Binocular lens treatment in tree shrews: Effect of age and comparison of plus lens wear with recovery from minus lens-induced myopia

Abstract

We examined normal emmetropization and the refractive responses to binocular plus or minus lenses in young (late infantile) and juvenile tree shrews. In addition, recovery from lens-induced myopia was compared with the response to a similar amount of myopia produced with plus lenses in age-matched juvenile animals. Normal emmetropization was examined with daily noncycloplegic autorefractor measures from 11 days after natural eye-opening (days of visual experience [VE]) when the eyes were in the infantile, rapid growth phase and their refractions were substantially hyperopic, to 35 days of VE when the eyes had entered the juvenile, slower growth phase and the refractions were near emmetropia. Starting at 11 days of VE, two groups of young tree shrews wore binocular +4 D lenses (n=6) or -5 D lenses (n=5). Starting at 24 days of VE, four groups of juvenile tree shrews (n=5 each) wore binocular +3 D, +5 D, -3 D, or -5 D lenses. Non-cycloplegic measures of refractive state were made frequently while the animals wore the assigned lenses. The refractive response of the juvenile plus-lens wearing animals was compared with the refractive recovery of an age-matched group of animals (n=5) that were myopic after wearing a -5 D lens from 11 to 24 days of VE. In normal tree shrews, refractions (corrected for the small eye artifact) declined rapidly from (mean±SEM) 6.6±0.6 D of hyperopia at 11 VE to 1.4±0.2 D at 24 VE and 0.8±0.4 D at 35 VE. Plus 4 D lens treatment applied at 11 days of VE initially corrected or over-corrected the young animals' hyperopia and produced a compensatory response in most animals; the eyes became nearly emmetropic while wearing the +4 D lenses. In contrast, plus-lens treatment starting at 24 days of VE initially made the juvenile eyes myopic (over-correction) and, on average, was less effective. The response ranged from no change in refractive state (eye continued to experience myopia) to full compensation (emmetropic with the lens in place). Minus-lens wear in both the young and juvenile groups, which initially made eyes more hyperopic, consistently produced compensation to the minus lens so that eyes reached age-appropriate refractions while wearing the lenses. When the minus lenses were removed, the eyes recovered quickly to age-matched normal values. The consistent recovery response from myopia in juvenile eyes after minus-lens compensation, compared with the highly variable response to plus lens wear in age-matched juvenile animals suggests that eyes retain the ability to detect the myopic refractive state, but there is an age-related decrease in the ability of normal eyes to use myopia to slow their elongation rate below normal. If juvenile human eyes, compared with infants, have a similar difficulty in using myopia to slow axial elongation, this may contribute to myopia development, especially in eyes with a genetic pre-disposition to elongate.

Figure 1

Experimental groups as a function of days of VE. The dark curve shows the normal axial growth of tree shrew eyes from 1 to 75 days of VE (curve fitted to the data of Norton and McBrien, 1992).

Figure 2

Refractive development of the eight animals in the young normal group (solid lines) and the five animals in the juvenile normal group (dashed lines). Right and left eyes of individual animals were averaged and are shown with the thin lines. The dark lines are the group averages. In tree shrews, an autorefractor value of +4D is estimated to be emmetropia based on a prior study that used visual evoked potentials to measure the refractive state (Norton et al., 2003). Therefore, in this and other figures, 4 diopters have been subtracted from the autorefractor value.

Figure 3

Refractive development of the six animals in the young plus-lens group compared with normal refractive development. A. Individual animals. B. Group average responses excluding the non-responding animal (0734).

Figure 4

Recovery from early plus-lens treatment. A. Group data for the 5 animals that compensated for the plus lens. B. Plots for animal 06109 showing the similar response in both eyes. After emmetropizing to the plus lenses, eyes were nearly emmetropic (filled symbols). When the lenses were removed at day 45 of VE, the eyes initially experienced hyperopia which rapidly decreased toward emmetropia (open symbols).

Figure 5

Refractive development of the animals in the juvenile plus-lens treatment groups compared with normal refractive development. A. plus 3 D lenses. B. plus 5 D lenses. Values are calculated with the lens in place.

Figure 6

Refractive development and recovery in the young binocular −5 D group. The young minus lens-treated animals began recovery at 24 days of VE. The −5 D values are with the lens in place.

Figure 7

Refractive development of the animals in the juvenile minus lens groups, compared with the normal groups. A. −3 D lenses. B. −5 D lenses. Values are with the lens in place.

Figure 8

Comparison of recovery from myopia induced by −5 D lens wear (data from Fig. 6) with the response to +5 D lens-wear in age-matched normal animals (Fig. 5B). Both the recovery and the plus lens wear began at 24 days of VE.

Figure 9

Refractive development of both eyes in the juvenile +5 D plus-lens treatment group showing the similar response of the two eyes in each animal.

Figure 10

Normalized change in axial length in the juvenile groups based on measures made at pedestal installation (21 days of VE) and at 35 days of VE. The change from 21 to 24 VE is based on data from previous experiments in which animals were measured at 21 and 24 VE (Siegwart, Jr. and Norton, 2005).