The effect of bifocal add on accommodative lag in myopic children with high accommodative lag.

The centuries-old association between near work and myopia dates back to the early 1600s.1 Within the past few decades, based on data from both humans and animal models of myopia, the proposed link between excessive accommodation and myopia has yielded a theory invoking high accommodative lag during near work as a cause of juvenile-onset myopia progression.2–4 Myopic children have a higher lag of accommodation than do emmetropic children. Therefore, myopic children experience a greater amount of hyperopic retinal blur during near work than do nonmyopic children.5,6 Lens-induced defocus is known to predictably alter eye growth across animal models, with negative lenses resulting in longer, myopic eyes and positive lenses resulting in shorter, hyperopic eyes.7–12 Because of the well-established ability of retinal defocus to guide eye growth in young animals and the higher accommodative lag found in myopic individuals, the link between accommodation and myopia in children is currently thought to be that excessive axial growth is caused by hyperopic retinal blur from a high lag of accommodation during near work.2–4 In an effort to reduce myopia progression in children, bifocal spectacles and progressive addition lenses (PALs) have been evaluated as a treatment in multiple clinical trials.13–16 The rationale for wearing bifocal spectacles is that they decrease accommodative lag during near work and therefore reduce hyperopic retinal blur, slowing myopia progression.15,16 The Correction of Myopia Evaluation Trial (COMET) found that myopic children with a high accommodative lag and near esophoria had the greatest 1- and 3-year PAL treatment effect (0.39 and 0.64 D), and children with high accommodative lag and low myopia (−2.25 D or less spherical equivalent myopia) had significant 1- and 3-year PAL treatment effects of 0.28 and 0.48 D, respectively.17 Although clinically meaningful treatment effects have been reported in children with a high accommodative lag, there has been limited success when myopic children with all levels of accommodative lag were included in interventions attempting to reduce progression by reducing lag.13–16 Unfortunately, because no trial has reported the change in accommodative lag in subjects wearing a bifocal add, it is difficult to confirm whether children with a high accommodative lag in previous studies have a greater treatment effect because of a greater reduction in accommodative lag when wearing a bifocal or PAL. Surprisingly little has been published on the effect of a bifocal spectacle add on measured accommodative lag in children. Nearly all published reports on the effect of a bifocal add on accommodative lag examine adults with emmetropia or myopia.18–23 Although interstudy variations exist in the testing methodology, including the dioptric amount of both the bifocal add and the test stimulus used, these studies consistently find that a bifocal add of +2.00 D or less either eliminates accommodative lag or results in a lead of accommodation. The effect of a bifocal add on accommodative lag in myopic children has been examined in only two studies, and their results conflict. Cheng et al.24 reported that accommodative lag for a binocular, 3-diopter stimulus was not eliminated in children with progressing myopia until they wore a bifocal add higher than +2.50 D.24 A +2.00-D add reduced accommodative lag from 1.00 to 0.22 D according to their modeled data, but did not eliminate accommodative lag. Sreenivasan et al.25 also examined the effect of a +2.00-D add on accommodative lag with a binocular, 3-diopter stimulus in myopic children and found that the bifocal essentially eliminated the initial 1.10-D accommodative lag.25 It is noteworthy that their sample included no children with near esophoria, because, as just described, children with near esophoria and a high lag of accommodation in COMET had the largest PAL treatment effect.17 Based on the literature, it is uncertain whether accommodative lag is significantly decreased in children who are reported to benefit most from PALs. It is important to understand the effect of a bifocal on accommodative lag in these children. Without knowing the reduction in accommodative lag in subjects wearing bifocal spectacles, it is not possible to state definitively whether the reduction in myopia progression found in previous trials using bifocal spectacles was due to decreased hyperopic retinal blur during near work or whether other mechanisms should be explored to explain the bifocal treatment effect. It is also not known how factors such as the accuracy of a child's myopic correction affects accommodative lag. Because myopia in childhood progresses on average by −0.50 D per year,14,15 myopic children spend a significant amount of time wearing a less than optimal distance correction. With the exception of one of the studies mentioned so far,22 accommodative lag was measured with the subject's full correction in place. The basis of the theory that accommodative lag during near work causes myopia progression rests on the amount of hyperopic retinal blur present during near work; therefore, it may be important to know both the “new” amount of accommodative lag present after updating a child's spectacles to the full correction and the habitual lag most recently experienced before updating the spectacle prescription. Longitudinal studies of juvenile-onset myopia progression typically measure accommodative lag in spectacle-wearing children with full correction in place.6,26,27 To the best of our knowledge, no studies have been conducted to examine the effect of measuring myopic children's accommodative lag with habitual correction versus their full correction to determine whether there are significant differences in lag in these two viewing conditions. The purposes of these analyses were to determine the effect of bifocal add on the amount of accommodative lag in children with progressing myopia with a high accommodative lag, who have been selected as most likely to benefit from PALs, and to determine the effect of correction type (habitual versus full manifest) on accommodative lag.