Reduced Image Quality Impairs Monocular Blur-Driven Accommodation in Keratoconus

The purpose of this study was to test the hypotheses that (1) monocular blur-driven accommodation is attenuated in keratoconus due to weaker defocus-blur signals arising from degraded image quality (IQ) compared with non-keratoconic controls, and (2) manipulating the strength of defocus-blur signal, either by correcting higher-order aberrations (HOAs) in keratoconus or by inducing keratoconic HOAs in controls, will correspondingly improve or degrade accommodative performance.

Monochromatic IQ degradation was computed from HOAs of 15 mild-to-moderate keratoconic participants (13-29 years) and 15 controls for a 4.5-mm pupil diameter. Participants monocularly accommodated to 0.5 to 4.0 diopter (D) hyperopic step changes whereas their lower-order aberrations (LOAs) or both LOAs and HOAs were corrected using an adaptive optics visual simulator. The protocol was repeated in 10 controls with induced HOAs from one moderate keratoconic eye.

IQ loss with induced blur was significantly lower in patients with keratoconus than in healthy controls (P < 0.001). Accommodative responses were reduced in keratoconus (0.6 ± 0.9 D across the 4-D range) compared with controls (2.6 ± 1.1 D, P < 0.01) and were correlated with the corresponding IQ loss (r = 0.75, P < 0.001). Although IQ improved with combined LOA + HOA correction in keratoconus, no significant change in accommodation was observed (P = 0.58). Inducing keratoconic HOAs, however, significantly attenuated accommodation in controls (0.27 ± 0.41 D, P = 0.001).

Reduced monocular accommodation is consistent with weakened defocus-blur signals available to drive these responses in keratoconic eyes. The absence of improvement in accommodation despite optical quality enhancement may indicate a limited role of pure defocus as an accommodative cue and/or reduced neural sensitivity to blur following chronically degraded retinal images.