Canonical Grading Scales of Corneal and Conjunctival Staining Based on Psychophysical and Physical Attributes.

Purpose: In this study, we apply psychophysical scaling principles based on physical (photometric) attributes of images to better understand the factors involved in clinician judgement of ocular surface staining and, using that knowledge, to develop photographic scales for the assessment of staining for dry eye (DE) and related conditions. Methods: Subjects with noninfectious ocular surface staining were enrolled at five clinical sites. Following instillation of fluorescein, photographs of corneal staining were taken every 30 seconds for at least 5 minutes. The same procedure was followed for conjunctival staining after instillation of 2 µl of 1% lissamine green. A subset of the best corneal and bulbar conjunctival staining images were anonymized and a spectroradiometer measured photometric attributes (luminance and chromaticity). The images were scaled psychophysically by study investigators, who participated in constructing grading scales based on physical and psychophysical analyses. The final grading scales were refined following consultation with outside DE experts. Results: Photographs were collected from 142 subjects (81% women), with an average age of 58 ± 17 years; 89% were diagnosed with DE. There was a monotonic relationship between between physical measurements and psychophysically scaled staining of both corneal (fluorescein) and bulbar (lissamine green) staining. Michelson contrast and u' (chromaticity) accounted for 66% and 64% of the variability in the psychophysically scaled images of fluorescein corneal and lissamine green conjunctival staining, respectively. Translational Relevance: This paper provides examples of the first ever clinically usable ocular surface staining scales validated using psychophysical scaling and the physical attributes (luminance and chromaticity) of the staining itself. In addition, it provides a generalizable method for the development of other clinical scales of ocular appearance.

Isoflurane and ketamine:xylazine differentially affect intraocular pressure-associated scotopic threshold responses in Sprague-Dawley rats.

PURPOSE: Amplitudes of electroretinograms (ERG) are enhanced during acute, moderate elevation of intraocular pressure (IOP) in rats anaesthetised with isoflurane. As anaesthetics alone are known to affect ERG amplitudes, the present study compares the effects of inhalant isoflurane and injected ketamine:xylazine on the scotopic threshold response (STR) in rats with moderate IOP elevation. METHODS: Isoflurane-anaesthetised (n = 9) and ketamine:xylazine-anaesthetised (n = 6) rats underwent acute unilateral IOP elevation using a vascular loop anterior to the equator of the right eye. STRs to a luminance series (subthreshold to -3.04 log scotopic cd s/m2) were recorded from each eye of Sprague-Dawley rats before, during, and after IOP elevation. RESULTS: Positive STR (pSTR) amplitudes for all conditions were significantly smaller (p = 0.0001) for isoflurane- than for ketamine:xylazine-anaesthetised rats. In addition, ketamine:xylazine was associated with a progressive increase in pSTR amplitudes over time (p = 0.0028). IOP elevation was associated with an increase in pSTR amplitude (both anaesthetics p < 0.0001). The absolute interocular differences in IOP-associated enhancement of pSTR amplitudes for ketamine:xylazine and isoflurane were similar (66.3 ± 35.5 vs. 54.2 ± 24.1 µV, respectively). However, the fold increase in amplitude during IOP elevation was significantly higher in the isoflurane- than in the ketamine:xylazine-anaesthetised rats (16.8 ± 29.7x vs. 2.1 ± 2.7x, respectively, p = 0.0004). CONCLUSIONS: The anaesthetics differentially affect the STRs in the rat model with markedly reduced amplitudes with isoflurane compared to ketamine:xylazine. However, the IOP-associated enhancement is of similar absolute magnitude for the two anaesthetics, suggesting that IOP stress and anaesthetic effects operate on separate retinal mechanisms.

Prevalence of visual impairment and uncorrected refractive error - report from a Canadian urban population-based study.

PURPOSE: The prevalence of visual impairment due to uncorrected refractive error has not been previously studied in Canada. A population-based study was conducted in Brantford, Ontario. METHODS: The target population included all people 40 years of age and older. Study participants were selected using a randomized sampling strategy based on postal codes. Presenting distance and near visual acuities were measured with habitual spectacle correction, if any, in place. Best corrected visual acuities were determined for all participants who had a presenting distance visual acuity of less than 20/25. RESULTS: Population weighted prevalence of distance visual impairment (visual acuity <20/40 in the better eye) was 2.7% (n = 768, 95% confidence interval (CI) 1.8-4.0%) with 71.8% correctable by refraction. Population weighted prevalence of near visual impairment (visual acuity <20/40 with both eyes) was 2.2% (95% CI 1.4-3.6) with 69.1% correctable by refraction. Multivariable adjusted analysis showed that the odds of having distance visual impairment was independently associated with increased age (odds ratio, OR, 3.56, 95% CI 1.22-10.35; ≥65 years compared to those 39-64 years), and time since last eye examination (OR 4.93, 95% CI 1.19-20.32; ≥5 years compared to ≤2 years). The same factors appear to be associated with increased prevalence of near visual impairment but were not statistically significant. CONCLUSIONS: The majority of visual impairment found in Brantford was due to uncorrected refractive error. Factors that increased the prevalence of visual impairment were the same for distance and near visual acuity measurements.

Human corneal adaptation to mechanical, cooling, and chemical stimuli.

PURPOSE: To psychophysically investigate adaptation in human corneas using the Belmonte pneumatic esthesiometer. METHODS: Twenty, 8, and 20 healthy subjects were enrolled in the mechanical, cool, and chemical experiments, respectively. Thresholds were estimated using an ascending method of limits and three intensities (subthreshold, threshold, and suprathreshold, in random order) were each presented 10 or 20 times, and subjects scaled the intensity of the stimuli (0-4 [no stimulus to very intense stimulus]). Friedman nonparametric ANOVA was used to analyze the rating data. RESULTS: There was measurable adaptation with both mechanical and cool stimuli. For both suprathreshold mechanical and cool stimuli, the earlier stimuli were rated more intensely than subsequent stimuli (both P < 0.05). However, this was not the case for subthreshold and threshold mechanical and cool stimuli (all P > 0.05). Paradoxically, for the chemical stimuli, there was adaptation to threshold stimuli (P = 0.03) but no adaptation for subthreshold and suprathreshold stimuli (P = 0.19 and 0.11, respectively). CONCLUSIONS: Both mechanical (mechanosensory or polymodal) and cold receptors on human corneas show adaptation to repeated suprathreshold stimuli with a reduction in perceived intensity after multiple exposures to the same physical stimulus intensity. This is in accord with the results found in electrophysiological and psychophysical experiments of somatosensation elsewhere in the body (and in other animals). The response to chemical stimuli was different, and this might reflect proximal and distal neural or stimulus-specific effects.

Corneal, limbal, and conjunctival epithelial thickness from optical coherence tomography.

PURPOSE: To compare human central corneal, limbal and bulbar conjunctival epithelial thickness in vivo using an Optical Coherence Tomographer (OCT). METHODS: Thirteen healthy human subjects participated in this study. An OCT (Carl Zeiss, Meditec, Dublin, CA) was used to image central cornea, temporal corneo-scleral limbus and bulbar conjunctiva of the left eye. Two images were taken at each location. Thirty central measurements were averaged from each image for quantifying epithelial thickness. RESULTS: In addition to the central cornea and limbal region, a band corresponding to bulbar conjunctival "epithelium" is apparent in OCT images, with respective thicknesses of 54.7 +/- 1.9 microm (mean +/- SD), 79.6 +/- 7.4 microm and 44.9 +/- 3.4 microm that are statistically significant different (repeated measures analysis of variance p < 0.01, post hoc test shows all p < 0.01). CONCLUSIONS: This study demonstrates that it is possible to image the epithelial tissue in humans in vivo using optical coherence tomography, and in these subjects, the corneo-limbal epithelium is the thickest, while the bulbar conjunctival epithelium is the thinnest and the corneal epithelium has intermediate thickness.

Corneal, limbal, and conjunctival epithelium of bovine eyes imaged in vitro by using a confocal laser scanning microscope.

PURPOSE: To provide a description of the distribution and cell morphology, by using fluorescent markers and confocal laser scanning microscopy, of the corneal, limbal, and conjunctival epithelium of bovine eyes in vitro. METHODS: Fresh enucleated bovine eyes were dissected within 2 hours postmortem. Central cornea, limbus, and bulbar conjunctiva were imaged with confocal microscopy after staining with acridine orange (AO) or calcein-acetoxymethyl and ethidium homodimer-1. Epithelial thickness, cell density, cell lamination, and cell morphology were evaluated at these 3 locations. RESULTS: Corneal epithelium was the thickest, and the conjunctival epithelium was the thinnest. The cell morphology was similar to that found in previous histologic studies, and the cell density gradually decreased from the basal to superficial layers. Nuclear AO staining particles increased from basal to superficial cells. Limbal superficial epithelial cells showed less AO staining than corneal and conjunctival superficial cells. CONCLUSIONS: Confocal results of the corneal central, limbal, and conjunctival morphology are similar to those found in traditional microscopic observations. Bovine central corneal epithelium is thicker than limbal epithelium. However, the nuclear AO staining pattern of unfixed ocular surface epithelium of bovine eyes in vitro might represent the cell differentiation status. With the aid of the fluorescence dye, confocal laser scanning microscopy can provide unique morphometric information about corneal, limbal, and conjunctival epithelial cells.

Objective and subjective responses in patients refitted to daily-wear silicone hydrogel contact lenses.

PURPOSE: Silicone hydrogel (SiH) lenses offer many physiological advantages for daily wear (DW) in addition to the continuous-wear modality for which they were originally developed. The purpose of this study was to investigate the clinical performance and physiological responses in a group of successful long-term wearers of conventional hydrogel lenses when refitted with DW SiH contact lenses. METHODS: Eighty-seven successful soft lens wearers (8.4+/-4.7 years of prior lens wear) participated in this study. Bulbar and limbal hyperemia were subjectively graded and digitally photographed for subsequent masked objective evaluation. Subjective symptoms were scored using visual analog scales. In addition, refractive error, corneal curvature, and corneal thickness were measured. All subjects were refitted with Focus Night & Day (lotrafilcon A) SiH lenses; however, to reduce the potential for bias, they were informed that they were being randomly assigned to wear either low oxygen permeability (Dk) lenses or high Dk SiH lenses and were "masked" as to their lens assignment. Subjects returned after 1 week, 1 month, and 2 months of DW, at which time all gradings, photographs, and measurements were repeated. End-of-day subjective symptoms were also graded periodically during the study. RESULTS: Ninety-three percent of subjects were successfully refitted. Both objective and subjective evaluations showed that bulbar and limbal hyperemia decreased significantly in all quadrants during the study (p<0.001), particularly for those subjects with greater baseline hyperemia (p<0.001). Subjects reported a concurrent reduction in end-of-day dryness and improved end-of-day comfort compared with their habitual lenses (p<0.001). No significant changes in refractive error, tarsal papillary response, corneal curvature, or corneal thickness were found during the study. CONCLUSIONS: Hyperemia in contact lens wearers may be attributed to a number of factors, including hypoxia. Refitting existing low Dk lens wearers with SiH lenses on a DW basis can result in a decrease in hyperemia, which may be significant for some subjects and also results in improvements in symptoms of dryness and discomfort.

Comparison of human central cornea and limbus in vivo using optical coherence tomography.

PURPOSE: The purpose of this study was to compare central corneal and limbal total and epithelial thickness using a commercially available optical coherence tomographer. METHODS: A Humphrey-Zeiss Optical Coherence Tomographer (OCT [Carl Zeiss, Meditec, Dublin, CA]) was used to obtain corneal images from 10 subjects. Central corneal and limbal total and epithelial thickness of both eyes were measured using the OCT. Each OCT image comprised 100 measurements, 10 nasal, 10 central, and 10 temporal measurements from each image were analysed. RESULTS: The central corneal and epithelial thickness of the right and the left eyes were 507.9 +/- 35.8 microm, 58.4 +/- 2.5 microm, 506.9 +/- 37.4 microm, and 58.5 +/- 2.5 microm, respectively. There were no differences between eyes (p > 0.05). The nasal and temporal limbal total and epithelial thickness of the right and left eyes were 703.8 +/- 32.1 microm, 704.9 +/- 31.0 microm, 76.8 +/- 3.5 microm, 77.9 +/- 2.9 microm, 704.4 +/- 31.8 microm, 706.3 +/- 32.5 microm, 77.5 +/- 2.8 microm, and 77.8 +/- 2.5 microm, respectively. There were no differences between the nasal and temporal total and epithelial thickness of both eyes (p > 0.05). However, there was a statistical difference between the central corneal and limbal total and epithelial thickness (both p < 0.05). CONCLUSIONS: Central cornea and limbus are measurably different using OCT. Central cornea is thinner than limbus for both total thickness and epithelial thickness. There is no difference between eyes of central corneal and limbal total and epithelial thickness. Optical Coherence Tomography is a useful instrument for in vivo human limbal morphometry.

The inhibitory interaction between human corneal and conjunctival sensory channels.

PURPOSE: To explore human corneal and conjunctival sensory channels at suprathreshold level. METHODS: Ten healthy human subjects participated in the study. The Belmonte pneumatic esthesiometer was used to apply mechanical and chemical stimuli to the central cornea and temporal conjunctiva of the left eye. Stimuli were applied in a paired and unpaired way for conjunctival stimulation. A 100-point visual analog scale (VAS) was used to rate the intensity of the stimulus. RESULTS: The magnitudes of the sensation evoked from the conjunctiva were different when using different methods for presenting stimuli to the ocular surface. When stimuli were applied to the conjunctiva alone, the magnitude of the sensation was stronger than when the stimuli were applied in pairs to the cornea and conjunctiva for both mechanical (P = 0.04) and chemical (P = 0.02) stimulation. CONCLUSIONS: The relatively strong discomfort evoked from the cornea appears to suppress partially the relatively weaker conjunctival stimulation. This manifested as the conjunctival sensory transducer function being shallower (less intense sensation) when immediately preceded by corneal stimulation than when the conjunctival sensory transducer functions were measured alone (unpaired). The underlying mechanism could be adaptation or some other inhibitory effect, such as diffuse noxious inhibitory control. At some level therefore, corneal and conjunctival sensory channels are not independent.

Characteristics of human corneal psychophysical channels.

PURPOSE: To characterize human corneal psychophysical channels. METHODS: Twenty subjects participated in this study. A Belmonte pneumatic esthesiometer was used to deliver stimuli, and the ascending method of limits and the method of constant stimuli were used to estimate thresholds. Sensation was characterized for different stimuli. Corneal mechanical and chemical thresholds were measured at different temperatures. RESULTS: The qualities of the sensations induced by stimuli with different temperatures were different, and the corresponding detection thresholds of the pneumatic stimuli at four temperatures gradually increased (repeated measures ANOVA (F(3,12) = 10.326, P = 0.000). There were no temperature effects on chemical thresholds (repeated measures ANOVA F(3,12) = 0.235, P = 0.870) or mechanical discomfort thresholds from 20 degrees C to 50 degrees C (paired t14 = -0.233, P = 0.818). There were strong interactions when chemical and mechanical stimuli were added. Chemical thresholds were progressively lower when the flow rate increased and mechanical thresholds went down as the percentage of added CO2 increased (repeated measures ANOVA F(3, 12) = 6.407, P = 0.007, F(4, 16) = 19.904, P = 0.000). CONCLUSIONS: This study suggests that humans sense corneal non-noxious cold and noxious mechanical and chemical stimuli, and that the sensitivity of some submodalities can be modulated by others. There are at least five psychophysical channels (non-noxious cold, noxious mechanical, noxious chemical-H+, noxious heat, and itching) processing corneal sensory information. Both decreased corneal chemical thresholds at high flow rates and decreased mechanical thresholds with an added chemical stimulation demonstrate that corneal psychophysical channels are not independent.

Nociceptive sensation and sensitivity evoked from human cornea and conjunctiva stimulated by CO2.

PURPOSE: To compare sensation and sensitivity evoked from human cornea and conjunctiva stimulated by CO2. METHODS: Twenty healthy participants were recruited for the study. Central corneal and temporal conjunctival chemical sensation and sensitivity of only one eye of each subject were evaluated. Air mixed with different concentrations of CO(2) was delivered by a modified Belmonte pneumatic esthesiometer. The ascending method of limits was used to determine the sensitivity and subjects were required to characterize the sensation at threshold. RESULTS: The sensations evoked by CO(2) in the cornea and conjunctiva were stinging or burning. The sensation evoked by mechanical stimulation was that of irritation. The corneal and conjunctival chemical thresholds were 31% +/- 2% and 54% +/- 5% CO(2) (mean +/- SE), respectively. The corneal and conjunctival mechanical thresholds were 80 +/- 6 and 140 +/- 10 mL/min (mean +/- SE), respectively. The corneal sensitivity was significantly higher for both mechanical and chemical stimuli (P < 0.05). CONCLUSIONS: The results suggest that CO(2) stimulates similar corneal and conjunctival nociceptors in that the interpretations were the same (i.e., nociceptive). The central cornea had a higher sensitivity to CO(2) than the temporal conjunctiva, which may reflect a different peripheral innervation, such as different nerve density or different receptor characteristics. Sensations evoked by mechanical and chemical stimulation were different, which suggests that at the peripheral level, the two modalities stimulate two different kinds of molecular receptors or channels and that this information is somehow retained within the nociceptive system.