Static anatomic risk factors do not fully explain the occurrence of ACG in patients with narrow angles. Many observations suggest that physiological risk factors are involved. Recently, some have suggested that the dynamic responses of the eye during pupil dilation, particularly iris changes, play a significant role in angle-closure pathogenesis. Quigley was one of the first to support this new concept. He recently demonstrated that iris cross-sectional area decreases less after pupil dilation in narrow-angle eyes, and that a smaller iris cross-sectional area change after pupil dilation may be a potential risk factor for angle closure. We developed a customized software method that allows measurement of the 3-dimensional volume of the iris from AS-OCT data (see figure 1).
Static anatomic risk factors do not fully explain the occurrence of ACG in patients with narrow angles
In our study, we found that iris volume increases after pupil dilation in narrow-angle eyes predisposed to acute angle closure, whereas it decreased significantly in the open-angle eyes. A greater iris volume increase was associated with angle closing despite the presence of a patent laser iridotomy. We used two multivariate models to determine factors related to change in iris volume with pupil dilation. The first model was constructed using relative change in iris volume as a dependent variable and diagnosis (fellow vs open-angle eyes), pupil size, iris color (brown vs others), axial length, anterior chamber diameter (spur to spur), age, sex, and refractive error as predictors. Because pupil dilation was significantly greater in the fellow eyes group (+ 3.30 mm) than in the open-angle eyes group (+ 2.78 mm), we constructed the second model using the ratio of change in iris volume to change in pupil diameter as dependent variable. The variables tested were diagnosis (fellow vs open-angle eyes), iris color (brown vs others), axial length, anterior chamber diameter (spur to spur), age, sex, and refractive error. In the narrow-angle eyes predisposed to angle-closure, we found that iris volume increased relatively linearly with increasing pupil diameter. For each millimeter increase in pupil size, iris volume increased of about 13.4 mm3. By contrast, the volume decreased with increasing pupil diameter in the open-angle eyes (9.6 mm3, per millimeter). It should be noted that axial length and refractive error were not significantly associated with iris volume change after pupil dilation in the multivariate analysis. The biometric and pupil size differences between the two groups therefore do not explain the disequilibrium behavior of the iris found in our study.
This differential response of the iris to pupil dilation may indicate a particular behavior of the iris stroma, especially a different extracellular fluid transfer, vascular tonus change, or both with pupil dilation, and is likely involved in the development of angle closure during mydriasis or semimydriasis. The findings were done in narrow-angle eyes predisposed to angle closure, not in asymptomatic narrow-angle eyes. We can assume that increase in iris volume could be one of the reasons leading a small number of narrow-angle eyes to develop an acute angle-closure, whereas the majority of them will not develop it. As suggested by our colleagues, it would be interesting to repeat the study in chronic ACG or in anatomically narrow angles without AC. We can also assume that asymptomatic narrow-angle eyes would have a lower increase in iris volume with pupil dilation than fellow eyes of acute angle closure. Similarly, iris behavior could explain that Asians are more likely to develop angle closure that Caucasians despite comparable biometric characteristics. Further longitudinal studies have to evaluate if iris volume change after pupil dilation is a significant predictor of angle-closure development. In this case, iris volume measurements using AS-OCT will become a good method to select which patients merit iridotomy among narrow-angle suspects. Iris volume measurement software could be easily integrated to the commercially available AS-OCT devices.