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Humma Shahid, Laura Sanchez Parra, Roger Buckley, Shahina Pardhan, Rupert Bourne; Dynamic iris volume characteristics in the physiological range of pupil size and reproducibility of measurements in eyes with occludable anterior chamber angles: The Investigating Management of Angle Closure and Treatment (IMPACT) study. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1895.
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Previous studies involving fellow eyes of patients in which the other eye has suffered an episode of acute angle closure (AAC) have reported increase in iris volume with pharmacological pupil dilation while normal irises lose volume- this ‘iris sponge hypothesis’ is thought to explain the predisposition of these eyes to AAC. This study aimed to investigate physiological changes in iris volume in eyes with occludable angles but no history of AAC using three-dimensional anterior segment ocular coherence tomography (AS-OCT).
69 eyes of 35 Caucasian patients with a gonioscopic diagnosis of Primary Angle Closure (PAC), Primary Angle Closure Suspect (PACS) or a combination of both conditions were included.The iris volume, angle opening distance (AOD),angle recess area (ARA), trabecular-iris space area (TISA) and trabecular-iris angle (TIA) were measured in superior, inferior, nasal and temporal meridiens using the novel non-contact three-dimensional AS-OCT (CASIA) in dark and light conditions. All images were acquired and evaluated by a single observer. Measurements were repeated to check for intra-observer reliability.Statistical analysis involved a comparison of the change in total iris volume in dark and light conditions. Single and multiple predictor linear regression was performed to determine whether there was a relationship between change in iris volume between light and dark conditions and dimensions of the drainage angle.
The total iris volume in dark conditions was always significantly lower than the volume in light conditions (P=0.008). Linear regression analysis showed that the change in iris volume between light and dark conditions was not influenced by differences in angle dimensions (P>0.05) in superior, inferior, nasal or temporal meridians. There was no significant interobserver error in iris volume measurements (P>0.05).
Three-dimensional anterior segment ocular coherence tomography is a reliable device for iris volume measurement with no significant intra-observer error. Eyes with occludable drainage angles but no history of acute angle closure exhibit a significant reduction in iris volume under physiological dark conditions. This observed change in iris volume is independent of the degree of angle narrowing.
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