From the Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology & Visual Science Key Lab, Beijing, China.
The authors have no financial or proprietary interest in the materials presented herein.
Address correspondence to Ningli Wang, MD, PhD, Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology & Visual Science Key Lab, Beijing, China. E-mail: firstname.lastname@example.org
Glaucoma is the third leading cause of bilateral blindness in China.1 Within the subgroups, primary angle-closure glaucoma (PACG) has a high prevalence.2,3 Laser peripheral iridotomy is effective for patients with PACG to open their appositionally closed angle and reduce the acute attack.4,5 Therefore, evaluation of anatomically narrow angles is important.
Although gonioscopy provides semiquantitative assessment of the angle width, it is a subjective technique and there are no uniform gonioscopic criteria for identifying angles that require treatment.6,7 Quantitative assessment of the anterior chamber angle is possible with ultrasound biomicroscopy or anterior-segment optical coherence tomography (AS-OCT) (Carl Zeiss Meditec, Dublin, CA).8 Yi et al. reported that the rotating Scheimpflug camera (Pentacam; Oculus, Wetzlar, Germany) can also provide good images and quantitative anterior chamber angle in eyes with normal open angles.9 All of these methods are less subjective than gonioscopy.
Ultrasound biomicroscopy requires a water bath and patients must lie on a bed to perform the procedure; therefore, the position of the iris may be changed and any excess pressure on the eye cup used while scanning can influence the angle configuration.10 In addition, there is a risk of corneal abrasion and ultrasound biomicroscopy cannot be employed shortly after trabeculectomy or phacoemulsification surgery because of the risk of endophthalmitis or infection, limiting its usefulness.
In contrast to ultrasound biomicroscopy, AS-OCT and the Pentacam allow noncontact imaging of the anterior chamber angle in a sitting position. They have been introduced for the measurement and evaluation of the anterior segment and provide a fast and convenient examination that is better tolerated by patients.
Anterior chamber angle measurements with AS-OCT have recently been demonstrated to be reproducible,11–13 and previous reports of the Pentacam focus on the corneal thickness and corneal curvature or the lens; however, few studies report the repeatability and the precision of angle measurements by the Pentacam.9
It remains uncertain whether the angle measurements obtained by the two instruments are comparable in healthy normal subjects and patients with narrow angles. The two instruments have great differences in the design and working principles, scan mode, and scan speed. Thus, use of Pentacam and AS-OCT may lead to measurement disparity in the anterior chamber angle measurements. The purpose of this study was to evaluate the agreement of narrow- and open-angle measurements obtained with the two instruments.
Patients and Methods
Patients with narrow angles and healthy normal volunteers at the Glaucoma Service of Beijing Tongren Hospital were enrolled consecutively in the study. One eye of 39 healthy normal volunteers (39 eyes) and 25 patients with narrow angles (37 eyes), who were diagnosed as having suspected PACG or PACG, was randomly selected for anterior chamber angle imaging. All patients with suspected PACG or PACG had used pilocarpine before the examination and underwent laser peripheral iridotomy immediately after the examination. The anterior chamber angles were measured with both AS-OCT and Pentacam in each patient. One physician (SNL), who was masked to the study group status and unaware of the diagnosis, marked the scleral spur for the AS-OCT and measured the anterior chamber angle with the built-in software.
The agreement between AS-OCT and Pentacam was evaluated. Healthy normal subjects were included in the open-angle group and patients with suspected PACG or PACG were included in the narrow-angle group.
All healthy normal subjects had no evidence of ocular disease and one eye was randomly selected. Any patients with a history of any previous intraocular surgery, laser trabeculoplasty, laser iridoplasty, or laser-assisted in-situ keratomileusis were excluded from the study. The study was conducted in accordance with the ethical standards stated in the 1964 Declaration of Helsinki and approved by the Clinical Research Ethics Committee (Beijing Tongren Hospital) and informed consent was obtained.
Subjects underwent gonioscopy by a second independent observer (FJ) with extensive experience in performing gonioscopy in a research setting and who was masked to the AS-OCT findings. The examination of all subjects was performed at a low level of ambient illumination with a Goldmann 1-mirror lens (model 902; HaagStreit, Koeniz, Switzerland). A 1-mm beam of light was reduced to a narrow slit. The vertical beam was offset horizontally for assessing superior and inferior angles and a vertically offset horizontal beam was used for nasal and temporal angles. Indentation gonioscopy with the Goldmann 1-mirror lens was used (except when the angle was wide open) to evaluate patients with angle closure.
Suspected PACG was defined as non-visibility of the filtering trabecular meshwork for 180° or more, an intraocular pressure less than 22 mm Hg, and no peripheral anterior synechiae in the angle.4 PACG was defined as the presence of glaucomatous optic neuropathy and compatible visual field loss associated with gonioscopic criteria for narrow angles and evidence of significant obstruction of the functional trabecular meshwork by the peripheral iris.7
Based on a recent study1 using AS-OCT and Pentacam, the average anterior chamber angle was found to be 45.41° with a standard deviation of 5.3°. Setting the level of significance at .05 and power at 80%, a sample size calculation indicates that a minimum of 19 subjects would be required to give 80% power at a 5% two-sided significance level for the statistical test to detect a mean difference of 10%.
AS-OCT can visualize anterior eye structures with high-resolution, direct cross-sectional images, which can be analyzed, measured, and used for evaluation. For AS-OCT, subjects were positioned with a head rest. All room lights were switched off and the room illumination was 10 ± 0.5 lux by a light meter (TES-1330A; TES Electrical Electronic Corp., Taipei, Taiwan). A single OCT examination was performed. To acquire images of each eye, the focus of an internal-fixation target was adjusted with reference to the subject’s refraction. A real-time charge-coupled device displaying the position of the scan and the position of the eye was available to enable scan alignment. AS-OCT scans were acquired with the protocol anterior segment single 0° to 180°. The image is horizontally composed of 256 A-scans in 16 mm with 1,024 points per A-scan in 8 mm of depth. Scanning at 2,000 axial scans per second, the machine needs approximately one-eighth of a second to scan one eye. The scan line was manually adjusted to bisect the pupil.13 Scanning the superior and inferior angles with the Visante OCT was difficult because manipulation of eyelids would be required to expose the limbus for imaging; thus, the vertical section was not scanned. The anterior chamber angle was calculated with built-in software using the scleral spur as the operator-defined landmark.
Pentacam is the first instrument to use a rotating Scheimpflug camera to take multiple images of the anterior segment and use these to generate three-dimensional images and calculate measurements of the eye. Pentacam comprises a rotating Scheimpflug camera and short-wavelength slit light, which scans and measures anterior and posterior cornea surfaces, corneal thickness, and anterior chamber depth within 2 seconds. The measurement wavelength is 475 nm (blue light-emitting diode laser). Pentacam is internal fixation by blue light-emitting diode (LED) light (475 nm ultraviolet free).
Pentacam calculates a three-dimensional model of the anterior eye segment from as many as 25,000 true elevation points. It takes a maximum of 2 seconds to generate 25 complete images of the anterior eye segment. Any eye movement is detected (and the results corrected) by a second camera.
When the examination is finished, all additional information including the anterior chamber angle, volume, and depth and corneal thickness and curvature at any location in the anterior chamber of the eye were automatically calculated by the built-in software.
To acquire images of the unaccommodated eye, subjects were positioned with a head rest and asked to open their eyes as widely as possible and the room light was switched off to get a reflex-free examination. The room illumination was 10 ± 0.5 lux by a light meter (TES-1330A). The Pentacam was aligned so that the patient’s pupil came into view and adjusted until the live Scheimpflug image appeared and the red fixation dot was visible. In this study, the 25-images-per-scan mode and the auto-measurement mode were chosen.
The Scheimpflug images taken during the examination are digitalized in the main unit and all image data are transferred to a personal computer. When the examination is finished, the personal computer calculates a three-dimensional virtual model of the anterior eye segment from which all additional information is derived.
It is difficult to measure the anterior chamber angle in 360° because of eyelid interference, so we only selected the horizontal meridian images.
Pentacam has built-in analysis software that depends on automatic measurements of the angle parameters. Two chamber angles in the horizontal section are calculated from the three-dimensional model to show the mathematically calculated chamber angle. The anterior chamber angle is calculated by lengthening the posterior cornea and the iris contour to compute the chamber angle using an interpolation method, because Pentacam cannot image the angle recess and scleral spur.
Statistical analyses were performed with commercial software (SPSS ver. 11.0; SPSS, Inc., Chicago, IL). Measurements from Pentacam and AS-OCT were compared with paired t tests and their agreements were evaluated with Bland–Altman plots. A P value of less than .05 was considered statistically significant.
The mean age of the 39 healthy normal subjects (20 men and 19 women) was 29.9 ± 6.1 years. The average values of nasal and temporal anterior chamber angle taken with the Pentacam were 40.67° ± 6.57° and 38.47° ± 10.8°, respectively, in healthy normal subjects. The average values of nasal and temporal anterior chamber angle taken with the AS-OCT were 38.47° ± 10.82° and 38.47° ± 11.39°, respectively, in healthy normal subjects. There was no significant difference in the nasal and temporal angle measurements between Pentacam and AS-OCT (P > .05; Table).
Table: ACA and Pupil Diameter Measurements by Pentacam and AS-OCT in Subjects with Normal and Narrow-Angle
A total of 37 eyes of 25 patients with narrow angles were included in the study (6 men and 19 women). Mean age was 63.5 ± 9.6 years (range: 45 to 79 years). However, average values of nasal and temporal anterior chamber angles taken by Pentacam were 25.5° ± 5.66° and 25.77° ± 5.15°, respectively, and the nasal and temporal anterior chamber angle value taken by AS-OCT were 13.40° ± 6.81° and 12.13° ± 6.47°, respectively. There was significant difference in the nasal and temporal angle measurements between Pentacam and AS-OCT (P < .001).
The pupil diameters measured by Pentacam and AS-OCT were different (3.12 ± 0.42 vs 5.34 ± 0.64 mm) in healthy normal subjects. However, pupil diameters measured by Pentacam and AS-OCT were similar in patients with narrow angles (Table).
The between-instrument comparison showed that the Scheimpflug and scanning-slit approaches produced similar anterior chamber angle results that agreed well in healthy normal subjects (Figs. 1 and 2). Pentacam measurement demonstrated best agreement with AS-OCT: 95% limits of agreement between −22.1° and 17.7° with a mean difference of −2.2° in nasal (Fig. 1) and 95% limits of agreement between −23.9° and 19° with a mean difference of −2.5° in temporal (Fig. 2).
Figure 1. Bland–Altman Analysis for Inter-Methods Agreement of the Anterior Chamber Angle (temporal) for Pentacam and Anterior-Segment Optical Coherence Tomography (AS-OCT) in Healthy Normal Subjects. The Mean Difference in Anterior Chamber Angle Between Instruments Was −2.2° (limits of Agreement: 17.7 to −22.1). SD = Standard Deviation. Pentacam Is Manufactured by Oculus, Wetzlar, Germany.
Figure 2. Bland–Altman Analysis for the Inter-Methods Agreement of Anterior Chamber Angle (nasal) for Pentacam and Anterior-Segment Optical Coherence Tomography (AS-OCT) in Healthy Normal Subjects. The Mean Difference in Anterior Chamber Angle Between Instruments Was −2.5° (limits of Agreement: 19 to −23.9). SD = Standard Deviation. Pentacam Is Manufactured by Oculus, Wetzlar, Germany.
The agreement of the two instruments was poor when measuring the anterior chamber angle in patients with narrow angles (Figs. 3 and 4). The Bland–Altman plots revealed that the anterior chamber angle measurement obtained from Pentacam was higher than that obtained from AS-OCT: a mean difference of 12.4° and the 95% limits of agreement were between −4.4° and 29.2° in temporal (Fig. 3) and a mean difference of 13.4° and the 95% limits of agreement were between −3.1° and 29.8° in nasal (Fig. 4).
Figure 3. Bland–Altman Analysis for the Inter-Methods Agreement of Anterior Chamber Angle (temporal) for Pentacam and Anterior-Segment Optical Coherence Tomography (AS-OCT) in Patients with Narrow Angles. The Mean Difference in Anterior Chamber Angle Between Instruments Was 12.4° (limits of Agreement, 29.2 to −4.4). SD = Standard Deviation.
Figure 4. Bland–Altman Analysis for the Inter-Methods Agreement of Anterior Chamber Angle (nasal) for Pentacam and Anterior-Segment Optical Coherence Tomography (AS-OCT) in Patients with Narrow Angles. The Mean Difference in Anterior Chamber Angle Between Instruments Was 13.4° (limits of Agreement, 29.8 to −3.1). SD = Standard Deviation.
Chamber angle measurements play an important role in angle closure research. The Pentacam is a rotating Scheimpflug camera that measures Scheimpflug images of the anterior segment. The Scheimpflug technique provides sharp and crisp images that include information from the anterior corneal surface to the posterior crystalline capsule.
The key advantages of the rotating imaging process are the precise measurement of the central cornea and the correction of eye movements. The Pentacam can also calculate a three-dimensional mathematical model of the anterior segment. In our study, we used the automatic release mode to rule out confounding operator-related variables, which increases comfort and guarantees a high reproducibility.14 Pentacam has proven its reliability in measuring anterior chamber angles.15
Excellent agreements (Figs. 1 and 2) for anterior chamber angle measurements were observed for Pentacam and AS-OCT in healthy normal subjects, which is consistent with Yi et al.’s study9 that found the mean differences in the anterior chamber angle, as measured with Pentacam and AS-OCT, were not statistically significant in healthy normal subjects and clinically negligible, approximately.
In our study, the different mean age between normal subjects and patients with suspected PACG has no influence on the purpose of this study. We only aimed to evaluate the agreement between the two instruments in measuring narrow and open angles.
We found that the agreement of the two instruments was poor when measuring the anterior chamber angle in patients with narrow angles (Figs. 3 and 4). The difference between Pentacam and AS-OCT measurements of the anterior chamber angle would be within 12.4° in 95% of observations (Table). A possible explanation is that the Pentacam has built-in analysis software that depends on automatic measurements of the angle parameters. The anterior chamber angle is calculated from measurements of the mathematically computed chamber angle by lengthening of the posterior cornea and the iris contour to compute the chamber angle using an interpolation method. In healthy normal subjects (Fig. 5), the anterior chamber angle is open and the iris is flat, so this interpolation method has little effect in measurements. However, in patients with narrow angles (Fig. 6), even in case of appositional angle closure, contact was found between the peripheral iris and angle wall anterior to the scleral spur; however, the anterior chamber angle was still calculated by lengthening the posterior cornea and the iris contour, which ignored the narrow configuration of the angle and resulted in a systematic overestimation of the anterior chamber angle. Therefore, this interpolation method accounts for the differences between the two instruments.
Figure 5. Pentacam Image of a Healthy Normal Subject. Pentacam Is Manufactured by Oculus, Wetzlar, Germany.
Figure 6. Pentacam Image of a Patient Diagnosed as Having Suspected Primary Angle-Closure Glaucoma. Pentacam Is Manufactured by Oculus, Wetzlar, Germany.
Compared with Pentacam, we found that sharp delineation of the scleral spur was obtained by AS-OCT, which makes measuring the angle more reliable and precise (Fig. 7). The light of the rotating Scheimpflug camera cannot penetrate the corneoscleral limbus, so it cannot image the sclera spur, which is an important anatomical landmark for the trabecular meshwork. The scleral spur in anterior segment imaging is marked by a prominent inner extension of the sclera (the thickest part)16,17 and is used in judging whether the anterior chamber angle was open or closed.
Figure 7. Image of a Healthy Normal Subject by Anterior-Segment Optical Coherence Tomography (arrow = Sclera Spur).
Although we attempted to standardize the lighting conditions for AS-OCT and Pentacam, it is possible that small variations in illumination occurred. We noticed that the pupil diameter taken by AS-OCT was significantly larger than Pentacam (P < .001, Table) in healthy normal subjects.
Both instruments have minimal illumination from the screen display. However, AS-OCT is internal fixation by diffuse infrared LED light (880 nm), whereas Pentacam is internal fixation by blue LED light (475 nm ultraviolet free). We presume that the blue LED light may affect the pupil diameter and poor agreement may be related to the difference of the LED light during the imaging process. The difference between Pentacam and AS-OCT measurements of pupil diameter would be within 2.2 mm in 95% of observations. In patients with narrow angles, the two instruments were in good agreement in pupil diameter (Table) because all of the patients had used pilocarpine before the examination, were meiotic, and the internal fixation sources could not affect the pupil diameter.
The Pentacam technology has an inherent limitation of being unable to visualize the angle recess or the scleral spur; thus, the anterior chamber angle measurements were obtained by extrapolation of the corneal and iris contours. This can explain why their agreement was poor when measuring narrow angles. Due to this, Pentacam has limited use in quantitative assessment of the angle. However, Pentacam images can help in qualitative assessment of whether an angle is wide open or not.
Our study was subject to limitations. First, the internal fixation light caused pupil constriction during Pentacam imaging and thus the illumination conditions were not the same when compared to AS-OCT. Second, in healthy normal subjects, one randomly selected eye was measured by two instruments; however, in patients with narrow angles, eyes were not randomly selected.
Both AS-OCT and Pentacam can reliably measure the anterior chamber angle in healthy normal subjects. In addition to the internal fixation light of Pentacam causing pupillary constriction when compared with AS-OCT, clinicians should also be aware of the great differences in the two instruments for imaging and measuring narrow anterior chamber angles.
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ACA and Pupil Diameter Measurements by Pentacam and AS-OCT in Subjects with Normal and Narrow-Angle
|Temporal ACA (°)||40.67 ± 6.57||38.47 ± 10.82||.186||25.77 ± 5.15||13.40 ± 6.81||<.001|
|Nasal ACA (°)||40.93 ± 7.4||38.47 ± 11.39||.166||25.51 ± 5.66||12.13 ± 6.47||<.001|
|Pupil diameter (mm)||3.12 ± 0.42||5.34 ± 0.64||<.001||1.74 ± 0.43||1.99 ± 0.91||.12|