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

Area of Science:

• Ophthalmology
• Biophysics
• Optics

Background:

• Human lens accommodation involves changes in lens shape and refractive index.
• Previous studies suggest water redistribution influences the lens gradient index (GRIN).
• In vivo studies on GRIN changes during accommodation are limited and yield mixed results.

Purpose of the Study:

• To investigate changes in human lens geometry and GRIN during accommodation using MRI.
• To quantify the contribution of GRIN changes to lens power increase.
• To compare accommodative mechanisms in young and middle-aged adults.

Main Methods:

• Magnetic resonance imaging (MRI) to measure lens geometry and GRIN.
• Zemax OpticStudio software for optical modeling.
• 10 young (20-27 years) and 14 middle-aged (48-55 years) adults under 3 Diopter (D) accommodation stimulus.

Main Results:

• Young lenses exhibited significant anterior GRIN smoothing, contributing 1.47-1.59 D to the 3.04 D accommodative power increase.
• Middle-aged (presbyopic) lenses showed no significant changes in geometry or GRIN.
• Presbyopic lenses only increased lens power by 0.62 D.

Conclusions:

• Findings support an intracapsular mechanism of accommodation (ICMA).
• Internal lens optics, specifically GRIN changes, play a crucial role in accommodation.
• Considering internal lens optics is vital for developing presbyopia treatments.