Chea-su Kee

Characteristics of astigmatism as a function of age in a Hong Kong clinical population.

Purpose. To characterize astigmatism as a function of age in a Hong Kong clinical population. Methods. All records from new clinical patients at a university optometry clinic in the year 2007 were used for the study. Only data from subjects with corrected visual acuity ≥6/9 in both eyes and with completed subjective refraction were analyzed. The subjects were divided into seven age groups by decade (i.e., 3 to 10 years, 11 to 20 years, …, >60 years). Refractive errors were decomposed into spherical-equivalent refractive error (M), J0, and J45 astigmatic components for analyses. Internal astigmatism was calculated by subtracting corneal astigmatism from refractive astigmatism (RA). Results. Of the 2759 cases that fulfilled our selection criteria, 58.9% had myopia (M ≥−0.75 D) and 28.4% had RA (Cyl ≥ 1.00 D). The prevalence of RA increased from 17.8% in the 3 to 10 years age group to 38.1% in the 21 to 30 years age group. It then dipped to 25.8% in 41 to 50 years age group but increased again to 41.8% in the >60 years age group. Among the astigmats, almost all 3- to 10-year-old children (92.6%) had with-the-rule (WTR) astigmatism, but a majority of the elderly (>60 years) had against-the-rule (ATR) astigmatism (79.7%). For a subset of subjects who had both subjective refraction and keratometric readings (n = 883), RA was more strongly correlated with corneal (r = 0.35 to 0.74) than with internal astigmatism (r = 0.01 to 0.35). More importantly, the magnitudes of both refractive and corneal J0 were consistent with synchronized decrements (−0.15 and −0.14 D per 10 years, respectively) after the age of 30 years, indicating that the shift toward more ATR astigmatism was related to corneal change. Conclusions. In this Hong Kong Chinese clinical population, the prevalence rates of both myopia and astigmatism increased during the first three decades and shared a similar trend before the age of 50 years. The manifest astigmatism was mainly corneal in nature, bilaterally mirror symmetric in axis, and shifted from predominantly WTR to ATR with age.

A comparison of refractive development between two subspecies of infant rhesus monkeys (Macaca mulatta)

PURPOSEnDifferent subspecies of rhesus monkeys (Macaca mulatta) that are derived from different geographical locations, primarily Indian and China, are commonly employed in vision research. Substantial morphological and behavioral differences have been reported between Chinese- and Indian-derived subspecies. The purpose of this study was to compare refractive development in Chinese- and Indian-derived rhesus monkeys.nnnMETHODSnThe subjects were 216 Indian-derived and 78 Chinese-derived normal infant rhesus monkeys. Cross-sectional data were obtained at 3 weeks of age for all subjects. In addition, longitudinal data were obtained from 10 Indian-derived (male=5, female=5) and 5 Chinese-derived monkeys (male=3, female=2) that were reared with unrestricted vision. Ocular and refractive development was assessed by retinoscopy, keratometry, video-based ophthalmophakometry, and A-scan ultrasonography.nnnRESULTSnAlthough the course of emmetropization was very similar in these two groups of rhesus monkeys, there were consistent and significant inter-group differences in ocular dimensions and refractive error. Throughout the observation period, the Chinese-derived monkeys were on average about 0.4D less hyperopic than the Indian-derived monkeys and the Chinese-derived monkeys had longer overall axial lengths, deeper anterior and vitreous chamber depths, thicker crystalline lenses, flatter corneas and lower powered crystalline lenses.nnnCONCLUSIONSnThe ocular differences observed in this study presumably reflect genetic differences between subspecies but could reflect the differences in the genetic pool between isolated colonies rather than true subspecies differences. Nonetheless, the substantial ocular differences that we observed emphasize that caution must be exercised when comparing and/or pooling data from rhesus monkeys obtained from different colonies. These inter-subspecies differences might be analogous to the ethnic differences in ocular parameters that have been observed in humans.

Impairment of retinal adaptive circuitry in the myopic eye

Previous studies have proposed that the inner retina is affected in myopes. This study aimed to investigate the changes in adaptive circuitry of the inner retina in myopia, using the global flash multifocal electroretinogram (global flash mfERG) with different levels of contrast (luminance modulation). Fifty-four myopes had global flash mfERG recorded with different contrasts. The direct component (DC) and the induced component (IC) of the mfERG response were pooled into six regions for analysis. The response amplitudes and implicit times at different contrasts were also analysed. Results showed that myopes had significant reduction in the paracentral DC amplitude for the 29% and 49% contrasts and in the paracentral IC amplitude at all contrasts measured. The peripheral IC amplitude for the 49% contrast was also reduced. No significant change was found in implicit time for either DC or IC response. Refractive error explained about 14% of the variance in DC and 16% of the variance in IC amplitude respectively; axial length could not account for additional variance in either paracentral DC or IC amplitudes in the hierarchical regression models used. We concluded that the paracentral retinal region in myopes showed signs of impaired retinal adaptation, suggesting a functional loss at the inner retinal layer. In addition, functions attributed to the outer retinal layer showed only small changes due to myopia.

Effects of hemiretinal form deprivation on central refractive development and posterior eye shape in chicks.

We determined effects of hemiretinal form deprivation (i.e., form-depriving half of the retina) on central refractive development and posterior eye shape in chicks. Seventy-seven White Leghorn chicks were randomly assigned to receive superior (SRD, Superior Retinal Deprivation or inferior visual field deprivation, same principle applies for the following abbreviations, n=17), inferior (IRD, n=14), temporal (TRD, n=23) or nasal hemiretinal (NRD, n=23) form deprivation monocularly from day 5 to day 26. Central refractive errors, expressed as interocular difference in spherical equivalent (M), J0 and J45 astigmatic components, were measured using Hartinger refractometer at the beginning and weekly after treatment for 3weeks. At the end of the treatment period, eyes of a subset of birds were enucleated and eye shape profile was photographed along four different meridians. These digital images were later processed to extract axial length (AL), equatorial diameter (ED), and AL/ED. For comparison purposes, the eye shape profile was also acquired from a separate group of birds reared with monocular full-retinal form deprivation (FRD, n=10). The four hemiretinal form deprivations altered central ametropia and posterior eye shape to different degrees. The biggest contrast in M was found between SRD and IRD groups (mean±SE after 3weeks: SRD=-4.14±0.71 D vs. IRD=+1.24±0.36 D; p<0.05), whereas subtle differences in J0 and J45 components were found across the four treatment groups (both p⩽0.03). SRD group also showed significantly higher AL/ED ratio compared to IRD and NRD groups (0.76±0.05 vs. 0.74±0.07 and 0.75±0.04; both p⩽0.03). Furthermore, M was significantly correlated with AL/ED ratio in the treated eyes of hemiretinal treated chicks (r=-0.55, p<0.001). Our results suggest that mechanism regulating central ametropia can be influenced by selectively interrupting the visual experience at different parts of visual field.

Sign-dependent changes in retinal electrical activity with positive and negative defocus in the human eye

The purpose of this study was to investigate the effect of optical defocus on changes of electrical response as a function of retinal region. Twenty-three subjects (aged 19-25 year) with normal ocular health were recruited for global flash multifocal electroretinogram (mfERG) recordings under control (fully corrected) condition, and short-term positive defocus (+2D and +4D) and negative defocus (-2D and -4D) conditions. The amplitudes and implicit times of direct (DC) and induced (IC) components of mfERG responses were pooled into six concentric rings for analyses. The mfERG responses demonstrated more significant changes in amplitude in paracentral retinal regions than in the central regions under defocused conditions. The paracentral DC amplitude showed a significant reduction under negative defocus conditions. In contrast, the paracentral IC amplitude showed a significant increment under positive defocus conditions. Interestingly, the central IC response showed significant reduction in amplitude only to negative defocus, while increasing its amplitude to positive defocus. However, the DC and IC implicit times were virtually unaffected under defocused conditions. Our findings suggest that human retina is able to differentiate defocused signals and to identify positive and negative defocus. It shows that paracentral retina reacts more vigorously to optical defocus than does central retina.

Blue-Light Filtering Spectacle Lenses: Optical and Clinical Performances

Purposes To evaluate the optical performance of blue-light filtering spectacle lenses and investigate whether a reduction in blue light transmission affects visual performance and sleep quality. Methods Experiment 1: The relative changes in phototoxicity, scotopic sensitivity, and melatonin suppression of five blue-light filtering plano spectacle lenses were calculated based on their spectral transmittances measured by a spectrophotometer. Experiment 2: A pseudo-randomized controlled study was conducted to evaluate the clinical performance of two blue-light filtering spectacle lenses (BF: blue-filtering anti-reflection coating; BT: brown-tinted) with a regular clear lens (AR) serving as a control. A total of eighty computer users were recruited from two age cohorts (young adults: 18–30 yrs, middle-aged adults: 40–55 yrs). Contrast sensitivity under standard and glare conditions, and colour discrimination were measured using standard clinical tests. After one month of lens wear, subjective ratings of lens performance were collected by questionnaire. Results All tested blue-light filtering spectacle lenses theoretically reduced the calculated phototoxicity by 10.6% to 23.6%. Although use of the blue-light filters also decreased scotopic sensitivity by 2.4% to 9.6%, and melatonin suppression by 5.8% to 15.0%, over 70% of the participants could not detect these optical changes. Our clinical tests revealed no significant decrease in contrast sensitivity either with (95% confidence intervals [CI]: AR–BT [–0.05, 0.05]; AR–BF [–0.05, 0.06]; BT–BF [–0.06, 0.06]) or without glare (95% CI: AR–BT [–0.01, 0.03]; AR–BF [–0.01, 0.03]; BT–BF [–0.02, 0.02]) and colour discrimination (95% CI: AR–BT [–9.07, 1.02]; AR–BF [–7.06, 4.46]; BT–BF [–3.12, 8.57]). Conclusion Blue-light filtering spectacle lenses can partially filter high-energy short-wavelength light without substantially degrading visual performance and sleep quality. These lenses may serve as a supplementary option for protecting the retina from potential blue-light hazard. Trial Registration ClinicalTrials.gov NCT02821403

Corneal shapes of Chinese emmetropes and myopic astigmats aged 10 to 45 years.

Purpose Myopia and astigmatism are highly prevalent in the Hong Kong Chinese. This study aimed to determine the effects of age and myopic astigmatism (MA) on the corneal shape factors in the Hong Kong Chinese. Methods One hundred subjects with compound MA or emmetropia (EM) were recruited from three age groups: 10 to 15 years (n = 32), 20 to 25 years (n = 37), and 40 to 45 years (n = 31). Refractive errors were measured by noncycloplegic subjective refraction. Corneal astigmatism and corneal shape factors were measured by the Scheimpflug-based Pentacam. The effects of age and refractive errors on the whole corneal shape (mean-P) and the semimeridian corneal shapes (semi-Ps) at the nasal, temporal, superior, and inferior corneal quadrants (from corneal apex to 3 mm peripheral cornea) were analyzed. Results Age had significant effects on the mean-P and semi-Ps (both p < 0.001), with both EM and MA showing less prolate corneal shapes in older age groups. Partial correlation analyses adjusted for age showed that mean-P and semi-Ps were correlated with multiple refractive-error components (Pearson r = −0.30 to −0.78, all p < 0.05), with higher correlations found along the horizontal semi-Ps in MA (Pearson r = +0.37 to −0.78, all p < 0.01). Compared with EM, MA had more prolate temporal semi-Ps in all the three age groups (p < 0.05). Strikingly, age and refractive errors also had significant impacts on the asymmetry of the corneal shape along the horizontal meridian. Conclusions Corneal shapes were influenced by age and MA in the Hong Kong Chinese. These results highlight the importance of controlling these factors when designing a study on corneal shape.

Could a cycloplegic agent be replaced by a fogging or a corrective lens in the biometric measurement of the crystalline lens

This study investigated whether a fogging or a corrective lens could be used to replace a cycloplegic agent in the ultrasonic measurement of crystalline lens thickness in myopia. A group of 28 Hong Kong Chinese adults with myopia was recruited. The crystalline lens thickness of the examined eye was measured by A‐scan ultrasonography while the fixating eye was in one of three conditions: fog (+2.00 D fogging lens), full corrective lens, or cycloplegia (50 minutes after instillation of 1% cyclopentolate HCl). We found that the mean lens thickness was significantly different between the three conditions in our myopic subjects. The mean crystalline lens thickness under fogging and corrective lens conditions was significantly greater than the cycloplegic condition by 0.09u2003mm and 0.11u2003mm, respectively. The 95% limits of agreement compared to cycloplegia (fogging: −0.32 to +0.14; corrective: −0.35 to +0.13) showed marked intersubject variability, indicating that there is a risk of overestimating the lens thickness when substituting cycloplegia with either a fogging or a corrective lens.

A novel instrument for logging nearwork distance

Citation information: Leung T‐W, Flitcroft DI, Wallman J, Lee TH, Zheng Y, Lam CS‐Y & Kee C‐S. A novel instrument for logging nearwork distance. Ophthalmic Physiol Opt 2011, 31, 137–144. doi: 10.1111/j.1475‐1313.2010.00814.x

Ocular aberrations and corneal shape in adults with and without astigmatism

Purpose To characterize and compare the corneal shapes and monochromatic aberrations in Chinese myopic adults with and without astigmatism. Methods Forty-six Hong Kong Chinese aged 50 to 70 years with compound against-the-rule myopic astigmatism (n = 18) or simple myopia (n = 28) were recruited. Corneal shapes were measured by a Scheimpflug-based corneal topographer: the semimeridian corneal shape factors at the nasal, temporal, inferior, and superior corneal quadrants measured from the corneal apex to 3 mm midperiphery were analyzed. The ocular aberrations were measured by the COAS (Complete Ophthalmic Analysis System) Shack-Hartmann wavefront aberrometer; the corneal aberrations were computed using the corneal topographic map data measured by the Medmont E300 corneal topographer; and the internal aberrations were calculated from the ocular and corneal aberrations. Results Compared with simple myopia, myopic astigmatism had more oblate nasal and temporal corneal shapes and showed significantly more negative Y trefoil and more positive vertical coma. The asymmetry in corneal shape along the vertical principal meridian (inferior − superior) was significantly associated with the Y trefoil and vertical coma of the cornea, suggesting that this regional asymmetry in corneal shape may contribute to the ocular aberrations. Conclusions The significant relationships found between astigmatism, corneal shapes, and monochromatic aberrations underscore the importance of taking corneal shape into account when correcting the optical defects in myopic Chinese adults with astigmatism.