Introduction
Although the links between near work and myopia development remain controversial there is a growing body of evidence linking accommodation responses, or more precisely accommodation errors, and myopia. Groups of children with myopia and young adult myopes exhibiting myopia progression have larger lags of accommodation (a lower response than the demand) than their emmetropic counterparts (discussed in Schmid and Strang, 2015). Although there is an association between accommodation errors and the worsening of myopia in young adults and myopia in children, causality has not been proven. It is unclear if higher lags of accommodation occur in emmetropes who go on to develop myopia and if reducing the lag of accommodation slows myopia progression. Some studies show that plus lenses worn at near have the greatest anti-myopia effects in children who have the higher lags of accommodation but other studies report no relationship between the effectiveness of treatment and the degree of the accommodation error.
Accommodation errors and demand
The data on accommodation errors is also confounded by the fact that the near working distance and temporal dynamics are important. The measured accommodation errors are greatest when the accommodation demand is produced using negative lenses while maintaining distance viewing compared to when the demand is created by decreasing the target distance – this presumably reflects the interactions between the accommodation and vergence systems and proximal cues to accommodation being less in the negative lens situation. Other situations that challenge the accommodation system, like reading at near for a prolonged period, would similarly produce larger accommodation errors.
Figure from Abbott et al. (1998). Comparison of accommodation stimulus response curves (ASRC), determined using the negative lens series (NLS), for adult emmetropes (EMM), progressing and stable myopes. The accommodation response to negative lenses is reduced in progressing myopes. The data of Gwiazda et al., (1993) for child myopes and emmetropes obtained using the same method (NLS) has been replotted for comparison. Error bars are standard errors of the mean.
Accommodation and optical treatments
There are many theories for the mechanism(s) by which the current optical treatments for slowing myopia, including orthokeratology and multifocal contact lenses, produce their beneficial effects (including peripheral defocus and modifications to the eye’s aberrations). One of the possibilities is alterations to the accommodation errors; if these treatments reduce accommodation lags and if the hyperopic defocus stimulates the myopia development then this could explain part of the effect. Although we have shown that accommodation accuracy at near was improved with positive lenses, multifocal spectacle lenses have not proven to be particularly effective at slowing myopia progression (except in particular groups and with particular lens designs, see Cheng D, Woo GC, Drobe B, Schmid KL. Effect of bifocal and prismatic bifocal spectacles on myopia progression in children: three-year results of a randomized clinical trial. JAMA Ophthalmology. 2014; 132 (3), 258–64).
Clinical relevance
These findings suggest that the accommodation system should be assessed, in particular that accommodation errors should be measured, in myopic patients and the impact of the proposed myopia treatment also assessed; this includes contact lens treatments. There is now evidence that young adult myopes wearing orthokeratology lenses have lower accommodative lags at near compared to matched single vision contact lens wearers (Gifford K, Gifford P, Hendicott PL, Schmid KL. Near binocular visual function in young adult orthokeratology versus soft contact lens wearers. Cont Lens Anterior Eye. 2017; 40(3),184-189). The accommodation response data of myopes wearing multifocal contact lenses varies across studies, with some reporting reduced accommodation errors and others greater errors; it is likely that the design of the multifocal contact lens will have a large impact. In clinical practices without free-space autorefractors, accommodation assessment could be done via the inclusion of near MEM retinoscopy, performed both with the distance prescription and then with positive lenses to assess the difference and then later with the multifocal contact lenses or during orthokeratology treatment.
Dr Katrina L Schmid BAppScOptom(Hons), PhD
Associate Professor, School of Optometry and Vision Science, Institute of Health and Biomedical Innovation, Faculty of Health, Queensland University of Technology.
References
Abbott ML, Schmid KL, Strang NC. Differences in the accommodation stimulus response curves of adult myopes and emmetropes. Ophthalmic Physiol. Opt. 1998; 18, 13–20.
Gwiazda J, Thorn F, Bauer J, Held R. Myopic children show insufficient accommodative response to blur. Invest. Ophthalmol. Vis. Sci. 1993; 34, 690–694.
