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Biomedical subjects

D F Edgar

Publications and source records attributed to D F Edgar.

27 records · Page 2Linked to original sources
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↗
PubMed1983

Local and systemic effects of pindolol eye drops.

Local (reduction of intraocular pressure) and systemic (inhibition of exercise-induced tachycardia) effects of 1% and 0.25% pindolol eye drops have been studied in a balanced cross-over double-blind trial in 12 healthy volunteers. Both concentrations produced a comparable reduction in intraocular pressure while the inhibition of exercise tachycardia was significantly less with the lower concentration.

Adult↗
PubMed1981

A noninvasive tonometer in the measurement of the effects of pindolol and timolol on intraocular pressure in normal subjects.

A noncontact tonometer was used to demonstrate in normal human volunteers the ocular hypotensive effects of single instillations of pindolol (1%) and timolol (0.25 and 0.5%) eye drops. Falls in intraocular pressure were significantly correlated with resting intraocular pressure. Evidence of systemic cardiovascular effects was seen in reductions in resting heart rate.

Adolescent↗
PubMed1981

The effect of 0.25% and 0.5% pindolol in intraocular pressure in normal human volunteers.

A study was carried out in 12 healthy volunteers to determine the effects of 0.25% and 0.5% pindolol eye drops on intraocular pressure. The results showed that both concentrations produced similar and highly significant falls in intraocular pressure after single instillation into the conjunctival sacs, the effect being measured with a non-contact tonometer. No significant changes were found in resting heart rate.

Adolescent↗