Peripheral image quality and myopia

Myopia (nearsightedness) is when the axial length of the eye is too long relative to its focal length. This typically occurs because of excessive eye growth in childhood, and results in blurred vision for distant objects. The elongation of the eye furthermore increases the risk for ocular sight-threatening diseases later in life. Myopia has increased globally in the past decades, but the underlying mechanisms behind myopic eye growth are not yet fully understood. Studies in animals have found that a peripheral focus behind or in front of the retina can signal to the eye to grow or to stop grow, respectively. More recently, various optical myopia control spectacles and contact lenses that modify peripheral image quality have been able to slow myopia progression in children, i.e., act as myopia control, though there are large individual variations that are unexplained. The aim of this thesis is to identify characteristics in the peripheral image quality related to myopia. In this thesis, peripheral image quality was predominantly measured with a dual angle wavefront aberrometer, employing two Hartmann-Shack wavefront sensors and connected relay systems. The properties of 4f relay systems, and an alternative non-4f relay system, were investigated, and the results used during development of the dual angle aberrometer. We investigated the effect of optical myopia control spectacles (one progressive design and two microlens designs) on peripheral image quality. We found that the progressive design induced a more negative relative peripheral refraction (RPR), i.e., shifted the peripheral image more in front of the retina. The two microlens designs did not change the relative peripheral refraction; instead, they made the peripheral image blurrier, irrespective of habitual RPR. This indicates that progressive and microlens spectacles have different myopia control mechanisms. We also studied changes in corneal aberrations during orthokeratology (rigid night lenses that reshape the cornea), and found that higher baseline myopia was correlated with better myopia control effect, and with larger corneal changes. As orthokeratology has been found to induce more negative RPR, this implies that the working mechanisms of keratology and progressive myopia control spectacles are similar. Additionally, we investigated differences in peripheral image quality in children, as well as adult myopes and emmetropes. We found that the children and adult non-myopes had asymmetric RPR profiles (nasal vs. temporal visual field), but not the adult myopes. The asymmetry strengthened during near-work, suggesting that RPR during near work could be important for myopia regulation. We also found that non-myopes had less well-defined peripheral foci (a broader 'depth-of-refraction') than myopes (in both adults and children), often with multifocal characteristics. Finally, we investigated the longitudinal development of the children over one year. Larger axial length growth was linked to more positive baseline RPR, particularly during near-work, although not to a significant degree. We will continue to monitor RPR, peripheral image quality, and axial growth in the children over the coming years.