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Biomedical subjects

D F Edgar

Publications and source records attributed to D F Edgar.

At least 19 recordsLinked to original sources
PubMed2001

Nonlinear behavior of certain optic nerve head parameters and their determinants in normal subjects.

OBJECTIVE: To investigate the relationship between the absolute area of certain optic disc parameters and a number of ocular dimensional characteristics and demographic factors in young, healthy subjects. DESIGN: Cross-sectional study. PARTICIPANTS: One hundred twenty-one subjects, aged 16.5 to 35.4 years participated, encompassing a wide range of refractive error from +4 diopters (D) to -25.75 D. One eye from each subject was randomly selected for examination. MAIN OUTCOME MEASURES: Absolute size of the optic disc parameters: optic disc area (DA), neuroretinal rim area (NRA), optic cup area (CA), and peripapillary atrophy area (PA). METHODS: Absolute sizes of optic disc parameters were determined from digitized photographic color transparencies, taking into consideration the magnification of the fundus camera and human eye. Relationships between the absolute size of the optic disc features and axial length, crystalline lens thickness, anterior chamber depth, ocular refraction, front surface keratometry, age, and gender were examined. RESULTS: Axial length, lens thickness, front surface keratometry, and age demonstrated positive associations with all optic disc parameters, and anterior chamber depth and ocular refraction demonstrated negative associations in all cases. Multiple regression analysis revealed that the associations observed univariately for age, anterior chamber depth, lens thickness, front surface keratometry, and ocular refraction were confounded by axial length for all disc parameters. After taking logs, DA and NRA exhibit an exponential relationship with axial length, whereas PA and CA exhibit a linear relationship with axial length. DA and NRA increase by approximately 3% per millimeter increase in axial length at 20 mm and up to 40% for DA and 50% for NRA at an axial length of 35 mm. PA and CA are estimated to increase by 26% (10%-44%) and 10% (5%-16%), respectively, per millimeter increase in axial length. Females have approximately 5% smaller DA and 7% smaller NRA than males, but these effects are not statistically significant (P > 0.2). From the multiple regression analysis, females have 31% larger optic cup areas (1%-57%, P = 0.03) and 24% smaller PA (42% smaller to 5% larger P = 0.06) than males, but these effects are of borderline statistical significance at 5%. CONCLUSIONS: Of the ocular biometric factors considered, axial length seems to be the most important predictor of the absolute area of the optic disc parameters. No association exists between any disc parameter and age in this sample of subjects less than 40 years of age. Females exhibited smaller values for DA, NRA, and PA than did males, differences that were not statistically significant, and larger CA (P = 0.03).

Adolescent↗
PubMed1999

Effects of dipivefrin and pilocarpine on pupil diameter, automated perimetry and LogMAR acuity.

BACKGROUND: A study was carried out to ascertain, in ophthalmologically normal subjects, the short-term effects of dipivefrin hydrochloride 0.1% on visual performance and make comparisons with pilocarpine. METHODS: Twelve normal volunteers aged 20-26 years attended on three occasions. One eye, randomly selected, received one drop of either pilocarpine 2%, dipivefrin or saline 0.9%. High- and low-contrast LogMAR acuity at 6 m and pupil diameter (measured by infra-red pupillometry) were recorded at baseline (T0) and at intervals up to 90 min following instillation of drops. Program 30-2 of the Humphrey Visual Field Analyzer (HFA) was run at T0 and at 60 min after treatment instillation (T60). Saline was always instilled at visit 1, to allow for learning effects. On visits 2 and 3 either pilocarpine or dipivefrin was randomly instilled into the treated eye. RESULTS: Pilocarpine significantly worsened the field global indices mean deviation (P < 0.001) and pattern standard deviation (P < 0.01) compared with T0. There was no significant change with dipivefrin. A significant (P = 0.01) pupil dilation from 5.44 mm (SD 0.79) at T0 to 6.19 mm (SD 1.09) at T90 occurred with dipivefrin. Pilocarpine caused significant miosis. No significant changes in LogMAR values were found with dipivefrin. Pilocarpine significantly (P < 0.01) increased LogMAR values (i.e. reduced acuity) compared with dipivefrin. At T30 the mean increase in LogMAR was 0.76 (SD 0.30) for high and 0.83 (SD 0.11) for low contrast. By T90 recovery of acuity was virtually complete. CONCLUSIONS: In normals dipivefrin causes mydriasis but does not affect the central visual field global indices (as assessed by STATPAC), or high- and low-contrast LogMAR acuity. Pilocarpine adversely affects the visual field and both measures of acuity. Knowledge of these effects is of value in glaucoma therapy and when monitoring the progression of visual loss.

Adult↗
PubMed1998

Magnification characteristics of fundus imaging systems.

OBJECTIVE: To compare the magnification properties of 11 different fundus cameras (including 1 stereo fundus camera), a Rodenstock (infrared) scanning laser ophthalmoscope (SLO), the Heidelberg Laser Tomographic Scanner (LTS), and the Heidelberg Retina Tomograph (HRT). DESIGN: A cross-sectional study of the relationship between the true size of a fundus feature and its photographic-computer image in 14 different fundus imaging devices. This relationship was evaluated for each instrument using a model eye adjusted for axial ametropia between +11 diopter (D) and -14 D. To simulate refractive ametropia, the "crystalline lens" was removed to render the model eye aphakic, and the axial length was adjusted to give aphakic ametropia from emmetropia to +20 D. MAIN OUTCOME MEASURES: A correction factor (p) was calculated for each instrument, which can be used in calculations for determining true retinal size. RESULTS: The following were found to be of telecentric construction, Zeiss Oberkochen (WS240 Heidelberg), Zeiss Oberkochen (UK), Zeiss Oberkochen (Cologne), Nikon NF505, Kowa RCXV, SLO prototype (UK), LTS, and the HRT, and each exhibited a constant relationship between p and degree of ametropia of the model eye. The Canon CF6OU, Canon CF6OS, Canon CR4-45NM, Nidek 3-DX, Olympus GRCW, and Carl Zeiss Jena Retinophot were found not to be telecentric and exhibited a linear relationship between p and degree of ametropia of the model eye. For all instruments, p remained unchanged for axial and refractive ametropias of the same degree. CONCLUSIONS: The study has shown that not all fundus imaging systems are telecentric, so the use of a single magnification correction value may not be appropriate. These findings have important implications for the way in which true retinal size calculations are performed. Examples are given to show how the tabulated values of correction factors can be used for both telecentric and nontelecentric cameras in image size calculations.

Fundus Oculi↗
PubMed1998

Comparison of the tolerability and efficacy of unit-dose, preservative-free topical ocular anaesthetics.

Several topical ocular local anaesthetics are available in preservative-free unit-dose applicators. There is little comparative data as to the efficacy and tolerability of these drugs. The purpose of this study was to compare the tolerability, and the depth and duration of corneal anaesthesia following instillation of one drop of 0.4% oxybuprocaine (benoxinate), 0.5% amethocaine, or 0.5% proxymetacaine. The tolerability of each anaesthetic was assessed using a linear ten point arbitrary comfort scale. A group of 14 healthy male subjects, with a wide variation in iris pigment levels, participated in the study (mean age 26.6 years, range 18-40 years). Corneal sensitivity was measured using a slit-lamp mounted Cochet-Bonnet aesthesiometer prior to instillation, and at 1, 2, 5, 10, 15, 20 and 30 min after instillation, and continued if necessary until corneal sensitivity had returned to pre-instillation levels. For each anaesthetic, complete anaesthesia occurred within 1 min of instillation and a return to baseline sensitivity levels occurred by 45 min. No significant difference in anaesthesia was found between the drugs at each time point. Tolerability profiles indicated that proxymetacaine was significantly better tolerated than either amethocaine (p < 0.01) or oxybuprocaine (benoxinate) (p < 0.001). There was considerable inter-subject variability in the duration of anaesthesia, and practitioners should be alert to this when allowing patients to leave the practice following the production of corneal anaesthesia. There seems little to choose clinically between the three active agents as regards clinical effectiveness. Proxymetacaine was significantly better tolerated than either amethocaine or oxybuprocaine.

Administration, Topical↗
PubMed1998

An overview of the current drugs scene.

Ophthalmic drugs, both diagnostic and therapeutic, play a central role in new developments in optometry, particularly as practitioners extend their involvement in primary care. Optometrists now dilate more patients in routine practice, and this brings increasing concern about the risk of provoking acute closed angle glaucoma (CAG) attacks. It would seem that the risk of inducing acute CAG is far outweighed by the potential benefit of examining the eye through a dilated pupil. Driving difficulties, reported subjectively to be the result of glare, are frequently encountered by patients after dilation, and advice from the College of Optometrists is welcomed. The local anaesthetic proxymetacaine 0.5% is now marketed in the UK in preservative-free unit-dose applicators both alone and in combination with fluorescein, and is a more comfortable alternative to existing anaesthetics in unit-dose form. The second half of this paper includes a summary of the background to recent developments, as the UK profession takes its first steps towards the use of ocular therapeutic agents. Finally, some exciting new therapeutic preparations for use in the treatment of glaucoma, allergic eye disease, and dry eye conditions are discussed.

Anesthetics, Local↗
PubMed1998

The effect of age on the light scattering characteristics of the eye.

The rising mean age of our population has increased the need for understanding the physiologic consequences of ageing on visual function. Changes due to age were evaluated using a new scatter test implemented on the P_SCAN 100 pupillometer apparatus (Barbur, 1991). The test yields the full scatter function of the eye and also permits simultaneous measurement of pupil size (Barbur et al., 1995). In addition, contrast sensitivity was measured using sine wave gratings. The 28 subjects had a spherical refractive error between +0.50 DS and -0.25 DS, and astigmatism of less than -0.50 DC, V.A. of at least 6/6, and were ophthalmologically normal. To facilitate statistical analysis, subjects were classified into five groups according to age. For younger subjects (under 45 years), k', the integrated straylight parameter, varied little with age. For this group, k' ranged from 4.9 to 8.1. For subjects aged over 45 years, k' increased with age, ranging from 10.7 to 19.7. One way analysis of variance showed Group 5 (60 year olds) to have significantly greater k' than 20, 30, 40 and 50 year olds (P = 0.000). A slight downward shift in the contrast sensitivity function was seen over the age of 45. Significant differences between older and younger subjects were found at spatial frequencies of 3, and 10 cpd (P = 0.081, P = 0.002, respectively). Pupil diameter was found to reduce with age, but there was no significant difference between groups. Therefore, above 45 years, the ageing eye reveals a more rapid increase in forward scatter, and a reduction in contrast sensitivity, despite apparently good visual acuity.

Adolescent↗
PubMed1997

Improving the prediction of visual field progression in glaucoma using spatial processing.

PURPOSE: The authors show how the predictive performance of a method for determining glaucomatous progression in a series of visual fields can be improved by first subjecting the data to a spatial processing technique. METHOD: Thirty patients with normal-tension glaucoma, each with at least ten Humphrey fields and 3.5 years of follow-up, were included. A linear regression model of sensitivity against time of follow-up determined rates of change at individual test locations over the first five fields (mean follow-up 1.46 years; standard deviation = 0.08) in each field series. Predictions of sensitivity at each location of the field nearest to 1 and 2 years after the fifth field were generated using these rates of change. Predictive performance was evaluated by the difference between the predicted and measured sensitivity values. The analysis was repeated using the same field data subjected to a spatial filtering technique used in image processing. RESULTS: Using linear modeling of the unprocessed field series, at 1 year after the fifth field, 72% of all predicted values were within +/- 5 dB of the corresponding measured threshold. This prediction precision improved to 83% using the processed data. At the 2-year follow-up field, the predictive performance improved from 56% to 73% with respect to the +/- 5 dB criterion. CONCLUSIONS: Predictions of visual field progression using a pointwise linear model can be improved by spatial processing without increased cost or patient time. These methods have clinical potential for accurately detecting and forecasting visual field deterioration in the follow-up of glaucoma.

Adult↗
PubMed1996

Motion discrimination of single targets: comparison of preliminary findings in normal subjects and patients with glaucoma.

BACKGROUND: Luminance, global motion and flicker sensitivities are affected in patients with primary open-angle glaucoma. Although no theoretical model has been put forward to explain the observed reduction in sensitivity in this patient group, these findings have often been attributed to diffuse and selective loss of large optic nerve fibres. METHODS: Movement processing was investigated using an optical projection system that generates smooth, continuous motion with control of speed, displacement and motion direction. Motion-displacement and direction-discrimination thresholds were measured in eight normal subjects and in three patients with diagnosed glaucoma. At each speed tested, targets were presented for a range of displacements and thresholds were extracted after probit analysis. The measurements were carried out both foveally and at 19 degrees in the periphery and provided the data necessary to develop and optimise a model of motion perception based on multiple time delays for the correlation of signals that map progressively more distant parts of the visual field. RESULTS: Our preliminary findings show that direction discrimination can be at chance level even for large displacements when motion is detected 80% of the time. Model simulations show that specific changes in the spatial sampling interval and the speed of transmission of the motion signals involved can cause the observed reduction in motion sensitivity and direction discrimination in patients with glaucoma. CONCLUSIONS: A model for motion detection and direction discrimination of single targets has been proposed to account for the measured functional relationship between motion displacement thresholds and target speed in normal subjects. Tested patients with glaucoma show reduced motion sensitivity and poor discrimination of motion direction. The type of degraded performance observed experimentally in glaucoma patients is also predicted by the model. Such predictions require specific changes in model parameters that may be indicative of changes in the retina caused by the disease.

Adult↗
PubMed1996

Automated static perimetry in myopes with peripapillary crescents--Part II.

Central visual fields were investigated by automated static perimetry, using programs 30-2 and 30-1 of the Humphrey Field Analyzer (HFA), in a group of healthy myopic individuals with peripapillary crescents and tigroid fundus changes only. Results were compared with 'non-myopic' and 'myopic subjects without crescents' control groups. The sample comprised 122 young, healthy volunteers between the ages of 18.5 and 35.4 years, free from any ocular or systemic disease, with refractive errors ranging from +4.00 to -25.75 D. Linear regression results are presented and may be useful in predicting the global indices mean deviation (MD) and mean sensitivity (MS), given either the axial length or ocular refraction, for medium and high myopes. Analysis of the HFA printouts revealed significantly depressed locations in all subject groups. However, locations identified at the 2, 1 and 0.5% probability levels were observed more frequently and more commonly in the myopes with peripapillary crescents than in the control groups. In this non-glaucomatous sample, four myopes had a borderline glaucoma hemifield test and two fell outside normal limits of this test. The superior hemifield, particularly the upper temporal quadrant, appeared to be most affected. Enlargement of the blind spot occurred in some subjects. Several possible explanations are suggested for the visual field sensitivity reduction in myopia.

Adolescent↗
PubMed1995

Automated static perimetry in myopes with peripapillary crescents--Part I.

Central visual fields were investigated by automated static perimetry in a group of healthy myopic individuals with peripapillary crescents and tigroid fundus changes only. Results were compared with 'emmetropic' and 'myopic subjects without crescents' control groups. The sample comprised 122 young, healthy volunteers between the ages of 18.5 and 35.4 years, free from any ocular or systemic disease, with refractive errors ranging from +4.00 D to -25.75 D. Central visual fields were examined using programs 30-2 and 30-1 of the Humphrey Field Analyzer (HFA), which in combination yield a test point resolution of 4.2 degrees within the central 30 degrees. Visual field analysis showed a decline in the differential light sensitivity in myopes with peripapillary changes only, compared with the control groups. These field changes became more pronounced as the degree of myopia, axial length and area of the peripapillary crescent increased. Results suggest that the sensitivity decline of the central field occurs in subjects with axial lengths above 26 mm and more than 5 D of myopia. Knowledge of the likely field defects encountered in myopia will assist practitioners to distinguish between physiological and pathological fields in myopes.

Adolescent↗
PubMed1995

Image processing of computerised visual field data.

BACKGROUND: Computerised perimetry is of fundamental importance in assessing visual function. However, visual fields are subject to patient response variability which limits the detection of true visual loss. METHODS: A method of improving the repeatability of visual field data was demonstrated by applying techniques used in image processing. An illustrative sample of nine normals and nine patients with field loss was used. Two successive Humphrey fields were selected for each subject. Repeatability was defined as the standard deviation of the pointwise differences between sensitivity values of the reference field and repeat field. The field data were then separately subjected to Gaussian and median image processing filters and the repeatability was compared with the unprocessed field results. RESULTS: Improvement in repeatability, by a factor of approximately 2, was demonstrated by both processes. CONCLUSION: These techniques may improve the reliable detection of loss of visual function using computerised perimetry.

Adult↗
PubMed1995

New approach to estimating variability in visual field data using an image processing technique.

AIMS: A new framework for evaluating pointwise sensitivity variation in computerised visual field data is demonstrated. METHODS: A measure of local spatial variability (LSV) is generated using an image processing technique. Fifty five eyes from a sample of normal and glaucomatous subjects, examined on the Humphrey field analyser (HFA), were used to illustrate the method. RESULTS: Significant correlation between LSV and conventional estimates--namely, HFA pattern standard deviation and short term fluctuation, were found. CONCLUSION: LSV is not dependent on normals' reference data or repeated threshold determinations, thus potentially reducing test time. Also, the illustrated pointwise maps of LSV could provide a method for identifying areas of fluctuation commonly found in early glaucomatous field loss.

Adult↗
PubMed1994

Improvements on Littmann's method of determining the size of retinal features by fundus photography.

Littmann's formula relating the size of a retinal feature to its measured image size on a telecentric fundus camera film is widely used. It requires only the corneal radius, ametropia, and Littmann's factor q obtained from nomograms or tables. These procedures are here computerized for practitioners' convenience. Basic optical principles are discussed, showing q to be a constant fraction of the theoretical ocular dimension k', the distance from the eye's second principal point to the retina. If the eye's axial length is known, three new methods of determining q become available: (a) simply reducing the axial length by a constant 1.82 mm; (b) constructing a personalized schematic eye, given additional data; (c) ray tracing through this eye to extend calculations to peripheral retinal areas. Results of all these evaluations for 12 subjects of known ocular dimensions are presented for comparison. Method (a), the simplest, is arguably the most reliable. It shows good agreement with Littmann's supplementary procedure when the eye's axial length is known.

Fundus Oculi↗
PubMed1993

A comparison of the efficacy and duration of action of topically applied proxymetacaine using a novel ophthalmic delivery system versus eye drops in healthy young volunteers.

A novel ophthalmic drug delivery system (NODS) has been developed to give precise and controlled delivery of a drug to the eye. The drug is incorporated into a polyvinyl alcohol flag attached to a carrier. When applied to the eye the flag detaches and gradually dissolves, releasing the drug. We investigated corneal anaesthesia produced by different concentrations of proxymetacaine NODS, and conventional eye drops. Subjects consisted of 28 normal males (mean age 25.3 (SD 3.9) years). Corneal touch sensitivity was measured with a biomicroscope mounted Cochet-Bonnet aesthesiometer. Each subject attended for two visits separated by 7 days. On each visit each eye randomly received one of four proxymetacaine preparations: 44 micrograms, 74 micrograms, 124 micrograms NODS, or 35 microliters of 0.5% proxymetacaine drops (175 micrograms). Corneal touch sensitivity was measured before, and at 1, 2, 5, 10, 15, 20, 30, 45, and 60 minutes following instillation. Complete anesthesia was achieved in the majority of subjects within 1 minute of instillation. The lowest NODS dose (44 micrograms) produced longer lasting anaesthesia than the 35 microliters drop (175 micrograms) (p < 0.05). Higher NODS doses produced a correspondingly greater increase in the duration of anaesthesia. The greater bioavailability achieved by this vehicle allows much lower drug concentrations to be used, reducing the likelihood of systemic adverse reactions.

Adult↗
PubMed1992

Repeatability, reproducibility and intersession variability of the Allergan Humphrey ultrasonic biometer.

We evaluated repeatability and reproducibility of the Allergan Humphrey model 820 ultrasonic biometer. Sixty eyes (30 subjects) free from ophthalmological abnormality were examined on two separate occasions by two experimenters, one experienced and one inexperienced. The 95% confidence limits for repeatability lie between +0.12/-0.13 mm and reproducibility between +0.15/-0.14 mm for AC depth, between +0.11/-0.13 mm and +0.12/-0.16 mm for lens thickness, between +0.17/-0.19 mm and +0.19/-0.18 mm for right axial length and between +0.26/-0.40 mm and +0.22/-0.25 mm for left axial length. The 40% gain used to measure left axial lengths produced poor retinal echospikes, which may account for reduced accuracy compared to the right eyes using 60% gain. Ten subjects (20 eyes) returned for 3 further visits and the variability over 5 sessions was examined using a two-factor ANOVA with repeated measures on one factor. There were no significant experimenter or session effects and no interaction effect. This biometer gives clinically repeatable and reproducible results and is convenient to use.

Anterior Chamber↗