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Light to dark physiological variation in irido-trabecular angle width

Gus Gazzard1,2,3  Paul J Foster1,2,3 D.S. Friedman4Peng T Khaw2,3 Steve KL Seah1


[1] Singapore National Eye Centre & Singapore Eye Research Institute
[2] Institute of Ophthalmology, University College London, England
[3] Glaucoma Research Unit, Moorfields Eye Hospital, London, England
[4] John Hopkins University, England



Correspondence to: Dr Steve K.L. Seah, Glaucoma Service, Singapore National Eye Centre, 11 Third Hospital Avenue, Singapore 168751. Fax: (65) 2277290
Email:snecss@pacific.net.sg


Accepted for publication:  August 1, 2004

 

 

Ultrasound biomicroscopy (UBM) is performed on a 70 year old Chinese woman with a history of episodic angle closure OD. Here, the asymptomatic fellow (left) eye is examined over three light-to-dark cycles. The iris configuration varies widely depending on illumination; the angle is seen to be wide open in light-on conditions, but changing to dark conditions rapidly results in appositional closure. The effect is reversible and reproducible through three light-to-dark cycles over 50 seconds.

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Introduction

The diagnosis of angle-closure is typically made on the basis of careful gonioscopic examination. Great emphasis has been placed on differentiating cases of angle-closure glaucoma from those with narrow but open angles. In clinical practice the division between these two entities is largely subjective, and dependent on the experience of the examiner. In epidemiological research, broad consensus has developed to make the diagnosis of angle-closure only if the posterior (usually pigmented) trabecular meshwork is seen for less than 90 to 180 degrees of the angle circumference. This is termed an "occludable angle". In two population-based studies of East Asian people, among whom angleclosure has a relatively high prevalence, we have shown that, while the odds of PAS were higher in people with narrower angles, there was a greater absolute number of people with primary PAS whose drainage angles were classified as "not occludable" than those classified "occludable". This suggests that the widely used epidemiological convention used to identify the probable mechanism of glaucoma excludes many people with angle-closure. The received wisdom is that careful gonioscopy can differentiate primary open-angle glaucoma (POAG) with narrow angles from primary angle-closure glaucoma (PACG). In light of emerging data, it is possible that current attitudes may be biased against the diagnosis of glaucoma with an element of angle-closure. It seems highly likely that definitional conventions under-emphasise the role of angle-closure in population surveys of glaucoma prevalence in Asian people.[1]

Observations

We present a video of a 70 year old Chinese woman undergoing ultrasound biomicroscopy (UBM) examination of the left eye in light and dark conditions, while in a supine position. She presented with an episode of symptomatic angleclosure in the right eye. There were no symptoms reported from the left eye, which at the time of this video was receiving no topical medication. The recording was made prior to prophylactic laser iridotomy. The sequence shows three complete light-to-dark cycles over a period of 50 seconds, with direct torch illumination into the contralateral eye. There are dramatic changes in the iris configuration depending on the illumination. With lights off, there was only diffuse, indirect illumination from the monitor of the UBM machine. In light conditions, the angle is wide open with a moderate anterior iris convexity. There is a zone of apparent contact between iris and lens at the pupil margin. The iris inserts into the mid-point of the anterior surface of the ciliary body, with a wellformed ciliary sulcus. There appears to be anterior rotation of the ciliary body attributable to crowding of the cilio-lenticular space, resulting in a moderate “plateau” configuration in the light.

In the dark, a pronounced increase in iris convexity accompanies limited pupillary dilatation, resulting in appositional closure anterior to Schwalbe’s line. Mapstone’s sulcus, an aqueous-filled sinus between iris and trabecular meshwork, is visible posterior to the area of appositional contact. There is a particular increase in mid-peripheral iris thickness occurring with pupillary dilatation. The most important and striking findings are the magnitude and rapidity of variation in drainage angle-width that accompanies changes in lighting conditions. The angle would be graded open by gonioscopy under photopic conditions, yet clearly closes in near darkness.


Comments

This video sequence illustrates the dynamic nature of the angle-closure process, and highlights diagnostic and classification dilemmas which must be addressed if we are to fully understand the aetiology of raised intraocular pressure (IOP) and development of glaucomatous optic neuropathy. We have previously described how linear (angle-opening distance at 250 and 500 microns from scleral spur) and area-related (angle-recess area) indices of angle width all increased significantly following iridotomy in fellow eyes of Asian people suffering symptomatic angle-closure. Increased illumination increased the angle measures, but induced a different configurational change in peripheral iris morphology. The illumination-induced changes were greater after iridotomy than before laser treatment. This led us to conclude that the mechanisms responsible for angle-closure in the dark are different, at least in part, from those associated with pupil-block.[2]

Provocative testing for angle-closure has gradually fallen into disuse. In 1967, Lowe summarized the methods of identifying people at risk of angle-closure, stating that “Present provocative tests are too inaccurate, too time consuming, or too dangerous for symptom-free eyes found to have shallow anterior chambers and narrow angles on routine examinations”. Instead, he recommended careful history taking and clinical examination.[3] Previously, provocative testing for angleclosure relied on changes in IOP following exposure to stimuli some of which were physiological (darkness or prone posture) and others not, such as pharmacological agents (often pilocarpine and phenylephrine). The rapid development in ocular imaging technology gives us much greater scope to conduct dynamic examinations of the angle-closure process. The use of an (arbitrary) IOP rise as a test end-point is no longer necessary. Imaging devices allow us to observe, in real time, the variations in anatomy that accompany changes in the environment. Given our current level of understanding, exposure to darkness and changes in posture would be important, common, physiological stimuli capable of inducing angle-closure. The UBM is a useful tool for assessing the effect of reduced lighting on angle width. Problems relating to the need for a water-bath or low-viscosity coupling medium make postural changes possible,but difficult, to examine.

Our findings illustrated here, and reported previously,[1] suggest that the current concern for carefully differentiating cases of angle-closure glaucoma from those with open but narrow angles may actually be an exercise in self-deception. We believe it is important for ophthalmologists treating patients with glaucoma to strongly consider whether there a possibility of angle-closure when angles are seen to be “narrow but open” using conventional gonioscopy. Gonioscopy should undoubtedly be carried out initially under very low lighting levels, at high magnification, with a very small (1 mm) spot of light reduced to a narrow beam. Light entering the pupil should be avoided. However, there is clearly a need to develop a rapid, non-contact method of imaging the angle in different levels of illumination and different postures. The challenge then would be to validate the current recommendation from experts in angle-closure management that any degree of appositional closure justifies at least a laser iridotomy, possibly with iridoplasty or topical pilocarpine, if closure persists.
 

Acknowledgements

This work was supported by a grant from the National Medical Research Council of Singapore. Professor Khaw is funded by the Medical Research Council (UK), grant number G9330070.


References

1. Foster PJ, Nolan WP, Aung T, Machin D, Baasanhu J, Khaw PT et al.Defining "occludable" angles in population surveys: Drainage angle width, peripheral anterior synechiae and glaucomatous optic neuropathy in East Asian people. Br J Ophthalmol 2004;88:486-90.

2. Gazzard G, Friedman DS, Devereux JG, Chew PT, Seah SK. A prospective ultrasound biomicroscopy evaluation of changes in anterior segment morphology after laser iridotomy in Asian eyes. Opthalmology 2003;110:630-8.

3. Lowe RF. Priamry angle-closure glaucoma. A review of provocvative tests. Br J Ophthalmol 1967;51:727-32.