The lens paradox is due to an age-related shift in optical centre of the lens

Abstract

Abstract Purpose : The loss of refractive power with age that occurs despite the increase in thickness and curvature of the crystalline lens is known as the lens paradox. The purpose of this study is to develop clinical Magnetic Resonance Imaging (MRI) protocols to investigate the hypothesis that an age-related anterior shift in the location of the highest point of refractive index along the optic axis, or lens optical centre (OC), contributes to the lens paradox. Methods : 34 human subjects were recruited under the University of Auckland Human Subjects Ethics Committee approval (#017162) and categorised into three groups: young (18-40), middle-aged (41-60) and old (>60). Ocular biometric parameters of left eyes were obtained with a LENSTAR LS 900 (Haag-Streit, USA). All subjects also underwent a T2-weighted MRI scan with a 3T MAGNETOM Skyra (Siemens, Germany) to measure lens water-to-protein ratio (T2), from which the gradient of refractive index (GRIN) was calculated. Anterior chamber depth (ACD), lens thickness (LT), lens anterior (Ra) and posterior curvatures (Rp), and OC were post-processed from these images using MATLAB v9.2.0 (R2017a). Results : Geometrical measurements obtained with MRI significantly correlated with those obtained with LENSTAR (LT: r= 0.941, p< 0.001; ACD: r= 0.859, p< 0.001). ACD decreased 0.013mm/yr (p< 0.001), LT increased 0.021mm/yr (p< 0.001), Ra steepened 0.063mm/yr (p< 0.001) and Rp steepened 0.014mm/yr (p= 0.003).The distance from anterior lens pole to OC was measured as a proportion of LT. The mean OC location was 44.83 ± 11.08% in the young group, 44.20 ± 8.13% in the middle-aged group, and 39.36 ± 14.22% in the old group. Overall, there was a trend for movement of the OC toward the anterior lens pole (r= -0.204, p= 0.123) with age. The refractive index of the OC tended to decrease with age (r= -0.133, p= 0.226) from 1.422 ± 0.013 to 1.419 ± 0.009. Conclusions : We observed a decline in OC refractive index alongside its anterior shift with age. We believe that the combination of these two factors causes the age-related hyperopic shift in lens power and contributes to the lens paradox. Our results suggest that the true asymmetrical nature of optics in the human eye, especially with aging, can only be modelled using full 3D shape and GRIN reconstruction of the lens, which is possible with MRI. This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.