Effect of inter-individual variations in ocular parameters on optical and mechanical accommodation efficiency

Abstract

Purpose : To determine the effect of inter-individual variations in ocular biometry on optical and mechanical accommodative efficiencies.

Methods : Following IRB approval, 29 subjects (21 to 68 years, spherical equivalent: -9.38 to +0.38 D) were imaged while focusing on a distant target using a custom-built extended depth OCT system (Ruggeri et al, Biomed Opt Exp 2012; 1506-1520). Images were segmented and corrected for distortions to obtain corneal thickness, anterior chamber depth (ACD), lens thickness (LT), vitreous depth (VD), anterior and posterior corneal and lens curvatures and lens equivalent refractive index (Chang et al, Biomed Opt Exp, 2019; 411-421). For each subject, an accommodation-dependent eye model was created assuming the following accommodative changes based on literature: LT = 0.05mm/D; ACD = -0.0375mm/D; VD = -0.0125 mm/D; ratio of anterior to posterior lens radius of curvature change = 3.5. The anterior radius was adjusted to simulate accommodation ranging from 0 to 10D in 2 D steps. The relationship between lens curvature, lens power, accommodation amplitude, and ocular biometry was quantified.

Results : Lens curvatures and total lens power increased linearly with accommodation. Optical accommodation efficiency (accommodation per lens power change) was 0.81 ± 0.03 D/D (0.73 to 0.90 D/D). It was dependent on ACD (p=0.02), but not VD (p=0.30), corneal power (p=0.17) nor age (p=0.20). Mechanical accommodation efficiency (accommodation per change in anterior lens curvature) was dependent primarily on the equivalent refractive index (p<0.001). It decreased with age (p<0.001) due to a decrease in equivalent refractive index with age (p<0.01).

Conclusions : The inter-individual variability of optical accommodation efficiency is small and caused primarily by variations in anterior chamber depth. The mechanical accommodation efficiency decreases with age due to a decrease in equivalent index with age. Together, these findings suggest that age-related changes in the optical properties of the lens contribute to presbyopia by increasing the force required to accommodate, independent of changes in lens stiffness.

This is a 2020 ARVO Annual Meeting abstract.

 

Figure 1: Curvature and lens power increase linearly with accommodation (Age = 21 years).

 

Figure 2: Left: Optical accommodation efficiency. Right: Mechanical accommodation efficiency. Red dots = outliers excluded from regression analysis.