Abstract
Purpose :
A cataract not only scatters the lights passing through the crystalline lens but also changes refractive power of the lens. The cataract induced refractive errors (CIRE) were attributed to a change in the refractive index of the lens, but, due to a complex gradient refractive index (GRIN) structure, the nature of the changes in lens GRIN was not well understood. The purpose of this study was to theoretically model the effect of the changes of lens GRIN profile on thr CIRE, and also the impacts on the measurement of axial length of the eye (AXL).
Methods :
A MatLab program was developed to ray-trace paraxial refractive power for a 50 year old GRIN model lens (Navarro, 2007; Atchison, 2009) with or without cataracts. The cataract induced change in GRIN was described with a Gaussian function (GF), with the height and width factors to represent the cataract severity and areas. The GF was peaked at lens center or lens cortex to model the nuclear or cortical cataracts. The cataract effects on AXL measurement were also estimated.
Results :
Comparing to a normal lens, the cataract lens power was increased when the cataract GF is added to the normal GRIN at the lens center, and thus induced myopia. But, as the GF peak was moved to lens cortex, the lens power was reduced and thus induced hyperopia. The change of cataract lens power was proportional to the height and width factors, but with the amounts depending on the cataract types. The changes in lens power were not symmetrical between the cataract types. For example, when the GF with a peak of 0.005 was located at the lens center, the lens power was increased 5.86D, but it was reduced only 1.43D if the GF was moved 2.0mm to lens cortex. The effect of GF on the measurement of AXL, was small, but, for severe cataract lens, it could be overestimated over 0.06mm.
Conclusions :
A cataract causes increases of the refractive index of the crystalline lens, which modify the GRIN profile, and thus induces refractive errors of the cataract eyes. While nuclear cataracts induce myopic refraction, cortical cataracts produce hyperopic refraction, with the cataract induced myopia more manifested over the hyperopia. The overestimate of the axial length of the eye due to cataracts is small in general, but for severe cataracts, it needs to be considered in cataract surgery design.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.