Animals raised wearing lenses which impose hyperopic retinal defocus (plane of focus located behind the retina) develop axial myopia. In line with these animal studies it has been suggested that focusing errors associated with prolonged accommodation, in particular lag of accommodation (plane of focus behind the retina), might explain the link between prolonged near work and the development of axial myopia in humans. Attempts to reduce accommodative lag by prescribing progressive addition lenses (PALs) to children in order to reduce myopia progression have had limited success. Although PALs may slow progression somewhat, the effect is insufficient to control myopia progression in the clinical situation. Whether results from animal models are directly applicable to naturally occurring myopia in humans is questionable.
A study of undercorrection of myopia found that myopia progressed significantly more rapidly in children who were undercorrected compared to those wearing a full correction, implying that myopic defocus in humans increases the rate of myopia progression. However, in animals myopic defocus slows elongation of the eye and causes hyperopia. Animal studies also predict that overcorrection of myopia might accelerate myopia progression in children. However, attempts to manage exotropia, or to slow myopia progression with overcorrection, do not appear to increase myopia progression.
Alternative theories linking near work and myopia development have proposed that intraocular forces associated with sustained accommodation might lead to eye enlargement, perhaps by “stretching” the sclera. The human eye elongates slightly during accommodation, suggesting that prolonged accommodation might lead to a permanent increase in eye length and myopia. On this basis, reducing accommodative effort might act to reduce myopia progression.
A prescription that is widely used to provide a near addition for presbyopic contact lens wearers is monovision, in which one eye is corrected for distance vision while the other is corrected for near vision. In principle, a monovision correction prescribed to children with myopia could reduce accommodative effort during near work and potentially slow myopia progression. Although some aspects of visual function may be compromised with monovision, most presbyopic monovision wearers perceive a clear image of the world at distance and at near and are unaware of the anisometropic blur.
The aims of this study were to determine whether children could successfully wear a monovision spectacle correction and whether it would reduce accommodative effort at near. A further aim was to investigate possible side effects of monovision wear, particularly whether it might induce some anisometropia over time. Monovision was prescribed as spectacles rather than contact lenses because the procedure was aimed at 11 year old children for whom spectacles are more universally applicable.
DISCUSSION
The primary reason for investigating a monovision prescription was its potential to reduce accommodation. Unexpectedly, children accommodated to read with the distance corrected eye, so accommodation was not appreciably reduced by monovision. A possible explanation for this finding is that the accommodation response followed accommodation demand in the dominant eye, as reported for perceptually rivalrous stimuli.
Another explanation (suggested by unpublished data from this laboratory) may be that accommodation was driven by the convergence necessary to maintain fusion while reading. Whatever the explanation, the result highlights the fact that undercorrecting one eye has quite different optical consequences than bilateral undercorrection. Bilateral undercorrection results in myopic defocus at distance but clear retinal images at near in both eyes. In contrast, unilateral undercorrection of the non-dominant eye results in continuous myopic defocus in the undercorrected eye at both distance and near. As expected, stereoacuity was reduced in some children with monovision but returned to 40 seconds of arc in all children with a balanced prescription. The best corrected acuity of all eyes remained at baseline levels (typically 6/5) throughout the study and based on these clinical tests there was no evidence of any change in visual function following monovision wear.
A significant finding was that the rate of myopia progression was slower in the near corrected eyes than in the distance corrected eyes. While participant dropout is of some concern, the demonstrated effect in 13 participants suggests that it can be generalised to at least 75% of the equivalent myopic population. Although it is probable that the difference in progression rates can be attributed to a slowing of progression in the near corrected eyes because of sustained myopic defocus, the possibility of some increase in progression rate in the distance corrected eyes cannot be ruled out. Progression is typically most rapid during the initial stages of myopia development and slows to a stable refraction over a number of years.
Accordingly, Grice et al reported a mean progression rate in the first year after myopia onset of -0.87 D/year in a group of 19 children, whereas children with longer standing myopia (for example, those wearing single vision lenses as controls in PAL studies) typically have progression rates between 0.5–0.7 D/year. Therefore, while the progression rate in distance corrected eyes of -0.72 D/year found in the present study is to be expected, that of -0.32 D/year in near corrected eyes is lower than expected for children who had only recently developed myopia and were receiving their first optical correction.
For all eyes myopia progression was closely correlated with changes in VCD. The slopes of the relations were comparable to the theoretical value of -2.70 D/mm suggesting that most of the difference in progression rate between the eyes could be accounted for by the difference in their vitreous chamber elongation rates.
In conclusion, monovision is not effective in reducing accommodation in juvenile myopia. However, the results suggest that myopic retinal defocus acts as an anti-myopiagenic stimulus that counters abnormal axial elongation of the human eye. This conclusion is the opposite of that reached after bilateral undercorrection of children with myopia but it is consistent with the results of animal studies.
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