OCT imaging has improved the diagnostic performance to detect angle closure. Defining a closed anterior chamber angle as the presence of any contact between the iris and angle wall anterior to the scleral spur, Sakata et al25
showed the Visante OCT detected a closed angle in at least one-quadrant in 59% of the eyes but only 33% by gonioscopy in a community clinic. In all, 71% of cases with closed angles on OCT but open angles by gonioscopy had a short irido-angle contact. The higher proportion of quadrants identified as closed angles on OCT imaging is likely related to the better visualization of the peripheral iris in contact with the trabecular meshwork. Half of the cases with open angles on OCT but closed angles by gonioscopy were found to have a steep iris profile. A false impression of angle closure may result in these eyes by gonioscopy and tilting of the goniolens often is needed to maximize the view of the angles.
OCT imaging can provide mechanistic insights into the pathophysiology of angle closure. The change from a forward bowing to a flattening configuration of the iris after laser iridotomy in eyes with angle closure demonstrates the relief of relative pupillary block and elimination of pressure gradient between the anterior and posterior chambers.3, 4, 5, 6
Widening of the angles has been consistently reported after lens extraction signifying the role of aging lens in contributing to angle closure and supporting lens extraction as a treatment option in management of primary angle closure glaucoma.26, 27, 28, 29
The measurement of the scleral spur to scleral spur distance with OCT, or the anterior chamber width, has been recently shown to be a risk factor of angle closure.30
Leung et al31
compared the anterior chamber width, anterior chamber depth, iris thickness, and angle width measured with an anterior segment OCT between Chinese and Caucasian eyes. In agreement with the finding from an epidemiology study,32
the axial length was not significantly different between the two ethnic groups. More important, the anterior chamber width was found to be shorter in Chinese than in Caucasian eyes even after adjustment of the anterior chamber depth. In other words, the iris-lens diaphragm is more anteriorly located in Chinese. This finding might explain in part the ethnic differences in development of primary angle closure and angle closure glaucoma.
In addition to the anterior chamber width, measurement of the iris has also shown to be important in determining the risk of angle closure.33, 34, 35, 36, 37
Aptel and Denis34
estimated the iris volume by capturing four cross-sectional images of the anterior segment at 45°-intervals. They showed that iris volume increased from ~45
after pharmacologic mydriasis in the fellow non-attacked eyes of patients with history of acute primary angle closure. In contrast, in normal eyes with open-angles, iris volume decreased from ~44
after pupil dilation. It has been proposed that differences in iris connective tissue and permeability of the iris stroma to aqueous may account for the differences in volume change in eyes with closed angles and open angles.38
The expansion in iris with dilation may block aqueous drainage at the angle and predispose to angle closure. Wang et al36
measured iris curvature, iris area, and iris thickness with the Visante OCT and showed that these parameters were independently associated with narrow angles. Cheung et al
studied the dynamic changes of iris configuration with real-time video capture using the Visante OCT. They found the iris in eyes with narrow or closed angles consistently remained in an anteriorly convex configuration in both light and dark conditions and that the iris curvature is an important determinant of the angle width. With the advent of the spectral-domain and swept-source OCT imaging systems, iris volume and iris configuration can be quantified in more details. This information would be valuable in studying iris dynamics in relation to development of primary angle closure.