HUMAN EYE RESPONSE TO SIMULTANEOUS OPTICAL DEFOCUS

Background The rapidly increasing global prevalence of myopia poses a significant public health challenge due to its associated risks of sight-threatening complications and reduced quality of life. Various intervention strategies are employed to control the progression of myopia including pharmacological, optical and environmental. Among these, optical interventions using spectacle lenses have been one of the most adopted approaches of myopia management due to its non-invasive nature and better compliance among children. Optical strategies, while correcting myopic refractive error, simultaneously impose myopic defocus using plus power at the retinal periphery to slow the progression of myopia. However, evidence from animal studies suggests that both simultaneous myopic defocus (positive defocus) and simultaneous hyperopic defocus (negative defocus) may regulate eye growth, raising important questions regarding the role of defocus polarity (myopic versus hyperopic) in human myopia control. Accordingly, this thesis aims to investigate how the simultaneous optical defocus of different polarity affects short-term ocular response, optical quality and visual acceptability in human myopic eyes. Methods This thesis adopted a stepwise experimental approach to investigate the short-term optical and clinical effects of simultaneous optical defocus in human myopic eyes, integrating evidence from animal models, short-term clinical trials, and bench-top optical characterization. First, a systematic review exploring the impact of simultaneous optical defocus in animal studies (Chapter 2) was conducted to establish the theoretical framework for how simultaneous optical defocus may influence ocular growth and to inform the hypotheses tested in subsequent human experiments. Building on this framework, a short-term clinical trial in young myopic subjects (Chapter 3) evaluated axial length and choroidal responses to simultaneous optical defocus using coaxial design prototype spectacle lenses imposing either simultaneous positive defocus (plano/+5.0) or simultaneous negative defocus (plano/-5.0) along with DIMS compared to plano control lens. To elucidate the optical mechanism underlying the observed clinical responses, a second experimental phase (Chapter 4) quantified and compared the optical quality and image displacement of coaxial and newly developed non-coaxial designs incorporating both positive and negative defocus, using a custom bench-top model eye and image displacement tester. Finally, a second human trial (Chapter 5) assessed static visual performance and subjective acceptance of coaxial, non-coaxial, and DIMS lenses, while examining whether the improved optical characteristics of non-coaxial designs translated into better visual tolerance without compromising efficacy. Secondarily, while visual performance and subjective acceptance is the primary objective of Chapter 5, this study included the additional evaluation of axial length response. Results In young adults aged 18-30 years, short-term wear non-dispensing clinical trials (Chapter 3) demonstrated that lenses utilizing coaxial optics, incorporating both simultaneous positive defocus and simultaneous negative defocus, resulted in significant reductions in axial length after 60 minutes of lens wear compared to single-vision control lenses. These findings suggest that both defocus polarities could inhibit axial elongation in humans, suggesting that the sign of defocus may be less critical than overall retinal image quality in eliciting compensatory axial responses in human eyes. However, changes in choroidal thickness were inconclusive in this trial. To better understand the optical mechanism underlying these short-term axial responses, Chapter 4 evaluated the image quality characteristics of the lens designs. Optical analysis using through-focus imaging, Modulation Transfer Function (MTF) and relative area under MTF (rAMTF) revealed that coaxial lenses produced two superimposed but laterally displaced star images at each focal plane. These lenses also exhibited oscillatory MTFs profile showing multiple contrast reversals, and falsely elevated rAMTF. Image displacement analysis further revealed significant image displacement (amount of prismatic deviation), indicating a high potential for visual disturbance during lens wear. In contrast, non-coaxial lenses yielded a single, spatially stable star pattern with consistent angular spacing of spokes across through-focus planes. Their MTF patterns and lower image displacement closely resembled those observed in DIMS lenses. Although slightly greater prismatic distortion was noted in image displacement analysis through these lenses compared to DIMS, the overall optical behaviour remained similar Importantly, within each design, both positive and negative defocus power showed comparable optical trends, indicating the lens geometry, rather than defocus sign, primarily influenced optical quality. Consistent with these optical findings, the short-term static vision and subjective acceptance assessment study (Chapter 5) indicated that central high-contrast vision was well preserved across all lens types; however, peripheral performance varied. All defocus lenses resulted in reduced low-contrast acuity and contrast sensitivity compared to single-vision lenses. Subjective rating obtained using visual analogue scale revealed more pronounced image doubling and ghosting with coaxial design lenses. Conversely, non-coaxial lenses were better tolerated, yielding ratings and clarity closer to DIMS lenses. Despite differences in visual comfort, both coaxial and non-coaxial lenses of both types of defocus demonstrated short-term reductions in axial length, emphasizing that lens design influences visual comfort and subjective tolerance more than efficacy. Conclusion This thesis demonstrates that short-term wear of certain design configurations in both simultaneous positive and negative defocus spectacle lenses can reduces axial length in human myopic subjects, suggesting that retinal image quality influences the axial changes more than the type of defocus. Further, lens design strongly influences visual quality and wearer acceptability. Coaxial lenses, despite their efficacy, compromise visual performance due to strong dual foci and image displacement (transverse displacement of image points), which limits their clinical applicability. In contrast, non-coaxial design lenses, balance efficacy with improved subjective acceptance and demonstrate optical similarities with DIMS, which has become a benchmark in clinical practice. Collectively, these findings provide evidence that both polarities of defocus can be effective in short-term axial length reduction and a cohesive framework for optimizing future myopia-control lens designs by integrating efficacy with optical quality and wearability.