The medical records of glaucoma patients were obtained from the database of the Catholic Medical Center Glaucoma Progression Study (CMC-GPS) which commenced in 2009 at Seoul St. Mary’s Hospital, Seoul, South Korea. The work was approved by our institutional review board of Seoul St. Mary’s Hospital and we followed all relevant tenets of the Declaration of Helsinki. We enrolled all consecutive eligible patients who were willing to participate, and all gave written informed consent.
All glaucoma patients enrolled in the CMC-GPS underwent a complete ophthalmic examination, including a review of medical history, measurement of best-corrected visual acuity, refraction assessment, slit-lamp biomicroscopy, gonioscopy, Goldmann applanation tonometry, measurement of central corneal thickness via ultrasound pachymetry (Tomey Corp., Nagoya, Japan), measurement of axial length with ocular biometry (IOL Master; Carl Zeiss Meditec, Dublin, CA, USA), dilated stereoscopic examination of the optic disc, red-free fundus photography (Canon, Tokyo, Japan), and Cirrus optical coherence tomography (OCT; Carl Zeiss Meditec). Starting from 2017, patient underwent additional OCT-A (DRI OCT Triton; Topcon, Tokyo, Japan) examinations. All disc hemorrhage detected on the color disc and fundus photography during follow-up were recorded.
Open-angle glaucoma was defined by the presence of a glaucomatous optic disc (exhibiting diffuse or localized rim thinning, a notch in the rim, or a vertical cup-to-disc ratio ≥ 0.2 than that of the other eye); a VF finding consistent with glaucoma (a cluster of ≥3 non-edge points on the pattern deviation plot with a probability of <5% of the normal population, with one of these points having a probability of <1%), a pattern standard deviation (PSD) with a P-value < 5%, or a Glaucoma Hemifield Test result consistently outside the normal limits on two VF examinations either on FDT perimetry or SAP as confirmed by two glaucoma specialists (H.Y.P. and C.K.P.); and an open angle evident on gonioscopy. Additional inclusions were: a best-corrected visual acuity ≥ 20/40, a spherical refraction within ±6.0 diopters (D), a cylinder correction within ±3.0 D, and mean deviation (MD) under -6dB on SAP. The exclusion criteria were: a history of any retinal disease, including diabetic or hypertensive retinopathy; a history of eye trauma or surgery with the exception of uncomplicated cataract surgery; any optic nerve disease apart from glaucoma; and a history of systemic or neurological diseases that might affect the VF. If both eyes of an enrolled patient met all inclusion and exclusion criteria, one eye was randomly chosen for study.
Visual field testing
All subjects underwent SAP using 24-2 SITA standard programs with a Humphrey field analyzer II 750i (Carl Zeiss Meditec, Dublin, CA). Goldmann size III targets with diameters of 0.43° were presented. FDT perimetry was performed using the 24-2 program with 5° stimuli, spatial frequency of 0.5 cycles/deg, and temporal frequency of 18Hz with the FDT Humphrey Matrix (Carl Zeiss Meditec). All the VF tests were done within no more than 3 months. The sequences of VF test were not standardized. Reliable tests were defined as <15% fixation losses, false positives, or false negatives.
To define SAP-FDT difference, we used MD and PSD of both SAP and FDT perimetry. We subtracted MD and PSD of FDT from those of SAP, respectively. Additionally, we compared the number of abnormal points depressed as P < 5% and P < 1% on the pattern deviation plot on both SAP and FDP perimetry. We compared the number of abnormal points of FDT from those of SAP, and eyes were classified to have significant SAP-FDT difference when the number of abnormal points were more than 4 points on FDT (Fig. 1).
Among all patients, eyes with peripheral nasal step (PNS) and parafoveal scotoma (PFS) were determined based on pattern deviation probability plots of FDT perimetry. PFS subjects had an isolated glaucomatous VF defect within twelve points of a central 10˚ radius. PNS subjects had an isolated glaucomatous VF damage within the nasal periphery outside 10˚ fixation.
Optic Nerve Head Parameters
According to the mechanical theory of glaucoma, posterior deformation of the LC is a principle pathologic event.45 A potential indicator of the LC morphology is the LC depth (LCD), because LCD is correlated with the magnitude of the posterior LC deformation.46,47 The efficacy in discriminating between glaucomatous eye and healthy eyes was better for LC curvature index (LCCI) than, therefore LC curvature may be a parameter for LC morphology.48,49
Heidelberg spectralis OCT provides up to 40,000 A-scans/s with a depth resolution of 7㎛ in tissues, and a transverse resolution of 14㎛ in images of ocular microstructures. Enhanced depth imaging (EDI)-OCT B-scans around optic nerve head (ONH) (6mm optic cube scans) were obtained using the spectralis OCT. Each section was obtained using eye tracking and incorporated an average of at least 35 OCT frames. Images with a quality score > 15 were obtained (~65–70 sections per eye). LCD and LCCI were measured based on the average images, as presented in previous studies.48,50 Measurements were done using caliper function of the OCT software by an observer (KSA). The measurements were performed in the superior mid-peripheral, center, and inferior mid-peripheral regions, which were scanned throughout the ONH.
LCD was determined by measuring the distance from a line connecting the edges of Bruch’s membrane opening (BMO), reference line, perpendicular to the level of the anterior LC surface. The average of three measurements of each region, as explained above was used to derive the LCD.
For measurement of LCCI, a reference line (LC surface reference line) was set in each B-scan by connecting the two points on the anterior LC surface that met with the lines drawn from each Bruch’s membrane termination point perpendicularly to the BMO reference line. The length of reference line was defined as width (W). The LCD was defined as the maximum depth from the reference line to the anterior LC surface. Then, LCCI was calculated as (LCD/W) X 100. LCCI and W were measured at three regions each, and then calculated LCCI respectively. The average of three LCCIs was used to derive the LCCI.
Peripapillary Vessel Density
Swept-source optical coherence tomography (SS-OCT) was performed using the deep-range imaging (DRI)-OCT system (Topcon, Tokyo, Japan) with a wavelength of 1050 nm and scan speed of 100 000 A-scans per second, using Topcon OCT-A ratio analysis algorithm. Optical coherence tomography angiography of the DRI OCT generated en face images via automated layer segmentation around the optic nerve head into 4 layers. The deep layer peripapillary choroidal microvasculature in the relevant region was evaluated using en face images generated via automated layer segmentation of signals from the retinal pigment epithelium. Clear images with quality score over 30 were analyzed. To measure choroidal VD, the boundaries of the optic disc and β-zone peripapilary atrophy (PPA) were delineated using ImageJ software (National Institutes of Health). Eyes without β-zone PPA were excluded. An 8-bit binary slab was created according to the mean threshold algorithm of ImageJ, which automatically computed the threshold value as the mean of the local grayscale distribution. After assigning white pixels as vessels and black pixels as background, parapapillary choroidal VD was defined as a percentage of vessel pixels within the β-zone PPA region relative to the total area of the β-zone PPA.
The deep-layer parapapillary microvasculature in the relevant region was evaluated using en face images generated by automated layer segmentation of signals from the retinal pigment epithelium, which extended to the outer scleral border. Microvascular dropout (MvD) was defined as focal, sectoral capillary dropout within the visible microvascular network. MvD was identified based on a dropout width > 2-fold that of the visible juxtapapillary microvessels. Two independent observers (H.Y.P and S.A.K.) blinded to the clinical data identified MvD. Disagreements were resolved by a third observer (C.K.P.). Only clear images (quality scores > 30 and no motion blurring) were analyzed.
Interobserver difference in MvD identifications were evaluated using к coefficients. Differences in continuous variables between two groups were compared using independent t-test, while categorial variables were compared using χ2 test. To determine the clinical variables associated with the difference of SAP and FDT, univariate and multivariate logistic regression analyses were performed. Variables with a P value less than 0.10 in univariate analysis were entered into multivariate analysis. Relationships between difference of MD and PSD between two VFs, deep peripapillary VD, and optic nerve head parameters, such as LCD, LCCI, and LC thickness were analyzed in linear regression analysis. P value < 0.05 indicated statistical significance. All statistical analysis was performed using SPSS software version 21.0 (IBM cop., Armonk, NY, USA).