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Video Report

Physiological changes of the angle with anterior segment OCT

Preston Eye Clinic, Melbourne, Australia

Correspondence: Dr Lance Liu
Email: drlsliu{at}hotmail.com Preston Eye Clinic, 268 Murray Rd, Preston, Australia 3072; Fax: +613 9478 0289

Date of acceptance: 29th December 2006

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Introduction

Open angle and angle closure glaucoma co-exist in the same populations with angle closure being more common amongst Asian people. The diagnosis of the type of glaucoma is based on gonioscopic findings – a subjective examination depending on the experience of the examiner and the type of lens and technique used. In clinical practice, gonioscopy is usually poorly performed, its interpretation can be variable and is usually not repeated.

To add to the subjective problems of gonioscopy, Gazzard et al.[1] recent ultrasound biomicroscopy video shows the physiological variation of the drainage angle in different illuminations in an Asian person. In the light, angles that look open or narrow can actually become closed in the dark. With the advent of newer technologies, like optical coherence tomography (OCT), we now have a method of objectively assessing the angle by non-contact means.

Observation

This video is of a 67-year-old Caucasian woman undergoing an OCT examination of the anterior segment of the right eye in light and dark conditions whilst in a supine position. She presented symptomatic angle closure with episodes of headaches around both eyes. Images were taken along the 0 to 180 degree axis. The video of the screen of the Visante® (Carl Zeiss, Germany) was taken prior to the installation of any topical medication or surgical intervention. The examination took place in a dark room with direct illumination of the examined eye using a penlight torch.

The video shows changes of the iris configuration and drainage angle depending on the type of illumination. In the light, the pupil is physiologically constricted and the height of the anterior border of the iris is level with Schwalbe’s line. Anatomically, the angle is open with the presence of the ciliary sulcus between the trabecular meshwork and the adjacent peripheral iris. The iris inserts into the anterior surface of the ciliary body, located near the lens equator, and is pushed forward by a large lens, resulting in a 'pseudo-plateau iris' type of configuration. There is contact between the iris and the lens at the pupil margin.

As the pupil becomes physiologically dilated in the dark, there is forward bowing and thickening (especially peripherally) of the iris causing it to move anterior. Now the angle is closed as iridotrabecular contact (ITC) is seen with loss of the ciliary sulcus. An aqueous sinus between the iris and trabecular meshwork (Mapstone’s sulcus) is just visible posterior to the area of ITC. The most important feature is the speed and change of the presence and absence (due to ITC) of the ciliary sulcus in different light settings. If one were to grade this angle by gonioscopy, the angle would be open in the light and closed in the dark.

Comments

This video illustrates the multi-mechanism aetiology of angle closure glaucoma[2] and the physiological changes of the angle in different lighting conditions in a Caucasian patient. It illustrates a 'pseudo-plateau iris configuration' of the iris in light conditions, pupil block with forward bowing of the iris in a dark environment, and a phacomorphic component. As the pupil becomes physiological dilated, the probability of ITC would depend on the height of the iris plane relative to the trabecular meshwork.

ITC is now being recognised as a risk factor for developing angle closure glaucoma[3]. This can lead to appositional or synechial closure. The rise in intraocular pressure (IOP) can be due to either a physical blockage of the aqueous to the trabecular meshwork and/or uveoscleral pathway by the peripheral iris or damage to the trabecular meshwork itself. This rise in IOP increases the risk of the patient developing a glaucomatous optic neuropathy.

This leads to another question - how does one define a closed angle? The current definition of being unable to see the posterior trabecular meshwork for 270 degrees is probably too strict and recent epidemiological studies suggest between 90 and 180 degrees of ITC may be more sensitive in detecting angle closure[3]. Given the subjective nature of gonioscopy and the cyclical nature of the angle, angle closure may be under diagnosed and the patient might be under treated.

The findings illustrated here, and those that have been reported previously[1], suggest that gonioscopy in a darkened room should be performed in patients of all ethnic populations to look for ITC. However, the advantage of the anterior segment OCT is that it is non-contact, allows an objective assessment of, in real time, the changes in the anatomy of the angle and can image up to 4 areas of the eye simultaneously. Unlike ultrasound biomicroscopy, it does not require a water bath or coupling fluid. In the future, one needs to look at the frequency of ITC in patients who are glaucoma suspects or who have 'open-angle' glaucoma. Long-term prospective studies are then needed to determine the efficacy of laser iridotomy or iridoplasty in these patients.

References

• Gazzard G, Foster P, Friedman D et al. Light to dark physiological variation in iridotrabecular angle width: Br J Ophthalmol Video Website, 2004.
• Ritch R, Lowe R. Angle-Closure Glaucoma: Mechanisms and Epidemiology. In: Ritch R, Shields M, Krupin T, editors. The Glaucomas, 2nd ed. St. Louis: Mosby, 1996:801.
• Foster P, He M, Liebmann J. Epidemiology, Classification and Mechanism. In: Weinreb R, Friedman D. Angle Closure and Angle Closure Glaucoma. The Hague: Kugler Publications, 2006:4.