Examining Anatomical and Physiological Characteristics in Hyperopic Eyes
Authors:   , Moukaddem Diana 
Published: July 2024

The series of experiments presented in this thesis examined the anatomical and physiological characteristics in young adult eyes. The overriding rationale is to gain an understanding of why hyperopic eyes have different growth characteristics to eyes that maintain emmetropia or develop myopia. In the first experiment, test-retest repeatability of choroidal thickness (ChT) manual measures in central and peripheral regions in young adult eyes (n=11) were determined. The measures were taken using a fovea-centred swept source optical coherence tomography (OCT) imaging protocol across the central 40° of the fundus. The results found levels of repeatable measurements in the sub-foveal (SF) and extra-macular regions to be ~3 and 12 μm, respectively. These values help gain an understanding of the instrument noise within the OCT imaging protocol and aids protocol development for subsequent chapters where small changes in ChT over time were being measured. In the second experiment, diurnal changes in multiple ocular biometric parameters, namely axial length (AL), ChT and intraocular pressure (IOP) were measured over a period of 12 hours in hyperopic and non-hyperopic young adult eyes (n=35). All groups showed similar diurnal variation phases and amplitudes in AL and IOP. Hyperopic eyes, specifically smaller eyes, showed a regional diurnal phase delay, reduced diurnal amplitude and thicker choroid, mostly concentrated in the superonasal fundus, compared to non-hyperopic eyes. Diurnal amplitudes in ChT were larger than repeatability values determined in the first experiment. In the third experiment, retinal vascular parameters were examined retrospectively and compared between refractive groups in young adults (n=272 eyes). Arteriolar and venular calibre, tortuosity and fractal dimension (FD) were extracted from a semi-automatic quantitative vessel measurement software using previously collected fundus photographs. Hyperopic eyes showed a reduced vascular calibre after accounting for optical magnification, greater arteriolar tortuosity and a reduced FD pattern compared to emmetropes and myopes. In the fourth experiment, ocular parameters most predictive of refractive error, and inferentially hyperopia, were determined in a group of young adults (n=112 eyes). A stepwise multivariable regression model was constructed using an extensive list of ocular biometric and vascular parameters, including novel parameters such as the foveal-to-macular thickness ratio and the retinal vascular parameters measured in the third experiment. AL and AL/CR were the biggest predictors of refractive error, with smaller prediction contributions found in average corneal radius (CR), anterior chamber depth (ACD), anterior chamber angle (ACangle) and FD values. This suggests that hyperopic eyes are characterised by a smaller AL and AL/CR ratio, flatter corneas, a shallower AC and a sparser retinal vasculature. In conclusion, these findings provide important insights into the disregarded and little-known morphology and physiology of hyperopic eyes. The novel finding of a localised circadian dysregulation in ChT and a sparser retinal vasculature may have important implications in understanding how hyperopic refractive error persists during human eye growth.