Ultrasound Biomicroscopy Dark Room Provocative Testing:: A Quantitative Method for Estimating Anterior Chamber Angle Width☆,☆☆
Introduction
Eyes with appositional angle closure are at risk for progressive trabecular damage, elevated intraocular pressure (IOP), peripheral anterior synechiae, and acute angle-closure glaucoma. It is generally agreed that eyes with occludable iridocorneal angles should undergo laser iridotomy.1, 2
Gonioscopy has been the most important step in assessing the iridocorneal angle. Accurate assessment of the potential for occlusion requires gonioscopy in a completely darkened room to allow the pupil to dilate physiologically. Nevertheless, being a subjective examination, gonioscopy does not guarantee determination of whether an angle is occludable, and even glaucoma subspecialists have disagreed as to the grading of a particular angle and its occludability.3 To compensate for this, a variety of provocative tests have been described for the detection of potential appositional angle closure in patients with normal vision and asymptomatic narrow angles.
Nonpharmacologic tests have been regarded as more accurately reproducing natural conditions than those that involve stimulating the sphincter and dilator muscles. Dark-room and prone dark-room provocative tests, considered the most physiologic of the provocative tests, have been much less frequently used since the advent of laser iridotomy. These tests require a dedicated room, and still rely on gonioscopy. An objective test for angle occludability would be useful and desirable.
With high frequency ultrasound biomicroscopy (UBM), high resolution imaging of the anterior segment in vivo can be attained, and this method is ideally suited for evaluation of the anatomy and pathophysiology of anterior segment diseases.4 In 1995, Pavlin et al reported the use of UBM as a helpful method in dark-room provocative testing to demonstrate angle occludability in a small series of patients.5 Sakuma et al6 performed UBM under dark-room conditions and reported two anatomic patterns of appositional angle closure. They subsequently found the topology of the iris root to be related to the pattern of the appositional angle closure.7
We have developed a software program to measure the iridocorneal angle recess area quantitatively, taking into account the topographic configuration of the anterior iris surface. We have used this program to evaluate factors associated with appositional angle closure during UBM dark-room provocation testing.
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Materials and Methods
All patients with clinically narrow angles referred for UBM dark-room provocative testing at the New York Eye and Ear Infirmary between September 1996 and March 1998 were enrolled in this study. We defined clinically narrow angles as those angles regarded by the examiner as being grade 1 or 2 using the Shaffer grading system. Patients who had undergone intraocular surgery, including laser treatment; who used topical drugs affecting pupillary diameter; and who had a history of ocular diseases
Results
All 178 eyes tested had anatomically open angles during UBM under light conditions. During dark-room provocation, 99 eyes (55.6%) were occluded (Group I) and 79 eyes (44.4%) were not (Group II eyes) (Table 1). Age, race, gender, refractive error, and axial length were similar in both groups.
ARA and y-intercept b were significantly smaller in group I eyes than in group II eyes under light conditions (Table 2). The difference in acceleration a between the two groups under light conditions did not
Discussion
A predictive and objective provocative test to elicit angle closure has been a long-sought goal. Elimination of the potential complications of intraocular surgery, particularly cataract formation, has created a de facto liberalization of criteria for performing iridotomy.1 Nevertheless, laser iridotomy is not entirely without complications.9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33
To perform laser iridotomy in all eyes with narrow angles
Acknowledgements
Supported in part by the Glaucoma Foundation, New York, NY; the New York Eye and Ear Infirmary Department of Ophthalmology Research Fund, New York, NY; and the New York Glaucoma Research Institute, NY, USA. Dr. Ishikawa is the recipient of the David Warfield Fellowship in Ophthalmology of the New York Community Trust and The New York Academy of Medicine.
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Presented in part at the Annual Meeting of the Association for Research in Vision and Ophthalmology, Fort Lauderdale, FL, 1996 and 1998.
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The authors have no proprietary interest in any device discussed in this article.