Sara Jouzdani

The Posterior Location of the Dilator Muscle Induces Anterior Iris Bowing during Dilation, Even in the Absence of Pupillary Block

PURPOSE To examine the effect of the posterior location of the dilator on iris anterior curvature during dilation. METHODS An in vivo human study, an ex vivo porcine experiment, and an in silico computational model were performed in parallel. Iris anterior curvature was measured in vivo before and after dilation by time-domain slit lamp optical coherence tomography (SL-OCT). All patients (n = 7) had undergone laser peripheral iridotomy to eliminate any pupillary block due to primary angle-closure glaucoma. In the ex vivo experiments, isolated porcine irides (n = 30) were secured at the periphery and immersed in an oxygenated Krebs-Ringer buffer. Dilation was induced pharmaceutically by the addition of 2.5% phenylephrine and 1% tropicamide. An in-house optical coherence tomography (OCT) system was used to obtain iris images before and after dilation. A finite element model was also developed based on typical geometry of the iris from the initial OCT image. The iris was modeled as a neo-Hookean solid, and the active muscle component was applied only to the region specified as the dilator. RESULTS An increase in curvature and a decrease in chord length after dilation were observed in both experiments. In both the in vivo and ex vivo experiments, the curvature-to-chord length ratio increased significantly during dilation. Computer simulations agreed well with the experimental results only when the proper anatomic position of dilator was used. CONCLUSIONS The posterior location of the dilator contributes to the anterior iris bowing via a nonpupillary block dependent mechanism.

Increased iris–lens contact following spontaneous blinking: Mathematical modeling

The purpose of this work was to study in silico how iris root rotation due to spontaneous blinking alters the iris contour. An axisymmetric finite-element model of the anterior segment was developed that included changes in the iris contour and the aqueous humor flow. The model geometry was based on average values of ocular dimensions. Blinking was modeled by rotating the iris root posteriorly and returning it back to the anterior. Simulations with maximum rotations of 2°, 4°, 6°, and 8° were performed. The iris-lens contact distance and the pressure difference between the posterior and anterior chambers were calculated. When the peak iris root rotation was 2°, the maximum iris-lens contact increased gradually from 0.28 to 0.34mm within eight blinks. When the iris root was rotated by 6° and 8°, the pressure difference between the posterior and anterior chambers dropped from a positive value (1.23Pa) to negative values (-0.86 and -1.93Pa) indicating the presence of reverse pupillary block. Apparent iris-lens contact increased with steady blinking, and the increase became more pronounced as posterior rotation increased. We conclude that repeated iris root rotation caused by blinking could maintain the iris in a posterior position under normal circumstances, which would then lead to the clinically observed anterior drift of the iris when blinking is prevented.

Contribution of Different Anatomical and Physiologic Factors to Iris Contour and Anterior Chamber Angle Changes During Pupil Dilation: Theoretical Analysis

PURPOSE To investigate the contribution of three anatomical and physiologic factors (dilator thickness, dynamic pupillary block, and iris compressibility) to changes in iris configuration and anterior chamber angle during pupil dilation. METHODS A MATHEMATICAL MODEL OF THE ANTERIOR SEGMENT BASED ON THE AVERAGE VALUES OF OCULAR DIMENSIONS WAS DEVELOPED TO SIMULATE PUPIL DILATION. TO CHANGE THE PUPIL DIAMETER FROM 3.0 TO 5.4 MM IN 10 SECONDS, ACTIVE DILATOR CONTRACTION WAS APPLIED BY IMPOSING STRESS IN THE DILATOR REGION. THREE SETS OF PARAMETERS WERE VARIED IN THE SIMULATIONS: (1) a thin (4 μm, 1% of full thickness) versus a thick dilator (covering the full thickness iris) to quantify the effects of dilator anatomy, (2) in the presence (+PB) versus absence of pupillary block (-PB) to quantify the effect of dynamic motion of aqueous humor from the posterior to the anterior chamber, and (3) a compressible versus an incompressible iris to quantify the effects of iris volume change. Changes in the apparent iris-lens contact and angle open distance (AOD500) were calculated for each case. RESULTS The thin case predicted a significant increase (average 700%) in iris curvature compared with the thick case (average 70%), showing that the anatomy of dilator plays an important role in iris deformation during dilation. In the presence of pupillary block (+PB), AOD500 decreased 25% and 36% for the compressible and incompressible iris, respectively. CONCLUSIONS Iris bowing during dilation was driven primarily by posterior location of the dilator muscle and by dynamic pupillary block, but the effect of pupillary block was not as large as that of the dilator anatomy according to the quantified values of AOD500. Incompressibility of the iris, in contrast, had a relatively small effect on iris curvature but a large effect on AOD500; thus, we conclude that all three effects are important.

Contribution of Different Physiological and Anatomical Factors to the Anterior Chamber Angle During Pupil Dilation

Angle closure is well documented to be more severe in dilation [1, 2]. In addition, many anatomical and physiological factors associated with dilation may also contribute to severity of angle-closure. For example, population-based studies have shown that the prevalence rates of primary angle closure glaucoma (PACG) are relatively high among Asian population, particularly older women. Three potential causes for dilation-induced angle-closure have been reported: iris volume change (or lack thereof), posterior location of the dilator muscle, and (dynamic) pupillary block.Copyright

Anterior Chamber Angle and Iris-Lens Contact Alteration During Pupillary Dilation

The aqueous humor (AH) provides oxygen and nutrients for the avascular ocular tissue specifically, the cornea and lens. AH is secreted by the ciliary body into the posterior chamber, passes through pupil, and drains into the anterior chamber (Fig. 1a). Resistance to the aqueous outflow generates the intraocular pressure (IOP), which is 15–20 mmHg in the normal eyes.Copyright

Patient-specific model of iris mechanics

Over 2 million Americans suffer from glaucoma [1]. Certain types of glaucoma are directly related to the iris contour. For example, some cases of primary angle-closure glaucoma (PACG) involved with pupillary block, which the iris is abnormally positioned towards the anterior [2]. Generally, the iris contour is determined by two factors: external stresses arising from the flow of the aqueous humor (AH) and internal stresses due to the passive and active components of the constituent tissues.Copyright

Spatial Heterogenity of Iris Elasticity Measured by Indentation

Angle-closure glaucoma, pigment dispersion syndrome, and intraoperative floppy iris syndrome (IFIS) are all ocular disorders that involve abnormal morphologies of the iris. In angle-closure glaucoma, for example, the iris bows anteriorly and impedes the natural flow of the fluid (aqueous humor), which then increases the intraocular pressure (IOP) leading to vision loss. The iris contour is determined by a combination of external stresses arising from the flow of the aqueous humor [1] and internal stresses due to the passive and active components of the constituent tissues. We have previously shown that the iris has a mechanical asymmetry [2] and that posterior positioning of the dilator muscle within the iris contributes to the anterior bowing during dilation [3]; however, the relative contributions of the individual components of the iris are unknown.Copyright