Structure-Function Relationship in Keratoconus: Spatial and Depth Vision

Purpose: The purpose of this study was to determine changes in spatial and depth vision with increasing severity of keratoconus and to model the structure-function relationship to identify distinct phases of loss in visual function with disease severity. Methods: Best-spectacle corrected, monocular high-contrast visual acuity, contrast sensitivity function (CSF) and stereoacuity of 155 cases (16–31 years) with mild to advanced bilateral keratoconus was determined using standard psychophysical tests. Disease severity was quantified using the multimetric D-index. The structure-function relationship was modeled using linear, positive exponential, negative exponential, and logistic nonlinear regression equations. Results: The logistic regression model explained the highest proportion of variance for spatial vision, without bias in the residual plots (R2 ≥ 66%, P 400 arc second); both were significantly poorer than controls (approximately 30 arc second). Conclusions: Vision loss in keratoconus varies with the visual function parameter tested. Contrast sensitivity may be an earlier indicator of spatial vision loss than visual acuity. Depth perception is significantly deteriorated from very early stages of the disease. Translational Relevance: The study outcomes may be used to forecast longitudinal vision loss in keratoconus and to apply appropriate interventions for timely preservation/enhancement of vulnerable visual functions

Relation between logMAR acuity, psychophysical best focus, psychophysical DOF and HORMS obtained from first control experiment.

<p>All other details are similar to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0148085#pone.0148085.g005" target="_blank">Fig 5</a>.</p

Image Quality Analysis of Eyes Undergoing LASER Refractive Surgery

<p>Average computational (panels A and B) and psychophysical (panels C and D) through-focus curves of all subjects obtained by plotting logVSOTF or logMAR acuity for each induced myopic and hyperopic lens power The solid circles indicate individual data points while the curve indicate the spline fit to the data. Panels A and C show through-focus curves for the first arm of the study while panels B and D show through-focus curves for the second arm of the study. Horizontal and vertical arrows in each panel indicate peak IQ and best focus location, respectively.</p

Suprathreshold contrast perception of resolvable high spatial frequencies remain intact in keratoconus

Contrast detection thresholds are elevated with optical quality loss in keratoconus. This study hypothesized that suprathreshold contrast perception is also impaired in keratoconus, with the impairment being predictable from the pattern of loss in threshold-level performance. Contrast detection thresholds were determined across a range of spatial frequencies in 12 cases with mild to severe keratoconus and 12 age-similar controls. These values were used to predict the contrast needed to achieve perceptual matches between reference and test spatial frequency pairs (peak of CSF Vs. 0.3x, 0.5x, 2x or 3x spatial frequency from the peak) for stimuli at 10% and 50% suprathreshold contrast. Contrast thresholds predicted a 1.5 to 6.7-fold increase in the test pattern's contrast to obtain a perceptual match with the reference pattern in keratoconus, relative to controls. Contrary to predictions, the empirical data of contrast matches between test and reference patterns were similar for higher than peak spatial frequencies at both contrast levels. However, as predicted, test patterns required higher contrast than the reference pattern for a perceptual match for lower than peak spatial frequencies. These results were similar to controls and invariant of disease severity, interocular asymmetry and short-term changes in optical quality. Unlike thresholds, suprathreshold contrast perception of resolvable high spatial frequencies appears immune to optical quality losses in keratoconus. These results are discussed in the context of the prevailing models of contrast constancy in healthy humans. Breakdown of contrast constancy at lower than peak spatial frequencies may reflect the properties of the testing paradigm employed here

Box and whisker plots of the DOF obtained from the second control experiment.

<p>Data obtained from the three different DOF criteria are shown in each panel. Details of the box and whisker plot are similar to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0148085#pone.0148085.g001" target="_blank">Fig 1C</a>.</p

Relation between peak logVSOTF, computational best focus, computational DOF and HORMS obtained from the first arm of the study.

<p>Panels A to C show data of peak logVSOTF, computational best focus and the computational DOF of both controls and cases plotted against their HORMS values, respectively. Panel D shows data of peak logVSOTF values plotted against the respective computational DOF.</p

Relation between peak high contrast logMAR acuity, psychophysical best focus, psychophysical high contrast DOF and HORMS obtained from the first arm of the study.

<p>All other details are similar to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0148085#pone.0148085.g003" target="_blank">Fig 3</a>.</p

Relation between peak logVSOTF, computational best focus, computational DOF and HORMS obtained from the second arm of study.

<p>All other details are similar to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0148085#pone.0148085.g003" target="_blank">Fig 3</a>.</p