Assessment of the Scleral Spur in Anterior Segment Optical Coherence Tomography Images

CLINICAL SCIENCES Assessment of the Scleral Spur in Anterior Segment Optical Coherence Tomography Images

Lisandro M. Sakata, MD, PhD; Raghavan Lavanya, DO; David S. Friedman, MD; Han T. Aung, MBBS; Steve K. Seah, FRCS; Paul J. Foster, PhD, FRCS; Tin Aung, MBBS, PhD, FRCS(Edin)

Objective: To assess visibility of the scleral spur in an- Results: Scleral spur location could be determined in 72% terior segment optical coherence tomography (AS-OCT) of the images of the right eye. Its location on AS-OCT im- images. ages was less detectable in quadrants with a closed angle on gonioscopy and also in images obtained in the superior and inferior compared with the nasal and temporal quad- Methods: This cross-sectional observational study in- rants (64%, 67%, 75%, and 80%, respectively; PϽ.001). cluded 502 participants aged 50 years or older who had no previous ophthalmic problems and were recruited from Conclusions: The inability to detect the scleral spur may a community clinic in Singapore. All participants under- hamper quantitative analysis of anterior chamber angle para- went gonioscopy and AS-OCT (Visante; Carl Zeiss Med- meters that are dependent on the location of this anatomi- itec, Dublin, California). Scleral spur location was as- cal structure, particularly in the superior and inferior quad- sessed in AS-OCT images by 2 examiners with glaucoma rants. New parameters independent of the scleral spur may subspecialty training and was defined as the point where be useful for detecting eyes at risk of angle closure. there was an inward protrusion of the sclera with a change in curvature of its inner surface. Arch Ophthalmol. 2008;126(2):181-185

NTERIOR SEGMENT OPTI- METHODS cal coherence tomography (AS-OCT) is a new Study participants were in a subset of consecu- technology for imaging the tive participants of a larger study conducted to anterior segment of the evaluate the performance of new imaging de- eye.A It is capable of obtaining real-time im- vices in screening for angle closure. Those in- ages of the anterior chamber angle and rep- cluded in the larger study were aged older than resents a rapid, noncontact method for the 50 years and were attending a community poly- detection of eyes at risk of angle clo- clinic in Singapore, for nonophthalmic rea- 1-5 sons from January to July 2006. Informed con- sure. As with ultrasound biomicros- sent was obtained from all participants. The copy, the evaluation of the anterior cham- study had the approval of the institutional re- ber angle in AS-OCT depends on view board of the Singapore Eye Research In- determining the location of the scleral spur, stitute and adhered to the tenets of the Decla- which anatomically represents the junc- ration of Helsinki. tion between the inner wall of the trabec- After being asked about his or her medical ular meshwork and the sclera. The scleral and ophthalmic history, each participant un- derwent the following examinations on the Author Affiliations: Singapore spur in anterior segment imaging is same day: visual acuity, AS-OCT (Visante; Carl Eye Research Institute and marked by a prominent inner extension Zeiss Meditec, Dublin, California), anterior Singapore National Eye Center, of the sclera (its thickest part)6-8 and rep- chamber depth and axial length measure- Singapore (Drs Sakata, Lavanya, resents an anatomical landmark for the tra- ments (IOL-Master, Carl Zeiss Meditec), slit- H. T. Aung, Seah, and T. Aung); becular meshwork, which is located ap- lamp biomicroscopy, Goldmann applanation Wilmer Eye Institute and Johns proximately 250 to 500 µm anterior to the tonometry, and gonioscopy. Participants were Hopkins Bloomberg School of scleral spur along the angle wall.6 The aim excluded if they had a history of intraocular sur- Public Health, Baltimore, gery or penetrating trauma in either eye, an- Maryland (Dr Friedman); and of this study was to evaluate the visibility terior segment laser treatment, or glaucoma. Institute of Ophthalmology and of the scleral spur in AS-OCT images and Gonioscopy was performed in the dark in Moorfields Eye Hospital, to assess the effect of this on the assess- all participants by a single examiner masked London, England (Dr Foster). ment of angle closure. to AS-OCT findings. A 1-mm light beam was

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©2008 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 10/01/2021 results (L.M.S. and T.A. working together). The examiners tried to locate the scleral spur on each AS-OCT image. The scleral spur was defined as the point where there was a change in curvature of the inner surface of the angle wall, often appearing as an inward protrusion of the sclera (Figure 1). The anterior chamber angle in a particular quadrant was classified as closed on gonioscopy if the posterior trabecular meshwork could not be seen (Scheie grade III or IV) during nonindentation gonioscopy. A closed anterior chamber angle on AS-OCT was defined by the presence of any contact anterior Figure 1. The location of the scleral spur in an anterior segment optical to the scleral spur between the iris and angle wall. coherence tomography image (arrows). The intraobserver reproducibility in detecting the scleral spur in AS-OCT images was assessed in a random subset of 33 eyes reduced to a very narrow slit, which was offset horizontally for (132 quadrants). Images of the 4 quadrants of the eye were as- assessing superior and inferior angles and vertically for nasal sessed in 2 sessions separated by a 1-week interval. A single and temporal angles. Nonindentation gonioscopy was per- examiner (L.M.S.), masked to the other test results, graded each formed using a Goldmann 2-mirror lens at high magnification quadrant in a dichotomous classification: detectable or unde- (ϫ16) with the eye in the primary position of gaze. Care was tectable scleral spur. In addition, a second analysis aimed to taken to keep the light from falling on the pupil and to avoid evaluate the reproducibility of determining the exact scleral spur accidental indentation during examination. The anterior cham- location in the same 132 AS-OCT images. Its location was plot- ber angle width in each quadrant was determined using the ted in the AS-OCT images using ImageJ software (NIH Im- Scheie grading system,9 which is based on the angle structures ageJ; National Institutes of Health, Bethesda, Maryland). A single visible during the examination. Slight tilting of the gonio- examiner (L.M.S.) obtained the x and y coordinates of the scleral scope lens was permitted in an attempt to gain a view over the spur location in 2 sessions separated by a 1-week interval. The convexity of the iris; the results obtained with this technique distance between the scleral spur (in each particular quad- were used in the analysis. Dynamic gonioscopy with a Suss- rant) was assessed in 78 quadrants, as the examiner could not man 4-mirror lens was also performed. The results of this dy- define its exact location in 54 quadrants. namic examination were not used in the analysis of our study. STATISTICAL ANALYSIS ANTERIOR SEGMENT OPTICAL COHERENCE TOMOGRAPHY Parametric and nonparametric tests were used to compare continuous variables according to data distribution. The ␹2 test was used to compare categorical data. Logistic regression analysis Details of AS-OCT technology have been described previ- was used to assess the factors related to the detectability of the ously.2,3 Briefly, this technology permits image acquisition at a scleral spur on AS-OCT images. For the logistic analysis, only rate of 8 frames/s (2000 A-scans/s) with a transverse resolution 1 quadrant of 1 eye per participant was randomly selected for of 60 µm and an axial resolution of 10 to 20 µm. Furthermore, analysis. The intraobserver reproducibility in determining the the use of wide-field scanning optics (16 mm) and deep axial presence of a detectable scleral spur was assessed using ␬ sta- scan range (8 mm) permit the AS-OCT to image the entire an- tistics.11 PϽ.05 was considered statistically significant. Statis- terior chamber in a single frame. After acquisition, the scanned tical analyses were performed using JMP5 (SAS Institute Inc, images are processed by a customized “dewarping” software, Cary, North Carolina) and MedCalc (MedCalc, Mariakerke, Bel- which compensates for the index of refraction transition at the gium) software. air-tear interface and the different group indices in air, cornea, and aqueous to correct the images’ physical dimensions.10 Participants were examined in the sitting position by a single RESULTS masked examiner (H.T.A.) before any procedure that in- volved contact with the eye. Anterior segment optical coher- A total of 504 consecutive participants were included in ence tomography images of the anterior chamber angle were this study, of whom 274 (54.6%) were women. Most par- obtained in dark conditions using the standard anterior segment single-scan protocol. To obtain the best quality image, ticipants were Chinese (463 [92%]), the rest being Ma- the examiner adjusted the saturation and noise and optimized lay (9 [2%]), Indian (20 [4%]), and other races (12 [2%]). the polarization for each scan during the examination. Three The mean (SD) age was 61.3 (7.6 [range, 51-93]) years. AS-OCT images of the anterior chamber angle of each eye were The mean (SD) anterior chamber depth of the partici- obtained: 1 scanning the angle at the 3- and 9-o’clock posi- pants’ right eyes was 3.12 (0.36) mm. Mean (SD) axial tions, 1 scanning the angle at the 12-o’clock position, and 1 length was 23.71 (3.25) mm. scanning the angle at the 6-o’clock position. Anterior segment optical coherence tomography im- Anterior segment optical coherence tomography image files ages of all 4 quadrants could be obtained in 502 pa- were exported to a personal computer and were evaluated using tients; it was not possible to obtain superior scans in both Microsoft Office Picture Manager software (Microsoft Corp, Red- eyes of 2 patients. The location of the scleral spur on AS- mond, Washington). Adjustments of the images’ contrast and brightness were performed to facilitate image evaluation in an OCT images could be determined in 1439 of 2008 (71.7%) attempt to improve the detection of the scleral spur. quadrants of the right eye and in 1415 of 2008 (70.5%) quadrants of the left eye (P=.44). In a quadrant-by- MAIN OUTCOMES VARIABLES quadrant comparison, the frequency of images with a detectable scleral spur in each of the quadrants of the right Scleral spur visibility in each of the 4 quadrants of the eye was eye was not significantly different from the correspond- assessed in AS-OCT images of both eyes by 2 examiners with ing quadrants of the left eye (Table 1). Further analy- glaucoma subspecialty training who were masked to other tests ses are for the right eye only.

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©2008 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 10/01/2021 Table 1. AS-OCT Images in Which Scleral Spur Location Table 2. Distribution of Eyes by Number of Quadrants Was Detectable in 502 Participants in Which Scleral Spur Location Was Undetectable on AS-OCT Images AS-OCT Images With Detectable Scleral Spurs, Eyes, No. (%) Undetectable Scleral Spur by Quadrant No. (%)

Quadrant Right Eye Left Eye P Value Eyes with undetectable scleral spur in 1 quadrant Superior 60 (17.9) Superior 322 (64) 296 (59) .10 Inferior 52 (15.5) Inferior 338 (67) 328 (65) .81 Nasal 38 (11.4) Nasal 375 (75) 369 (73) .72 Temporal 23 (6.9) Temporal 404 (80) 422 (84) .16 Eyes with undetectable scleral spur in 2 quadrants Total 1439 (72) 1415 (70) .44 Superior/inferior 38 (11.4) Superior/nasal 20 (6.0) Abbreviation: AS-OCT, anterior segment optical coherence tomography. Superior/temporal 13 (3.9) Inferior/nasal 11 (3.3) Inferior/temporal 15 (4.5) Nasal/temporal 7 (2.1) A Eyes with undetectable scleral spur in 3 quadrants Superior/inferior/nasal 18 (5.4) Superior/inferior/temporal 7 (2.1) Inferior/temporal/nasal 9 (2.7) Superior/temporal/nasal 10 (3.0) Eyes with undetectable scleral spur in 4 quadrants Superior/inferior/nasal/temporal 13 (3.9) Total 334 (100)

Abbreviation: AS-OCT, anterior segment optical coherence tomography.

B Table 3. Logistic Regression of Factors Associated With Undetectable Scleral Spur Location on AS-OCT Imagesa

Factor Odds Ratio (95% CI) P Value Right eye 1.18 (0.78-1.79) .42 Female sex 1.34 (0.88-2.05) .17 Age 1.00 (0.29-3.40) .99 Spherical correction 1.54 (0.26-9.81) .64 Figure 2. Anterior segment optical coherence tomography images in which Cylinder correction 1.90 (0.45-7.91) .38 the location of the scleral spur could not be determined. A, The right Open angle on gonioscopy 0.54 (0.33-0.91) .02 quadrant shows a cross-sectional image of the anterior segment where the Quadrant internal surface of the sclera formed a smooth continuous line, with no Temporal 1 [Reference] internal protrusion of the sclera. B, An anterior chamber angle of the superior Superior 2.80 (1.41-5.52) .003 quadrant showing an atypical contour of the inner corneoscleral wall. Inferior 2.00 (1.00-3.96) .046 Nasal 0.43 (0.19-0.90) .03

The visibility of the scleral spur was significantly lower Abbreviations: AS-OCT, anterior segment optical coherence tomography; in the superior quadrant compared with the temporal and CI, confidence interval. nasal quadrants (PϽ.001) and in the inferior compared a R2=0.052. with the temporal and nasal quadrants (P=.01). Most of the cases in which the scleral spur could not be detected occurred in images in which the internal surface of the Logistic regression showed that superior and inferior sclera formed a smooth continuous line (with no in- quadrants as well as quadrants with angle closure (diag- ward protrusion of the sclera or change in its curvature) nosed on gonioscopy) were more likely to have an un- or in images with suboptimal quality. Less frequently, the detectable scleral spur on AS-OCT images (Table 3). scleral spur was difficult to identify owing to an atypical Although the identification of the scleral spur location contour of the inner corneoscleral wall (Figure 2). was not possible in 569 of 2008 right-eye quadrants im- In the analysis by eye, the scleral spur location could aged by AS-OCT, it was still possible to grade the ante- not be determined in at least 1 quadrant of 334 of 502 rior chamber angle status as open or closed in 490 of 569 (66.5%) eyes. In most of these eyes (52%), the scleral spur (86%) of these images (Table 4). Figure 3 shows ex- could not be detected in just 1 of the 4 quadrants; but in amples of quadrants imaged by AS-OCT that were graded 4% of these eyes, its location was undetectable in all 4 as open or closed even though the scleral spur location quadrants. The superior and inferior anterior chamber could not be determined. angles were the locations with the highest rates of un- The intraobserver agreement in detecting the scleral detectable scleral spur for eyes in which the scleral spur spur (132 quadrants) was moderate to substantial in all was not detectable in 1 or 2 quadrants (Table 2). 33 eyes of the random subset (␬=0.65; 95% confidence

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©2008 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 10/01/2021 cess area, and trabecular-iris space area are determined rela- Table 4. Ability to Grade Anterior Chamber Angle (ACA) tive to the scleral spur.6-8,10,12,13 In our study, we observed Status by Quadrant and Scleral Spur Detectability that scleral spur location could not be detected in approxi- mately 30% of the anterior chamber angle images ob- ACA Open or Closed Status, No. (%) tained with the Visante AS-OCT. This information is clini- cally relevant because software algorithms for quantitative No. of Not measurements of the anterior chamber angle rely entirely Quadrant Images Gradable Gradable on the scleral spur as the principal angle landmark. Dif- Detectable scleral spur ficulty in determining scleral spur location in AS-OCT Superior 322 316 (98.1) 6 (1.9) imaging could adversely affect the accuracy and perfor- Inferior 338 337 (99.7) 1 (0.3) mance of future imaging programs that aim to automati- Nasal 375 372 (99.2) 3 (0.8) Temporal 404 404 (100) 0 cally detect this anatomical structure. Total 1439 1429 (99.3) 10 (0.7) The rates of detection were better in AS-OCT images Undetectable scleral spur obtained in the nasal quadrant and worse in those ob- Superior 180 156 (86.7) 24 (13.3) tained in the superior and inferior quadrants compared Inferior 164 134 (81.7) 30 (18.3) with images taken in the temporal quadrant. In addition, Nasal 127 109 (85.8) 18 (14.2) detection was worse in images obtained in quadrants with Temporal 98 91 (92.9) 7 (7.1) Total 569 490 (86.1) 79 (13.9) narrow anterior chamber angles. Although logistic regression analysis is a mere attempt to measure the percentage of variance explained by the model, the R2 value of 0.05 observed in this study suggests that other factors may be A accounting for the limited detection of the scleral spur on AS-OCT, such as technical difficulties in imaging the superior and inferior quadrants (imaging artifacts from eyelids and/or manipulation of the eyelids), anatomical varia- tions between participants and/or the quadrants of the eye, relatively low lateral resolution of the AS-OCT, and other factors related to image acquisition or imaging process- ing (eg, optical correction factors). The worse visibility of the scleral spur in narrow anterior chamber angles may also be because of the proximity of 2 high-reflectivity struc- B tures on AS-OCT imaging: the trabecular meshwork and peripheral iris. However, the scleral spur location could not be determined in many AS-OCT images with an open anterior chamber angle (251 quadrants). Notably, manipulation of the image contrast and/or brightness could not improve visibility in most of the images. While the interpretation of anterior chamber angle images obtained with ultrasound biomicroscopy is also dependent on the identification of the scleral spur, no study Figure 3. Anterior segment optical coherence tomography images in which the anterior chamber angle could be graded as open or closed, though the location has directly addressed this issue. In a previous study that of the scleral spur could not be determined. A, An open anterior chamber angle compared ultrasound biomicroscopy with a prototypical with a wide open angle. B, A clearly closed anterior chamber angle. version of the AS-OCT, the authors stated that scleral spurs were more distinct in AS-OCT images, though no further information was provided.3 In our previous study,1 when interval, 0.52-0.78). In the assessment of the reproduc- the anterior chamber angle was imaged using a prototypi- ibility of the exact scleral spur location, the distance be- cal AS-OCT device, the visibility of the scleral spur seemed tween the scleral spur localized in the same image across better than in images obtained with the Visante AS-OCT. 2 sessions was within 10 µm in 83% of the 78 quadrants Although this difference in performance was not evalu- assessed and within 20 µm in 90% (median difference, 4 ated formally here, we believe that better resolution of the [range 0-69] µm). Fifty-four quadrants were excluded AS-OCT is in part attributable to higher sampling of the from this analysis, as the exact location of the scleral spur anterior chamber angle images with the prototypical de- was considered undetectable. vice. The prototype used frame averaging of 3 images of the anterior chamber angle, whereas the Visante AS-OCT COMMENT relies on a single image. Based on feedback we (and prob- ably other users) provided, the manufacturers of the de- Scleral spur location represents an important anatomical vice have introduced a new algorithm for angle imaging landmark in imaging the anterior chamber angle, as it is a that also uses averaging of 3 anterior chamber angle im- reference point for the relative position of the trabecular ages. It will be interesting to evaluate whether this new meshwork. The scleral spur is also used as a landmark for algorithm will improve the assessment of the scleral spur. quantitative measurements of the anterior chamber angle; Notably, the Visante AS-OCT has a high-resolution scan parameters such as the angle-opening distance, angle re- protocol, which permits the acquisition of cross-sectional

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©2008 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 10/01/2021 images of a single anterior chamber angle at a time. While vature of the inner (posterior) surface of the sclera, it is possible that the visibility of the scleral spur may be there was no reference standard to confirm where this better with this high-resolution scan, the images obtained structure is actually located in the eye. with this scan are not corrected for the different indices of In summary, our study showed that almost 90% of the refraction of the air, cornea, and aqueous humor (ie, not Visante AS-OCT images could be qualitatively graded by “dewarped”) and so were not used in this study. 2 glaucoma specialists (working together) as having either The intraobserver reproducibility in determining the an open or closed anterior chamber angle, though the lo- presence of a detectable scleral spur in this study was sub- cation of the scleral spur could not be determined in ap- stantial. Its exact location could not be determined in 41% proximately 30% of images. This problem of determining of the quadrants included in the reproducibility analy- the scleral spur location may hamper quantitative evalu- sis; however, among the remaining quadrants where it ation of anterior chamber angle parameters and the de- was considered detectable, the distance between scleral termination of eyes at risk for closure. New anterior cham- spur locations (determined in 2 independent sessions) ber angle parameters for assessing angle closure that are was within 10 µm in 83% of the quadrants. It is impor- independent of locating the scleral spur may improve the tant to note that the effect of the variability of the scleral performance of AS-OCT imaging in detecting eyes at risk spur location in measuring anterior chamber angle para- of angle closure. meters (such as angle-opening distance) was not evaluated here; it will be the subject of future studies. Submitted for Publication: April 4, 2007; final revision Although the location of the scleral spur could not be received July 6, 2007; accepted July 15, 2007. determined in 30% of AS-OCT images, the presence or Correspondence: Tin Aung, MBBS, PhD, FRCS(Edin), absence of angle closure could still be assessed qualita- Singapore National Eye Center, 11 Third Hospital Ave, tively in more than 90% of eyes evaluated. Some eyes had Singapore 168751 ([email protected]). deep and open angles with the iris base clearly going into Financial Disclosure: Carl Zeiss Meditec loaned the the ciliary body. In these cases, the examiners who graded AS-OCT for the study and provided technical support. Dr the AS-OCT images understood that scleral spur loca- T. Aung has received financial support and honoraria for tion was not necessary for diagnosis of an open angle. travel to conferences from Carl Zeiss Meditec and Dr Similarly, some angles seemed clearly closed, as they Friedman has acted as a consultant to Carl Zeiss Meditec. showed a large area of iris contact to the angle wall an- Funding/Support: This study was supported by an unre- terior to the insertion of the iris. Only 10% of images could stricted grant from Singhealth Foundation, Singapore. not be classified by the examiners owing to lack of visu- alization of the scleral spur. It is important to consider REFERENCES that the results of the open/closed status of the anterior chamber angle in images with an undetectable scleral spur 1. Nolan WP, See JL, Chew PT, et al. 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