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Jennifer Helen Tweedy, Mariia Dvoriashyna, Rodolfo Repetto; Aqueous Humor Flow in the Posterior Chamber of the Eye in the Case of Iridotomy. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3540.
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© ARVO (1962-2015); The Authors (2016-present)
The choice of optimal size and location of an iridotomy is still poorly understood. We study aqueous flow driven by production in the ciliary body in the posterior chamber of the eye, and also consider the effect of miosis on the flow. We use a theoretical approach that allows the problem to be solved semi-analytically. We use the results find both the pressure difference between anterior and posterior chambers, which gives insight into the risk of angle closure glaucoma, and also the stress generated on the surrounding tissues. We use it to predict the range of iridotomy sizes that maintain the pressure within safe limits and avoid large stresses.
We derive the shape of the posterior chamber from ultrasound images. Since its geometry is long and thin, we use lubrication theory to simplify the problem and we justify a quasi-steady approach to model miosis. In our model we treat the iridotomy as a point sink in the iris and we assume the flux through it is proportional to the pressure drop across it. We can achieve a numerically tractible model by mathematically regularising the pressure, and we implement it using a finite difference method.
The geometry of the posterior chamber significantly influences the pressure and flow, and in particular the height and length of the iris-lens channel and the diameter of the iridotomy. Conversely, the location of the iridotomy on the iris and the velocity of the iris during miosis do not have a significant effect. We find the iridotomy diameter that maximizes the velocity of the jet of aqueous passing through the iridotomy, and show it is much higher during miosis than otherwise. In the case of pupillary block an iridotomy with a diameter of at least 20 um is needed to avoid dangerously high pressures.
The ideal size and location of an iridotomy is influenced by various geometrical and fluid mechanical factors. We find the most significant ones are the size of the hole, the width and height of the narrow iris-lens channel and the possible presence of pupillary block. For certain iridotomy diameters, we cannot rule out the possibility that the jet velocity through the iridotomy during miosis might become large enough so as to cause corneal damage.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
Pressure distribution and depth-averaged velocity vectors of the flow in the posterior chamber in the presence of an iridotomy with diameter 100 um.
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