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Biomedical subjects

G C Woo

Publications and source records attributed to G C Woo.

At least 37 records · Page 2Linked to original sources
PubMed1986

Effect of hemodialysis on contrast sensitivity in renal failure.

After a brief review of the ocular aspects of hemodialysis treatment for renal failure, we report on 10 patients who underwent this treatment. Contrast thresholds over a wide range of spatial frequencies and Snellen visual acuities were measured before and after treatment. Subtle changes in vision can be monitored by the contrast sensitivity function (CSF) technique. We conclude that there are measurable changes in contrast sensitivity after hemodialysis treatment for each of the 10 patients (20 eyes).

Adult↗
PubMed1986

Effect of Fresnel prism dispersion on contrast sensitivity function.

Fresnel prisms are often used in orthoptics for binocular anomalies and in low vision for visual field defects. These prisms are made of optical polyvinyl chloride (PVC) and this material increases chromatic dispersion and produces a loss of contrast. In this study, the effect of chromatic dispersion on contrast sensitivity is determined. Contrast thresholds over a wide range of spatial frequencies are obtained on a number of subjects with the use of Fresnel prisms ranging from 5 to 30 prism dioptres. As predicted there is a decrease in visual acuity and cut-off spatial frequency threshold as prism power increases. Similarly, a decrease in contrast sensitivity at high and medium spatial frequencies is recorded.

Form Perception↗
PubMed1986

The spectacle magnification of focal telescopes.

A generalized expression for spectacle magnification is derived and used to determine the magnification of focal telescopes used to correct ametropia and/or to view objects at finite distances. When applied to low-powered hand-held telescopes, the expression predicts a clinically negligible difference between spectacle and nominal magnification.

Eyeglasses↗
PubMed1985

A distance visual acuity chart incorporating some new design features.

Standardized visual acuity assessments are possible when consideration is given to legibility, acuity row legibility, letter spacing, optotype, row arrangement, and letter size gradation. The relative optotype legibilities reported by Sloan were verified experimentally under simulated conditions of optical defocus. Acuity row component letters were selected to equalize aggregate row legibilities and were arranged to minimize contaminative sequencing dependency. We describe a new acuity chart format with vertical acuity row presentations to eliminate bias from previous reading experience, and testing procedures using this chart.

Humans↗
PubMed1985

Contrast sensitivity function as a diagnostic tool in low vision.

In this case report, a patient's visual loss associated with both cataract and senile macular degeneration is described. The contrast sensitivity function (CSF) as well as Retinometer and Visometer findings indicate that the additional visual loss over a year's time was lenticular, rather than retinal, in origin.

Aged↗
PubMed1985

Contrast attenuation characteristics of iris clipped intraocular lens implants in situ.

In this study contrast thresholds were monitored over a wide range of spatial frequencies for patients with iris clipped implants or Fyodorov and Binkhorst types. These results were compared with those of an age matched normal group and attenuation characteristics derived for the implants and media environment alone. These results show that lenticular implants of this type can produce large reductions in contrast with a wide variety of transfer functions, for example, low pass, band reject, and band pass. These findings highlight the potential inadequacies of the assessment of the visual capabilities of these devices by acuity measures alone. We suggest that a more realistic and abbreviated approach should be based upon supplementing acuity assessment with monitoring of threshold sensitivity for a low, medium, and high spatial frequency sinewave grating.

Humans↗
PubMed1985

Fresnel prism correction for trauma-induced diplopia.

Fresnel prisms placed on the spectacle lenses of a patient with optic atrophy resulting from an accident eliminated constant diplopia in the remaining visual field. A reduction in vision due to Fresnel prisms was found to be more tolerable than diplopia.

Diplopia↗
PubMed1984

Effect of target configuration on the measurement of astigmatism.

In this study, five dissimilar crossed cylinder targets were used to determine the cylindrical component of the refractive error of 30 subjects. Our purpose is to see if variations in target configuration affect the results of the test. Although there are statistical differences when targets with different configurations are used in determining astigmatism subjectively, these differences are not clinically significant if we assume that the least amount of error required for clinical significance is +/- 0.25 D. We conclude that dissimilar crossed cylinder targets yield similar astigmatic findings clinically.

Adult↗
PubMed1983

Fresnel prism therapy for right hemianopia.

Fresnel prisms placed on the spectacle lenses of a patient with right hemianopia provide an easier means for the patient to become aware of objects located in the blind field. Exact location of the prisms is important.

Eyeglasses↗
PubMed1983

An evaluation of the Arden grating test.

The Arden grating test is found to be a statistically reliable tool which provides an index of contrast sensitivity. Although no statistically valid mean score can be assigned to each plate, a range of normals can be established for this particular age group. In this study, after each individual's initial scores were established for each plate they remained constant each time the test was repeated. The scores found for each plate are somewhat less than those established by Arden which may in part be related to the effect of age and to some variation in the mode of presentation by the test administrator. In summary, the Arden grating test when used properly should be a rapid and effective screener. It can reveal visual loss in instances where other clinical techniques fail to show significant findings. It is a reasonable clinical substitution of the contrast sensitivity function test in monitoring or detecting subtle changes in low and medium spatial frequency thresholds. The Arden grating test has recently been updated and is being marketed by the American Optical Company. Use of this test can add another dimension to the clinical ophthalmic examination although clinical trials should first be made against the original AGT.

Adult↗
PubMed1981

A pilot study of contrast sensitivity assessment of the CAM treatment of amblyopia.

Contrast thresholds of varying spatial frequencies were obtained on 3 amblyopic patients before and after the CAM treatment of amblyopia. Results of this study confirm previous findings that in the absence of change of visual acuity, contrast sensitivity function can be changed with the use of this technique. Improvement of contrast sensitivity can occur in older patients and that the sensitivity sometimes can deteriorate when treatment is stopped.

Amblyopia↗