City Research Online

Abstract

Background and aims: Keratoconus is an ocular condition that causes a change in corneal shape and consequently degrades the retinal image quality and visual function. Despite extensive studies on the impact of the disease on corneal structure and visual function in isolation, the precise relationship between structure and function in keratoconus has not yet been determined. A structure-function relationship in a progressive condition such as keratoconus can provide valuable insight and evidence that can inform its treatment. The overarching aim of this thesis was to understand the structure-function relationship in keratoconus. Specifically, an attempt was made to determine the structure-function relationship for threshold and suprathreshold levels of visual performance, with the aid of spectacles and contact lenses.

Methods: For the three main studies reported in this thesis, threshold level spatial visual performance was assessed using high contrast visual acuity (COMPlog ® software) and the contrast sensitivity function (quick CSF). Random dot stereograms and Howard-Dolman apparatus were used to assess stereoacuity. Corneal curvature, elevation, and thickness-based indices such as maximum keratometry, index of surface variance, vertical asymmetry, height decentration, keratoconus index and D-index were assessed using Scheimpflug imaging. The D-index was used as an indicator of disease severity. In the first study, these parameters were determined for varying severity of keratoconus in a total of 155 participants (310 eyes) and the structure-function relationship investigated for four different mathematical models using regression analysis. In the second study, the effect of best corrected spectacles and different contact lenses, namely, conventional and Rose K2 rigid gas permeable contact lenses, a custom-designed soft toric contact lens (Kerasoft), and scleral contact lenses, on threshold level visual performance was determined using the tests already listed. In the final study, suprathreshold visual performance was determined in twelve subjects with keratoconus and twelve age-similar controls with a contrast matching paradigm implemented in Matlab.

Results: Threshold level visual functions progressively deteriorated with increasing severity of keratoconus. The loss in all three functional measures was best explained by the five-parameter sigmoid-shaped logistic regression model (r2 ≥ 0.66; p<0.001). The high contrast visual acuity showed a distinct ceiling phase and a steeper loss rate with increasing D-index (1.8units/D-index) relative to the CSF (0.28units/D-index). Stereoacuity was poorly correlated with D-index (p≤0.2, for both inter-ocular difference and the D-index of the best eye). Assessment with contact lenses revealed that all the outcome variables improved with all forms of lenses relative to spectacles (p<0.05). This improvement was smaller with Kerasoft lenses but higher and comparable with the other three lens designs, across the disease severity. Interestingly, none of the visual function measures or the optical quality reached the level of controls for any correction modality. The assessment of suprathreshold performance showed that the perceptual contrast match for eyes with mild to moderate keratoconus across spatial frequencies were found at a comparable level to that of the controls, complying with the well-established phenomenon of contrast constancy in the literature.

Conclusions: Results suggest that the vision loss in keratoconus cannot be generalized and is critically dependent on the overall disease severity, interocular symmetry and the type of function being evaluated. The presence of constancy in contrast perception irrespective of the disease severity, inter-ocular asymmetry and short-term change in optical quality may indicate an active gain control mechanism across spatial frequency channels for the visual system at a suprathreshold level in keratoconus.

Publication Type:	Thesis (Doctoral)
Subjects:	R Medicine > RE Ophthalmology
Departments:	School of Health & Psychological Sciences > Optometry & Visual Sciences School of Health & Psychological Sciences > School of Health & Psychological Sciences Doctoral Theses Doctoral Theses

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