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The impact of uncoordinated growth of the optical components of the eye may stimulate different levels of monochromatic aberrations in the growing eyes of the children. This thesis aimed to examine the impact of age, visual acuity and refractive error on higher order aberrations as well as to determine the relationship between them. Hartman Shack images taken with the Welch Allyn® SureSight Autorefractor were calibrated in order to determine the Zernike coefficients up to the 8th order for a pupil diameter of 5mm. The MATLAB code proposed by Thibos et al that follows the standard for reporting the optical aberrations of the eye was the basis of code written for this study. Modification was required to suit the specific needs of the Welch Allyn® SureSight Autorefractor. After calibration the lower order aberrations could then be compared with the results from cyclopledged retinoscopy. RMS values of aberrations and Strehl ratios were computed to examine the optical performance of the eye. A total of 834 Hartmann-Shack images of 436 children (mean age 3.94± 0.94 years, range 3 to 6 years) were examined in this study (right eyes 436; left eyes 398).The sample had a mean (± STD) spherical equivalent of 1.19 ± 0.59D, a mean with-the-rule astigmatism (J0) of 0.055 ± 0.22D, and a mean oblique astigmatism (J45) of 0.01±0.14D. Visual acuity varied from 6/6 to 6/18. Moderate mirror symmetry was found between the eyes. Like refractive error, higher order aberrations declined with age in this sample. There was an impact of higher order aberrations on refractive error. Significantly higher ocular aberrations were found in the higher hyperopic group (SE>+2.0D) compared to emmetropic (-0.5<SE<+0.5D) and low hyperopic groups (+0.5<SE<+2.0D). The Strehl ratio was significantly lower in the high hyperopic group. Higher Strehl ratios were observed for better acuity groups but the average Strehl ratios among the different visual acuity groups were not statistically significant. In conclusion, there was an impact of age on the ocular aberrations. A wider range of age from birth to adolescence is required for further investigation. This could be indirectly influenced by the age related changes in refractive error as the correlation between refractive error and the higher order aberrations were significant. This finding also concludes that Strehl Ratio alone is not capable of perfectly describing the visual acuity of the eye; other metrics such as the neural transfer function and neural noise are necessary to describe the resultant visual performance of the eye.

The impact of uncoordinated growth of the optical components of the eye may stimulate different levels of monochromatic aberrations in the growing eyes of the children. This thesis aimed to examine the impact of age, visual acuity and refractive error on higher order aberrations as well as to determine the relationship between them. Hartman Shack images taken with the Welch Allyn® SureSight Autorefractor were calibrated in order to determine the Zernike coefficients up to the 8th order for a pupil diameter of 5mm. The MATLAB code proposed by Thibos et al that follows the standard for reporting the optical aberrations of the eye was the basis of code written for this study. Modification was required to suit the specific needs of the Welch Allyn® SureSight Autorefractor. After calibration the lower order aberrations could then be compared with the results from cyclopledged retinoscopy. RMS values of aberrations and Strehl ratios were computed to examine the optical performance of the eye. A total of 834 Hartmann-Shack images of 436 children (mean age 3.94± 0.94 years, range 3 to 6 years) were examined in this study (right eyes 436; left eyes 398).The sample had a mean (± STD) spherical equivalent of 1.19 ± 0.59D, a mean with-the-rule astigmatism (J0) of 0.055 ± 0.22D, and a mean oblique astigmatism (J45) of 0.01±0.14D. Visual acuity varied from 6/6 to 6/18. Moderate mirror symmetry was found between the eyes. Like refractive error, higher order aberrations declined with age in this sample. There was an impact of higher order aberrations on refractive error. Significantly higher ocular aberrations were found in the higher hyperopic group (SE>+2.0D) compared to emmetropic (-0.5<SE<+0.5D) and low hyperopic groups (+0.5<SE<+2.0D). The Strehl ratio was significantly lower in the high hyperopic group. Higher Strehl ratios were observed for better acuity groups but the average Strehl ratios among the different visual acuity groups were not statistically significant. In conclusion, there was an impact of age on the ocular aberrations. A wider range of age from birth to adolescence is required for further investigation. This could be indirectly influenced by the age related changes in refractive error as the correlation between refractive error and the higher order aberrations were significant. This finding also concludes that Strehl Ratio alone is not capable of perfectly describing the visual acuity of the eye; other metrics such as the neural transfer function and neural noise are necessary to describe the resultant visual performance of the eye.

Background: Uncorrected Refractive Errors (UREs) are the most common cause of vision loss globally. The burden is particularly worse in low- and middle-income countries like Zambia, where access to Refractive Error Correction Services (RECS) is limited. This study aimed to assess the user's accessibility to RECS in selected Zambian Hospitals. Methods: Twenty (20) public health facilities offering RECS were conveniently selected using a crosssectional design. These represented 20 districts in eight provinces of Zambia. A questionnaire-based on access to health care services framework was administered. The framework assessed service accessibility in terms of availability, geographical accessibility, and affordability. Facility managers completed and submitted the questionnaire via email. Results: Completed questionnaires were received from 20 facilities. Nineteen facilities were located in rural areas whilst one facility was located in an urban area. Most facilities (84%) had the Ministry of Health recommended equipment, though essential equipment such as tonometers were lacking in most facilities (70%). Fifteen facilities (75%) reported having Optometry Technologists as the main staff offering services. Only two facilities (10%) had an Ophthalmologist each and no facility had an Optometrist. School-based programmes were not carried out in all facilities. Only one (5%) facility was able to dispense spectacles soon after refraction as it had a spectacle manufacturing workshop. For some facilities (60%), a poor road network posed a challenge to geographical accessibility. Insufficient funding limited access to RECSs. Facility representative stated that not all patients could meet the cost of services in all the facilities. Conclusion: Access to refractive error correction services in the 20 facilities was limited due to a combination of eye health programme deficiencies and general challenges typical in low- and middle-income countries. Funding, human resources and equipment were insufficient. Inadequate road network and infrastructure undermined service delivery. The accessibility shortcomings identified should be used to improve user accessibility of refractive services.

No description available.

Aim; This study looks at the effect of orthokeratology on a number of biometric parameters and refractive error in an adult population. Method; Forty three myopic subjects were recruited to a twelve month study into the effects of orthokeratology on ocular biometry and refractive error. Two different back surface lens designs were applied right eye) pentacurve and left eye) aspheric. The aspheric design was chosen to more closely mimic the cornea¿s natural shape. Anterior and posterior apical radii and p-values; corneal thickness and anterior chamber depth were measured using the Orbscan IIz; together with ocular biometry by IOL Master and a standard clinical refraction. All measurements were repeated at one night, one week, one, three, six and twelve months. Refractive changes were analysed against biometric changes. Results; Twenty seven participants completed one month of lens wear. Twelve subjects completed twelve months of lens wear. Subjects with myopia ¿ -4.00DS were successfully treated with orthokeratology. Both anterior and posterior apical radii and p values were altered by orthokeratology. Corneal thickness changes were in agreement with previously published studies. Axial length and anterior chamber depth were unaffected by the treatment. Conclusion; Orthokeratology should be available as an alternative to laser refractive surgery. It is best restricted to myopes of up to -4.00DS with low levels of with the rule corneal astigmatism. The use of an aspheric back design contact lens did not produce a significant benefit over that of a pentacurve.

Yes / Purpose: To investigate whether a genetic risk score (GRS) improved performance of predicting refractive error compared to knowing a child’s number of myopic parents (NMP) alone. Methods: This was a retrospective analysis of data from the Avon Longitudinal Study of Parents and Children (ALSPAC) birth cohort study. Refractive error was assessed longitudinally between age 7–15 using non-cycloplegic autorefraction. Genetic variants (n=149) associated with refractive error from a Consortium for Refractive Error And Myopia (CREAM) genome-wide association study were used to calculate a GRS for each child. Using refractive error at ages 7 and 15 years as the outcome variable, coefficient of determination (R2) values were calculated via linear regression models for the predictors: NMP, GRS and a combined model. Results: Number of myopic parents was weakly predictive of refractive error in children aged 7 years, R2=3.0% (95% CI 1.8–4.1%,p<0.0001) and aged 15 years, R2=4.8% (3.1–6.5%,p<0.0001). The GRS was also weakly predictive;age 7 years, R2=1.1% (0.4–1.9%,p<0.0001) and 15 years R2=2.6% (1.3–3.9%,p<0.0001). Combining the 2 variables gave larger R2 values at age 7, R2=3.7%(2.5–5.0%,p<0.0001) and 15, R2=7.0% (5.0–9.0%,p<0.0001). The combined model improved performance at both ages (both p<0.0001). Conclusion: A GRS improved the ability to detect children at risk of myopia independently of knowing the NMP. We speculate this may be because NMP captures information concerning environmental risk factors for myopia. Nevertheless, further gains are required to make such predictive tests worthwhile in the clinical environment. / The UK Medical Research Council and Wellcome. Grant Number: 102215/2/13/2 The University of Bristol provide core support for ALSPAC PDF. Grant Number: 459KB The College of Optometrists NIHR Senior Research Fellowship. Grant Number: SRF‐2015‐08‐005 23andMe

Myopia is a common ocular condition worldwide and the mechanism of myopia is still not clear. A number of animal models of myopia and refractive error development have been proposed. The fact that form deprivation myopia could be induced in tilapia fish, as shown previously in my research, suggests the possibility that tilapia could be a new animal model for myopia research. In the first part of this thesis the tilapia model was perfected and then, based on this model, the effect of systemic hormones (thyroid hormones) associated with eye and body development was investigated during refractive error development. Lastly, the physiological and morphological changes on the retina were further studied with optical coherence tomography (OCT). In these experiments, significant amounts of myopia, and hyperopia were induced within two weeks using goggles with lens inserts as in other higher vertebrate animal models, e.g. chicks. The results from form deprivation treatment also show that the sensitivity of tilapia eyes may be an age related effect during the emmetropization process. The larger the fish, the less hyperopic the fish eye, though the small eye artefact may be a factor. The susceptibility of the refractive development of the eye to the visual environment may be also linked to plasma hormone levels. It was found that induced refractive errors could be shifted in the hyperopic direction with high levels of thyroid hormones. Also, after 2 weeks of treatment with negative or positive lens/goggles, the tilapia retina becomes thinner or thicker, respectively. When the goggles are removed, the thickness of the retina changes within hours and gradually returns to normal. However, the circadian retinomotor movement is a complicating factor since it affects the retinal thickness measurement with OCT at some time points. In conclusion, tilapia represent a good lower vertebrate model for myopia research, suggesting a universal mechanism of myopia development, which may involve systemic hormones and immediate, short term retinal responses.

Myopia is a common ocular condition worldwide and the mechanism of myopia is still not clear. A number of animal models of myopia and refractive error development have been proposed. The fact that form deprivation myopia could be induced in tilapia fish, as shown previously in my research, suggests the possibility that tilapia could be a new animal model for myopia research. In the first part of this thesis the tilapia model was perfected and then, based on this model, the effect of systemic hormones (thyroid hormones) associated with eye and body development was investigated during refractive error development. Lastly, the physiological and morphological changes on the retina were further studied with optical coherence tomography (OCT). In these experiments, significant amounts of myopia, and hyperopia were induced within two weeks using goggles with lens inserts as in other higher vertebrate animal models, e.g. chicks. The results from form deprivation treatment also show that the sensitivity of tilapia eyes may be an age related effect during the emmetropization process. The larger the fish, the less hyperopic the fish eye, though the small eye artefact may be a factor. The susceptibility of the refractive development of the eye to the visual environment may be also linked to plasma hormone levels. It was found that induced refractive errors could be shifted in the hyperopic direction with high levels of thyroid hormones. Also, after 2 weeks of treatment with negative or positive lens/goggles, the tilapia retina becomes thinner or thicker, respectively. When the goggles are removed, the thickness of the retina changes within hours and gradually returns to normal. However, the circadian retinomotor movement is a complicating factor since it affects the retinal thickness measurement with OCT at some time points. In conclusion, tilapia represent a good lower vertebrate model for myopia research, suggesting a universal mechanism of myopia development, which may involve systemic hormones and immediate, short term retinal responses.

Aim: This study looks at the effect of orthokeratology on a number of biometric parameters and refractive error in an adult population. Method: Forty three myopic subjects were recruited to a twelve month study into the effects of orthokeratology on ocular biometry and refractive error. Two different back surface lens designs were applied right eye) pentacurve and left eye) aspheric. The aspheric design was chosen to more closely mimic the cornea's natural shape. Anterior and posterior apical radii and p-values; corneal thickness and anterior chamber depth were measured using the Orbscan IIz; together with ocular biometry by IOL Master and a standard clinical refraction. All measurements were repeated at one night, one week, one, three, six and twelve months. Refractive changes were analysed against biometric changes. Results: Twenty seven participants completed one month of lens wear. Twelve subjects completed twelve months of lens wear. Subjects with myopia ≤ -4.00DS were successfully treated with orthokeratology. Both anterior and posterior apical radii and p values were altered by orthokeratology. Corneal thickness changes were in agreement with previously published studies. Axial length and anterior chamber depth were unaffected by the treatment. Conclusion: Orthokeratology should be available as an alternative to laser refractive surgery. It is best restricted to myopes of up to -4.00DS with low levels of with the rule corneal astigmatism. The use of an aspheric back design contact lens did not produce a significant benefit over that of a pentacurve.