By: Dr Tin Aung
Dynamic indentation gonioscopy remains the current reference standard for assessing the anterior chamber angle, but a variety of recently introduced objective imaging techniques may become useful adjuncts in clinical care, said Dr Tin Aung, PhD.
"The new technologies, which include methods of crosssection angle imaging using optical coherence tomography (OCT) and angle photography, are all easy to perform and each has unique advantages," said Dr Aung, consultant ophthalmologist, Singapore National Eye Centre, and associate professor of ophthalmology, National University of Singapore, Singapore.
"While each of these modalities has disadvantages as well, [we hope] they will lead to exciting improvements in angle imaging.
"After all, we use a variety of imaging methods in [the care of patients with] glaucoma... for assessing the optic disc, retinal nerve fibre layer, and corneal thickness," he
"Perhaps we should also include angle assessments among our imaging techniques in glaucoma."
A role for objective imaging techniques for angle assessment is underscored by consideration of the limitations of gonioscopy.
Its performance requires training, interpretation is subjective and observerdependent, and the findings vary depending on room lighting conditions, lighting on the pupil, and the type of lens used.
In addition, recording of the gonioscopy findings by some physicians is less than optimal, Dr Aung
"However, the problem with using ASOCT for diagnosing angle closure is that one has to identify contact between the iris and the trabecular meshwork, but the trabecular meshwork is not visible in most images," Dr Aung
"Therefore, one must use the scleral spur, which lies behind the trabecular meshwork, as a landmark in both qualitative and quantitative assessments.
Angle closure is determined by identifying contact between the iris and angle wall anterior to the sclera spur."
A study performed by Dr Aung
and colleagues comparing ASOCT and gonioscopy for diagnosing angle closure demonstrated that the imaging technology had high sensitivity (98%) but lower specificity (54.4%) because its use resulted in diagnosis of angle closure in some patients who were considered to have an open angle on gonioscopy.
"Possible reasons accounting for the discrepancy between gonioscopic and ASOCT diagnosis include differences in the lighting conditions used for the two examinations, distortion of the angle configuration when performing gonioscopy, and use of different landmarks for the two methods," Dr Aung
In addition to its inability to visualize the trabecular meshwork in most cases, other disadvantages of ASOCT include the fact that scleral spur definition is difficult in up to 30% of images overall, and according to a study by Dr Aung
and colleagues, at an even higher rate when assessing the inferior and superior quadrants.
Dr Tin Aung
Articles by Dr Tin Aung