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Editors Selection IGR 21-3

Examination methods: AS-OCT and angle illumination

Comment by Tin Aung on:

19602 Dynamic analysis of dark light changes of the anterior chamber angle with anterior segment OCT, Leung CK; Cheung CY; Li H et al., Investigative Ophthalmology and Visual Science, 2007; 48: 4116-4122

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Gonioscopy is the current reference standard for angle assessment and a key component in the clinical examination of glaucoma patients. However, gonioscopy is difficult to perform in a reproducible fashion and angle assessment may be affected by many variables, such as inadvertent pressure on the cornea, and by light exposure onto the pupil during the examination. Previous studies have shown that even experienced examiners demonstrated only moderate agreement in determining angle width. In this interesting study by Leung et al. (942) the new non-contact anterior segment OCT (AS-OCT) system was used to evaluate changes in the angle in response to illumination. The dynamic dark-light changes were captured with real time video recording and serial images were analyzed to investigate changes in angle opening distance (AOD) and trbecular iris space area (TISA) in relation to pupil size. The study found increases in the AOD and TISA with illumination and decrease in these values with increasing pupil diameter. However, these differences varied substantially among individuals. The study results confirm the variability of gonioscopy findings induced by changes in illumination and emphasizes the need to standardize the background light and to minimize the amount of light entering the pupil when performing gonioscopy. AS-OCT was able to detect small differences in the angle in response to light stimuli, making AS-OCT a useful tool in assessing subtle changes in the angle.

There is a need to standardize the background light and to minimize the amount of light entering the pupil when performing gonioscopy

However, image processing with the described angle analysis software requires the operator to accurately define the position of the scleral spur on the image before the angle variables can be calculated. This step introduces some variability. In some eyes, the scleral spur can be difficult to identify with AS-OCT. Only the nasal quadrants were analyzed in the study as imaging of the superior and inferior quadrants may be difficult to perform with AS-OCT. The video images were not 'dewarped', a process which compensates for the index of refraction transition at the air-tear interface and the different refractive indices in air, cornea, and aqueous. There is a need to standardize the background light and to minimize the amount of light entering the pupil when performing gonioscopy.

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