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Biomedical subjects

G C Woo

Publications and source records attributed to G C Woo.

At least 19 recordsLinked to original sources
PubMed2005

Inter-relationships between DNA damage, ascorbic acid and glycaemic control in Type 2 diabetes mellitus.

AIMS: The onset of complications in Type 2 diabetes mellitus (DM) patients cannot be predicted in individuals. Evidence suggests a link between complications and hyperglycaemia, oxidative stress and antioxidants, but causality is unclear. This study investigated baseline (entry) fasting plasma ascorbic acid, lymphocytic DNA damage and glycaemic control in Type 2 DM as part of a long-term study, the aim of which is to explore a biomarker profiling approach to identify and improve outcome in high-risk subjects. METHODS: A cross-sectional study, in which DNA damage, glycated haemoglobin (HbA(1c)), fasting plasma glucose (FPG) and ascorbic acid (AA) were measured on fasting blood samples collected from 427 Type 2 DM subjects. RESULTS: DNA damage was significantly (P < 0.0001) and directly correlated to both FPG (r = 0.540) and HbA(1c) (r = 0.282), and was significantly (P < 0.0001), independently and inversely correlated to plasma AA (r = -0.449). In those subjects with both poor glycaemic control and low AA (< 48 microm, the overall mean value for the study group), DNA damage was significantly (P < 0.005) higher compared with those subjects with a similar degree of hyperglycaemia but with AA above the mean. CONCLUSIONS: The novel finding of a significant inverse relationship between plasma AA and DNA damage in Type 2 DM indicates that poorly controlled diabetic subjects might benefit from increased dietary vitamin C. The data also have important implications for biomarker profiling to identify those subjects who might benefit most from intensive therapy. Longer-term follow-up is underway.

Ascorbic Acid↗
PubMed2001

Vision of low astigmats through thick and thin lathe-cut soft contact lenses.

Distance and near visual acuity of 13 low astigmats were determined in a double-masked experiment through thick and thin (centre thickness 0.12 mm and 0.06 mm, respectively) spherical lathe-cut soft lenses. For each lens type, distance and near LogMAR VA and over-refraction were assessed with different logMAR VA charts. For 70% of the subjects, the residual astigmatism was significantly lower than the refractive astigmatism with thicker lenses. No statistically significant differences in the distance and near logMAR VA was found between the two lens types using any of the charts used, though, in general, logMAR VA obtained through the thicker lens was better than logMAR VA through the thinner lens. The variabilities in distance and near logMAR VA between the two lens types increased with decreased contrast. The variabilities in distance logMAR VA were greater with Chinese charts than with English charts, and LogMAR VA with Chinese charts were significantly worse for both lens types. Based on the results of this study, we concluded that thicker spherical lathe-cut soft lenses provide better vision in low astigmats. The Snellen acuity test is inadequate for vision assessment of soft contact lens wearers. When a patient wearing thin soft contact lenses complains of poor vision in spite of 6/6 or 6/5 Snellen acuity, changing to thicker lenses may be considered.

Journal Article↗
PubMed2001

The effect of conventional CR39 and Fresnel prisms on high and low contrast acuity.

The effect of conventional CR39 and Fresnel prisms on high and low contrast letter acuity was studied. Visual acuity of the fully corrected better eyes of 15 subjects was measured with the high (90%) and low (10%) contrast logMAR letter charts, while they wore prisms of varying power. The results showed that when the power of the conventional CR39 prism and the Fresnel prism reached 10 prism dioptres and 5 prism dioptres, respectively, significant reduction (1 tail t-test, p < 0.05) of the high and low contrast acuity occurred. The Fresnel prism caused a significantly greater acuity reduction than the conventional CR39 prism for powers ranging from 5 to 30 prism dioptres for both contrasts. The rate of acuity reduction with increasing prism power was greater with the low contrast targets than with the high contrast targets for both prisms. In addition, the rate of acuity reduction with increasing prism power was greater with the Fresnel prism than with the conventional CR39 prism for both contrasts. The conventional CR39 prism reduced acuity by a ratio of about 0.8-0.9 of that of the Fresnel prism for powers ranging from 5 to 20 prism dioptres and by about 0.7 for 30 prism dioptres. These ratios applied for both high and low contrast acuity, and therefore were independent of the level of contrast used.

Adult↗
PubMed2000

The calibration of a 2.5x Galilean focusable telescope as an optometer for refraction.

A 2.5x Selsi achromatic Galilean focusable telescope was calibrated for refraction at 6 m. In its calibration, minus power trial lens was placed at the objective of the telescope to simulate vergence of a target at a finite distance, before the back vertex power (BVP) of the telescope at each setting (telescope length) was measured by a focimeter. By using a graphical presentation of the results, the BVP of the telescope at each setting could be determined at different selected target distances. For a target vergence of -0.167 D or a distance of 6 m, the common testing distance in clinical practice, this calibrated telescope had a BVP or refraction measuring range of -7.27(-)+7.52 D. When this telescope was used to measure simulated manifest refractive errors at 6 m, it yielded a mean error of +0.13 D with a 95% confidence limit of agreement of -0.38(-)+0.64 D. These results indicated that the accuracy and precision of telescopic refraction were comparable to that of retinoscopy. Therefore, the calibrated telescope could be considered as a reliable and inexpensive instrument for determining spherical refractive errors. Telescopic refraction is applicable in refracting economically disadvantaged population in underserved areas where modern equipment and electricity are not available. In addition, it provides an alternative subjective refraction method for low vision population because the magnification of this calibrated telescope has the advantage of allowing low vision patients to be refracted at the common 6 m testing distance in clinical practice.

Adult↗
PubMed1998

Scattering properties of Bagolini lenses and their effects on spatial vision.

The effect of a Bagolini lens on spatial vision was investigated by studying its far-field diffraction pattern as produced by a coherent beam of laser light, and its effect on the contrast sensitivity function (CSF) in human subjects. For lenses of the main type studied, which were crossed by a series of slightly-irregular striated bands, each consisting of fine, parallel, etched lines of various widths and separations, the diffraction pattern consisted of undiffracted light giving a bright central spot and wide-angle, diffracted light giving a dim streak. The latter was due to the sum of the diffraction patterns associated by the irregular fine etched lines. The streak produced by a single striated band was modulated by a series of regular maxima and minima related to the width of the band. Analysis of this pattern gave the width of the band as 0.6 mm, in close agreement with direct microscopical measurements. When four bands were illuminated by a beam of about 3 mm diameter, similar to the diameter of the photopic pupil, the diffraction pattern showed no obvious maxima and minima, due to irregularity in the width and separation of the bands. The central spot contained more than 90% of the total light in the diffraction pattern. Thus the Bagolini lens, with its relatively weak far-field diffraction pattern lacking regular maxima and minima when areas > or = 3 mm in diameter were used, was expected to have only a small effect on the apparent contrast of the targets in CSF experiments. This was confirmed by the measurements: Bagolini lenses showed no significant effect on either the monocular or binocular CSF. Further similar measurements with lenses of slightly different design from another manufacturer confirmed these findings. Therefore Bagolini lenses do not disrupt vision when they are used to determine the presence of suppression and anomalous retinal correspondence.

Contrast Sensitivity↗
PubMed1997

The validity of current clinical tests of contrast sensitivity and their ability to predict reading speed in low vision.

PURPOSE: Contrast sensitivity (CS) testing using chart tests of CS is becoming increasingly common in low vision assessment. Yet we know little about the validity of these charts, i.e. which region of the spatial frequency spectrum is being measured. In this study we aimed to determine the validity of currently available CS charts by comparison against oscilloscope-based CS. We also determined their relative ability to predict reading speed. METHODS: CS was measured with five commercially available charts and the contrast sensitivity function was determined with sinusoidal gratings presented on a Joyce screen using a two-alternative forced choice staircase technique in 36 observers with low vision and 3 with normal vision. Reading rate was also measured with the subject reading with his or her own optical low vision aid. RESULTS: The results show that the Pelli-Robson chart and the Cambridge gratings are good measures of medium to low spatial frequencies, as would be predicted from their design, while the Regan and UW charts correlated with medium to high frequencies. The Vistech chart was a good predictor of CS at each spatial frequency. CONCLUSIONS: The best chart test of CS depends on which region of the CS curve is of interest. All the charts were good predictors of reading rate.

Adult↗
PubMed1993

Notch in contrast sensitivity function of optical origin: diffraction effects of acrylic filters.

A series of retinal image degrading filters was evaluated by measuring the contrast sensitivity function of four human subjects through the filters (residual CSF). The acrylic filters, with regularly spaced cross-hatches, produced progressively more reduction in the residual CSF as the density of the cross hatching increased. For some of the filters there was a selective loss of a narrow band of spatial frequencies as a result of diffraction effects. This experiment serves to further emphasize the need to rule out optical causes of such notches in the CSF before making a diagnosis of neurological dysfunction.

Acrylic Resins↗
PubMed1992

Normal values of eye position in the Chinese population of Hong Kong.

Exophthalmos, interpupillary distance (IPD), interobital distance (IOD), and inner intercanthal distance (ICD) were measured in an adult Chinese population from Hong Kong (HKC). Mean values and normal range for 95% of the population were determined and the relation with head size and body height examined. Values of exophthalmos and IPD in our Chinese population were similar to those given for Caucasian groups. IOD and ICD were larger in the Chinese than in adult Caucasians. Our findings show generally larger values in the HKC than has been found for other populations in mainland China.

Adolescent↗
PubMed1990

Current methods of treating and preventing myopia.

In this review, we discuss and compare current methods of treating and preventing myopia including radial keratotomy, keratomileusis, keratophakia, epikeratoplasty, keratokyphosis, scleral reinforcement, phakoemulsification, and heat application. Among the visual training methods are such procedures as biofeedback and behavior modification. The use of drugs, orthokeratology, spectacles, bifocals, prisms, intraocular lenses, contact lenses, and ultrasound are described.

Humans↗
PubMed1988

Effect of magnification and field of view on reading speed using a CCTV.

Reading speeds were measured in 18 subjects with normal vision and 10 with low vision for each of 20 experimental conditions with different magnifications and field sizes on the Closed Circuit Television System (CCTV). There was a significant difference between the results for the two groups of subjects. These results suggested that in low-vision patients with faster reading speeds, minimum magnification for maximum field size on the CCTV would be valid advice. For low-vision patients who read more slowly, reading speed may improve at higher magnifications despite reduced field size.

Adult↗
PubMed1987

Optical management of posterior chamber lens tilt.

A pseudophakic patient reported blurred vision and diplopia one year after the surgery. We suspected that the origin of both these problems was subluxation of the intraocular lens. His blurred vision was eliminated by a cylindrical spectacle lens at an oblique axis. Monocular diplopia could only be eliminated occasionally. The displacement of the IOL is shown photographically in this report.

Adult↗
PubMed1987

Telescopic scanning and age-related maculopathy.

We report the management of various aspects of the care provided over a span of 12 years for a patient with age-related maculopathy (ARM). The use of a low-power telescope proved to be particularly helpful because, with it, she learned eccentric viewing and made best use of her remaining vision.

Aged↗
PubMed1987

Tolerance to visual defocus.

Low-resolution optical systems are more tolerant to defocus than are high-resolution systems. We wished to determine whether this principle applies to human vision. We used psychophysical methods to measure the effects of defocus in normal eyes under low-resolution conditions. Modulation transfer of sine-wave gratings was measured as a function of dioptric defocus at low and medium spatial frequencies. We defined the depth of focus at a given spatial frequency to be the dioptric range for which the modulation transfer exceeds 50% of its peak value. For dilated pupils, depth of focus increased from about 2.5 diopters (D) at 3.5 cycles/deg to about 17 D at 0.25 cycles/deg. From our results we predicted that tasks requiring only low spatial frequencies will be more tolerant to defocus than tasks requiring higher spatial frequencies. This prediction was confirmed in a letter-recognition experiment. The increasing tolerance to defocus at low spatial frequencies also implies that individuals with low acuity will be more tolerant to defocus than people with normal vision. We confirmed this prediction by measuring tolerance to defocus in 30 low-vision eyes.

Humans↗