Adaptation model of nearwork-induced transient myopia

A nearwork model was developed to determine whether variation in accommodative adaptation gain, KA, can account for the differences in the dynamic decay timecourse following near work in hyperopes (HYP), emmetropes (EMM), early-onset myopes (EOM), and late-onset myopes (LOM). The model incorporated a proximal component into a previously-developed adaptation model of accommodation and vergence. It was used to simulate the nearwork-induced transient myopia (NITM) response following 10 min of congruent binocular near viewing (5 D, 5 MA). The accommodative adaptation gain, KA, value was varied from 1.0 to 6.0 in increments of 0.5. For the hyperopes, an additional constraint was imposed wherein the accommodative response was biased on the under-accommodated side of the deadspace operator (i.e., depth-of-focus). In addition, the effect of prolonged nearwork was simulated by alternating between 1 hr of congruent near viewing (3 D, 3 MA) and 5 min of congruent far viewing (0.25 D, 0.25 MA) over a 160 hr period representing one work-month with 40 hours of nearwork per week. The steady-state rms value of the accommodative error was measured as a function of KA. It was found that the NITM timecourses for HYP, EMM, EOM, and LOM could be simulated accurately using KA values of 2.0, 2.5, 4.0 and 5.5, respectively. The long-term final steady-state rms of the accommodative error was found to increase from 0.182 D to 0.188 D as KA increased from 1 to 6. This indicated a small and progressive increase in residual accommodative error for higher KA values, which was associated with EOM and LOM. Thus, NITM for the different refractive groups could be quantified by the accommodative adaptation gain element, with KA for the HYP, EMM, and EOM and LOM groups having lower, intermediate, and higher values, respectively. The larger rms for higher KA values suggests 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.