Accommodation is a dioptric change in power of the crystalline lens resulting from ciliary muscle contraction that leads to an increase in lens surface curvatures and thickness and changes in the position of lens surfaces. Previous studies have used A-scan ultrasound to measure changes in the position of lens surfaces with voluntary accommodation, but have not simultaneously measured the change in refraction. The goal of this study is to simultaneously measure and correlate refractive and biometric changes in the lens during voluntary accommodation in humans.
Refraction was measured off-axis in the right eye and biometry on-axis in the left eye simultaneously during voluntary accommodation in 22 human subjects between the ages of 21 and 30 years (mean ± standard deviation: 25.8 ± 2.3 years). Subjects viewed a distant target and four near targets spanning the full accommodative range available to evaluate refraction and lens surface position at each accommodative state.
Maximum objectively measured accommodative amplitude of all subjects was 5.64 ± 0.21 D (mean ± standard error of mean). Biometric and refractive changes during accommodation were linearly correlated. The mean ± standard error of mean decrease in anterior chamber depth was 0.051 ± 0.008 mm/D, increase in lens thickness was 0.067 ± 0.008 mm/D, and increase in anterior segment length was 0.017 ± 0.005 mm/D during accommodation. There was a net anterior movement of the lens center of 0.017 ± 0.005 mm/D.
Anterior chamber depth, lens thickness, and anterior segment length change linearly with refraction during accommodation. Per-diopter changes in the lens were greater in the current study compared with previous studies in which only accommodative demand was measured, which overestimates the accommodative response.