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Biomedical subjects

D F Edgar

Publications and source records attributed to D F Edgar.

7 recordsLinked to original sources
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↗
PubMed1992

Construction of a model eye and its applications.

Construction details are given of a model eye, based on the Bennett and Rabbetts schematic eye. It incorporates a cornea, lens, and spherical fundus. Distilled water filled the anterior and vitreous chambers. By means of a micrometer screw, the vitreous chamber depth can be precisely varied to produce axial ametropia from +11 to -17 D. Readings taken over the greater part of this range with a Topcon Autorefractor RM-A6500 were found to be repeatable and reproducible within +/- 0.25 D. The model eye was used to investigate the relationship between the actual size of a fundus feature and its photographic image in two different fundus cameras. With a Zeiss Oberkochen camera of telecentric design the magnification was found to remain constant whatever the degree of axial ametropia, whereas with a Carl Zeiss Jena camera the magnification varied linearly with ametropia. A technique developed by Littmann for determining the actual size of a retinal feature when using a fundus camera of telecentric design is discussed briefly.

Eye↗
PubMed1992

Effects of different ocular fixation conditions on A-scan ultrasound biometry measurements.

A-scan readings must be taken along the visual axis for accurate axial length measurement. Various fixation targets, which ideally should not induce accommodation, are used to limit eye movements. In this study results using the Humphrey biometer under different fixation conditions were compared to establish the most suitable target. We also investigated corneal applanation effects by comparing optical and ultrasonic measurements of anterior chamber depth. We examined 12 randomly selected normal eyes of 12 young subjects. Three A-scans were recorded for each of five fixation conditions: spotlight at 6 m; Snellen letter at 6 m; solid probe's built-in green fixation target; Snellen letter at 6 m viewed through a -4.00 DS lens; spotlight at 6 m under cycloplegia (the comparison condition). Variability 'within' each fixation condition, estimated by the standard deviation for each subject, was equal throughout the 12 subjects for each fixation condition. Also variation was independent of size of the ocular dimension. The good agreement between the comparison condition and optical anterior chamber depth measurements indicated that the solid probe produced little applanation. Differences 'between' fixation conditions were investigated. It was concluded that a spotlight or letter at 6 m provides a suitable fixation target for biometry. Built-in fixation targets can produce significant errors.

Adult↗
PubMed1989

Influence of thymoxamine eye-drops on the mydriatic effect of tropicamide and phenylephrine alone and in combination.

In a preliminary experiment in 12 healthy volunteers, one drop of thymoxamine 0.5% instilled into the conjunctival sac completely reversed the mydriasis produced by phenylephrine 2.5%, 5% and 10% after 20 minutes. In a second study in eight volunteers, thymoxamine 0.5% completely prevented the mydriasis produced by phenylephrine 2.5% and produced a miosis. It also completely reversed the mydriasis produced by tropicamide 0.5%. The mydriatic effect of tropicamide 0.5% plus phenylephrine 2.5%, however, was not completely reversed by thymoxamine 0.5% over a period of 180 minutes. Phenylephrine, tropicamide and thymoxamine are freely available for use by registered optometrists.

Adolescent↗
PubMed1988

Reversal of tropicamide-induced mydriasis by thymoxamine eye drops.

In a study in 12 healthy volunteers, local instillation of thymoxamine eye drops (0.5%), completely reversed the mydriasis produced by tropicamide (0.5%), but only incompletely that by tropicamide (1.0%). The difference between these effects was statistically significant (p less than 0.025). The thymoxamine eye drops were well tolerated.

Adult↗