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I. Grierson, K.P. Cracknell, P. Appleton, N. Pfeiffer, P.A. Hogg; Iris Darkening after Short Term Exposure to Latanoprost Is Due to a Small Increase in Melanin Granule Size . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4231.
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Purpose: The aim of this study is to investigate the morphological changes that are causing the latanoprost (Xalatan®) induced iris-darkening (LIID) side effect after short-term exposure. Methods: Patients requiring bilateral trabeculectomies provided peripheral iridectomy specimens for this study. The patients were unilaterally treated with 50µg/ml latanoprost daily for 6 months prior to surgery, with the fellow untreated eye as a control. Of an original study group of seventeen, two underwent iris colour change. For the color change iredectomies, light microscopic (LM) investigation of the cellularity and quantative electron microscopic (EM) investigation of the melanocytes, and melanin granules were undertaken. The melanin granules within the melanocytes were analysed with regard to, their number, maximum diameter, area occupied, and incidence of immature melanosomes. Results: Comparison of the paired data sets enabled changes within the latanoprost treated tissue to be observed. LM analysis showed no significant difference in cellularity. EM analysis of the melanocyte melanin granules showed no change to either the number of granules or immature melanosomes. However, within the anterior border of the iris, there was an increase in the diameter of the melanin granules, area of granules, and the total melanin content within the cells. Data was subjected to a t test and these changes were found to be statistically significant at p<0.05. Examination of the melanin profile found that this increase in size was across the population, and not due to a sub population of large granules. Within the stromal region a similar but less marked trend was observed to that in the anterior border. There was no change in the melanin granules in the posterior pigmented eplithilium. Conclusions: Short-term exposure to latanoprost has no discernable effect on the tissue at a cellular level or on melanosomes turnover. Iris darkening after short-term treatment does appear to be associated with a modest increase in volume of melanin within the deep stromal and anterior border melanocytes.
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