Fourier-domain Optical Coherence Tomography Research Articles

Central corneal thickness measurement by Fourier domain optical coherence tomography, ocular response analyzer and ultrasound pachymetry

PurposeTo assess the repeatability, reproducibility, and agreement of central corneal thickness (CCT) measured by non-contact Fourier domain optical coherence tomography (FD-OCT; OptoVue) with the other two contact devices, ocular response analyzer (ORA; Reichert Ophthalmic Instruments) and Ultrasound Pachymetry (USP; DGH Technologies). MethodsThis observational cross-sectional study measured CCT sequentially using FD-OCT, ORA and USP. The first 16 volunteers (32 eyes) received three measurements by two independent examiners in a single session to determine intra-observer repeatability and inter-observer reproducibility. An additional 27 volunteers (54 eyes) received one measurement by the same examiner. The measurements of all 86 eyes were analyzed for the difference, correlation, and agreement among the three devices. ResultsFD-OCT measured the thinnest while USP measured the thickest CCT (548.6 ± 28.3 μm, 556.9 ± 28.8 μm, and 560.0 ± 28.8 μm by FD-OCT, ORA, and USP, respectively, p < 0.001). The mean differences (lower/upper limit of agreement) for CCT measurements were 8.4 ± 7.6 μm (−6.5/23.2) between ORA and FD-OCT, 11.4 ± 7.3 μm (−2.8/25.7) between USP and FD-OCT, and 3.1 ± 5.1 μm (−6.9/13.1) between ORA and USP. The intra-class correlation coefficients were above 0.98 for all tested groups. FD-OCT had the lowest intra-examiner variability (coefficient of repeatability of 0.64%) and lowest inter-examiner variability (coefficient of reproducibility of 1.16%). ConclusionFD-OCT, ORA, and USP demonstrated good inter-observer reproducibility and intra-observer repeatability. The three measurements were highly correlated; however, systematic differences between the three tested devices did exist. FD-OCT was a reliable and examiner-independent method in CCT measurement.

Correlation between the ganglion cell complex and structural measures of the optic disc and retinal nerve fiber layer in glaucoma.

To correlate the ganglion cell complex (GCC) parameters with structural measures of the optic nerve head (ONH) and retinal nerve fiber layer (RNFL) as evaluated by Fourier-Domain optic coherence tomography (OCT). This retrospective study included patients with glaucoma, ocular hypertensive patients and glaucoma suspects who had previously undergone OCT examination with the RTVue-100. The parameters of GCC (average, superior, inferior, focal loss volume [FLV], global loss volume [GLV]) were correlated with the values of the ONH (cup volume, cup area, horizontal cup-to-disk ratio, vertical cup-to-disk ratio, and rim area) and RNFL (average, superior, and inferior) using Pearson's correlation coefficient. The sample included 74 eyes of 37 patients. All correlations between GCC parameters and RNFL were strong (r>0.60). The correlation between GCC parameters and ONH were good for most parameters, except that for FLV and cup volume (r=0.13), GLV and cup volume (r=0.09), and GLV and cup area (r=0.21). The GCC parameters can be used as structural measures of the glaucomatous optic neuropathy.

TCT-382 Safety and Feasibility of Optical Coherence Tomography: A Single Center Experience

Optical coherence tomography (OCT) is increasingly used in the catheterization laboratory. Previous smaller studies have reported the safety of OCT in different clinical settings, however large datasets are still lacking. We report safety of intracoronary Fourier Domain OCT (FD-OCT) imaging in a

Epithelial remodeling after partial topography-guided normalization and high-fluence short-duration crosslinking (Athens protocol): Results up to 1 year

To compare epithelial remodeling in keratoconic eyes that had photorefractive keratectomy and corneal collagen crosslinking (Athens protocol) with that in untreated keratoconic eyes and healthy eyes. Private clinical practice, Athens, Greece. Comparative case series. Fourier-domain anterior segment optical coherence tomography (AS-OCT) was used to obtain in vivo 3-dimensional epithelial thickness maps and center, superior, inferior, maximum, minimum, mean, midperipheral, and variability data. Group A comprised 175 treated keratoconic eyes (Athens protocol); Group B, 193 untreated keratoconic eyes; and Group C, 160 healthy eyes. The 1-year mean center epithelial thickness in Group A was 47.78 μm ± 7.36 (SD) (range 33 to 64 μm). At the first clinical visit, it was 52.09 ± 6.80 μm (range 36 to 72 μm) in Group B and 52.54 ± 3.23 μm (range 45 to 59 μm) in Group C. The mean thickness range in Group A at 1 year was -19.94 ± 7.21 μm (range -6 to -34 μm). It was -21.83 ± 12.07 μm (range -4 to -66 μm) in Group B and -6.86 ± 3.33 μm (range -3 to -29 μm) in Group C. The mean topographic thickness variability in Group A at 1 year was 4.64 ± 1.63 μm (range 1.6 to 8.1 μm) (P<.05). It was 5.77 ± 3.39 μm (range 1.3 to 17.8 μm) in Group B and 1.59 ± 0.79 μm (range 0.6 to 5.6 μm) in Group C. Anterior segment OCT indicated a thinner and more homogeneous remodeled epithelium in the keratoconic eyes treated using the Athens protocol. Dr. Kanellopoulos is a consultant to Alcon Surgical, Inc.; Wavelight Laser Technologie AG; Avedro, Inc.; and i-Optics Optikgeräte GmbH. Dr. Asimellis has no financial or proprietary interest in any material or method mentioned.

Fourier-domain optical coherence tomography evaluation of clear corneal incision structure according to blade material

To use Fourier-domain optical coherence tomography (OCT) to evaluate the wound characteristics of clear corneal incisions (CCIs) created with a metal or diamond blade in cataract surgery. Department of Ophthalmology, Institute of Vision Research, Yonsei University College of Medicine, Seoul, South Korea. Prospective comparative observational study. Patients who had cataract surgery were randomized into 2 groups based on whether a metal blade (Group 1, 37 eyes) or diamond blade (Group 2, 33 eyes) was used to create a 2.8 mm temporal CCI. One day, 1 week, and 1 month postoperatively, structural characteristics of the CCI were analyzed using RTVue-100 Fourier-domain OCT. Parameters included incision angle, corneal thickness, epithelial or endothelial gaps, and Descemet membrane detachment. Visual acuity, surgically induced astigmatism (SIA), and ocular aberrations were evaluated. There was a significant difference in corneal thickness at the 1.0 mm temporal side from midpoint. The mean uncorrected distance visual acuity in Group 2 (33 eyes) improved significantly over time. In both groups, corneal thickness at the midpoint, 1.0 mm temporal side, and 1.0 mm nasal side from the midpoint of the incision significantly decreased over time. At all timepoints, temporal and nasal thickness in Group 2 was significantly greater than in Group 1 (37 eyes), with the exception of temporal thickness at 1 month. In both groups, wound healing was reliable over time. There were no significant between-group differences in SIA or changes in aberrations. Corneal thickness at the incision site showed a significant difference between the 2 groups. Both groups achieved structural stabilization. No author has a financial or proprietary interest in any material or method mentioned.

Chemically Etched Axicon Fiber as a Sensor Probe of Optical Coherence Tomography

In this paper, we investigated a high lateral resolution common-path Fourier domain OCT system with the use of a chemically etched single mode fiber. In our experiments, single mode optical fiber for 840nm was used for preparing the tapered tips. Our system used a conical microlens that was chemically etched by selective chemical etching technique using an etching solution of buffered hydrofluoric acid (BHF). From experimental results, we verified that our proposed optical coherence tomography system could operate as a common-path Fourier domain OCT (FD-OCT) system. Furthermore, we experimentally investigated the chemical etching process of silica-based optical fiber probes by controlling the etching time when applying chemical etching techniques using hydrofluoric acid. We evaluated the beam profile from fiber probe. From these investigations, it was found that etching time was important parameter for sensor probe of optical coherence tomography.

Generic pixel-wise speckle detection in Fourier-domain optical coherence tomography images.

We present a generic phase-domain processing method for detecting speckles in Fourier-domain optical coherence tomography (OCT) images. The physics behind the interferometry is revisited and analytically along with simulation results it indicates that the speckle formation comes with phase distortion to the complex OCT signal. The first and the second derivatives of phase along the imaging depth are then calculated for speckle identification. The phase-domain processing method was applied to images acquired by both spectral-domain OCT and swept-source OCT systems, and the experimental results show that this method enables pixel-wise speckle identification.

THE DOSE-DEPENDENT MACULAR THICKNESS CHANGES ASSESSED BY FD-OCT IN PATIENTS WITH RETINITIS PIGMENTOSA TREATED WITH CILIARY NEUROTROPHIC FACTOR

To evaluate the effect of intravitreal ciliary neurotrophic factor (CNTF) implant on mean macular thickness (MMT) in eyes with retinitis pigmentosa using high-resolution Fourier domain optical coherence tomography imaging. A cohort of 8 patients (CNTF-3: n = 5; CNTF-4: n = 3) enrolled in Neurotech sponsored Phase 2 clinical trial underwent Fourier domain optical coherence tomography imaging. A ≥3% change in MMT from baseline or fellow eye was considered as a measurable change. Two patients enrolled in the CNTF-3 study received low-dose implant. At 18 months, a change in MMT from -4.47 μm to 6 μm from baseline was noted. Six patients received high-dose implant (CNTF-3: n = 3; CNTF-4: n = 3). In CNTF-3 group, 1 eye showed an increase in MMT by 19.25 μm (+7.6%) from baseline at 18 months. In CNTF-4 group, 1 eye had an increase in MMT of 27.08 μm (+11%) from baseline at 30 months; second eye had increase in MMT of 31.36 μm (+12%) from contralateral eye. Amongst these 3 responsive high-dose implant eyes, overall thickening of the retina could not be attributed to any specific retinal layer. A heterogeneous dose-dependent response on MMT was noted in eyes treated using intravitreal CNTF implant for retinitis pigmentosa. We recommend corroboration of our findings with Neurotech sponsored clinical trial results.

Assessment of total retinal blood flow using Doppler Fourier Domain Optical Coherence Tomography during systemic hypercapnia and hypocapnia.

The purpose of this study was to investigate changes in total retinal blood flow (RBF) using Doppler Fourier Domain Optical Coherence Tomography (Doppler FD‐OCT) in response to the manipulation of systemic partial pressure of CO2 (PETCO2). Double circular Doppler blood flow scans were captured in nine healthy individuals (mean age ± standard deviation: 27.1 ± 4.1, six males) using the RTVue™ FD‐OCT (Optovue). PETCO2 was manipulated using a custom‐designed computer‐controlled gas blender (RespirAct™) connected to a sequential gas delivery rebreathing circuit. Doppler FD‐OCT measurements were captured at baseline, during stages of hypercapnia (+5/+10/+15 mmHg PETCO2), return to baseline and during stages of hypocapnia (−5/−10/−15 mmHg PETCO2). Repeated measures analysis of variance (reANOVA) and Tukey's post hoc analysis were used to compare Doppler FD‐OCT measurements between the various PETCO2 levels relative to baseline. The effect of PETCO2 on TRBF was also investigated using linear regression models. The average RBF significantly increased by 15% (P < 0.0001) with an increase in PETCO2 and decreased significantly by 10% with a decrease in PETCO2 (P = 0.001). Venous velocity significantly increased by 3.11% from baseline to extreme hypercapnia (P < 0.001) and reduced significantly by 2.01% at extreme hypocapnia (P = 0.012). No significant changes were found in the average venous area measurements under hypercapnia (P = 0.36) or hypocapnia (P = 0.40). Overall, increased and decreased PETCO2 values had a significant effect on RBF outcomes (P < 0.002). In healthy individuals, altered end‐tidal CO2 levels significantly changed RBF as measured by Doppler FD‐OCT.

Nanosensitive optical coherence tomography for the study of changes in static and dynamic structures

We briefly discuss the principle of image formation in Fourier domain optical coherence tomography (OCT). The theory of a new approach to improve dramatically the sensitivity of conventional OCT is described. The approach is based on spectral encoding of spatial frequency. Information about the spatial structure is directly translated from the Fourier domain to the image domain as different wavelengths, without compromising the accuracy. Axial spatial period profiles of the structure are reconstructed for any volume of interest within the 3D OCT image with nanoscale sensitivity. An example of application of the nanoscale OCT to probe the internal structure of medico-biological objects, the anterior chamber of an ex vivo rat eye, is demonstrated.

Optical Coherence Tomography for the Study of Polymer and Polymer Matrix Composites

ABSTRACTBased on low‐coherence interferometry, a hybrid optical coherence tomography (OCT) system has been built. It coupled time‐domain OCT (TD‐OCT) and Fourier‐domain OCT (FD‐OCT) into one system. TD‐OCT can take the advantage of a large axial scan range while FD‐OCT has superior performance in fast imaging as no axial scan is needed. The two imaging modalities shared a broad bandwidth light source with a centre wavelength of 1550 nm, which is less scattering and can give better penetration depth in the polymer‐based material than the shorter wavelength used for biomedical applications. 2D translation stages were incorporated in the system to make cross‐sectional and volume imaging available. It can provide larger scan range as well as less image distortion compared with galvo scanners. Finally, we reported on successfully characterized specimens such as polymer coatings and glass‐fibre composites. The cross‐sectional and volumetric images obtained clearly show the microstructure of the materials. The thickness as well as the defects, e.g. microcrack and delamination can be determined.

Potential new diagnostic tool for Alzheimer's disease using a linear discriminant function for Fourier domain optical coherence tomography.

We calculated and validated a linear discriminant function (LDF) for Fourier domain optical coherence tomography (OCT) to improve the diagnostic ability of retinal and retinal nerve fiber layer (RNFL) thickness parameters in the detection of Alzheimer's disease (AD). We enrolled AD patients (n = 151) and age-matched, healthy subjects (n = 61). The Cirrus and Spectralis OCT systems were used to obtain retinal measurements and circumpapillary RNFL thickness for each participant. An LDF was calculated using all retinal and RNFL OCT measurements. Receiver operating characteristic (ROC) curves were plotted and compared among the LDF and the standard parameters provided by OCT devices. Sensitivity and specificity were used to evaluate diagnostic performance. A validating set was used in an independent population to test the performance of the LDF. The optimal function was calculated using the RNFL thickness provided by Spectralis OCT, using the 768 points registered during peripapillary scan acquisition (grouped to obtain 24 uniformly divided locations): 18.325 + 0.056 × (315°-330°) - 0.122 × (300°-315°) - 0.041 × (285°-300°) + 0.091 × (255°-270°) + 0.041 × (225°-240°) + 0.183 × (195°-210°) - 0.108 × (150°-165°) - 0.092 × (75°-90°) + 0.051 × (30°-45°). The largest area under the ROC curve was 0.967 for the LDF. At 95% fixed specificity, the LDF yielded the highest sensitivity values. Measurements of RNFL thickness obtained with the Spectralis OCT device differentiated between healthy and AD individuals. Based on the area under the ROC curve, the LDF was a better predictor than any single parameter.

Retinal and Choroidal Thickness Changes after Single Anti-VEGF Injection in Neovascular Age-related Macular Degeneration: Ranibizumab vs Bevacizumab

To evaluate and compare the effects of single intravitreal injection of ranibizumab and bevacizumab on central retinal and choroidal thickness in patients with neovascular age-related macular degeneration (AMD). Forty eyes of 40 patients with neovascular AMD that underwent intravitreal injection of vascular endothelial growth factor inhibitors (anti-VEGFs) were included. Patients were randomized into 2 groups: 20 eyes received ranibizumab and 20 eyes received bevacizumab injection. Central retinal and choroidal thicknesses of all eyes at baseline and 1 month postinjection scans were measured with Fourier-domain optical coherence tomography (OCT). Student t test and Mann-Whitney U test were used to compare the data. The mean central retinal thickness (CRT) showed significant decrease after single injection of ranibizumab (from 345.0 µm to 253.5 µm, p&lt;0.01) and bevacizumab (from 329.5 µm to 251.0 µm, p&lt;0.01) at the first month, respectively. There was no significant difference regarding the CRT change between groups (p = 0.39). The mean choroidal thickness decreased from 158.6 µm (115-317) to 155.5 µm (111-322) in the ranibizumab group and from 211.5 µm (143-284) to 201.5 µm (93-338) in bevacizumab group. The decrease was not significant between groups (p = 0.35). Intravitreal injection of both ranibizumab and bevacizumab provided a significant decrease in CRT; however, the agents caused no significant change in choroidal thickness. Additionally, no difference between ranibizumab versus bevacizumab was observed related to macular edema inhibition.

Retinal nerve fibre layer and macular thickness analysis with Fourier domain optical coherence tomography in subjects with a positive family history for primary open angle glaucoma

AimTo detect early structural changes of retinal nerve fibre layer (RNFL) and macular ganglion cell complex (GCC) in subjects with a positive family history for primary open angle glaucoma (POAG)...

The impact of lumen size and microvascular resistance on Fourier-domain optical coherence tomography (FD-OCT) coronary measurements

[1] Yarmohammadi H, Dalfardi B, Rezaian J, Ghanizadeh A. Al-Akhawayni's description of pulmonary circulation. Int J Cardiol 2013;168:1819–21. [2] Yarmohammadi H, Dalfardi B, Kalantari Meibodi M, Ghanizadeh A. Ibn al-Quff (1233–1286 AD), genius theorist of the existence of capillaries. Int J Cardiol 2013;168: e165. [3] Dalfardi B, Mahmoudi Nezhad GS, Mehdizadeh A. How did Haly Abbas look at the cardiovascular system? Int J Cardiol 2014 Mar 1;172(1):36–9. [4] Daneshfard B, Yarmohammadi H, Dalfardi B. The origins of the theory of capillary circulation. Int J Cardiol 2014 Mar 15;172(2):491–2. [5] Khan IA, Daya SK, Gowda RM. Evolution of the theory of circulation. Int J Cardiol 2005;98:519–21. [6] Joosse NP, Pormann PE. Archery, mathematics, and conceptualizing inaccuracies in medicine in 13th century Iraq and Syria. J R Soc Med 2008;101:425–7. [7] Shahawi MM. Al-Tibb min al-Kitab wa-al-Sunna (Medicine from the Book and the Life of the Prophet) by Abd al-Latif al-Baghdadi. Beirut: Alem-o al-Kotob; 2005163 [In Arabic]. [8] Raupp P, Hassan JA, Varughese M, Kristiansson B. Henna causes life threatening haemolysis in glucose-6-phosphate dehydrogenase deficiency. Arch Dis Child 2001;85:411–2. [9] Mohkam M, Roozrokh M. Medicine and nephro-urology in ancient Iran: Part II: Avicenna and his art of uroscopy. J Pediatr Nephrol 2013;1:41–3. [10] Esfahani MM. The Key to the Mujiz by Jamal al-Din Muhammad ibn Muhammad Aqsara'i. Tehran: Tehran University Press; 200895–6 [In Arabic].

Simultaneously measuring the refractive index and thickness of an optical sample by using improved fiber-based optical coherence tomography

An improved structure for the optical coherence tomography (OCT) scheme based on a 4×4 fiber coupler for simultaneously measuring the refractive index and thickness of optical samples is presented. The proposed structure incorporates the optical path length difference of interference light and is used to calculate the refractive index and thickness of an unknown sample without any prior knowledge of the sample parameters. Two methods (time-domain and Fourier-domain OCT) of obtaining information about an unknown sample are proposed, and a high-speed high-resolution OCT system was developed.

Doppler Optical Coherence Tomography

Optical Coherence Tomography (OCT) has revolutionized ophthalmology. Since its introduction in the early 1990s it has continuously improved in terms of speed, resolution and sensitivity. The technique has also seen a variety of extensions aiming to assess functional aspects of the tissue in addition to morphology. One of these approaches is Doppler OCT (DOCT), which aims to visualize and quantify blood flow. Such extensions were already implemented in time domain systems, but have gained importance with the introduction of Fourier domain OCT. Nowadays phase-sensitive detection techniques are most widely used to extract blood velocity and blood flow from tissues. A common problem with the technique is that the Doppler angle is not known and several approaches have been realized to obtain absolute velocity and flow data from the retina. Additional studies are required to elucidate which of these techniques is most promising. In the recent years, however, several groups have shown that data can be obtained with high validity and reproducibility. In addition, several groups have published values for total retinal blood flow. Another promising application relates to non-invasive angiography. As compared to standard techniques such as fluorescein and indocyanine-green angiography the technique offers two major advantages: no dye is required and depth resolution is required is provided. As such Doppler OCT has the potential to improve our abilities to diagnose and monitor ocular vascular diseases.

Reliability and validity of Cirrus and Spectralis optical coherence tomography for detecting retinal atrophy in Alzheimer’s disease

To evaluate and compare the ability of two Fourier-domain optical coherence tomography (OCT) devices to detect retinal and retinal nerve fibre layer (RNFL) atrophy in patients with Alzheimer's disease (AD) compared with healthy subjects; to test the intra-session reliability of two OCT devices in AD patients and healthy subjects. AD patients (n=75) and age-matched healthy subjects (n=75) underwent three Macular Cube 200 × 200 protocols using the Cirrus and Spectralis OCT devices and three 360° circular scans centred on the optic disc using the Cirrus OCT device, the classic glaucoma application, and the new Nsite Axonal Analytics application of the Spectralis OCT instrument. Differences between healthy and AD eyes were compared, and measurements provided by each OCT protocol were compared. Reliability was measured using intraclass correlation coefficients and coefficients of variation. Correlations between OCT measurements and disease duration and severity were also analysed. Retinal thinning was observed in AD eyes in all areas except the fovea using both OCT devices. RNFL atrophy was detected in AD eyes with all three protocols, but the Nsite Axonal application was the most sensitive. Measurements by the two OCT devices were correlated, but differed significantly. Reliability was good using all protocols, but better with the glaucoma application of Spectralis. Mean RNFL thickness provided by the Nsite Axonal application correlated with disease duration. Fourier-domain OCT is a valid and reliable technique for detecting subclinical RNFL and retinal atrophy in AD, especially using the Nsite Axonal application. RNFL thickness decreased with disease duration.

Predicting transepithelial phototherapeutic keratectomy outcomes using Fourier domain optical coherence tomography.

The aim of this study was to use Fourier domain optical coherence tomography to predict transepithelial phototherapeutic keratectomy outcomes. This is a prospective case series. Subjects with anterior stromal corneal opacities underwent an excimer laser phototherapeutic keratectomy (PTK) combined with a photorefractive keratectomy using the VISX S4 excimer laser (AMO, Inc, Santa Ana, CA). Preoperative and postoperative Fourier domain optical coherence tomography images were used to develop a simulation algorithm to predict treatment outcomes. Main outcome measures included preoperative and postoperative uncorrected distance visual acuities and corrected distance visual acuity. Nine eyes of 8 patients were treated. The nominal ablation depth was 75 to 177 μm centrally and 62 to 185 μm peripherally. Measured PTK ablation depths were 20% higher centrally and 26% higher peripherally, compared with those for laser settings. Postoperatively, the mean uncorrected distance visual acuity was 20/41 (range, 20/25-20/80) compared with 20/103 (range, 20/60-20/400) preoperatively. The mean corrected distance visual acuity was 20/29 (range, 20/15-20/60) compared with 20/45 (range, 20/30-20/80) preoperatively. The MRSE was +1.38 ± 2.37 diopters (D) compared with -2.59 ± 2.83 D (mean ± SD). The mean astigmatism magnitude was 1.14 ± 0.83 D compared with 1.40 ± 1.18 D preoperatively. Postoperative MRSE correlated strongly with ablation settings, central and peripheral epithelial thickness (r = 0.99, P < 0.00001). Central islands remained difficult to predict and limited visual outcomes in some cases. Optical coherence tomography measurements of opacity depth and 3-dimensional ablation simulation provide valuable guidance in PTK planning. Post-PTK refraction may be predicted with a regression formula that uses epithelial thickness measurements obtained by optical coherence tomography. The laser ablation rates described in this study apply only to the VISX laser.

Development in anterior segment imaging for glaucoma

Recent developments in anterior segment imaging enable high-resolution imaging with higher speeds than previous systems. This review will focus on advances in optical coherence tomography (OCT) imaging of the anterior segment with an emphasis on angle evaluation. Anterior segment imaging with OCT is possible with both time domain and Fourier domain devices, and with light sources of different wavelengths. Recent studies have identified new risk factors for angle closure and prediction models incorporating quantitative angle width parameters have been developed for detection of gonioscopically defined angle closure. Swept source OCT is a variation of Fourier domain OCT that allows three-dimensional imaging of the angle analogous to gonioscopy and novel quantitative parameters have been described for angle measurement with this technology. Fourier domain OCT devices have higher resolution and sampling density than time domain OCT and enable visualization of more angle structures such as the Schwalbe's line and Schlemm's canal. OCT technology is expanding our understanding of angle closure disease. Although it does not replace gonioscopy in angle evaluation, it may help us identify predictors of progression along the angle closure disease spectrum such that treatment may be better targeted.