Comparison of dynamic changes in anterior ocular structures examined with anterior segment optical coherence tomography in a cohort of various origins

Purpose. To identify risk factors associated with primary angle closure (AC) using anterior segment optical coherence tomography (ASOCT) measurements of the iris and to determine if these risk factors differ according to geographic origin. Methods. Anterior segment OCT images were collected on 267 persons (eyes) whose family origin was determined by a standardized method. In the 257 eyes with pupil diameter increase in the dark of 0.5 mm or more, findings were compared between bright light conditions and those in a dark room. In 130 eyes, comparison was made after pharmacological pupil dilation. After marking the position of the scleral spur, an automated program quantified many angle and iris parameters, with use of a manual method for a minority that the software could not analyze. Results. Iris area in bright light was larger with increasing age (univariate regression, P = 0.0005), largest in European and African-derived, and smallest in Korean and Chinese eyes (multivariable regression, P = 0.0001), and was significantly larger in AC groups compared with normal and open angle glaucoma groups (univariate regression, P < 0.0001). The absolute iris area loss per mm pupil dilation was significantly less in Chinese persons than African-derived persons (multivariable regression, P < 0.05 adjusted Tukey). Furthermore, in persons with past acute AC attack, the baseline iris area was not different from others, but their iris area lost per millimeter dilation was significantly less than in persons without past acute AC attack (multivariable regression, P = 0.04). The odds of AC disease significantly increased in eyes with smaller percent iris area lost and percent iris area lost per millimeter pupil increase, but when adjusted for geographic origin, this was significant only in persons of Chinese origin (interaction regression model). Apparent gain of iris volume on pupil dilation, due to an artifact in calculation from iris area loss, may indicate a detrimental shift in iris tissue toward the angle. Conclusions. Chinese persons in this cohort had relatively low baseline iris area, but less loss of iris area on pupil dilation than other groups, a feature also associated with greater prevalence of past acute AC attack. Disproportionate peripheral redistribution of iris area on dilation may contribute to AC.