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S. K. Dorairaj, C. Oliveira, V. H. Barocas, J. M. Liebmann, R. Ritch; Dynamic Changes in Posterior Chamber Volume During Pupil Dilation. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3930.
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To determine the change in posterior chamber volume during dilation, as a means to assess the role of aqueous humor pressurization in the dark room provocative test.
Subjects with anatomically narrow angles were imaged by, ultrasound biomicroscopy in a room initially lit to a standard level. A video was made of the iris contour, and the room was darkened while continuously recording images of the posterior chamber. The imaged region included the entire posterior chamber and both sides of the pupillary margin. Pupillary diameter was measured directly from the image for each frame. Posterior chamber volume was calculated by tracing the posterior chamber in each frame; the area was rotated around the pupillary axis, which was constructed as the perpendicular bisector of the pupillary diameter.
In most cases, the dilation of the pupil in response to room darkening was easily recognized in the video, corresponding to a 40-60% increase in pupillary diameter. Posterior chamber volume was much more variable, with no clear trend. In some cases, it increased with dilation, whereas in others it decreased slightly.
The results suggest that 10-20% error in measurement of posterior chamber volume (e.g., because the ultrasound image was slightly off the radius of the eye, or because of error in tracing the posterior chamber area) was enough to obscure any transient period of constant volume, which would be expected if incompressibility is important in determining iris curvature during dark-room tests. We can, however, conclude that posterior chamber volume changes relatively little during dilation (~ 20% at most). The hypothesis that (incompressible) aqueous humor is trapped in the posterior chamber and causes anterior bowing of the iris seen in dark-room testing cannot be confirmed or rejected by the current study.
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