Authors Sara J. McCullough, Ulster University

Reference Is binocular vision a critical component of myopia management? By Dr Sara McCullough. Originally published in Optometry Today 10 October 2020

Biography

 Dr Sara McCullough is a lecturer in Optometry and Vision Science at Ulster University.  She is a coinvestigator on the Northern Ireland Childhood Errors of Refraction (NICER) Study, a large, prospective study investigating the progression of refractive errors in children and young adults in the UK with a particular interest in the risks and progression of myopia development.  She has presented her research findings and clinical expertise at numerous national and international conferences and CET events.  Sara also provides undergraduate lectures and clinical tutorials on Binocular Vision and Orthoptics at Ulster University and is a research advisor and examiner for the College of Optometrists, UK.

This article describes the elements of binocular vision assessment that are important with regard to the risk of myopia development and progression in children and how binocular vision might influence your choice of myopia control strategy.  This will cover:

  • What tests of binocular vision are important for myopes and for detecting those at risk of myopia?
  • What binocular vision features are commonly encountered among myopic patients?
  • How might binocular vision affect your myopia management strategy?
  • How might myopia management affect your patient’s binocular vision status?

Introduction

 A thorough binocular vision assessment is an important feature of any eye examination and particularly for children.  The pertinent binocular vision assessments are within the scope of practice for a primary care optometrist and should, at a minimum, include an evaluation of accommodative function and an assessment of ocular motor balance.

Essentially it is important to establish whether there is a phoria present at distance and near fixation and how well it is compensated (i.e. does it recover well and is it likely to be comfortable and asymptomatic) or if there is a tropia present.  This can be simply evaluated using a distance and near cover test.  The degree of compensation/fixation disparity can be assessed using the Mallett Unit as an example, and a measurement of the patient’s fusional ranges.  An evaluation of the interaction between the accommodative and vergence systems can also be achieved by calculating the accommodative convergence to accommodation ratio (AC/A ratio).  This describes the patient’s convergence ability per unit of accommodation exerted. A typical response is generally between 2-5:1 depending on the method of measurement1; an assessment by the heterophoria method using a comparison of the size of phoria between distance and near fixation will result in a higher ratio than when comparing the size of the phoria when accommodation is stimulated or relaxed by concave or convex lenses respectively (gradient method).

Habitual binocular status is important to consider prior to changing to a different refractive correction as the change in correction method can alter the vergence state and resultantly the binocular status.  It is also important to consider if there is a tropia present, as it is likely that management of the binocular vision problem will be prioritised before commencing myopia management. These will be discussed in more detail later in the article.

While the recent International Myopia Institute guidelines state that currently there is no consensus on the gold standard techniques for assessing binocular vision prior to instigating myopia management, at a minimum they suggest an evaluation of both the accommodative and vergence systems as described above.  They would also suggest that the same tests are used at subsequent visits to monitor any change in binocular function with myopia management.2 A summary of the binocular vision tests discussed within this article and their significance can be found in Table 1.

Accommodative Response

It has been reported that myopes tend to have larger accommodative lags than their emmetropic peers3 and that a lag of accommodation, and thus hyperopic defocus on the central retina may be a stimulus to eye elongation and myopia development (Figure 1).  This theory has been supported by studies whereby myopia has been promoted in eyes with induced hyperopic defocus.4 An assessment of accommodative response accuracy is therefore a useful test to help identify those children who may be at increased risk of developing myopia.  An accommodation lag occurs when there is a significant reduction in accommodative response compared to the accommodative demand.  For example, if you ask a patient to look at a target at 25cm (a 4D demand), an accommodative response of less than 4D represents an accommodative lag.  Larger accommodative lags lead to a greater amount of central hyperopic defocus and thus potentially a greater stimulus to myopia development.  However, it is currently unclear whether the accommodative lag found in myopes is a precursor to myopia development (i.e. present in children at risk of developing myopia, the pre-myope) or is a consequence of the changes within the eye that are resultant from myopia development.5,6  Similarly, the evidence is conflicting on whether myopic progression is faster among those with lags of accommodation or larger accommodative lags with some supporting the theory 5,7,8 while others do not.9-12 Although the evidence is conflicting with regard to accommodative lag and myopia progression, measurement of accommodative response is a quick and easy test to perform and can be helpful to include as a further risk indicator for myopia development.

Illustration of accommodative lag.

Ocular Motor Status

It has been suggested that near esophoria is associated with more rapid myopia progression and there is a shift towards esophoria during myopic progression.13-15 Those with near esophoria are thought to relax their accommodation in an attempt to relax their convergence at near to help maintain single vision.  The reduced accommodation as a result would in theory promote axial elongation as previously described above. In some studies, children who developed myopia also showed an elevated AC/A ratio in the several years prior to myopia development compared to their age-matched peers who remained emmetropic.5,16,17 However, the evidence surrounding near esophoria and myopia development and progression is conflicting.  Those who support the theory have reported that progression was slightly faster in those with near esophoria (approximately 30%) compared to those with ortho/exophoria, although this difference was small at only 0.09D greater progression per year.13 Other authors have reported annual progression to be statistically significantly greater in those with near exophoria compared to near esophoria however, again, this difference wasn’t clinically relevant.18

Unpublished data from the Northern Ireland Childhood Errors of Refraction (NICER) Study, a large, 9-year evaluation of longitudinal changes in refractive error among white, UK based children and young adults aged between 6-7- and 15-16-years and 12-13- and 21-22-years19 found that the majority of those in the study who were myopic or developed myopia during the study period (n=211) were either orthophoric (74%) or exophoric (18%), with the minority being esophoric (6%) on near cover test (with correction).  Similarly, the majority of myopic participants from the MiSight® 1 day randomised controlled clinical trial20 were orthophoric (67%) or exophoric (23%) at near compared to only a small number presenting with esophoria (8%).

Accommodative Lag & Near Esophoria

As a result of reports that those with near esophoria and accommodative lag were likely to have faster myopic progression, researchers investigated whether these individuals would have less myopic progression when bifocal or progressive addition lenses were prescribed compared to single vision distance spectacles.  The treatment effects in studies that specifically targeted children with these conditions show some, but not obviously different results compared to those where these restrictions were not employed.  There is also an indication that executive-type bifocals may provide the best results for the optical treatment of myopia progression over other types of multifocal lenses, particularly when those with near esophoria and accommodative lag are considered.21 However, caution must be taken that this is only in one study population and further replication is required before clear cut advice can be given.

For clinicians, we can therefore say it is likely that the majority of myopic patients will either be orthophoric or exophoric at near with fewer presenting with esophoria (approximately one in ten).  Children presenting with near esophoria, an accommodative lag and elevated AC/A ratio may be potentially at greater risk of myopia development or show faster progression of myopia once myopia has developed than their orthophoric/exophoric peers, however the results are still inconclusive.  Other risk factors for myopia development such as family history of myopia, East Asian ethnicity and visual environment (low time outdoors, intense near work)2 should be evaluated and are likely to outweigh the binocular vision risk factors discussed.22 Those rare myopes who exhibit both near esophoria and accommodative lag could be prescribed multifocal lenses such as executive bifocals (where other treatment options such as myopia control contact lenses might be potentially contraindicated) as they may respond favourably to this treatment modality compared to the majority without these binocular vision features.

How might a patient’s binocular vision be affected by myopia control strategies?
It is important to check the ocular motor status of a myopic patient before changing from single vision spectacles to a contact lens correction.  When myopes look through a spectacle lens at near, there is some element of base in prism induced; therefore they need to exert less vergence in spectacles than they do in contact lenses.  The off-centre power of the minus lens is also less during near tasks and therefore accommodative demand is less with spectacles compared to contact lenses (Figure 2).  However, moderate levels of myopia are required for this to be significant.  For example, using Prentice’s rule (P=cF) with an inward eye movement of 2mm in each eye for near vision, the patient would need to be a -5D myope to obtain 1Δ base in.  So, if a child has a well-compensated exophoria that shows brisk recovery on cover test, no fixation disparity and normal fusional ranges, there should be no issue of changing from spectacles to a contact lens correction, particularly when low levels of myopia are present.  However, if a child has a large exophoria, that shows poor/sluggish recovery on cover test, an exo-slip on fixation disparity and/or a reduced positive fusional range then changing to contact lenses may cause decompensation of the exophoria.  In this instance, it would be beneficial to deal with the binocular vision issues first by providing orthoptic exercises to improve convergence and positive fusional reserves (e.g. pen to nose exercises, dot cards, positive/near stereograms) before changing the refractive correction modality. An easy way to confirm that you have not disrupted the patient’s binocular status when changing refractive error correction methods is to perform a cover test with the new correction to ensure you have not significantly increased the size of the phoria present or caused it to decompensate.

Illustration of change in phoria and accommodation in a myope when wearing spectacle lenses and contact lenses when viewing at near.

A recent study by Ruiz-Pomeda et al.23, conducting a randomised control trial of MiSight® 1 day lenses in children aged 8-12 years compared binocular vision and accommodative characteristics between those wearing single vision spectacles and MiSight lenses and followed them over a period of two years.  Those in the MiSight® 1 day group became slightly more exophoric at near over the two-year period, however the change did not differ significantly on average to the group of children wearing the single vision spectacle lenses.  This change in near phoria may be explained by the children using the near addition to relax their accommodation and therefore their convergence or from the effect of changing from spectacle lenses to contact lenses as described above.  The near point of convergence, accommodative amplitudes and responses were within normal limits for those wearing the MiSight® 1 day contact lenses, did not change over time and did not differ significantly from those wearing the spectacle lenses. When changing children from single vision spectacles to myopia management contact lenses it is unlikely to cause clinically significant changes to binocularity or accommodation, nonetheless it is important to confirm the ocular motor status with the new correction to ensure you haven’t disrupted binocularity or accommodative function.

Contact lenses in those with compromised binocular vision
Cases of children with anisomyopic amblyopia are rare and were reported in less than 1% of myopic European children aged between 5-16 years.24   A recent report by Bullimore (2017)25 also reported that there was no increased risk of corneal infiltrative events in children compared to young adults, in fact the risk was markedly lower as parents supervised their younger children with cleaning regimes. However, the final binocular vision consideration that may be important prior to commencing a myopia management strategy is the case of a myopic child with amblyopia as a result of significant anisometropia or a manifest strabismus. If the child is amblyopic or strabismic, treatment should be first initiated to manage the binocular vision issues prior to myopia control.  Where the child remains amblyopic after treatment, it is important to consider the potential consequences of fitting contact lenses to a child with good vision in only one eye.  The risks of contact lens induced keratitis should be discussed with the parents and balanced with the risks and likelihood of myopia progression and future visual difficulties.    Children with significant anisomyopia but good vision in both eyes may benefit from a contact lens myopia management strategy to negate the unwanted effects of aniseikonia. Fast-paced development may see alternative options to contact lenses coming to market in the future such as Defocus Incorporated Multiple Segments (DIMS)26 or low dose atropine therapy.27

Conclusions

Binocular vision is important to consider for the young myopic patient, particularly when the clinician is planning on initiating a myopia management strategy.  At a minimum, a measure of accommodation and ocular motor status should be considered prior to commencing myopia management and the same tests should be repeated on subsequent visits.  It is also of interest to glean information on signs of accommodative lag, the presence of near esophoria and a high AC/A ratio as children with these features, although at a low incidence, may be at a higher risk of developing myopia or having faster progression once myopia has developed.  Although strategies such as myopia control contact lenses are unlikely to significantly alter the binocular and accommodative status of a child, it is important to check these clinical features before and after fitting these refractive modalities, as with any refractive error correction,  to ensure they have not been compromised as a result.

Table 1. Summary of the pertinent binocular vision tests and their relevance to myopia control strategies.

References

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