Imaging the Iris with Swept Source Optical Coherence Tomography - Impact of Iris Volume Measurement on Primary Angle Closure

Abstract

 

Purpose

 

To measure iris volume and anterior segment parameters using a swept-source anterior segment optical coherence tomography (OCT) and investigate factors associated with iris volume and iris volume change after pupil dilation in eyes with open-angles and primary angle closure (PAC).

 

Methods

 

A total of 73 eyes, including 31 eyes from 21 PAC or PAC-suspect patients, 22 eyes from 14 primary open-angle glaucoma (POAG) patients, and 20 eyes from 13 normal subjects were analyzed as a cross-sectional study. The anterior segment was imaged by the Casia SS-1000 OCT (Tomey, Nogaya, Japan) in room light, dark, and after pharmacological dilation. Iris volume and anterior segment parameters were measured by the instrument software. Linear mixed models were used to examine the association between iris volume and change in iris volume after dilation and each of the following: age, gender, anterior chamber volume (ACV), axial length, pupil diameter, and angle width.

 

Results

 

The mean iris volume significantly reduced from light to dark and after pharmacological dilation in angle closure (40.0±5.2mm³, 38.8±5.4 mm³ and 32.5±4.5 mm³, respectively), POAG (39.6±4.3 mm³, 38.7±4.4 mm³ and 33.4±3.4 mm³, respectively) and normal eyes (39.0±3.9 mm³, 37.0±3.7 mm³ and 30.4±3.3 mm³, respectively). Iris volume was negatively associated with ACV and positively associated with axial length. The change in iris volume/mm change in pupil diameter was 2.11, 2.19 and 2.34 mm³/mm in the angle closure, POAG and normal groups, respectively (p≥0.278). A smaller ACV and an older age was associated with a smaller change in iris volume/mm change in pupil diameter. A larger iris volume, smaller ACV and larger pupil diameter were significant determinants of smaller angle widths (all with p≤0.001).

 

Conclusions

 

The mean iris volume reduced after pupil dilation in eyes with open-angle as well as in eyes with angle closure, although the degree of reduction was less in eyes with a small ACV. Both iris volume and ACV were important determinants of the anterior chamber angle.

 

 

An example illustrating the change in iris volume after pharmacological dilation in an (A) open-angle and (B) angle closure eye.

 

 

Shown above are 12 of 64 total B-scans analyzed, illustrating the automated detection by the instrument software of both anterior and posterior boundaries of the iris and posterior boundaries of the cornea.

 

Keywords: 571 iris • 550 imaging/image analysis: clinical • 421 anterior segment