Visual performance after correcting higher order aberrations in keratoconic eyes

Abstract

Keratoconic eyes are affected by an irregular optical blur induced by significant magnitude of higher order aberrations (HOAs). Although it is expected that correction of ocular aberrations leads to an improvement in visual performance, keratoconic eyes might not achieve the visual benefit predicted by optical theory because of long-term adaptation to poor retinal image quality. To investigate this, an adaptive optics (AO) system equipped with a large-stroke deformable mirror and a Shack-Hartmann wavefront sensor was used to correct the aberrations and measure high contrast tumbling E visual acuity (HCVA) in 8 keratoconic eyes. Eight normal eyes were employed as control. Aberrations were dynamically corrected with closed-loop AO during visual acuity testing, with residual root-mean-square error of around 0.1 microm in both groups over 6-mm pupil (p = 0.11). With AO correction, the HCVA in logMAR was -0.26 +/- 0.063 in normal eyes, and in keratoconic eyes, it was -0.07 +/- 0.051 (p = 0.0001) for the same pupil size. There was no correlation in the AO-corrected HCVA for normals with the magnitudes of their native HOA. However, within keratoconic eyes, poorer AO-corrected HCVA was observed with an increase of the native magnitudes of HOA (R(2) = 0.67). This may indicate that long-term visual experience with poor retinal image quality, induced by HOA, may restrict the visual benefit achievable immediately after correction in keratoconic eyes.

Figure 1

Verification of measurement and correction with large-stroke AO system. (A) Theoretical point-spread function computed from aberrations measured with Shack–Hartmann wavefront sensor and experimental point-spread function measured in the AO system. (B) Measured point-spread function obtained after AO correction of the normal and the KC phase plate.

Figure 2

Time course of (A) total RMS and (B) higher order RMS during visual acuity testing for normal (GY) and one moderate KC eye (MT). The two curves represent the evolution of total RMS and higher order RMS for the normal (red) and moderate KC eye (blue) after the AO was switched on. The peaks in the time course correspond to blinks during the visual acuity test at which point the correction is suspended.

Movie 1

Convolved images of the letter “E” based on the measured aberrations calculated under the normal viewing condition including chromatic aberrations, to simulate retinal image quality for (A) normal subject GY and (B) moderate KC subject MT. The letter size used in this simulation corresponds to a 20/20 Snellen letter.

Figure 3

Total RMS and higher order RMS before and after AO correction for normal and KC eyes over a 6-mm pupil. The RMS shown for both groups before AO correction is the value after precompensation of second order aberrations.

Figure 4

High contrast visual acuity for normal and keratoconic eyes after AO correction of all aberrations. The AO correction was performed over a 6.5-mm pupil and visual acuity was measured for a 6-mm pupil.

Figure 5

AO-corrected visual acuity versus theoretical retinal image quality calculated from native higher order aberrations in normal and KC eyes. Retinal image quality is represented as the area under the MTF calculated up to 60 cycles/deg. The coefficient of correlation for a linear fit and an exponential fit for normal and KC eyes, respectively, is also indicated.

Figure 6

Theoretical point-spread functions for an advanced keratoconic eye SC computed from aberrations over pupil sizes of 3 mm, 4 mm, 5 mm, and 6 mm. All figures have the same resolution as indicated. Normalized color bars between 0 and 1 are also shown.