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Editors Selection IGR 17-4

Clinical Forms of Glaucoma: LPI in pigment dispersion syndrome

Comment by Syril Dorairaj on:

46339 Biometric analysis of pigment dispersion syndrome using anterior segment optical coherence tomography, Aptel F; Beccat S; Fortoul V et al., Ophthalmology, 2011; 118: 1563-1570

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The main objective of this study is to compare anterior chamber volume (ACV), iris volume, and irido-lenticular contact (ILC) area before and after laser peripheral iridotomy (LPI) in eyes with pigment dispersion syndrome (PDS), using anterior segment optical coherence tomography (AS OCT). Aptel et al. (1422) have drawn two main conclusions from this prospective case control study. The first conclusion is that PDS eyes do not have an iris that is abnormally large, relative to the anterior segment size, and the second is that in PDS eyes there is a weakly resistant iris that is stretched and pushed against the lens when there is a pressure difference across the iris. This study expands our knowledge and understanding of the anatomical and dynamic iris factors that predispose towards development of PDS and indicate that iris stroma, muscles, or both in PDS eyes have abnormal biomechanical properties that predispose these eyes for pressure gradient changes. Interestingly, the author's first conclusion is in line with previous reports shown by other studies, where only the cross-sectional area and thickness were estimated. Authors took the analysis a step further by using a mathematically validated method of estimating the three-dimensional volume differences in iris and anterior chamber and successfully showed that there is no difference in these biometric parameters in PDS patients and in normal subjects. They also showed that ACV is the most significant predictor of greater ILC.

It would have been interesting if the authors had evaluated stress factors like accommodation and prevention of blinking that cause pressure gradient across iris while documenting the second conclusion. Previous investigations performed with ultrasound biomicroscopy by preventing blinking documented increased ILC and reverse pupillary block effect. Normal blinking appears to create transient vector forces, which promote aqueous humor flow from the posterior to anterior chamber. Accumulation of aqueous humor in the posterior chamber, when blinking is prevented, alters iris position in PDS and healthy eyes and increases distances in irido-zonular and irido-ciliary process. However, the pressure gradient across the iris increases the ILC in PDS patients causing reverse pupillary block effect. In this context it would be interesting to document the irido-zonular distance and volume differences across the iris. It is the mechanical chaffing between the mid posterior iris surface and the zonular bundles that causes pigment dispersion. Since ASOCT cannot visualize the structures located behind the iris pigment epithelium for now, the authors could have documented the peripheral AC depth, which indirectly provides information regarding the changes in irido-zoular distance, if other physiologic factors are kept constant. It should also be noted that there is a time-dependent nature of iris contour response, with significant differences observed in dynamic iris parameters between the initial observation after accommodation and the observation at the end of ASOCT examination.

In conclusion, this study embarks on an important step in understanding and identification of the anatomical factors that predispose towards development of PDS. Since not all PDS patients develop glaucoma, it becomes imperative to identify these factors to understand the phenotypic variability.

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