What happens to binocular vision during cycloplegia?

The use of cycloplegic agents may be employed in examining myopic children to ensure the most accurate refraction possible.¹ What does a cycloplegic agent do to binocular vision function? Typically, under these conditions we would expect a less minus refraction, and at near a reduced or absent accommodation response and a likely exophoric shift. However, LM presented this fascinating case where a patient with a history of exophoria exhibited a significant esophoric shift under cycloplegia. Here is the case.

The IMI Clinical Management Guidelines suggest that, when cycloplegia is used, to instill two drops of 1% tropicamide or cyclopentolate, given 5 minutes apart.¹ In younger children especially, the accuracy of autorefraction is improved with cycloplegia.²

Lyu et al found an increase in esodeviation under cycloplegia in 34 Korean children with hyperopia and esotropia.³ The cycloplegic agent used was a mix of 0.5% tropicamide and 0.5% phenylephrine. The authors proposed two explanations. Firstly, a decrease in fusional divergence effort may occur, as patients experience blurred vision due to cycloplegia and stop attempting fusion. This could make sense for children with pre-existing esodeviation, but could apply less to this case where the child has a history of significant exodeviation.

The second explanation was that the 0.5% tropicamide and 0.5% phenylephrine combination has been reported as insufficient to achieve complete accommodation paralysis in children with darkly pigment irides and high amplitude of accommodation.^4,5 Hence a young patient may attempt to accommodate to overcome the near blur under incomplete cycloplegia and by doing so, drive more convergence.

There have been case studies reported in India that 0.01% atropine induces an esodeviation in children with convergence excess esotropia, after switching from 1% atropine for myopia control.^7,8 

A single case report7 involved a 10-year-old with a small near esophoria before 1% atropine treatment, which was changed to 0.01% after 18 months, due to complaints of photophobia. At 6 month follow up, the patient exhibited a poorly controlled near esophoria - despite being prescribed progressive addition spectacles for near support. This resolved on cessation of 0.01% atropine. Recommencement of 1% atropine, with a +3.00 Add for near, resulted in well-controlled exophoria at distance and near.

Another case series⁸ reported three myopes with a mean age of 5.3 years who developed convergence excess esotropia following use of 0.01% atropine eye drops. All had undergone surgery previously for intermittent exotropia. The authors pointed to the hypo-accommodation side effect driving excessive convergence.

These side effects are unexpected given that low-dose atropine has been shown to have minimal impact on accommodation amplitude and is generally well tolerated. In the LAMP study,⁹ amps were reduced by only around 2D in 0.025% and 0.05% atropine and barely at all with 0.01%. In children with pre-existing vergence disorders, these case studies highlight that the slight reduction in accommodation function could influence the accommodation-convergence relationship and lead to reflex over-convergence and increased esodeviation.

When considering spectacle and contact lens options for myopia control, spectacles can be more beneficial for exophores as shifting from spectacles to contact lenses in myopia creates an exophoric shift.10 When myopes view at near with spectacles, they experience base-in prism and less accommodation demand due to off-axis spectacle lens effects. These effects don't exist when viewing at near in contact lens wear, requiring increased convergence and accommodative effort by comparison. In patients with normal binocular vision function this may be inconsequential, but for some the resultant exophoric shift and increase in accommodative lag could cause difficulty.

Even though this child has reportedly well controlled exophoria, his binocular vision history led his eye care practitioner LM to preference myopia controlling spectacles. Read more in this clinical article What About the Exophores? 

While cycloplegia may reveal the 'true' refractive state, it may not necessarily give us a picture of the true binocular vision state when accommodation is active. The dynamic binocular visual system can compensate when accommodation is influenced by partial or full cycloplegia, leading to unexpected alteration in vergence function.

At baseline, prior to cycloplegia and on follow-up examinations, this case study and the literature confirm the importance of measuring binocular vision function to assess for any impacts due to low-dose atropine treatment.¹

1. Cycloplegic agents may result in an esophoric shift due to the binocular vision system attempting to overcome near blur by driving excessive convergence.
2. Exophores can benefit from myopia controlling spectacles, which have less accommodative and vergence demand at near compared to when wearing contact lenses.
3. It is important to check the binocular vision status of a child who is being treated with low-dose atropine.

• What is pseudomyopia? Avoiding overcorrection in children

• How to achieve accurate refractions for children

• Four reasons why binocular vision matters in myopia management

• What about the exophores?