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Refractive error, particularly myopia, is a significant visual problem considered to result from a complex interplay between genetics and environmental influences. While there has been a worldwide increase in the prevalence of myopia in recent years, the aetiology of myopia is not yet known. Emmetropisation is the process of normal eye growth during development such that refractive error stabilises at, or close to, emmetropia. This has been shown to be an active process influenced by the visual environment. Previous animal studies have shown that emmetropisation can be disrupted by introducing abnormal optical factors into the central and/or peripheral visual field. Differences in the optical and biometric properties of the emmetropic, myopic, and hyperopic eye have previously been found. The aim of this thesis was to extend previous work by investigating several important biometric and optical characteristics, in emmetropic, myopic and hyperopic eyes, in order to establish any systematic refractive group differences, which may explain the failure of the emmetropisation process. The results presented in this thesis show clear differences between the shape and refractive characteristics of the peripheral retina between myopic, emmetropic and hyperopic subjects. These results suggest that refractive error group dependent differences in the peripheral retina could be a key correlate of refractive error development. Optical properties of the eye, specifically pupil diameter, monochromatic higher order aberrations, and chromatic aberration were shown not to vary significantly between refractive groups, suggesting that these factors are unlikely to play any major role in the development of myopia. It could be, however, that the optical properties of the eye are a consequence of myopia development. Alternatively, the optical properties of the eye vary before myopia develops, or reaches adult levels.

Optometrists play an important part in delivering eye care in the United Kingdom; however opportunities for practitioners to extend their role through co-management of patients with ophthalmologists vary across the country. Devolution in Scotland and Wales has led to greater emphasis on community based care in these regions. This thesis reviews the current situation and, by examining ophthalmic outpatient clinic data, discusses further opportunities to reduce demands on secondary care and the cost savings that can be made. To assess whether the profession is currently in a position to adopt an extended clinical role, changes in the availability of optometric instrumentation are assessed over a two year period. An increased prevalence of fundus cameras and contact tonometers places optometrists in a good position to take on further responsibilities in glaucoma management, however future investment could be impacted by the current economic climate as value for money became increasingly important to practitioners looking to purchase equipment. Methods of training optometrists in the necessary skills to utilise new technology to extend their role are evaluated in terms of both learning and cost effectiveness. Interactive distance learning is proposed as a convenient and effective method to deliver continuing professional development. Any changes to optometric practice must take account of the need for a sustainable business and the importance of attracting and retaining patients. The views of patients are assessed through a validated service quality questionnaire, SERVQUAL. The questionnaire is found to be valid for use in an optometry setting. Patients have a generally positive view of the service quality they receive from their optical practice and consider the intangible aspects, in particular responsiveness and empathy, most important. Optometrists are well placed to increase their role in patient management; however a viable business model must exist to enable investment in instrumentation and training.

This investigation involved a number of related studies with the principal aim of assessing the variability in chromatic sensitivity in "normal" trichromats and colour deficient observers. An important outcome was the development of a new method for accurate and efficient measurement of chromatic sensitivity and the establishment of reliable statistical limits that describe the distribution of redgreen(RG) and yellow-blue (YB) chromatic sensitivity in normal trichromats. Chromatic sensitivity was assessed using a computer-based psychophysical procedure that employs spatiotemporal luminance contrast (LC) masking techniques to isolate the use of colour signals. The colour-defined stimuli were buried in dynamic LC noise and moved diagonally across a square. The subject's task is to indicate one of the four possible directions of stimulus motion by pressing the corresponding response button. The Colour Assessment and Diagnosis (CAD) test was optimised for stimulus size and background luminance level to provide an efficient measure RG and YB chromatic sensitivity. Colour detection thresholds are assessed along 16 directions in chromaticity space, selected to yield maximum information on RG and YB chromatic sensitivity loss and to distinguish between deutan and protan deficiencies. The CAD test was used to measure chromatic discrimination thresholds in 472 observers. The results made it possible to evaluate the screening and diagnostic efficiency of the CAD test in comparison with a number of established clinical colour vision tests, e.g., Nagel anomaloscope and Ishihara pseudoisochromatic plates. The specificity and sensitivity of the CAD test versus the Nagel anomaloscope (223 subjects) was found to be 100% and 97.5%, respectively. The diagnostic agreement with respect to the Nagel anomaloscope was 0.99; only two subjects showed inconsistent classification deficiency. Chromatic thresholds measured in normal trichromats were also examined and the variability amongst normal colour vision was investigated. The data obtained were used to establish the statistical limits for the standard normal observer on the CAD test. A template based on these limits was generated and provides an efficient way of separating accurately colour deficient from normal trichromats. The high specificity found is consistent with the correct detection of minimal colour deficiencies that sometimes go undetected in other colour vision tests. Molecular genetic analysis was also carried out in a small group of subjects with unusual colour deficiencies in an attempt to understand the genotype-phenotype relationship between predicted and observed colour vision losses. A comparison of genetic results from DNA sequencing and chromatic detection thresholds measured on the CAD test suggests that the separation of 30 nm between peak sensitivity of L and M normal photoreceptor pigments is not an absolute requirement for an observer to exhibit normal colour vision as assessed with conventional colour vision tests. The macular pigment (MP) optical density was also measured in 23 subjects selected for their higher than normal levels of pigment as a result of their participation in a carotenoid supplementation trial. This pre-receptoral filter absorbs light preferentially in the short wavelength region of the visible spectrum and has been shown to affect colour matches. Contrary to expectations, the findings of this investigation showed no correlation between YB thresholds and MP optical density levels. A model that explains this finding was produced confirming the observed experimental findings. In addition, the model also predicts a small improvement in RG chromatic discrimination sensitivity with increased MP optical density in the eye.

Thesis (MBA.) --University of Limpopo, 2008 / Refer to document