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Rupert Bourne, Laura Sanchez Parra, Humma Shahid, Roger Buckley, Shahina Pardhan; Dynamic iris volume characteristics pre and post laser peripheral iridotomy (LPI) in eyes with occludable anterior chamber angles measured with 3 dimensional swept-source Ocular Coherence Tomography (OCT): The Investigating Management of Angle Closure and Treatment (IMPACT) study. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1875. doi: https://doi.org/.
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Previous studies involving fellow eyes (post-LPI) of patients in which the other eye has suffered acute angle closure (AAC) have reported a gain in iris volume with pharmacological dilation while irises of normal controls lose volume- this ‘iris sponge’ hypothesis may explain a predisposition of these eyes to AAC. This study investigated the hypothesis that LPI affects iris volume change but in patients with no history of AAC and within the physiological range of pupil size.
32 Caucasian patients with bilateral Primary Angle Closure (PAC), Primary Angle Closure Suspect (PACS) or PAC/PACS were recruited. A randomly chosen eye of each patient received LPI. OCT (CASIA) measurements were measured in light (170-200 lux) and dark (0.3-0.5 lux) prior to and 6 weeks after LPI by the same examiner. Change in iris volume between light and dark conditions was measured at each of these 2 timepoints, and association with pre-laser presence of peripheral anterior synechiae (PAS) was explored.
Prior to LPI, total iris volume was greater in light (mean, 36.5 mm3) than in dark conditions (mean, 35.8 mm3; paired t test, p=0.05). Following LPI, a similar volume relationship existed (light: mean, 36.4 mm3; dark: mean, 35.5 mm3, p=0.01). Light minus dark iris volume change (LDVC) was no different before and after LPI (p=0.59). LDVC was not significantly different between eyes with PAS (n=9; mean LDVC, 0.61mm3) compared to those without (n=23; mean LDVC, 0.75mm3) prior to LPI (P=0.86). However, following LPI, eyes with PAS demonstrated negative LDVC values (mean LDVC, -0.49mm3) compared to eyes without PAS in which the relationship was reversed (mean LDVC, 1.54mm3, P<0.01).
This study has demonstrated reduction in iris volume in dark conditions within the physiological range of pupil size for a condition (PACS/PAC) which has a much higher prevalence in the population than AAC. The finding that those eyes with PAS exhibited less volume change or an actual increase in iris volume in dark conditions lends support to the ‘iris sponge hypothesis’, and suggests that there is a spectrum of pathology of dynamic iris behavior in eyes with occludable angles.
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