Over the past decade, new anterior chamber (AC) imaging techniques, such as ultrasound biomicroscopy (UBM) and optical coherence tomography (OCT), have provided numerous valuable insights into the pathogenesis of angle-closure glaucoma, and demonstrated that the well-identified anatomic risk factors are not sufficient to produce angle closure.
1 Clearly, anatomic conditions, such as shallow AC, shorter axial length, larger lens thickness and volume, or greater lens vault, are statistically demonstrated risk factors of angle closure, angle-closure glaucoma, or acute angle closure.
2–7 However, the anatomic mechanisms taken alone do not explain that 80%–90% of the eyes with such predisposing characteristics never have angle closure, that Asians have a much higher prevalence of angle closure and angle-closure glaucoma than Caucasians despite often comparable biometric characteristics, and that the numerous available biometric measurements have a poor predictive power to identify eyes a priori that will have further development of angle closure and need iridotomy.
7–12 Recent studies have investigated the role of the dynamic response of some intraocular structures to physiologic conditions, particularly the response of the iris to pupil dilation, and have suggested that angle closure could be due to abnormal dynamic behavior of the iris occurring in anatomically predisposed eyes. First, Quigley et al., using anterior segment OCT (AS-OCT), found that the iris cross-sectional area is nearly two times smaller after physiologic or pharmacologic pupil dilation in healthy eyes, and that a lower reduction of the iris cross-sectional area after pupil dilation may be a potential risk factor for angle closure.
13 They hypothesized that the normal iris loses volume in the dark or after pharmacologic pupil dilation, and that eyes with angle closure lose less iris volume on dilation, contributing to irido-trabecular apposition. In addition, we demonstrated previously, using customized software allowing us to estimate the whole iris volume from AS-OCT data, that iris volume increases after pupil dilation in narrow-angle eyes predisposed to acute angle closure, whereas it decreases in healthy open-angle eyes.
14 We suggested that these changes could result from a change in vascular tonus leading to venous outflow decrease and intravascular volume increase. We compared the fellow eyes of patients with acute angle closure with healthy open-angle eyes. By extrapolation, we and others have hypothesized that the behavior of the iris may explain that a small proportion of narrow-angle eyes develop angle closure, whereas the majority of narrow-angle eyes with comparable biometric characteristics do not develop angle closure.
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