, including the plateau iris associated with synechial contact and the anterior bowing of the iris in appositional contact, are consistent with our computer model results. The role of other factors, particularly eye anatomy, remain to be explored.
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