The influence of low to moderate myopia on retinal nerve fiber layer as assessed by scanning laser polarimetry with variable corneal compensator

PURPOSE: To evaluate the influence of different degrees of myopia on retinal nerve fiber layer (RNFL) as measured by scanning laser polarimetry (SLP) with variable corneal compensator (VCC) in healthy eyes. METHODS: One hundred and seventy-four healthy age-matched subjects with low to high myopic and emmetropic eyes underwent RNFL measurement by means of GDxVCC. The GDx parameters included in the analysis were: Temporal-superior-nasal-inferior thickness (TSNIT) average, Superior average, Inferior average, TSNIT standard deviation, Inter-eye symmetry, NFI, Symmetry, Superior ratio, Inferior ratio, Superior/nasal, Maximum modulation, Superior maximum, Inferior maximum, Ellipse modulation, Normalized superior area, Normalized inferior area, Ellipse standard deviation, Ellipse average. The mean value of each SLP parameter of myopic eyes was compared with the mean value of the same parameter of emmetropic eyes. RESULTS: Mean myopia was 3.9 +/- 1.5 dpt (range: -2 to -8.25 dpt). TNSIT average was higher in myopic group (p = 0.0111), together with Superior average (p = 0.0244), Symmetry (p < 0.0001) and Ellipse average (p = 0.0111). Two ratio parameters, Superior ratio and Inferior ratio, were higher in emmetropic eyes (p = 0.0179 in both cases). In both the myopic and the emmetropic group, all the SLP assessments of the RNFL fell within the normal range, according to the GDx database. CONCLUSIONS: Low to high myopia is not related to clinically relevant variations of SLP parameters, as assessed with GDx-VCC.     

Reproducibility of retinal nerve fiber layer thickness measurements using scanning laser polarimetry in pseudophakic eyes

OBJECTIVE: To assess the reproducibility of retinal nerve fiber layer measurements in pseudophakic normal and glaucomatous eyes using scanning laser polarimetry (GDx, Laser Diagnostic Technologies, Inc., San Diego, CA). PATIENTS AND METHODS: Normal and glaucomatous patients with polymethylmethacrylate posterior chamber intraocular lenses that satisfied entry criteria underwent imaging by two experienced operators. Eyes with posterior capsule opacification or vision less than 20/30 were excluded. Baseline images (mean pixel SD less than 8 mm) were obtained on 3 separate days within a 7-week period. Reproducibility, defined as the pooled within eye variance of these 3 measurements and the coefficient of variation for 12 retardation parameters generated by GDx software were calculated. RESULTS: Eighteen eyes (11 glaucoma, 7 normal) of 15 patients (7 female, 8 male) were enrolled (mean age 78 +/- 6 years). Among glaucomatous eyes, the average mean deviation and corrected pattern standard deviation using achromatic automated perimetry (Zeiss-Humphrey, Dublin, CA) was -3.8 +/- 1.5 dB (range, -1.89 to -5.04 dB) and 4.9 +/- 3.3 dB (range 0 to 11.05 dB), respectively. Coefficient of variation was 10% or less for all retardation parameters except ellipse modulation (20.2%) and neural network number (12.4%). Glaucomatous and normal eyes had similar variability for 8 of 12 (66.7%) retardation parameters. Inferior ratio, ellipse modulation, and superior ratio were significantly less variable in glaucomatous eyes (P = 0.007, 0.02, and 0.04 respectively) than normal eyes. Superior integral was more variable in glaucomatous eyes (P = 0.03). CONCLUSION: Retardation measurements may be obtained in pseudophakic eyes with acceptable reproducibility. Normal eyes and eyes with mild glaucomatous damage have similar variability for most retardation parameters.     

Effect of laser in situ keratomileusis on retinal nerve fiber layer thickness measurements by scanning laser polarimetry

PURPOSE: To determine the effect of laser in situ keratomileusis (LASIK) on retinal nerve fiber layer measurements by scanning laser polarimetry in a large clinical trial. SETTING: Asan Medical Center, University of Ulsan, Department of Ophthalmology, Seoul, Korea. METHODS: Scanning laser polarimetry measurements were performed in 59 eyes of 59 consecutive patients who had LASIK. The mean retardation values of the whole measurement ellipse and of the superior, inferior, temporal, and nasal sectors of the ellipse before and after LASIK were compared statistically. RESULTS: The mean spherical equivalent refractive error was -6.10 diopters (D) +/- 2.34 (SD) (range -1.19 to -15.50 D) preoperatively and -0.31 +/- 0.52 D (range +0.68 to -2.13 D) postoperatively. The mean ablation depth was 94.0 +/- 30.49 microm (range 28.0 to 161.1 microm). The mean postoperative retardation values of all the sectors and of the superior, temporal, inferior, and nasal sectors showed reduced nerve fiber layer thickness (P =.000, P =.000, P =.011, P =.000, and P=.002, respectively). CONCLUSION: Laser in situ keratomileusis affected nerve fiber layer thickness measurements by scanning laser polarimetry. The change in nerve fiber layer thickness was probably due to modification of the corneal architecture. This effect of keratorefractive surgery should be considered when using scanning laser polarimetry as a diagnostic and follow-up tool for glaucoma.     

Effect of corneal polarization axis on assessment of retinal nerve fiber layer thickness by scanning laser polarimetry

PURPOSE: Scanning laser polarimetry uses an anterior segment compensating device that assumes a fixed axis of corneal birefringence, which we call the corneal polarization axis. The purpose of this investigation was to establish the distribution of corneal polarization axes among a population of normal eyes and to evaluate the relationship between corneal polarization axis and posterior segment retardation. METHODS: We constructed a noninvasive slit lamp-mounted device incorporating two crossed linear polarizers and an optical retarder in order to measure the slow axis of corneal birefringence. Normal subjects underwent corneal polarization axis measurement. A subset of eyes underwent scanning laser polarimetry of the peripapillary retinal nerve fiber layer (n = 32) and macula (n = 29), and retardation measurements were evaluated in each group. RESULTS: One hundred eighteen eyes of 63 normal subjects (35 female, 28 male) underwent corneal polarization axis measurement (mean age, 45.5 +/- 17.1 years). Six eyes (5.1%) demonstrated unmeasurable corneal polarization. In the remaining 112 eyes, the mode of the corneal polarization axis distribution was 10 to 20 degrees nasally downward (range, 90 degrees nasally downward to 54 degrees nasally upward). A significant (P <.0001) correlation was observed between fellow eyes (R(2) =.52), with a mean difference of 11.2 +/- 10.5 degrees (range, 0-52 degrees). Corneal polarization axis was significantly associated (R(2) =.52-.84) with retinal nerve fiber layer and macula summary retardation parameters (average thickness, ellipse average, superior and inferior average, superior and total integral; P <.0001 for all groups). CONCLUSIONS: The mean corneal polarization axis among normal corneas is nasally downward; however, considerable intraindividual and interindividual variability exists. The linear relationship between corneal polarization axis and posterior segment retardation parameters is responsible, in part, for the wide distribution of retinal nerve fiber layer thickness data generated by scanning laser polarimetry.     

Correlation between optic disc area and retinal nerve fiber layer thickness: a study on scanning laser polarimetry with variable corneal compensation

Purpose To evaluate if a significant increase of the peripapillary retinal nerve fiber layer (RNFL) thickness can be measured in a sample of healthy eyes by means of scanning laser polarimetry with variable corneal compensation (GDx-VCC) as the optic disc (OD) area increases. Methods One eye each of 232 healthy subjects (mean age: 57.8 years; range:40–70) was considered. Temporal-superior-nasal-inferior-temporal average (TSNIT Avg) and OD area (area within the ellipse placed on inner border of peripapillary scleral ring) values were collected. Ellipse horizontal and vertical diameters provided on printout were used to estimate OD area using the equation: OD area = π × horizontal radius×vertical radius. TSNIT Avg values were plotted against OD area and a multiple linear regression including age calculated. Results Mean OD area was 2.19 mm²±0.45 (range: 1.23–3.59) and mean TSNIT Avg was 54.3 μ ± 3.2 (range: 45.8–61.8). Multiple linear regression equation was TSNIT Avg=57.7−0.096×OD Area−0.055×Age (Pearson r=−0.146: p=0.086) Conclusion In our sample of healthy eyes, no significant correlation was found between TSNIT Avg and OD area. In spite of a shorter distance between OD and measurement ellipse margins, larger discs did not necessarily show a thicker RNFL. Probably the large inter-subject variability for RNFL thickness, and then for axonal count, was a predominant factor over OD area. None of the authors have any financial or proprietary interest with products cited in the text.     

Effect of cataract surgery and foldable intraocular lens implantation on retinal nerve fiber layer as measured by scanning laser polarimetry with variable corneal compensator

PURPOSE: Scanning laser polarimetry (SLP) enables the measurement of retinal nerve fiber layer (RNFL) thickness in vivo. As SLP measurements can be influenced by several conditions such as corneal birefringence, the new version GDx Access is implemented by the variable corneal compensator (VCC). The aim of this study was to evaluate the influence of cataract extraction and foldable intraocular lens (IOL) implantation on SLP parameters measured by GDx Access. METHODS: Sixty-eight eyes of 68 patients undergoing phacoemulsification with foldable IOL implantation (silicone and acrylic) were examined by SLP before and after surgery. SLP was performed using GDx Access (with VCC). Mean values for SLP parameters were compared before and after 30 days from surgery with paired two-tailed Student's t-test. RESULTS: No statistically significant differences were found between SLP parameters before and after cataract surgery, and regardless of the type of IOL implanted. CONCLUSIONS: Phacoemulsification and foldable IOL implantation do not influence RNFL parameters, as measured by GDx Access.     

The effect of laser-assisted in situ keratomileusis on retinal nerve fiber layer measurements obtained with scanning laser polarimetry

PURPOSE: Scanning laser polarimetry provides indirect measurements of retinal nerve fiber layer thickness by measuring the retardation of polarized laser light as it passes through the retinal nerve fiber layer. Laser-assisted in situ keratomileusis is a refractive technique by which corneal stromal photoablation is achieved by the structural and refractive changes induced by the excimer laser. Both techniques are becoming widely performed and there is some evidence that scanning laser polarimetry measurements are significantly changed after laser-assisted in situ keratomileusis. The authors performed a larger study to determine whether laser-assisted in situ keratomileusis induces predictable and consistent alterations in the measurement of retinal nerve fiber layer thickness. METHODS: Retinal nerve fiber layer thickness measurements using scanning laser polarimetry were performed 1 week before and 1 week after laser-assisted in situ keratomileusis in 30 consecutive eyes (16 patients) undergoing the procedure at The Eye Institute. RESULTS: Mean +/- SD preoperative spherical equivalent refraction was -4.66 +/- 2.40 D, mean ablation depth was 73.0 +/- 36.23 microm, and mean patient age was 40.6 +/- 8.7 years. Six of 12 retinal nerve fiber layer thickness measurements showed significant change (P < 0.05) after laser-assisted in situ keratomileusis. There was no correlation with corneal ablation depth (P > 0.05) for all parameters. CONCLUSION: Laser-assisted in situ keratomileusis significantly affects retinal nerve fiber layer thickness measurements obtained by scanning laser polarimetry. Sequential images should be interpreted with caution and new postoperative baseline images should be obtained.     

Nerve fiber layer measurement using scanning laser polarimetry with fixed corneal compensator in normal cynomolgus monkey eyes

PURPOSE: The purpose of this study was to examine retinal nerve fiber layer thickness in normal cynomolgus monkeys using a scanning laser polarimeter with a fixed corneal compensator (GDx FCC), and to clarify the reproducibility and symmetries (right-left differences) between both eyes for the GDx parameters. METHODS: GDx parameters were measured in 36 normal eyes of 18 cynomolgus monkeys aged 4.0-5.5 years. The retardation values (thickness parameters) at peripapillary and macular areas obtained from the GDx FCC were measured and calculated thickness, ratio, and modulation parameters in the superior and inferior quadrants. Mean and standard deviation (SD), coefficient of variation (CV), and binocular differences were obtained for each parameter from three independent measurements made during a 1-week period. Correlation between both eyes in macular retardation and baseline values, which indicated the combined minimum retardation values for the nasal and temporal quadrants, and between macular retardation and baseline values were analyzed. RESULTS: The intraocular pressure values (mean +/- SD, n = 18) obtained for the right and left eyes were 20.7 +/- 3.8 and 20.0 +/- 3.2 mm Hg, respectively (no significant differences in both eyes). No significant differences between right and left eyes were detected for any GDx parameters. All parameters showed small right-left differences. The CVs (SD/mean x 100) for all parameters were less than 10%. Highly significant correlations were seen between bilateral eyes for macular retardation (r = 0.936, p < 0.0001) or baseline values (r = 0.946, p < 0.0001). A significant correlation (r = 0.883, p < 0.0001) was also seen between macular retardation and baseline values. CONCLUSIONS: Considering individual differences in corneal birefringence, GDx parameters obtained from a GDx FCC may be useful for the objective evaluation of time-related changes in individual eyes or for binocular comparisons in cynomolgus monkeys.     

Assessment of nerve fiber layer thickness before and after laser in situ keratomileusis using scanning laser polarimetry with variable corneal compensation

PURPOSE: To determine the effect of laser in situ keratomileusis (LASIK) on retinal nerve fiber layer (RNFL) thickness measurements obtained by scanning laser polarimetry with variable corneal compensation (SLP-VCC). SETTING: Gimbel Eye Centre, Calgary, Alberta. METHODS: Retinal nerve fiber layer thickness measurements were performed in both eyes of 25 consecutive healthy patients the day of LASIK surgery and 1 month after by trained examiners using the GDx-VCC nerve fiber analyzer. Thickness measurements and all other parameters provided by the software of the machine before and after LASIK were analyzed using the paired Student t test. RESULTS: Mean age of the patients was 39 years +/- 9.6 (SD) (range 24 to 57 years). The mean preoperative spherical equivalent was -4.15 +/- 1.76 diopters (D) (range -1.0 to -7.50 D) and the mean postoperative spherical equivalent, 0.12 +/- 0.39 D (range -0.75 to +1.00 D). Mean ablation depth was 62 +/- 23 mum. No statistically significant difference was found in SLP parameters after LASIK (P<.05). No clinically significant difference in RNFL thickness measurements was noted in any eye. CONCLUSION: These data suggest that SLP-VCC mean thickness measurements are not influenced by LASIK-induced alterations in corneal architecture. Measurements obtained with SLP-VCC before surgery may be used for future comparisons.     

Retinal nerve fiber layer thickness measurements with scanning laser polarimetry predict glaucomatous visual field loss

PURPOSE: To assess whether baseline retinal nerve fiber layer (RNFL) measurements obtained with a scanning laser polarimeter, the GDx Nerve Fiber Analyzer, (Laser Diagnostic Technologies Inc., San Diego, California) are predictive of development of repeatable glaucomatous visual field damage in glaucoma suspect eyes. DESIGN: Cohort study. METHODS: Participants were recruited from the UCSD longitudinal Diagnostic Innovations in Glaucoma Study (DIGS). One eye from each of 160 glaucoma suspects with normal standard automated perimetry (SAP) visual fields at baseline was studied. Study eyes were divided into convert and nonconvert groups based on the development of three consecutive glaucomatous visual fields during follow-up. SLP parameters, IOP, vertical cup disk ratio, stereophotograph assessment as glaucoma or normal, corneal thickness, and visual field indices were included in univariate and multivariate Cox proportional hazards models to determine which SLP RNFL and ocular parameters were predictive of visual field conversion. RESULTS: Sixteen (10%) eyes developed repeatable visual field damage (converts) and 144 (90%) did not (nonconverts). Mean (95%CI) follow-up time until visual field conversion for convert eyes was 2.7 (1.7, 3.6) years. Mean total follow-up of nonconvert eyes was 3.8 (3.5, 4.1) years. Four out of thirteen examined baseline SLP parameters and baseline SAP Mean Deviation (MD), SAP Pattern Standard Deviation (PSD), and glaucomatous stereophotograph assessment were significant univariate predictors of visual field conversion. In multivariate models adjusted for age, IOP and CCT, SLP parameters inferior ratio, ellipse modulation, and UCSD linear discriminant function (LDF) were significant predictors of visual field conversion. When SAP PSD and stereophotograph assessment were also included in the multivariate model inferior ratio and UCSD LDF remained independently predictive of visual field loss. CONCLUSIONS: Thinner baseline SLP RNFL measurements were independent predictors of visual field damage. In addition to thinner SLP RNFL measurements, higher baseline SAP PSD, and baseline glaucomatous stereophotograph assessment each contributed to an increased risk of the development of abnormal visual fields in glaucoma suspect patients. SLP RNFL measurements were independently predictive of future visual loss even when age, IOP, CCT, vertical cup disk ratio, and SAP PSD were included in the model.     

The effect of ageing on retinal nerve fibre layer thickness: an evaluation by scanning laser polarimetry with variable corneal compensation

PURPOSE: To evaluate whether a significant age-related thinning of the retinal nerve fibre layer (RNFL) is measurable by means of scanning laser polarimetry with variable corneal compensation (GDx-VCC) in a sample of healthy eyes. METHODS: A sample of 324 eyes of 324 healthy subjects (mean age 56 +/- 14 years, range 21-85 years) underwent a complete ophthalmic evaluation, standard automated perimetry and RNFL scan with the GDx-VCC. Temporal-superior-nasal-inferior-temporal (TSNIT), superior and inferior average (SA and IA) values provided on the printout were collected and their mean value averaged from all eyes and also after separating the eyes by gender and by age decades. The values of the three parameters were plotted against age and linear regression was calculated. RESULTS: Ageing is associated with a significant RNFL thinning (0.08 micro, 0.16 micro and 0.12 micro per year for TSNIT, SA and IA, respectively; p < 0.001). Analysis showed a similar association with age decade (p < 0.001 on anova). For the TSNIT average, a 9.5% thinning from baseline values was estimated for a 65-year lifespan. Corresponding values for SA and IA were 16.2% and 11.7%, respectively. CONCLUSIONS: Analysis by GDx-VCC confirmed previous reports about significant age-related RNFL thinning. However, a lower rate per year was found, probably because GDx-VCC measurements are much more reliable than those obtained with the previous generation of polarimeters.