City Research Online

Abstract

Increased lower- and higher-order wavefront aberrations and the associated loss in image quality due to corneal distortions are demonstrated in keratoconus. This loss in image quality is bilaterally asymmetric due to the asymmetric nature of the disease. Loss of monocular functions is well established in keratoconus, whereas studies reporting the changes in binocular functions are limited. It is important to understand the binocular functions in the presence of asymmetry because previous literature on asymmetric image quality (induced anisometropic blur) has shown losses in binocular vision. Given this background, the present thesis investigated the effect of bilateral asymmetry in keratoconus on binocular functions. The first experiment quantified the depth of suppression using the binocular contrast rivalry paradigm. The dwell time of each eye at different contrast in the stronger eye was estimated to determine the depth of suppression. Results revealed that the weaker eye is suppressed in asymmetric keratoconus and binocular viewing is dominated by the eye with lesser disease severity. Depth of suppression is increased with an interocular difference in acuity (r=0.85; p<0.001) and an interocular difference in disease severity in keratoconus (r=0.70; p<0.001). Depth of suppression is greater for higher spatial frequency targets that encounter greater loss of image quality [20% (8 – 41.5%)] compared to lower spatial frequency targets [68% (27 – 86.75%)] (Z=5.4, p<0.001). Depth of suppression with spectacles [37% (12 – 72.84%)] is greater compared to contact lens correction [80.05% (60 – 91%)], (Z=3.8, p=0.001) for the same reason as the spatial frequency factor described above. Suppression of the eye is also associated with stereoacuity loss. As the Blur is a combination of both contrast losses and phase shifts, the effect of this contrast and phase shifts on stereo acuity is estimated in the next two experiments. The second experiment investigated the hypothesis that contrast balancing improves stereoacuity in keratoconus and the percentage improvement is dependent on the baseline interocular difference in contrast. The stereoacuity was tested using a Randot stereogram at different contrast levels in the stronger eye. Results revealed that interocular contrast balancing improved stereoacuity by a median value of 34.6% (19.0 – 65.1%) in bilateral asymmetric keratoconus, independent of their baseline contrast difference (ρ=0.2, p=0.26). The third experiment investigated the effect of phase shifts of keratoconic eyes on stereoacuity by inducing them in normals. Stereoacuity was tested in normals by inducing the HOAs of keratoconic subjects with natural phase and phase rectified conditions. This was done in normals because it is not possible to independently manipulate phase shifts in keratoconics. Phase reversals due to HOAs in keratoconus deteriorated stereoacuity (t>3.479, p<0.007) and this deterioration is again dependent on the severity of the disease. The phase correction improved the stereoacuity in all the induced conditions by 30% (20.44 to 38.15%). In conclusion, the eye with lesser disease severity dominates binocular viewing in keratoconus. The suppression of the weaker eye depends on the bilateral disease severity, optical correction modality, and the target spatial frequency. The correction of sensory factors (contrast and phase shifts) could partially restore the stereoacuity in keratoconus.

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

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