Myopia and exotropia – what’s the connection?

Intermittent exotropia (IXT) is characterised by an intermittent outward deviation of one or both eyes. It is the most common type of strabismus in childhood, occurring in approximately 1% of healthy children in the US and more prominently in Asia, at 0.62-4.7%.1-3

Myopia commonly presents with exotropia as a co-morbidity, as IXT has been associated with myopia onset. The prevalence of myopia is higher in children with exotropia than children without strabismus, as seen in 12-year-old Australian children (57% vs 12%) and in children up to 6 years old in the US (12% vs 4%).4,5 A population-based study noted that almost 47% of children with IXT had developed myopia by age 10, and over 90% by age 20.6

On the other hand, the relationship between exotropia and myopia progression (among myopic children) is not as well established. One recent study suggests that there is no difference in natural myopia progression among children with IXT and those without strabismus.7 Interestingly, undergoing corrective strabismus surgery does not have any impact on a child’s rate of myopia progression.6,7

It is unknown whether myopia development or progression could trigger IXT, or whether the reverse is true. 

If myopia comes first, then myopia lends to lower accommodative demand, resulting in reduced convergence. For predisposed individuals, this may cause decompensation into significant exophoria or IXT. 

If the IXT comes first, then the increased accommodative demand to maintain accurate near point focus could lead to pseudo-myopia at distance, and drive myopia progression. If this is the case, then correcting IXT may slow the progression of myopia (or vice-versa). However – as previously mentioned, surgical correction of IXT was found to have no effect on myopia progression in multiple studies.6,7

Disrupted fusion from anisometropic myopia in early childhood can lead to exotropia. The reverse can also occur – suppression from exotropia can cause sensory strabismus and form deprivation myopia.8

For every child, choosing between spectacles and contact lenses does more than correct myopia – it can also affect their binocular vision function.

A myopic individual who reads through glasses experiences base-in prism at near, moving the image further away. Looking away from the optical centre of the lens also reduces the power, effectively reducing the accommodative demand. 

When switched to a single vision contact lens, they must increase their accommodative effort, and they will also show an exophoric shift (as the base-in effect is lost) and have to converge more by themselves.9

Shifting from spectacles to LASIK correction in young adults has also been associated with exophoric shifts.10

Ajay was referred for orthokeratology (OK) at age 11, after his myopia progressed by R -0.75 and L -2.50 over one year. Initially at age 10, his spectacle refraction was R-2.00/-0.75x90 and L -2.50/-1.25x90. On examination, I noted that Ajay exhibited a left eye almost-constant convergence insufficiency exotropia (near worse than distance). 

For Ajay, shifting from a spectacle correction where an IXT – which was nearly constant – could lead to decompensation into a constant exotropia. Despite being referred for OK fitting, Ajay's binocular vision status made him an unsuitable candidate. Thus, it was planned for Ajay to continue wearing glasses for the next year – with prism correction to help fusion, along with vision training to improve his binocularity. 

While a -0.50 shift in both R&L eyes has occurred over the subsequent year, this is significantly less than the previous year. Additionally, the sensory component of Ajay's strabismus has improved. In time, he may be suitable to be fit with OK or alternatively a myopia control soft contact lens.

Mary had unfortunately progressed to high anisometropic myopia by the time I first saw her at age 11. She had a similar binocular vision status to Ajay with left IXT, though not as severe as Ajay. 

Because she was very motivated to wear contact lenses, she was initially fit with OK correction. Full correction was achieved in her right eye and a -1.75DS residual occurred in her L eye. I was satisfied with this outcome, as studies have shown that partial correction of high myopia can achieve a similar myopia control effect to full correction.11 Mary also needed to wear glasses anyway, as base-in prism was prescribed to improve fusion.

After one year, Mary's myopia in her right eye stable – but her left eye had progressed by -1.50DS. A terrible outcome! What has occurred?

It turned out that having her right eye fully corrected by OK had disincentivized her to wear glasses. As a result, her left eye had spent a significant time undercorrected, which is known to increase myopia progression.12 Poor compliance with her glasses also led to more inconsistent binocular fusion. Overall, the left eye had suffered a double whammy of undercorrection and intermittent suppression. 

Mary was discontinued from OK and fit into full correction, centre-distance toric multifocal soft contact lenses with +2.50 add. She has since worn these successfully for over a year now, with stable myopia. Vision training was provided to improve her binocular stability and base-out (convergent) fusional reserves, as her IXT needed to be reduced for her to continue wearing contact lenses.

• Intermittent exotropia can increase the risk of developing myopia, but once myopic, the rate of myopia progression is comparable to peers without strabismus.
• Switching a myopic child from spectacles to contact lenses often increases accommodative demand and produces an exophoric shift, which can potentially destabilize binocular fusion.
• Assess binocular vision in individuals with myopia, and consider binocular vision status when tailoring treatments. 

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