Abstract
Purpose: :
To study changes in iris configuration and anterior chamber angle during and after blinking, accommodation, and miosis.
Methods: :
We developed a mathematical model of the anterior segment to predict changes in iris contour during and after blinking, accommodation, and miosis. Geometric and mechanical parameters were based on published data. Blinking was modeled by posteriorly rotating the iris root in 80 ms and returning it to its original position in 200 ms every three seconds. Accommodation was modeled by 0.2 mm reduction in the anterior chamber depth in 0.5 second via anterior movement of the lenticular surface. Physiological miosis was modeled by imposing a circumferential stress in the sphincter region to reduce pupil diameter. Accommodation and miosis were performed simultaneously after 10 seconds of blinking. The iris was modeled either as compressible or incompressible. Iris curvature and iris-lens contact were calculated.
Results: :
Before blinking, the iris was slightly bowed toward the posterior and the pressure in the posterior was slightly higher than that in the anterior chambers for both compressible and incompressible models. After blinking, both iris curvature and iris-lens contact changed slightly. As accommodation and miosis began, the pressure was elevated in the anterior chamber (reverse pupillary block), causing more pronounced posterior bowing of the iris. In the incompressible model, immediately after accommodation and miosis, iris-lens contact and AOD500 increased by 0.85 mm and 60.62 μm, respectively while the pressure drop across the iris decreased by 42.26 Pa compared to post-blinking condition. In the compressible model, immediately after accommodation and miosis, the iris-lens contact and AOD500 increased by 0.80 mm and 53.48 μm, respectively, and the pressure drop decreased by 36.66 Pa compared to post-blinking condition. As the system reached steady state, the pressure in the posterior chamber became higher than that in the anterior chamber for both models.
Conclusions: :
Clinical studies have shown that accommodation leads to posterior bowing of the iris, particularly in patients suffering from pigment dispersion syndrome (PDS) that have flaccid and smaller irides. The combination of pupil constriction and accommodation produced significant posterior bowing of the iris. The effect was more pronounced when the iris was incompressible, suggesting that changes in iris volume could play a role in PDS similar to that seen in angle closure (Quigley et al., J Glaucoma, 18:173-9, 2009)
Keywords: accommodation • anterior segment • iris