City Research Online

Abstract

The light scattering characteristics of the eye were determined for contact lens wearers, normals, and patients with keratoconus and corneal dystrophies, using a computerised straylight program - the P SCAN 100 scatter apparatus. This system utilises extended annular sources to assess the full scatter function, permitting the calculation of k' from the integral function of the scatter function. This parameter shows reduced subject variability compared to n (the scatter index) and k (the straylight parameter) and is proportional to the total amount of light scattered. Contrast sensitivity was assessed at seven spatial frequencies.

A normative database was created using 31 subjects (16 to 60 years) with little refractive error, k' showed no significant change up to age 45, but increased rapidly above age 45. There was no significant change in n up to 45 years of age, however, n was significantly lower is subjects over 45.

Diurnal and longitudinal variations were investigated. One subject had a highest k' value (7.60) at 08:00 hrs, falling to a steady (minimum) value of approximately 6.10. Similar results were found on retesting. Results on three eyes following patching suggested that the immediate post-patching peak in k', and its subsequent reduction, were related to changes in corneal thickness.

Longitudinal variations in light scatter with a possible cyclical pattern were noted in one female across four menstrual cycles. Light scatter changes in the menstrual cycle were tested in a double-blind masked study. The average between-day variability for three females was 2.97 (s.d.) compared to 0.39 (s.d.) for three males. Precise cyclical trends were unclear, beyond a fall in scatter levels during menses. Variations in corneal thickness and pupil diameter appeared unrelated to changes in light scatter. Changes in mucus levels in the tear film may be a contributory factor, because administration of acetylcysteine 5% reduced light scatter during menses. Pre-menopausal females were excluded from subsequent studies.

Quantification of the effects of a range of comeal abnormalities on light scatter and the CSF allowed later comparisons with the magnitude of the effects of contact lens wear. A database of values of k' and CS was established for 11 subjects with keratoconus (18 eyes) and eight subjects (11 eyes) with a range of anterior segment abnormalities. Long-term contact lens wearers (29 subjects, 54 eyes) had significantly greater light scatter than non-lens wearing subjects of the same age. Eight eyes had values of k' within the keratoconic range. CSF was reduced in most adapted contact lens wearers, compared to age-matched controls. Thirteen eyes revealed no CS loss, whilst 19 eyes showed loss at all spatial frequencies.

When 14 non-lens wearing subjects were fitted with either a hydrogel or RGP contact lens, neither lens type caused a statistically significant change in intraocular light scatter. Hydrogel contact lenses did not, in general, adversely affect CS. For RGP lenses, results were more variable, both within and between subjects, than for hydrogel lenses. However, no definite trends in CS emerged. Therefore, the increase in light scatter in the long-term contact lens-wearing population is unlikely to be due to the physical presence of the contact lens. Instead, it is probably the result of a number of physiological changes that are known to occur in response to contact lens wear.

Publication Type:	Thesis (Doctoral)
Subjects:	R Medicine
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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