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Biomedical subjects

D B Henson

Publications and source records attributed to D B Henson.

17 recordsLinked to original sources
PubMed1992

The effect of orthoptic treatment upon the vergence adaptation mechanism.

This paper is a review of the research work that has been carried out over the past few years investigating the ability of the oculomotor system to adapt to prism-induced heterophoria. Our results show that subjects with normal binocular vision can adapt to horizontal and vertical prism-induced heterophorias whether fixating at distance or near. Further studies have shown that subjects with symptomatic abnormal binocular vision have an abnormal adaptation mechanism. Finally, we have found that when orthoptic treatment results in relief from the symptoms, there is an associated improvement in the subjects' ability to adapt to prism-induced heterophoria.

Adaptation, Ocular↗
PubMed1989

Visual field screening and the development of a new screening program.

The diagnosis of chronic open-angle glaucoma requires diligent and careful screening. Intraocular pressure measures and disc evaluations, when used in isolation, give unacceptably high numbers of false positives and false negatives, while full visual field analysis, as conducted with kinetic or full threshold static perimetry, takes too long for routine use. Screening of the visual field may offer a viable alternative. The effect of changing certain parameters of a visual field screening test are evaluated. These include the location of the stimuli, the number of stimuli and their suprathreshold increment. The results of an optimized visual field screening test are then evaluated with respect to the theoretical predictions. The results indicate that high levels of sensitivity and specificity can be achieved with visual field screening tests which are short enough to be included as part of the routine optometric examination.

Glaucoma, Open-Angle↗
PubMed1988

Cluster analysis in visual field quantification.

The central visual fields of 2165 normal and 106 glaucoma eyes were measured using a threshold related suprathreshold strategy. The effects of altering the cluster radius in normals and glaucoma eyes sheds light on the nature of defects in these two groups. It is estimated that approximately 13% of normals have clusters; the great majority of these individuals have one cluster of two defects. Most clusters in normals are formed artefactually due to angioscotoma and/or physiological variations in the blind spot position. Clusters due to other factors occur rarely. Clusters are found with equal frequencies in the superior and inferior fields in normal eyes, but with a greater frequency in the superior field in glaucoma eyes. The use of clusters in quantification is both sensitive and specific. Using results from this large sample and looking at other visual field properties, it is possible to devise weighted probability indices to score visual fields.

Adult↗
PubMed1988

Clinical evaluation of the Non-Contact Tonometer Mark II.

The purpose of this investigation was to test the reliability of the American Optical Non-Contact Tonometer Mark II (NCT II) using the Goldmann Applanation Tonometer (GAT) as the validating instrument. The sample contained 102 consecutive patients from our University Eye Clinic, of whom one-half had 4 NCT II measurements first, followed by 4 GAT measurements; the other one-half had 4 GAT measurements first, followed by 4 NCT II measurements. No significant change in intraocular pressure (IOP) was noted over the measurement sequence with either instrument. There was no significant difference between paired NCT II and GAT readings when the NCT II was used first; however, a highly significant difference between paired readings was obtained when the GAT was used first, indicating that the GAT measurement produced a delayed reduction in the IOP. This effect did not occur with the NCT II. Although the NCT II is shown to have a good overall reliability when compared to the GAT in both protocols, the agreement between any two tonometers may be influenced greatly by the very process of taking a measurement and by the dynamic nature of the IOP.

Adult↗
PubMed1988

Screening for glaucomatous visual field defects: the relationship between sensitivity, specificity and the number of test locations.

The visual field results from 104 eyes with early glaucomatous loss and 1860 normal eyes have been used to derive the relationship between sensitivity, specificity and the number of test locations in a visual field screening test. A logarithmic relationship was found between sensitivity and the number of test locations and a linear relationship between specificity and the number of test locations. The results indicate that large numbers of test locations are not required to reach high levels of sensitivity and that clinically it might be better to screen all patients with a few stimuli quickly than to examine carefully those patients who meet a precondition such as raised intraocular pressure or abnormal disc appearance.

Computers↗
PubMed1986

Frequency distribution of early glaucomatous visual field defects.

The optimization of visual field screening programs for glaucoma requires precise knowledge of where glaucoma defects occur and whether any relation exists between the size of defects and the associated retinal disease. The visual field results from 109 eyes of patients with early glaucomatous damage were investigated with a Friedmann Mark II Visual Field Analyser. The frequency distribution of scotomas as a function of location demonstrated that the tested field could be divided into four zones, one where scotomas occur frequently, one where scotomas occur a reasonable number of times, one where scotomas occur rarely, and one which corresponds to the blind spot (BS) region. The distribution of scotoma size in the first three zones was found to be about the same. Differences among the distributions of defects measured in this study and those of previous studies are attributed to different examination techniques.

Glaucoma↗
PubMed1986

Effects of prolonged forced vergence upon the adaptation system.

In previous studies the ability of the oculomotor system to adapt to prism-induced heterophoria has been shown. The 'phoria that occurs immediately after a prism has been placed before one eye gradually reduces and returns to its original baseline value as the subject is allowed binocular experience while wearing the prism. Little work has been carried out to determine when the adaptation is complete. In this study we report on the results of two experiments designed to determine when adaptation is complete. In the first, the rate of decay of adaptation, i.e. loss of slow fusional vergence, is measured after varying periods of forced vergence. In the second experiment the rate of adaptation to an additional disparity is measured; this should be the same as that before the induced effect if adaptation is complete. The results of this study indicate that although adaptation may appear complete by the criterion that the 'phoria has returned to its baseline value, the adaptation is unlikely to be truly complete for a much longer period of time has passed.

Adaptation, Ocular↗
PubMed1985

Informational content of visual field location in glaucoma.

Information Theory has been applied to produce estimates of how valuable each stimulus location on the Friedmann Visual Field Analyser mk. II is in detecting early glaucomatous visual field defects. The results indicate that stimuli in the superior arcuate region of the visual field and in the inferior nasal quadrant give the maximum amount of information. Stimuli in the extreme superior field (beyond 20 degrees), the inferior temporal quadrant and around the physiological blind spot give the least amount of information. It is suggested that these results can be used to develop computerised techniques for the analysis of visual field data.

Adult↗
PubMed1985

Vergence-adaptive change with a prism-induced noncomitant disparity.

Oculomotor adaptation to a magnified motor field in one eye can occur as a result of the vergence system responding either to (1) the magnification of the field, or (2) via small adaptive fields. This was investigated by monitoring adaptation with the whole oculomotor field magnified in one experiment, and just the central motor field magnified in another experiment. It was shown that adaptation in the periphery occurred at a much slower rate when it was occluded. It is concluded that adaptation in the periphery occurs via adaptive spread from central positions and not via a direct response to the magnification of the oculomotor field.

Adaptation, Physiological↗
PubMed1985

Adaptation to lens-induced heterophorias.

In previous studies we have shown that subjects with normal binocular vision adapt to prism-induced heterophorias. In this study we have continued the work by investigating whether the oculomotor system can also adapt to lens-induced heterophorias. It was found that subjects gradually adapted to the esophoria induced when viewing a distance chart through negative lenses. Adaptation also occurred to lens-induced esophoria and exophoria at near, while at distance, where the visual feedback was unaltered, the phoria remained virtually constant. These findings demonstrate the flexibility of the oculomotor system.

Adaptation, Ocular↗
PubMed1979

Normative and clinical data with a new type of dark adaptometer.

Dark adaptation has been measured with a new type of dark adaptometer in a group of 25 normal subjects, and in a group of 10 patients suffering from a variety of retinal pathologies. The results indicate that this instrument is capable of distinguishing between normal and pathological dark adaptation on the basis of data obtained in two clinical samples.

Adult↗
PubMed1979

Measurement of corneal sensitivity and thickness with PMMA and gas-permeable contact lenses.

Corneal sensitivity and thickness were measured in nine subjects. Two different types of hard contact lenses were fitted to each subject; a traditional polymethylmethacrylate (PMMA) lens on one eye and a gas-permeable cellulose acetate butyrate (CAB) lens on the other eye. Corneal sensitivity did not differ significantly between the two eyes after 6 and 10 hr of wear, but PMMA lenses tended to induce slightly more corneal edema than CAB lenses. As oxygen tension behind such contact lenses is different, whereas mechanical stimulation is nearly the same, this would appear to support the view that corneal sensitivity changes are related to mechanical stimulation. However, careful scrutiny of the data does not support this view unequivocally, and other suggestions are made.

Acetates↗
PubMed1979

Dark adaptation in diabetes mellitus.

The effect of diabetes mellitus on the normal dark adaptation curve is investigated. It has been found that diabetic patients take longer to adapt and that their absolute thresholds are raised. The degree of elevation in the final threshold correlates with the duration of diabetes.

Adolescent↗
PubMed1977

A new dark adaptometer.

A dark adaptometer that was designed for student-laboratory use is described and advocated for use in optometric practice because of its simplicity, accuracy, and low cost. Representative dark-adaptation results obtained from 2 naive and 2 sophisticated subjects are presented.

Dark Adaptation↗