From the Hamilton Glaucoma Center (SD, LA, GV, FM, RNW), University of California, San Diego, La Jolla, California; the Department of Ophthalmology and Visual Sciences (SD), University of Kentucky, Lexington, Kentucky; and the Department of Ophthalmology and Visual Sciences (GV), University of Texas Medical Branch, Galveston, Texas.
Presented as a poster at the Association for Research in Vision and Ophthalmology annual meeting, May 3–7, 2009, Ft. Lauderdale, Florida.
The authors have no financial or proprietary interest in the materials presented herein.
Address correspondence to Sunil Deokule, MD, Department of Ophthalmology and Visual Sciences, University of Kentucky, E-328, Kentucky Clinic, S Limestone, Lexington, KY 40536. E-mail: email@example.com
Anterior segment optical coherence tomography (AS-OCT) is a non-contact method of imaging the anterior segment and is increasingly being used to evaluate and measure anterior chamber angle parameters.1–6 The angle parameters produced by this imaging technology have been shown to be reproducible.7–9 Visante AS-OCT (Carl Zeiss Meditec, Dublin, CA) is one such commercially available machine. Although fast and easy to use, the built-in Visante AS-OCT software is limited by the image quality, horizontal resolution, and subjective user input.10,11 In addition, the instrument relies on the operator to properly identify the scleral spur to obtain angle measurements. A new scan acquisition protocol is available that enables averaging of four images for a single location (enhanced scan). This enhanced scanning protocol is deemed to improve the signal-to-noise ratio compared with a single image (unenhanced scan) that was available originally, therefore allowing for an easier identification of the scleral spur. It is not known whether this change in scan acquisition protocol results in a more accurate and reproducible angle assessment.
The purpose of this study is to evaluate the reproducibility and intraobserver and interobserver agreement of anterior chamber angle measurements acquired using enhanced and unenhanced anterior chamber angle imaging protocols for Visante AS-OCT.
Patients and Methods
All subjects were consecutively recruited at the Hamilton Glaucoma Center, University of California San Diego. All subjects had normal ophthalmic examinations with no history of glaucoma, eye disease, prior laser therapy, raised intraocular pressure, or intraocular surgery. Informed consent was obtained from all subjects and the University of California San Diego Institutional Review Board approved all protocols. The methods described adhere to the tenets of the Declaration of Helsinki. Health Insurance Portability and Accountability Act authorization forms were obtained from all participants.
The details of AS-OCT imaging technology have been described previously.8 All subjects were imaged using the Visante AS-OCT. Images of nasal and temporal angles (3- and 9-o’clock meridians) were acquired using radial scans with a scan length of 16.0 mm. All images were acquired with subjects in the sitting position using an unenhanced imaging protocol followed by an enhanced imaging protocol (Fig. 1). Each scan was centered on the undilated pupil and was accepted when the corneal reflex was visible as a vertical white line along the corneal center. This was ascertained by employing internal fixation for each eye after correcting the refractive error. All images were captured in the dark within a single session by a single examiner.
Figure 1. Unenhanced (A) and enhanced (B) scan acquisition protocols with angle measurements.
Two masked glaucoma specialists then analyzed each image independently on two separate occasions with a 30-day interval between sessions. The built-in software of the Visante AS-OCT was used for angle measurements that involved initial identification of the scleral spur by the observers, followed by automatic angle measurements by the software (Fig. 1) for both the nasal and temporal angles. In addition to the currently accepted angle parameters, such as the angle-opening distance (AOD), the trabecular-iris space area (TISA), and the angle recess area (ARA) at 500 and 750 μm distance from the scleral spur, Visante AS-OCT also calculates the anterior chamber angle (scleral spur angle) as an angular parameter. The details of these parameters have been previously described.3,12,13 The description of each parameter is as follows.
For the AOD at 500 and 750 μm (AOD 500 and AOD 750, respectively), the scleral spur is first identified and a point is marked 500 μm anterior to it. From this point, a line is drawn perpendicular to the plane of the trabecular meshwork to the opposing iris and the distance between these last two points was defined as the AOD 500. A similar point is marked at 750 μm anterior to the scleral spur to calculate AOD 750. Scleral spur angle measurement (also known as anterior chamber angle measurement) is an angular instead of a linear measurement, with the same reference points as AOD 500.
For the ARA at 500 and 750 μm (ARA 500 and ARA 750, respectively), the boundaries of this triangular area are the AOD 500 or AOD 750 (the base), the angle recess (the apex), and the iris surface and inner corneoscleral wall (sides of the triangle). The ARA takes into account the whole contour of the iris surface rather than measuring at a single point on the iris, as is the case with the AOD.
For the TISA at 500 and 750 μm (TISA 500 and TISA 750, respectively), the trapezoidal area has the following boundaries: anteriorly, the AOD 500 or AOD 750; posteriorly, a line drawn from the scleral spur perpendicular to the plane of the inner scleral wall to the opposing iris; superiorly, the inner corneoscleral wall; and inferiorly, the iris surface.
All continuous values are reported as mean ± standard deviation (SD). Intraobserver reproducibility was estimated by calculating coefficients of variation (CV) and within-subject standard deviation (Sw) for each observer for each angle parameter. Intraobserver and interobserver agreement was evaluated using Bland–Altman plots. The limits of agreement (LOA), defined as mean difference ± double SD of the differences, of the Bland–Altman plot were computed. Statistical analyses were performed using JMP software version 6.0.2 (SAS institute, Cary, NC) and SPSS software version 13.0 (SPSS, Inc., Chicago, IL).
Thirty eyes (15 right eyes and 15 left eyes) of 15 healthy subjects (mean age: 33.8 ± 13.0 years, 8 women) were included in the study. Because scleral spur was identified in all images, none were excluded. Table 1 presents intraobserver reproducibility for all parameters, including overall mean, Sw, and within-subject CV. The anterior chamber angle was the most reproducible parameter for both observers for both unenhanced or enhanced scans. For example, the mean anterior chamber angle for observer 1 was 53.9 ± 9.9 and 51.8 ± 9.5 degrees and the CV was 5.6% and 6% for the unenhanced and enhanced scan, respectively. ARA 500 was the least reproducible parameter with enhanced scan for observer 1 and with unenhanced scan for observer 2. For example, the mean ARA 500 for observer 1 was 382 ± 116.3 and 316.5 ± 114.5 μm2 and the CV was 13.1% and 15.1% for the unenhanced and enhanced protocols, respectively. The intraobserver and interobserver reproducibility was similar regardless of the imaging protocol used.
Table 1: Intraobserver Reproducibility for All Parameters at 500 and 750 μm Acquired Using Enhanced and Unenhanced Scan Acquisition Protocol
Table 2 presents intraobserver and interobserver agreements using Bland–Altman plots. Intraobserver and interobserver agreement was similar for all parameters evaluated between the scan protocols (eg, intraobserver and interobserver 95% LOA for unenhanced anterior chamber angle were −7.0 to 13.9 and −16.3 to 7.5, respectively). Figures 2 and 3 present Bland–Altman plots for anterior chamber angle parameters.
Table 2: Intraobserver and Interobserver Agreements for All Angle Parameters Using Bland–Altman Plots
Figure 2. Bland–Altman plots for the agreement between the scleral spur angle parameter by two observers (interobserver) using unenhanced (A) and enhanced (B) scan protocol. The lines represent the mean difference (central line) with 95% limit of agreements (upper and lower line).
Figure 3. Intraobserver (observer 1) Bland–Altman plots for the scleral spur angle parameter using unenhanced (A) and enhanced (B) scan protocol. The lines represent the mean difference (central line) with 95% limit of agreements (upper and lower line).
This study evaluates the anterior chamber angle parameter measurements using Visante AS-OCT. Our results suggest fair to good intraobserver and interobserver reproducibility for all parameters with the anterior chamber angle being the most reproducible parameter, a finding that agrees with other studies.1,7–9 In the study by Müller et al., the anterior chamber angle was detected to have high intraobserver reproducibility with intraclass correlation coefficient of 0.94 for observer 1, and intraclass correlation coefficient of 0.91 for observer 2.8 In our study, the anterior chamber angle was also shown to have excellent repeatability, with Sw ranging between 2.6 and 2.9 for observer 1 and 3.4 and 3.7 for observer 2. Thus, the anterior chamber angle can be a useful parameter for longitudinal angle assessment.
ARA 500 was the least reproducible parameter in our study for both observers. This could be due to the need for additional user input to calculate this parameter over and above identification of the scleral spur. ARA 500 and ARA 750 calculation involves identifying the endpoint of the angle recess after identifying the scleral spur, thus inducing further variability. Therefore, this parameter may be of limited value in serial angle assessment.
In this study, it also is notable that there are similar reproducibility values for enhanced and unenhanced scan acquisition protocol. Thus, there may not be a particular advantage to either protocol in eyes with open angles, provided the same protocol is used for each examination of an individual patient. Identification of the scleral spur remains a source of significant variability in angle parameter calculation, because it requires subjective input. In nearly one-third of scans, scleral spur may not be accurately identified and marked on the OCT image.11 Observers in our study also experienced difficulties in identifying the scleral spur in some cases. Theoretically, the relative ease of identifying the scleral spur with the enhanced scan should favor the use of enhanced scan protocol over unenhanced scan protocol. However, this study shows that there is no clear benefit in using one protocol over another in open angles.
There are some limitations in this study. The findings from this study may not be applicable in eyes with narrow or closed angles. All eyes in this series had open angles, which may have contributed to similar performance of both scan protocols. In a study by Console et al., the CV for all angle parameters except TISA 500 was greater in narrow angles compared with wide angles.10 In addition, the SD on repeated scleral spur placement was significantly higher in narrow angles in that study, which may have contributed to higher CV. The performance of enhanced scanning needs to be studied further in eyes with narrow angles. Also, the observers in this study are highly trained glaucoma specialists who may have overcome the difficulty in identifying the scleral spur using unenhanced scan protocol, thus neutralizing the purported benefit of enhanced scan over unenhanced scan. In a recent study, Tan et al. demonstrated similar reproducibility for analyzing an AS-OCT image between nonexperts and experts using the enhanced scan protocol.9 In our study, the image was acquired once by a single operator. Therefore, the variability that could result from multiple image acquisitions during the same session was not evaluated. However, the use of different images for the identification of the scleral spur may not contribute significantly to the variability in the angle parameter measurements, providing all images are of adequate quality.9
Our study confirmed good intraobserver and interobserver reproducibility of anterior chamber angle measurements using Visante AS-OCT. The enhanced scan protocol was not found to have any significant benefit over the unenhanced scan protocol in this series of eyes with open angles.
- Wirbelauer C, Karandish A, Häberle H, Pham DT. Noncontact goniometry with optical coherence tomography. Arch Ophthalmol. 2005;123:179–185. doi:10.1001/archopht.123.2.179 [CrossRef]
- Su DH, Friedman DS, See LJ, et al. Degree of angle closure and extent of peripheral anterior synechiae: an anterior segment OCT study. Br J Ophthalmol. 2008;92:103–107. doi:10.1136/bjo.2007.122572 [CrossRef]
- Rhadhakrishnan S, Goldsmith J, Huang D, et al. Comparison of optical coherence tomography and ultrasound biomicroscopy for detection of narrow anterior chamber angles. Arch Ophthalmol. 2005;123:1053–1059. doi:10.1001/archopht.123.8.1053 [CrossRef]
- Nolan WP, See JL, Chew PT, et al. Detection of primary angle closure using anterior segment optical coherence tomography in Asian eyes. Ophthalmology. 2007;114:33–39. doi:10.1016/j.ophtha.2006.05.073 [CrossRef]
- Sakata LM, Lavanya R, Friedman DS, et al. Comparison of gonioscopy and anterior segment ocular coherence tomography in detecting angle closure in different quadrants of the anterior chamber angle. Ophthalmology. 2008;115:769–774. doi:10.1016/j.ophtha.2007.06.030 [CrossRef]
- Dada T, Sihota R, Gadia R, et al. Comparison of anterior segment optical coherence tomography and ultrasound biomicroscopy for assessment of the anterior segment. J Cataract Refract Surg. 2007;33:837–840. doi:10.1016/j.jcrs.2007.01.021 [CrossRef]
- Li H, Leung CKS, Cheung CYL, et al. Repeatability and reproducibility of anterior chamber angle measurement with anterior segment optical coherence tomography. Br J Ophthalmol. 2007;91:1490–1492. doi:10.1136/bjo.2007.118901 [CrossRef]
- Müller M, Dahmen G, Pörksen E, et al. Anterior chamber angle measurement with optical coherence tomography: intraobserver and interobserver variability. J Cataract Refract Surg. 2006;11:1803–1808. doi:10.1016/j.jcrs.2006.07.014 [CrossRef]
- Tan AN, Sauren LD, Brabander J, et al. Reproducibility of anterior chamber angle measurements with anterior segment optical coherence tomography. Invest Ophthalmol Vis Sci. 2011;52:2095–2099. doi:10.1167/iovs.10-5872 [CrossRef]
- Console JW, Sakata LM, Aung T, Freidman DS, He M. Quantitative analysis of anterior segment optical coherence tomography images: the Zhongshan angle assessment program. Br J Ophthalmol. 2008;92:1612–1616. doi:10.1136/bjo.2007.129932 [CrossRef]
- Sakata LM, Lavanya R, Friedman DS, et al. Assessment of the scleral spur in anterior segment optical coherence tomography images. Arch Ophthalmol. 2008;126:181–185. doi:10.1001/archophthalmol.2007.46 [CrossRef]
- Pavlin CJ, Harasiewicz K, Foster FS. Ultrasound biomicroscopy of anterior segment structures in normal and glaucomatous eyes. Am J Ophthalmol. 1992;113:381–389.
- Ishikawa H, Liebmann JM, Ritch R. Quantitative assessment of the anterior segment using ultrasound biomicroscopy. Curr Opin Ophthalmol. 2000;11:133–139. doi:10.1097/00055735-200004000-00012 [CrossRef]
Intraobserver Reproducibility for All Parameters at 500 and 750 μm Acquired Using Enhanced and Unenhanced Scan Acquisition Protocol
|Variable||Observer 1||Observer 2|
|Mean ± SD||Sw||CV (%)||Mean ± SD||Sw||CV (%)||Mean ± SD||Sw||CV (%)||Mean ± SD||Sw||CV (%)|
|Anterior chamber angle (degree)||53.4 ± 9.9||2.6||5.6||51.8 ± 9.5||2.9||6.0||.21||49.0 ± 11.0||3.4||8.3||47.4 ± 10.9||3.7||10.1||.39|
|AOD 500 (μm)||726.8 ± 247.7||63.0||10.0||683.0 ± 222.9||68.6||10.6||.16||627.2 ± 240.3||82.0||14.0||591.7 ± 213.9||63.1||12.2||.23|
|AOD 750 (μm)||987.3 ± 323.3||72.7||7.7||930.1 ± 310.0||84.5||9.1||.16||860.8 ± 314.4||102.7||13.4||827.9 ± 288.5||71.7||10.0||.40|
|TISA 500 (μm2)||261.4 ± 80.4||23.6||10.2||240.8 ± 80.5||26.3||11.9||.06||226.9 ± 80.5||44.6||18.7||221.9 ± 80.9||40.4||14.7||.14|
|TISA 750 (μm2)||475.9 ± 152.8||39.7||9.3||442.1 ± 145.9||44.4||10.6||.08||407.7 ± 148.3||52.4||13.6||387.4 ± 136.4||39.8||11.5||.27|
|ARA 500 (μm2)||348.2 ± 116.3||41.0||13.1||316.5 ± 114.5||43.3||15.1||.03||282.0 ± 116.4||75.5||20.4||245.1 ± 97.3||36.5||15.6||.20|
|ARA 750 (μm2)||562.7 ± 182.0||56.1||11.1||519.6 ± 179.6||63.2||12.9||.07||437.3 ± 171.4||65.7||16.1||423.8 ± 156.0||55.6||14.2||.31|
Intraobserver and Interobserver Agreements for All Angle Parameters Using Bland–Altman Plots
|MD||95% LOA||MD||95% LOA||MD||95% LOA||MD||95% LOA|
|Anterior chamber angle (degree)||3.4||−7.0–13.9||3.0||−8.0–13.9||.54||−4.4||−16.3–7.5||−4.0||−15.4–7.4||.60|
|AOD 500 (μm)||72.7||177.4–322.7||56.1||−178.0–290.2||.30||−99.5||−359.7–160.7||−91.3||−327.8–145.3||.61|
|AOD 750 (μm)||70.3||−291.9–432.4||68.4||−213.1–350.0||.93||−126.5||−492.2–239.3||−102.2||−393.1–188.7||.27|
|TISA 500 (μm2)||38.7||−123.3–200.8||24.4||−67.0–115.7||.10||−32.2||−173.9–109.5||−29.4||−125.6–66.7||.73|
|TISA 750 (μm2)||48.5||−101.1–198.2||37.8||−107.9–183.4||.27||−68.2||−231.3–94.9||−54.7||−198.8–89.3||.18|
|ARA 500 (μm2)||51.9||−86.1–189.9||37.3||−99.1–173.6||.10||−85.4||−242.4–71.6||−71.4||−204.7–61.8||.15|
|ARA 750 (μm2)||64.5||−121.6–250.6||53.3||−153.2–259.7||.39||−117.4||−318.6–83.7||−95.8||−300.0–108.3||.11|