Natalie Hutchings

Swelling of the Human Cornea Revealed by High-Speed, Ultrahigh-Resolution Optical Coherence Tomography

PURPOSE To evaluate the change in thickness of the anterior, stromal, and posterior corneal laminae in response to hypoxia-induced corneal swelling, by means of ultrahigh-resolution optical coherence tomography (UHR-OCT). METHODS A UHR-OCT system, operating in the 1060-nm range, was used to acquire in vivo cross-sectional images of human cornea with a 3.2x10-microm (axial x lateral) resolution in corneal tissue. Corneal edema was induced by inserting a thick, positive-powered, soft contact lens, over which the eye was closed and patched for 3 hours. Tomograms were acquired from eight non-contact-lens wearers. Baseline images were obtained before inducing corneal edema, immediately after removal of the patch and the lens, and then every 15 minutes for approximately 2 hours. All images were postprocessed with a segmentation algorithm to identify the laminae visible in the image. The apical thickness of the laminae (epithelium [EPI], epithelial-Bowmans membrane [Ep-BM] complex, stroma, and endothelial-Descemets membrane [En-DM] complex) were determined at each time interval. RESULTS There was an interaction between time after removal of the hypoxic stimulus and deswelling of the layers (RM-ANOVA; P<0.001). The epithelial and stromal thickness reduced significantly with time (P=0.001; P<0.001, respectively), whereas the Ep-BM and En-DM complexes did not (P>0.50). All layers except the En-DM complex exhibited a biphasic pattern of recovery. CONCLUSIONS UHR-OCT showed regional differences in swelling due to hypoxic provocation. On removal of the hypoxic stimulus, the rate of recovery varied between layers, and all layers except the En-DM complex exhibited a biphasic recovery.

Pointwise Univariate Linear Regression of Perimetric Sensitivity Against Follow-up Time in Glaucoma

PURPOSE The authors compared pointwise univariate linear regression (ULR) of sensitivity against follow-up as an indicator of visual field progression with that of the corresponding ULR of mean deviation (MD) and with the Glaucoma Change Probability (GCP) analysis. The authors determined the influence of the number and sequence of prior examinations on the slope of the pointwise function. METHODS Univariate linear regression was undertaken at each stimulus location on the arbitrarily assigned left eyes of 38 patients with glaucoma examined with the Humphrey Field Analyzer Programs 30-2 or 24-2 (stimulus size III, Humphrey Instruments Inc, San Leandro, CA). The mean age was 59.0 years (standard deviation [SD] = 12.9), the mean number of fields per patients was 12.0 (SD = 2.8), and the mean duration of follow-up was 6.0 years (SD = 1.6). RESULTS Four patients showed statistically significant MD slopes. Of the 34 patients exhibiting a nonsignificant MD slope, 15 exhibited clusters of at least two contiguous progressing locations. Less than half of these locations were designated as progressing by GCP. The GCP detected less than one third of the locations considered progressing by ULR for the last six fields in the series: this was attributed to the nonlinear nature of the decline in sensitivity. CONCLUSIONS The degree of agreement between the outcomes of ULR and GCP was dependent on the quality of the collected data, the magnitude of the baseline sensitivity, the extent and type of the subsequent visual field progression, and the position of the fields within the examination series. Good agreement was illustrated at those locations where the deterioration fell outside the limits of expected variability in stable glaucoma.

The use of fractal analysis and photometry to estimate the accuracy of bulbar redness grading scales.

PURPOSE To use physical attributes of redness to determine the accuracy of four bulbar redness grading scales, and to cross-calibrate the scales based on these physical measures. METHODS Two image-processing metrics, fractal dimension (D) and percentage of pixel coverage (% PC), as well as photometric chromaticity were selected as physical measures, to describe and compare grades of bulbar redness among the McMonnies/Chapman-Davies scale, the Efron Scale, the Institute for Eye Research scale, and a validated scale developed at the Centre for Contact Lens Research. Two sets of images were prepared by using image processing: The first included multiple segments covering the largest possible region of interest (ROI) within the bulbar conjunctiva in the original images; the second contained modified scale images that were matched in size and resolution across scales, and a single, equally-sized ROI. To measure photometric chromaticity, the original scale images were displayed on a computer monitor, and multiple conjunctival segments were analyzed. Pearson correlation coefficients between each set of image metrics and the reference image grades were calculated to determine the accuracy of the scales. RESULTS Correlations were high between reference image grades and all sets of objective metrics (all Pearsons r >or= 0.88, P <or= 0.05); each physical attribute pointed to a different scale as being most accurate. Independent of the physical attribute used, there were wide discrepancies between scale grades, with almost no overlap when cross-calibrating and comparing the scales. CONCLUSIONS Despite the generally strong linear associations between the physical characteristics of reference images in each scale, the scales themselves are not inherently accurate and are too different to allow for cross-calibration.

In vivo volumetric imaging of the human corneo-scleral limbus with spectral domain OCT

The limbus is the structurally rich transitional region of tissue between the cornea on one side, and the sclera and conjunctiva on the other. This zone, among other things, contains nerves passing to the cornea, blood and lymph vasculature for oxygen and nutrient delivery and for waste, CO2 removal and drainage of the aqueous humour. In addition, the limbus contains stem cells responsible for the existence and healing of the corneal epithelium. Here we present 3D images of the healthy human limbus, acquired in vivo with a spectral domain optical coherence tomography system operating at 1060nm. Cross-sectional and volumetric images were acquired from temporal and nasal locations in the human limbus with ~3µm x 18µm (axial x lateral) resolution in biological tissue at the rate of 92,000 A-scans/s. The imaging enabled detailed mapping of the corneo-scleral tissue morphology, and visualization of structural details such as the Vogt palisades, the blood and lymph vasculature including the Schlemm’s canal and the trabecular meshwork, as well as corneal nerve fiber bundles. Non-invasive, volumetric, high resolution imaging reveals fine details of the normal human limbal structure, and promises to provide invaluable information about its changes in health and disease as well as during and after corneal surgery.

A comparison of cup-to-disc ratio measurement in normal subjects using optical coherence tomography image analysis of the optic nerve head and stereo fundus biomicroscopy

Purpose:  To determine the relationships between morphometric optic nerve head (ONH) characteristics defined using optical coherence tomography (OCT) and clinical judgments of cup‐to‐disc (CD) ratios.

The perceived bulbar redness of clinical grading scales.

Purpose. To use a psychophysical scaling method to estimate the perceived redness of reference images of the McMonnies and Chapman-Davies (six reference levels), Institute for Eye Research (four), Efron (five), and Validated Bulbar Redness (five) bulbar redness grading scales. Methods. Regions of interest were cropped out of the grading scale reference images; three separate image sets (color, grayscale, and binarized) were created for each scale, combining to a total of 20 images per image set. Ten naïve observers were asked to arrange printed copies of the 20 images per image set across a distance of 1.5 m on a flat surface, so that separation reflected their perception of bulbar redness; only start and end point of this range were indicated. The position of each image was averaged across observers to represent the perceived redness for this image, within the 0 to 100 range. Subjective data were compared with physical attributes (chromaticity and spatial metrics) of redness. Results. For each image set, perceived redness of the reference images within each scale was ordered as expected, but not all consecutive within-scale levels were rated as having different redness. Perceived redness of the reference images varied between scales, with different ranges of severity being covered by the images. Perception of redness severity depended on the image set (repeated-measures analysis of variance; all p ≤ 0.0002). The perceived redness was strongly associated with the physical attributes of the reference images. Conclusions. Subjective estimates of redness are based on a combination of chromaticity and vessel-based components. Psychophysical scaling of perceived redness lends itself to being used to cross-calibrate these four clinical scales.

Grading bulbar redness using cross-calibrated clinical grading scales.

PURPOSE To determine the between-scale agreement of grading estimates obtained with cross-calibrated McMonnies/Chapman-Davies (MC-D), Institute for Eye Research (IER), Efron, and Validated Bulbar Redness (VBR) grading scales. METHODS Modified reference images of each grading scale were positioned on a desk according to their perceived redness (within a 0 to 100 range) as determined in a previous psychophysical scaling experiment. Ten observers were asked to represent perceived bulbar redness of 16 sample images by placing them, one at a time, relative to the reference images of each scale. Only 0 and 100 were marked on the scale, but not the numerical position of the reference images. Perceived redness was taken as the measured position of the placed image from 0 and was averaged across observers. RESULTS Overall, perceived redness depended on the sample image and the reference scale used (repeated measures ANOVA; P = 0.0008); six sample images had a perceived redness that was significantly different between at least two of the scales. Between-scale correlation coefficients of concordance ranged from 0.93 (IER vs. Efron) to 0.98 (VBR vs. Efron). Between-scale coefficients of repeatability ranged from five units (IER vs. VBR) to eight units (IER vs. Efron) of the 0 to 100 range. CONCLUSIONS The use of cross-calibrated reference grades for bulbar redness grading scales allows comparison of grading estimates obtained with different scales. Perceived redness is dependent on the dynamic range of the reference images of the scale, with redness estimates generally being found to be higher for scales with a shorter dynamic range.

Outcomes of wavefront-guided laser in situ keratomileusis for hyperopia.

PURPOSE: To determine the impact of wavefront‐guided laser in situ keratomileusis (LASIK) for hyperopia on corrected distance visual acuity (CDVA), contrast sensitivity, and higher‐order aberrations (HOAs). SETTING: Centre for Contact Lens Research, University of Waterloo, Waterloo, Ontario, Canada. DESIGN: Case series. METHODS: This study comprised patients who had LASIK for hyperopia. The HOAs up to the 4th order (5.0 mm pupil) in right eyes were analyzed. RESULTS: The study enrolled 31 patients. The preoperative mean values were sphere, +2.60 diopters (D) ± 1.15 (SD) (range +1.00 to +5.00 D); astigmatism, −0.87 ± 0.74 D (range 0.00 to −3.75 D); and spherical equivalent (SE), +2.16 ± 1.00 D (range +0.63 to +4.50 D). The postoperative refractive SE was within ±0.50 D in 71.0% of eyes. The uncorrected distance visual acuity was 20/20 or better in 87.2% of eyes. There was no significant change in CDVA or contrast sensitivity (all P>.05). There was a significant change in Zernike coefficients Z(0,4), Z(−1,3), Z(−3,3), and Z(3,3) (range of mean absolute change ± SEM: 0.092 ± 0.013 μm to 0.208 ± 0.018 μm; all P<.001). Coefficients Z(−1,3) and Z(3,3) increased and Z(0,4) and Z(−3,3) decreased in magnitude; each had a reversal of sign. The only significant postoperative association was between low‐contrast CDVA and Z(0,4) (r = 0.617, P<.001). CONCLUSIONS: Low‐contrast CDVA after LASIK surgery for hyperopia was correlated with Z(0,4). There were no significant changes in CDVA or contrast sensitivity. The relative risk for losing low‐contrast CDVA increased if more than 1 Zernike term had a sign change. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned.

Eye and head movement alterations in naïve progressive addition lens wearers.

Aims:  To examine adaptation to progressive addition lenses (PALs) by investigating eye and head movement strategies of individuals naïve to PAL wear; to determine whether adaptation is a short‐term change in strategy or is maintained with continued PAL wear; to determine if adaptation strategies are different between PAL designs.

A comparison of cup‐to‐disc ratio evaluation in normal subjects using stereo biomicroscopy and digital imaging of the optic nerve head

The cup‐to‐disc (CD) ratio evaluated using stereoscopic biomicroscopy was compared with that evaluated by viewing a non‐stereo digital image of the optic nerve head. Twenty normal subjects (mean age 26 ± 4 years) were evaluated by two observers. The average CD ratio was 0.29 with a range of 0–0.80. The intra‐class correlations between the observers ranged from 0.82 to 0.96, when comparing horizontal and vertical CD ratios for both evaluation techniques. The mean CD ratio was significantly smaller when viewing the digital image (0.25 ± 0.01) compared with stereoscopic observation (0.33 ± 0.03, p < 0.0001). Although the mean differences between the two techniques were small, the percentage of CD ratios that differed by ≥0.2 were between 5 and 25% of evaluations. Caution should be exercised when using stereoscopic and non‐stereo digital evaluations of CD ratio interchangeably to assess longitudinal progression in a multi‐clinician setting.