Retinal Nerve Fiber Layer Area Research Articles

Minimising retinal vessel artefacts in optical coherence tomography images

Optical coherence tomography (OCT) is commonly used to investigate the layers of the retina including retinal nerve fiber layer (RNFL) and retinal pigment epithelium (RPE). OCT images are altered by vessels on the retinal surface producing artefacts. We propose a new approach to compensate for these artefacts and enhance quality of OCT images. A total of 28 (20 normal and 8 glaucoma subjects) OCT images were obtained using Spectralis (Heidelberg, Germany). Shadows were detected along the image and compensated by the A-Scan intensity difference from surrounding non-affected areas. Images were then segmented and the area and thickness of RNFL and RPE were measured and compared. 10 subjects were tested twice to determine the effect of this on reproducibility of measurements. Shadow-suppressed images reflected the profile of the retinal layers more closely when assessed qualitatively, minimising distortion. The segmentation of RNFL and RPE thickness demonstrated a mean change of 2.4% ± 1 and 6% ± 1 from the original images. Much larger changes were observed in areas with vessels. Reproducibility of RNFL thickness was improved, specifically in the higher density vessel location, i.e. inferior and superior. Therefore, OCT images can be enhanced by an image processing procedure. Vessel artefacts may cause errors in assessment of RNFL thickness and are a source of variability, which has clinical implications for diseases such as glaucoma where subtle changes in RNFL need to be monitored accurately over time.

Influence of Refractive Error on Optic Disc Topographic Parameters: The Singapore Malay Eye Study

To examine the relationship between refractive error and optic disc topographic parameters and retinal nerve fiber layer (RNFL) thickness. Population-based cross-sectional study. All 3,280 (78.7% response rate) persons of Asian Malay ethnicity 40 to 80 years of age underwent a standardized interview, ocular examinations, and optic disc imaging with the Heidelberg Retina Tomography II (HRT; Heidelberg Engineering). Associations between spherical equivalent (SE) and HRT parameters were evaluated by partial correlation analysis. There were 2,457 (74.9%) right eyes with good-quality HRT images for analyses. When compared with emmetropic eyes, highly myopic eyes (SE < -6.0 diopters) had greater disc area, rim area, rim volume, whereas hyperopic eyes (SE > 0.5 diopters) had significantly smaller disc area, rim area, and cross-sectional RNFL area (all P < .005). In partial correlation after adjustment for age, sex, body mass index, intraocular pressure, and optic disc area, increase of SE (toward hyperopia) was related significantly with greater cup area (partial correlation coefficient, r = 0.058; P = .004) and cup-to-disc area ratio (r = 0.058; P = .004) and with smaller optic disc area (r = -0.058; P = .004), rim area (r = 0.058; P = .004), and rim volume (r = -0.091; P < .001). This population-based study shows that refractive error is significantly (but weakly) associated with HRT parameters of disc area, cup area, rim area, cup-to-disc area ratio, and rim volume. The clinical significance of these associations need to be determined in further studies.

Retinal Nerve Fiber Layer Assessment: Area versus Thickness Measurements from Elliptical Scans Centered on the Optic Nerve

An evaluation of the retinal nerve fiber layer (RNFL) provides important information on the health of the optic nerve. Standard measurements of the RNFL consider only thickness, but an accurate assessment should also consider axial length, size of the optic nerve head (ONH), blood vessel contribution, and distance of the scan from the ONH margin. In addition, although most primate ONHs are elliptical, the circular scan centered on the ONH is the mainstay in both clinical and research analyses. The purpose of this study was to evaluate thickness and area measures of RNFL cross sections when axial length and ONH shape are included. Circular, raster, and radial scans of left eye optic nerves were acquired from 40 normal rhesus monkeys (Macaca mulatta) using spectral domain optical coherence tomography. The disc margin was identified by manually selecting the RPE/Bruch's membrane opening and ONH border tissue. With a pixel-to-micrometer conversion computed from a three-surface schematic eye, RNFL scans were interpolated at 300 to 600 μm (50-μm increments) from the edge of the ONH. The thickness and area of the RNFL at each distance were obtained by custom programs. Blood vessels in the RNFL were selected and removed from the overall RNFL measures. The average RNFL thickness decreased systematically from 149 ± 12.0 μm for scans 300 μm from the disc margin to 113 ± 7.2 μm at an eccentricity of 600 μm (P < 0.05). In contrast, the cross-sectional areas of the RNFL did not vary with scan location from the disc margin (0.85 ± 0.07 mm(2) at 300 μm compared with 0.86 ± 0.06 mm(2) at 600 μm). Blood vessels accounted for 9.3% of total RNFL thickness or area, but varied with retinal location. On average, 17.6% of the superior and 14.2% of the inferior RNFL was vascular, whereas blood vessels accounted for only 2.3% of areas of the temporal and nasal RNFL regions. In nonhuman primates, with appropriate transverse scaling and ONH shape analysis, the cross-sectional area of the RNFL is independent of scan distance, up to 600 μm from the rim margin, indicating that the axonal composition changes little over this range. The results suggest that, with incorporation of transverse scaling, the RNFL cross-sectional area, rather than RNFL thickness, provides an accurate assessment of the retinal ganglion cell axonal content within the eye.

Retinal Nerve Fiber Layer Imaging with Spectral-Domain Optical Coherence Tomography: Pattern of RNFL Defects in Glaucoma

To characterize the distribution pattern, angular width, and area of retinal nerve fiber layer (RNFL) defects in glaucoma using spectral-domain optical coherence tomography (OCT). Prospective, cross-sectional study. We included 113 normal subjects and 116 glaucoma patients. One eye from each individual was randomly selected for Cirrus HD-OCT (Carl Zeiss Meditec Inc., Dublin, CA) RNFL imaging of the 6 × 6-mm² parapapillary region. The RNFL defects were identified in the RNFL thickness deviation map as superpixels coded in red. The angular location and the angular width of RNFL defects were measured. The proportion of area with RNFL measurements within the normal ranges in the RNFL thickness deviation map was expressed as the RNFL area index (RAI): 1 - [area of superpixels coded in red/(6 × 6 - optic disc and parapapillary atrophic area)]. The diagnostic performance between RAI and average RNFL thickness was compared with the area under the receiver operating characteristic curve after adjusting refraction, signal strength, optic disc, and parapapillary atrophic areas. Frequency distribution profiles and distribution patterns of RNFL defects, diagnostic sensitivity and specificity of RAI, and average RNFL thickness. The RNFL defects in glaucoma were most frequently found at the inferotemporal meridian at 284° (80.4%), followed by the superotemporal meridians at 73° (54.2%). The respective proportions of localized (angular width ≤ 30°) and diffuse (angular width > 30°) RNFL defects were 11.4% and 70.5% in mild glaucoma (MD ≥ 6 dB), and 4.2% and 94.5% in moderate to advanced glaucoma (MD < -6 dB). The RAI was 90.2 ± 6.4% and 83.6 ± 7.4% in the mild and moderate to advanced glaucoma groups, respectively. At a specificity of 90.0%, the respective diagnostic sensitivity of RAI and average RNFL thickness was 95.7% (95% confidence interval, 92.2-99.1%) and 94.0% (90.1-99.1%). Analysis of the pattern of RNFL defects with spectral domain OCT imaging offers important insights in understanding the characteristics of RNFL damage. As RNFL defects expand in size as the disease progresses, measurement of the angular width and area of RNFL defects can provide an additional dimension for evaluation of glaucoma.

Longitudinal Variability of Optic Disc and Retinal Nerve Fiber Layer Measurements

To evaluate the longitudinal variability of optic disc and retinal nerve fiber layer (RNFL) measurements obtained from optical coherence tomography (OCT), scanning laser polarimetry (SLP), and confocal scanning laser ophthalmoscopy (CSLO). Forty-five normal and 43 glaucomatous eyes of 88 subjects were analyzed in this longitudinal study. RNFL thickness was measured by OCT (StatusOCT; Carl Zeiss Meditec, Dublin, CA) and SLP (GDx VCC; Carl Zeiss Meditec). Neuroretinal rim area was measured by CSLO (HRT 3; Heidelberg Engineering, GmbH, Dossenheim, Germany). Three separate measurements taken over an average period of 8.8 +/- 1.2 months were used to evaluate measurement variability. Reproducibility coefficient, coefficient of variation, intraclass correlation coefficient (ICC), and sensitivity to change [(97.5 percentile value - 2.5 percentile value)/2 x within-subject standard deviation (Sw)] of the global measures were calculated. The association between RNFL and rim area measurement variability and visual field MD (mean deviation) was evaluated with regression analysis. Low variability was found for global and sectorial rim area and RNFL measurements. The reproducibility coefficient, ICC, and sensitivity to change for OCT average RNFL thickness, GDx VCC TSNIT average, and HRT global rim area were 11.7 microm (95% confidence interval [CI]: 10.5-12.9 microm), 0.97 (0.96-0.98), 10.2 (9.2-11.4); 4.7 microm (4.2-5.1 microm), 0.98 (0.97-0.99), 11.3 (10.2-12.6); and 0.22 mm(2) (0.19-0.24 mm(2)), 0.97 (0.95-0.98), 9.3 (8.4-10.4), respectively. No association was found between OCT (r = 0.010 [95% CI: -0.200-0.219], P = 0.924) and SLP (r = -0.034 [95% CI: -0.241-0.177], P = 0.756) RNFL thickness variances and visual field MD. The association between CSLO rim area variance and visual field MD became insignificant after adjustment for reference height variance. Longitudinal RNFL and neuroretinal rim measurements obtained with OCT, SLP, and CSLO have low variability. As the measurement variability does not change with the severity of glaucoma, these parameters are useful for assessment of glaucoma progression.

Relationship Between Humphrey 30-2 SITA Standard Test, Matrix 30-2 Threshold Test, and Heidelberg Retina Tomograph in Ocular Hypertensive and Glaucoma Patients

To evaluate the relationship between global indices of Humphrey standard automated perimetry (SAP, 30-2 SITA standard test), Humphrey Matrix frequency doubling technology (FDT, 30-2 threshold test), and Heidelberg Retina Tomograph (HRT II) parameters and measure the level of agreement among these 3 tests in classifying eyes as normal or abnormal. The study included 1 eye of 29 ocular hypertensive and 56 glaucoma patients with a mean age of 60.9+/-10.5 years. All subjects had reliable visual fields and HRT measurements performed within a 2-week period. The eyes were classified as normal/abnormal according to visual field criteria and Moorfields regression analysis (MRA). Correlations between visual field indices (mean deviation and pattern standard deviation) and HRT parameters were analyzed using Spearman correlation coefficient (r) and the agreement between the tests in classifying eyes was defined with kappa value. FDT Matrix mean deviation and pattern standard deviation parameters were found to be highly correlated with those of SAP (r=0.66 and 0.69, respectively). Visual field indices showed statistically significant correlations with cup area, rim area, cup/disc (C/D) area, linear C/D, cup shape, mean retinal nerve fiber layer thickness and retinal nerve fiber layer area parameters (P<0.05). Fifty-eight patients (68.2%) had abnormal results at least with 1 of the tests and 21 subjects (24.7%) had abnormal results with all 3 tests. The kappa values were 0.6 for SAP and Matrix (P<0.001), 0.33 for SAP and MRA (P=0.002), and 0.31 for Matrix and MRA (P=0.004). FDT Matrix results are highly comparable with SAP in the assessment of glaucoma. Visual field global indices show statistically significant, but low-moderate correlations with most of the HRT parameters. The agreement among MRA and visual fields for abnormality is fair. Either HRT or visual fields may show the first evidence of glaucomatous damage; therefore, the combination of optic nerve head parameters and visual field results could improve glaucoma diagnosis and follow-up.

Analysis of Retinal Nerve Fiber Layer and Macular Thickness Measurements in Healthy Taiwanese Individuals Using Optical Coherence Tomography (Stratus OCT)

To investigate the reproducibility of peripapillary retinal nerve fiber layer (RNFL) and macular thickness measurements in healthy Taiwanese subjects using optical coherence tomography (Stratus OCT). Fifty-two eyes of 52 healthy Taiwanese subjects (32 females and 20 males) were enrolled in the cross-sectional study. A randomly chosen single eye from each healthy subject underwent thickness measurements using OCT, before and after pupillary dilation, by 3 trained and experienced operators. Average measurements of peripapillary RNFL and macular thickness were calculated. Comparisons of thickness measurements before and after pupillary dilation, and among the 3 operators were performed. The RNFL thickness measurements were higher in the inferior peripapillary area by RNFL scan (average: 107.4+/-17.8 microm) and the macular thickness measurements showed a ring-shaped hump in the 3 mm perifoveal area by macular scan (average: 252.8+/-8.9 microm). Comparing the peripapillary RNFL and macular thickness before and after pupillary dilation, there was no significant difference (P>0.05) in average, superior, inferior, temporal, or nasal peripapillary areas, in total macular volume and foveal thickness, nor in 1, 3, and 6-mm perifoveal areas, whether before or after pupillary dilation and irrespective of which operator performed the measurements. The inferior RNFL area and the 3-mm perifoveal area showed higher thickness measurements in the peripapillary region and macular region, respectively. The thickness measurements performed using OCT showed no significant differences before and after pupillary dilation, and showed good reproducibility.

Diffuse nerve fiber layer loss in normal tension glaucoma.

14 eyes of 14 normal tension glaucoma (NTG) patients with the maximum intraocular pressure (IOP) > or = 19 mmHg and 16 eyes of 16 NTG patients with the maximum IOP < 19 mmHg were examined. All patients had a scotoma confined to the upper or lower hemifield. Eyes with the maximum IOP > or = 19 mmHg showed significantly diffuse retinal nerve fiber layer (RNFL) loss in the RNFL area corresponded to the spared visual hemifield as compared to those with the maximum IOP < 19 mmHg. The results suggests that, even in NTG, IOP may be an important factor causing optic nerve damage.

The retinal nerve fiber layer, neuroretinal rim area, and visual evoked potentials in MS.

In a prospective study of 57 patients with clinically definite, probable, or possible multiple sclerosis (MS), one-half of whom had a history of optic neuritis, retinal nerve fiber layer (RNFL) defects and the neuroretinal rim (NRR) area were quantitatively determined and compared with the visual evoked potential (VEP). There were abnormal VEP latencies in 63% of all patients (definite and probable = 68%, possible = 50%); local or diffuse RNFL defects in 54%, (definite and probable = 54%, possible = 50%); and an abnormally small NRR area in 30% (definite and probable = 32%, possible = 25%). Abnormalities in one or more of the VEP, RNFL, or NRR area occurred in 86% of all patients (definite and probable = 90%, possible = 75%), thus considerably increasing the yield of optic nerve abnormalities over that of the VEP alone. The predominance and extent of the diffuse RNFL defects, which are axonal abnormalities, suggest a more diffuse optic nerve pathology in MS than can be accounted for by a "plaque" pathology and indicate that extensive axonal loss commonly occurs in the optic nerves of MS.