Abstract
Purpose: To study accommodation behaviour of early presbyopes with the full suite of accommodative stimuli, and to monitor changes in spherical aberration, pupil size and image quality that accompany the accommodative response.
Methods: Using a high resolution Shack-Hartmann aberrometer, we measured refractive state as a binocularly viewed 0.30 logMAR (6/12 or 20/40) letter E was moved from 2 m to 20 cm and simultaneously monitored pupil diameter and spherical aberration for 19 subjects (mean age: 42 +/- 7.18 years). Refractive state was defined using three standard criteria: minRMS, paraxial, and optimum image quality using the Visual Strehl ratio metric (VSOTF).
Results: Because of changes in spherical aberration, accommodative gain measured with the paraxial criterion is generally greater (on average by 34%) than the minRMS gain in early presbyopes. The slope of stimulus/response curve is relatively stable up to age 41 and then declines progressively in spite of a full binocular set of accommodative cues. Reduced accommodative gain appears once accommodative amplitude has dropped to below 3D, and gain drops to < 0.5 as amplitude drops to 1.5D. This decline happens at different ages in different presbyopes.
Conclusion: Prior to the early 40s, changes in accommodation are restricted to a reduction in amplitude, but during the 40s the continued loss of accommodative amplitude is accompanied by a concurrent drop in accommodative gain. Therefore, reduced near image quality in early presbyopes is caused by lower accommodative amplitudes and gains, which may explain the apparent acceleration in symptoms and near add prescriptions during the mid to late 40s.