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Abstract
The aims of this thesis were to investigate peripheral refraction in different refractive groups and verify the use of peripheral refraction as a measure of ocular shape. Furthermore, manipulation of peripheral refraction with orthokeratology (OK) and soft contact lenses (SCLs) for potential myopia control were explored.
Characteristic peripheral refraction profiles were measured using the Shin-Nippon NVision-K 5001 autorefractor in emmetropes (relative peripheral myopia) and myopes (relative peripheral hyperopia). East Asian moderate myopes had greater amounts of relative peripheral hyperopia compared to Caucasians with a similar central refractive error. This was interpreted to reflect a more prolate ocular shape in the myopic Asian eye.
Calculated axial length from peripheral refraction was found to comparable to direct axial length measurements with the IOLMaster in the nasal retina and to underestimate axial length in the temporal retina. Although not entirely comparable, peripheral refraction is still able to give information on the shape of eyes with different refractive errors.
Conventional corrections of central myopia induce hyperopia onto the peripheral retina. However, OK lenses were found to cause a hyperopic shift in the central visual field (VF) in both myopic children and adults. This caused peripheral refraction which was initially relatively hyperopic to become relatively myopic compared to the central refraction. Furthermore, changing the optic zone diameter or steepening the periphery of the OK lens was found to cause no significant effect.
Relative hyperopic peripheral refraction was measured along the horizontal VF with under (+0.75DS), full, over (-0.75DS) SCL correction and multifocal SCLs with a distance centre and plus power (+2.00) periphery were found to induce less peripheral hyperopia compared to single vision (SV) SCL correction.
OK allows the peripheral retina to experience myopic defocus while central myopia is corrected, and multifocal SCLs are able to reduce the amount of relative peripheral hyperopia compared to SV SCLs. This may explain the reduced myopia progression reported with OK and multifocal SCLs. The effects of manipulating other OK lens parameters for potential customised myopia control are yet to be explored, and the impact of myopic peripheral defocus on myopia progression in children is yet to be ascertained.