In vivo changes in the gradient of refractive index distribution in the accommodating human lens

Previously, we observed that accommodative rounding of the young human lens was accompanied by a redistribution in its internal free water content that resulted in a more linear water gradient across the anterior region of the lens. We hypothesised that this would produce a smoother gradient of refractive index (GRIN) which would, in turn, increase the internal lens refractive contribution to lens power during accommodation. However, in vivo studies of the human lens GRIN changes during accommodation remain limited and report mixed findings. To gain further insight into this, we employed our established magnetic resonance imaging (MRI) protocols to measure the changes in lens geometry and GRIN for 10 young (aged 20-27 years) and 14 middle-aged (aged 48-55 years) adults under a 3 Dioptre (D) accommodative stimulus. The MRI-derived lens measurements were then modelled in Zemax OpticStudio software to quantify the refractive contribution of the GRIN changes to the overall lens power increase during accommodation. In young participants, we observed a significant smoothing of the anterior GRIN, which contributed some 1.47-1.59 D of the total 3.04 D increase in lens power under the 3 D accommodative stimulus. In contrast, presbyopic lenses showed no significant changes in geometry or GRIN, culminating in only a total 0.62 D increase in lens power. These findings support the presence of an intracapsular mechanism of accommodation (ICMA) and highlight the need to consider internal lens optics alongside mechanical properties when developing new treatments for presbyopia.