Forward light scatter and contrast sensitivity in keratoconic patients

Abstract

Purpose

The aim of this study was to obtain intraocular forward light scatter (FLS) data in keratoconus patients and to explore possible relationships with contrast sensitivity.

Method

Intraocular straylight was assessed in the right eye of 10 visually-normal subjects (31 ± 5 years) and 5 keratoconus patients (34 ± 9 years) using the van den Berg straylight meter. Contrast sensitivity was also measured in both groups, with and without a glare source provided by the Brightness Acuity Tester (BAT), using a Pelli–Robson chart. Inclusion criteria for the keratoconic patients were chosen to avoid natural causes of increased scatter and to limit the influence of factors that could affect the straylight measurements.

Results

Intraocular light scatter was significantly greater in the keratoconic patients than in normal subjects for each of the three eccentricities tested (Mann–Whitney U test (2-tailed): Z ≤ −2.2, p ≤ 0.027). Baseline Pelli–Robson contrast sensitivity was also found to be lower in the keratoconus group than in the normal subjects (Mann–Whitney U test: Z = −2.2, p = 0.023). Under glare conditions contrast loss was significantly greater in the keratoconic patients than in the normal subjects (Mann–Whitney U test: Z = −2.5, p = 0.013). High levels of negative correlation were found between the magnitudes of intraocular straylight and baseline contrast sensitivity in the keratoconus group (Spearman's correlation coefficient (R_S) ≤ −0.87, p ≤ 0.027).

Conclusion

The high values of straylight measured in the keratoconic patients cannot be explained by age or scarring and could be related to the structural degradation of the cornea. Further investigation of intraocular scatter in keratoconus is warranted to improve our current understanding of the visual deficits associated with this pathology.

Introduction

Keratoconus is a common, non-inflammatory ectatic disorder in which the cornea adopts a steepened, conical profile [1]. These modifications can severely degrade the optical quality of the eye and the associated retinal image. Several studies have shown that both ocular [2], [3], [4] and corneal [5], [6], [7] higher-order aberrations are significantly elevated in keratoconic eyes compared to normal eyes. Vision correction can often be difficult to achieve using spectacles or contact lenses in patients with keratoconus. The literature suggests that best-corrected visual performance in non-scarred eyes with keratoconus, irrespective of the method of correction, is reduced compared to normal [8], [9], [10]. Several authors have suggested that large magnitudes of uncorrected, residual higher-order aberrations (persisting even with rigid gas permeable (RGP) lenses in place) are a probable cause of this reduced visual performance [11], [12], [13], [14].

Besides higher-order aberrations, changes in the transparency of the cornea could also impact on visual performance. The transparency of the cornea is related to its highly organised structure [15], and when this complex configuration becomes altered (e.g. oedema [16], [17], [18], with penetrating keratoplasty [19] or after corneal refractive surgery [20], [21]) light scatter is increased. Scattered light which is deviated more than 90° (referred to as backward light scatter) mainly results in a reduction of the amount of light reaching the retina. On the contrary, scattered light deviated less than 90° (referred to as forward light scatter (FLS) or straylight) results in a veiling luminance superimposed upon the retinal image, leading to a reduction in retinal image contrast and possible disability glare [22], [23].

In keratoconus, changes to the corneal structure such as alterations to the epithelia [24], [25], [26], stromal keratocytes [27], [28], [29], stromal collagen lamellae [30], [31] and Bowman's layer [32], [33], [34], [35] can occur. In view of these corneal structural alterations, it seems reasonable to assume that some of the decrease in contrast sensitivity experienced by the keratoconic patient [36], [37] can perhaps be attributed to FLS. Although the measurement of FLS in patients with keratoconus could provide useful information, this has only previously been investigated in one study using Hartmann-Shack spot images [38]. To date, FLS has not been explored in patients with keratoconus using the straylight meter developed by van den Berg and co-workers [39].

The aim of this study was to measure FLS in patients with keratoconus and to investigate possible relationships with contrast sensitivity measured with and without a glare source. Keratoconic data were compared to measurements made in visually-normal, control subjects. As the concept of aberration-controlling contact lenses has already been proposed [40], [41], knowledge of how keratoconic eyes scatter light could improve our current understanding of the limit in visual improvement afforded by successfully correcting higher-order aberrations alone. The results of this study may provide useful information to enhance our current understanding of the visual deficits associated with keratoconus.