Adaptation model of nearwork-induced transient myopia

A nearwork model was developed to determine whether variation in accommodat ive adaptation gain, K-A, can account for the differences in the dynamic de cay timecourse following near work in hyperopes (HYP), emmetropes (EMM), ea rly-onset myopes (EOM), and late-onset myopes (LOM). The model incorporated a proximal component into a previously-developed adaptation model of accom modation and vergence. it was used to simulate the nearwork-induced transie nt myopia (NITM) response following 10 min of congruent binocular near view ing (5 D, 5 MA). The accommodative adaptation gain, K-A, value was varied f rom 1.0 to 6.0 in increments of 0.5. For the hyperopes, an additional const raint was imposed wherein the accommodative response was biased on the unde r-accommodated side of the deadspace operator (i.e., depth-of-focus). In ad dition, the effect of prolonged nearwork was simulated by alternating betwe en 1 hr of congruent near viewing (3 D, 3 MA) and 5 min of congruent far vi ewing (0.25 D, 0.25 MA) over a 160 hr period representing one work-month wi th 40 hours of nearwork per week. The steady-state rms value of the accommo dative error was measured as a function of K-A If was found that the NITM t imecourses for HYP, EMM, EOM, and LOM could be simulated accurately using K -A values of 2.0, 2.5, 4.0 and 5.5, respectively. The long-term final stead y-state rms of the accommodative error was found to increase from 0.182 D t o 0.188 D as K-A increased from 1 to 6. This indicated a small and progress ive increase in residual accommodative error for higher K-A values, which w as associated with EOM and LOM. Thus, NITM for the different refractive gro ups could be quantified by the accommodative adaptation gain element, with K-A for the HYP, EMM, and EOM and LOM groups having lower, intermediate, an d higher values, respectively. The larger rms for higher K-A values suggest s that a myopic individual may have a predisposition to exhibit a slightly larger long-term accommodative error, which may stimulate axial elongation and in turn promote the progression of axial myopia. (C) 1999 The College o f Optometrists. Published by Elsevier Science Ltd. All rights reserved.