Biomedical subjects

George C Woo

Publications and source records attributed to George C Woo.

10 recordsLinked to original sources

PubMed2007

Myopia prevalence in Chinese-Canadian children in an optometric practice.

PURPOSE: The high prevalence of myopia in Chinese children living in urban East Asian countries such as Hong Kong, Taiwan, and China has been well documented. However, it is not clear whether the prevalence of myopia would be similarly high for this group of children if they were living in a Western country. This study aims to determine the prevalence and progression of myopia in ethnic Chinese children living in Canada. METHODS: Right eye refraction data of Chinese-Canadian children aged 6 to 12 years were collated from the 2003 clinical records of an optometric practice in Mississauga, Ontario, Canada. Myopia was defined as a spherical equivalent refraction (SER) equal or less than -0.50 D. The prevalence of myopia and refractive error distribution in children of different ages and the magnitude of refractive error shifts over the preceding 8 years were determined. Data were adjusted for potential biases in the clinic sample. A questionnaire was administered to 300 Chinese and 300 Caucasian children randomly selected from the clinic records to study lifestyle issues that may impact on myopia development. RESULTS: Optometric records of 1468 children were analyzed (729 boys and 739 girls). The clinic bias adjusted prevalence of myopia increased from 22.4% at age 6 to 64.1% at age 12 and concurrently the portion of the children that were emmetropic (refraction between -0.25 and +0.75 D) decreased (68.6% at 6 years to 27.2% at 12 years). The highest incidence of myopia for both girls ( approximately 35%) and boys ( approximately 25%) occurred at 9 and 10 years of age. The average annual refractive shift for all children was -0.52+/-0.42 D and -0.90+/-0.40 D for just myopic children. The questionnaire revealed that these Chinese-Canadian children spent a greater amount of time performing near work and less time outdoors than did Caucasian-Canadian children. CONCLUSIONS: Ethnic Chinese children living in Canada develop myopia comparable in prevalence and magnitude to those living in urban East Asian countries. Recent migration of the children and their families to Canada does not appear to lower their myopia risk.

Asian People↗

PubMed2005

Diversity in optometric education within and across China: challenges for harmonization.

The development of optometric education and the establishment of formal tertiary educational programs in the Chinese mainland, Hong Kong and Taiwan are described. The various types of degree and diploma programs from medical and technological universities are differentiated. The definition of optometry, as it is understood by the regulatory bodies and the populace, is discussed. There is more diversity in optometric programs within and across Greater China than among some nations. However, due to cultural and linguistic similarities, interactions and interchanges among institutions/associations at regular intervals are of immense value in harmonizing the differences in educational programs and professional practices. Exchange programs, conferences and seminars further enhance the educational standards by exchanging technical knowledge and good clinical practices.

China↗

PubMed2004

Repeatability of the Waterloo Four-Contrast LogMAR Visual Acuity chart and Near Vision Test card on a group of normal young adults.

PURPOSES: To investigate the repeatability of logMAR visual acuity (VA) with the Waterloo Four-Contrast LogMAR Visual Acuity (FCLVA) chart and the Near Vision Test (NVT) card. The differences and agreements between near logMAR VA using horizontally- and vertically-presented letters were also determined. METHODS: Visual acuity of one eye (55 subjects) was first assessed by using the FCLVA chart, comprising four charts of varying contrast, and then with the NVT card (comprising four charts of two contrasts and two presentations). Measurements were repeated after 3 or 4 weeks. RESULTS: No significant between-visit differences were observed for any of the charts used. The repeatability coefficient for the distance 90, 60, 30 and 10% charts were 0.07, 0.11, 0.15 and 0.16 log units, respectively. The repeatability coefficients of the near vision charts were 0.06, 0.04 (high contrast), 0.11 and 0.10 (low contrast) log units. The agreements between horizontally- and vertically-presented letters were good. The differences were similar to the repeatability coefficient of each chart. CONCLUSIONS: The repeatability coefficient of the high contrast distance and near chart was about half a line. Repeatability coefficients increase with decreasing contrasts for both distance and near charts, with coefficients of one line or more for low contrast charts. Variabilities in both distance and near VA increase with decrease in contrast. The presentation of the letters does not affect near VA and the agreement between horizontally- and vertically-presented letters (both high and low contrast) was within the repeatability coefficient of each chart.

Adult↗

PubMed2003

Objective method to measure corneal clarity before and after laser in situ keratomileusis.

PURPOSE: To develop, evaluate, and use an objective method to determine the effect of laser in situ keratomileusis (LASIK) on corneal clarity. SETTING: Centre for Myopia Research, The Hong Kong Polytechnic University, and the Hong Kong Laser Eye Center, Hong Kong, China. METHODS: Color photographs of corneal sections were taken using a digital camera and converted to 8-bit gray-scale images. The desired area of the photograph was isolated using a preset mask, and a gray-scale or corneal clarity index of the desired area was obtained by averaging the "intensity" indices of individual pixels within the area. The reliability of the clarity index measures was determined by comparing test and retest measures. The sensitivity of the method was quantified by its ability to identify a small (clinically undetectable) decrease in corneal clarity produced by tight-fitting soft contact lenses worn for 30 minutes. Finally, corneal clarity was measured and compared in 24 patients before and 1 day, 1 week, and 1 month after LASIK. RESULTS: The reliability value was 4.11 corneal clarity units, and the change in corneal clarity due to soft contact lens use was 16.24 units. In the LASIK patients, there were statistically significant decreases in corneal clarity from preoperatively to 1 day and 1 week but not to 1 month. CONCLUSIONS: The method measured changes in corneal clarity that were undetectable clinically and were 4 times greater than 95% of the differences between test and retest measures. The method is therefore reliable and sensitive. Corneal clarity decreased after LASIK and recovered within approximately 1 month.

Calibration↗

PubMed2002

Contrast sensitivity after laser in situ keratomileusis. one-year follow-up.

PURPOSE: To determine whether contrast sensitivity measurement, a more sensitive test of visual function than visual acuity, better characterizes visual outcomes after laser in situ keratomileusis (LASIK). SETTING: Hong Kong Laser Eye Centre, Hong Kong, China. METHODS: Contrast sensitivity was monitored in 41 LASIK patients for 1 year. Seven spatial frequencies (0.3, 0.8, 1.5, 3.4, 6.9, 10.3, and 20.5 cpd) were tested with 15 sequences per spatial frequency, and a staircase technique was used for target presentation. RESULTS: There was a general depression in the contrast sensitivity function after LASIK; 1.5 cpd and 3.4 cpd were the most affected frequencies. Recovery took at least 6 months. The reduction in contrast sensitivity was greater for higher amounts of myopia. CONCLUSION: The post-LASIK nonpermanent depression in contrast sensitivity was probably due to optical factors.

Adult↗

PubMed2002

Intra-observer and inter-observer repeatability of anterior eye segment analysis system (EAS-1000) in anterior chamber configuration.

BACKGROUND: Anterior chamber configuration can be assessed via optical or ultrasonic techniques. Scheimpflug photography is a non-invasive method measuring the anterior segment. The Anterior Eye Segment analysis system, EAS-1000, utilises the Scheimpflug principle and was found to have good repeatability. Previous repeatability studies, however, have had limitations in their design. The current study investigated the intra-observer and interobserver repeatability of the EAS-1000. METHODS: Twenty-five healthy young subjects were recruited. The anterior chamber angles in different quadrants were measured by two examiners for interobserver analysis. The first examiner repeated the measurement at another session for intra-observer analysis. The 95% limits of agreement and intra-class correlation coefficient (ICC) were calculated. The anterior chamber depth was also measured and compared with ultrasound biometry. RESULTS: The anterior chamber angle assessment demonstrated good intra-observer (ICC ranging from 0.77 to 0.90 for different quadrants) and interobserver (ICC ranging from 0.68 to 0.81 for different quadrants) repeatability. The 95% intra-observer limits of agreement were within +/-5 degrees. The 95% interobserver limits of agreement were within +/-6 degrees. There was no significant difference between male and female subjects or among angles at different quadrants. The anterior chamber depth was found to be repeatable (ICC > 0.90) with 95% limits of agreement +/-0.1 mm. The anterior chamber depth was shallower than that obtained from ultrasound biometry. CONCLUSIONS: The EAS-100 is a non-invasive instrument which is repeatable for measuring the anterior chamber angle and depth. It provides quick results and is good for screening purposes. There is an under-estimation of anterior chamber depth, as previously reported.

Adolescent↗

PubMed2002

Are ethnic differences in the F-M 100 scores related to macular pigmentation?

BACKGROUND: It is known that the macular pigment can significantly affect colour matching and other aspects of colour vision tests. The difference in macular pigmentation between Asians and Caucasians may lead to different colour discrimination. METHODS: This study compared chromatic discrimination between Asians and Caucasians using the Farnsworth-Munsell 100 Hue test. Fifty Asians who were ethnically Chinese and 50 Caucasians served as subjects, ranging in age from 30 to 59 years. RESULTS: The partial blue-yellow square root error score of the Asian subjects was significantly higher than that of the Caucasian subjects (p = 0.022) and the difference appeared to increase with age. DISCUSSION: There was a difference in the F-M 100 scores between the two groups. The difference was confined in the blue-yellow region, producing a tritan-like bias for the Asian group in the test.

Adult↗

PubMed2001

The term magnification.

Many types of magnification described by different authors have dissimilar meanings. Basic principles of magnification such as relative size magnification, relative distance magnification and angular magnification used commonly in low vision are clarified in this paper. The three frequently-used terms of magnification, including apparent magnification, two relative magnification formulae, namely, M = F/4 and M = F/2.5, and iso-accommodative magnification are described. The concepts of equivalent viewing power (EVP) and equivalent viewing distance (EVD) are reviewed as an alternative means of specifying magnifying power. They represent an intrinsic property of an optical system that corresponds to the resolution afforded by the system in terms of dioptric power.

Journal Article↗

PubMed2001

Changes in fixation in the presence of prism monitored with a confocal scanning laser ophthalmoscope.

BACKGROUND: The main optical effect of an ophthalmic prism is to deviate uniformly the entire field of view seen through the prism, resulting in an equal eye movement, if fixation on the same object is to be maintained. Conversely, it has been suggested that, in the case of low vision due to central scotoma, the eye changes its eccentric viewing behaviour when a prism is introduced, that is, the eye remains stationary with a change of retinal image location. METHOD: A new method is described in which retinal image position was recorded with a confocal scanning laser ophthalmoscope in four experimental conditions. RESULTS: Results showed that the normally-sighted eye rotates rapidly to compensate for the introduction of a prism. CONCLUSION: Future studies are required involving subjects with central scotoma but it is likely that many subjects with low vision will also refixate behind a prism and that the prescribing and wearing of bilateral prism in cases of central vision loss will have limited effectiveness in the demonstration or training of eccentric viewing.

Journal Article↗

PubMed2001

Determining the power of a negative lens field expander.

In 1984, Kozlowski, Mainster and Avila derived a formula for the power of the lens needed in the negative lens field expander. Their method is based on the principle of telescopic magnification. In this technical note, we use the principle of spectacle magnification and obtain the power of the same lens. Both methods are presented for comparison. Practitioners may choose either method in determining the power of the negative lens field expander for their low vision patients, but the spectacle magnification method has the advantages of simplicity and familiarity to ophthalmic practitioners.

Journal Article↗