Age-related changes in the accommodation mechanism.

Presbyopia is caused by changes in the visco-elastic properties of the lens or the choroid or both. Far-to-near (FN) accommodation dynamics are dominated by the properties of the lens, whereas near-to-far (NF) accommodation dynamics depend on the properties of the lens as well as the choroid. The aim of this study was to gain more insight into the cause of presbyopia by analyzing how FN and NF accommodation dynamics change as a function of age. Changes in axial lens thickness during accommodation were measured in vivo using continuous ultrasonographic biometry. The measurements were analyzed with the use of a biomechanical model of the mechanism of accommodation. Fitting the model to the responses yields time constants of FN and NF accommodation. The time constants FN and NF increase with age at an approximately equal rate: 7 ms/year (+/- 1.8 SEM) and 6 ms/year (+/- 1.6 SEM), respectively. This result supports a lenticular cause of presbyopia. In addition, estimations of the lens damping coefficients at different ages are given for the first time: the damping coefficient of the lens increases 20-fold between 15 and 55 years of age.