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V.H. Barocas, E.C. Huang; Computer Modeling of Pupil Dilation: Is Aqueous Humor Trapped in the Posterior Chamber? . Invest. Ophthalmol. Vis. Sci. 2006;47(13):223.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate the hypothesis that dark–room provocative testing for narrow angles leads to trapping of aqueous humor in the posterior chamber, which in turn pushes anteriorly on the iris and narrows the angle.
A previous finite–element computer model was extended to include transient forces in the sphincter and/or dilator to dilate the pupil and simulate the response to room darkening. The resulting motion and contour change of the iris were modeled for different dilation rates and initial pupil diameters.
The pupil dilation model showed a slight trapping effect in which the posterior chamber volume decreased more slowly than in steady–state simulations. This corresponded to a large (25–50%) decrease in iris–lens contact, a slight (10%) rise in posterior–anterior pressure difference, and a slight (∼ 10 um) increase in iris curvature.
The finite–element model predicts a relatively small trapping effect, which may contribute to angle narrowing during dark–room testing but is not sufficient to explain the entire phenomenon. Our results suggest that increased pressure in the posterior chamber should quickly cause anterior deflection of the iris (i.e., the decrease in contact area) and allow the aqueous humor to drain into the anterior chamber. If the pupil blocking force were stronger, the observed effects would be more pronounced.
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