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DM Silver, HA Quigley; Aqueous Flow Dynamics Across the Iris-lens Channel as the Origin of Anterior-posterior Differential Pressure . Invest. Ophthalmol. Vis. Sci. 2002;43(13):988.
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Purpose: To explore the hypothesis that differential pressure between the anterior and posterior chambers arises from the dynamics of aqueous flow across the iris-lens channel. Methods: Using principles of fluid dynamics, the Navier-Stokes equations are derived for a viscous homogeneous isotropic fluid (aqueous) passing through an iris-lens channel, which is a disk-shaped region surrounding the pupil where the iris conforms to the lens curvature while maintaining a separation distance (gap height) over a certain disk width (channel length). Upstream and downstream of the iris-lens channel, aqueous flow is assumed to be unobstructed. The fluid flow field and pressure drop along the iris-lens channel are determined from numerical evaluation of the equations, parameterized by the channel length, gap height, pupil size, and aqueous outflow rate. Results: The magnitude of the pressure drop from posterior to anterior is greater with increases in the channel length, decreases in the gap height, decreases in the pupil diameter, and increases in the aqueous flow rate. For example, with a channel length of 0.5 mm, a gap height of 7 microns, and an aqueous outflow of 2.2 microliters/min, the pressure drop changes from 0.2 to 0.9 mm Hg with a change in pupil size from 6 to 1 mm. With a pupil diameter of 1 mm, the pressure drop increases from 0.9 to 10 mm Hg when the gap height decreases from 7 to 3 microns. Conclusion: The iris-lens channel configuration and aqueous flow dynamics comprise a regime dominated by viscous forces in the fluid. The flow of aqueous across the iris-lens channel is driven by the pressure differential between the posterior and anterior chambers. With a constant aqueous outflow rate, narrowing the channel gap height below 5 microns causes pressure differences that are high enough to be of concern. Tonometry on the cornea measures the anterior chamber pressure and does not estimate the true pressure in the posterior chamber and vitreous cavity. Hence, depending on the aqueous fluid dynamics, the operative pressure at the optic nerve head may be incorrectly estimated in some eyes when measured at the cornea (for instance, in normal tension glaucoma eyes with narrow iris-lens channel gap heights), thereby obscuring an additional risk factor in glaucoma damage.
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