Intermittent exotropia with a distance–near difference in a 5-year-old
A 5-year-old boy is brought by his mother because the right eye ‘turns outward’, noticed since about 18 months of age. It appears mainly when he is tired or daydreaming, and she says it usually returns to straight when he blinks or is spoken to. He has no glasses, no previous treatment, and no developmental or neurological problems. Visual acuity is 6/9 in each eye with a picture chart and there is no fixation preference. Cycloplegic refraction is +1.00 DS in both eyes. Cover test shows an intermittent exotropia of 30 prism dioptres at 6 metres and 12 prism dioptres at 33 cm; near stereoacuity is 100 seconds of arc and Worth four-dot is fused at both distances. After 45 minutes of monocular occlusion the distance deviation remains 30 prism dioptres but the near deviation increases to 25 prism dioptres. Anterior segments and fundoscopy are normal.
1. Classify this exotropia using the occlusion test, and explain why the classification changes the surgical plan.
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The occlusion test is what separates the two main possibilities. Before occlusion the patient had a distance deviation of 30 prism dioptres and a near deviation of 12 - a distance–near difference that superficially suggests divergence excess. After 45 minutes of monocular occlusion the near deviation rose from 12 to 25 prism dioptres while the distance deviation stayed at 30. That is the signature of simulated divergence excess, not true divergence excess.
The interpretation is that the small near measurement before occlusion was an artefact of proximal convergence and fusional effort during near fixation; when fusion is suspended by patching, the true near deviation is revealed. In true divergence excess the near deviation stays small after occlusion.
Why it matters surgically: true divergence excess is treated by operating on the distance deviation alone, typically bilateral lateral rectus recession. Simulated divergence excess behaves as a basic exotropia once the mask is removed, and operating only for the distance angle risks undercorrection and a residual near exotropia. The plan should therefore be based on the post-occlusion near measurement and may require surgery on both the lateral and medial recti (for example lateral rectus recession with medial rectus resection on the non-dominant eye) or larger bilateral recessions than the distance angle alone would suggest.
I would also repeat the measurements at a second visit, because control and angles fluctuate, and document the post-occlusion angle as the number I would operate for.
2. What non-surgical options are reasonable, and would you prescribe the +1.00 DS refraction?
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This child has good near stereoacuity and fusion, no amblyopia and no significant refractive error, so surgery is elective and non-surgical measures are a legitimate first step.
- Refraction: +1.00 DS is within the normal range and does not need correcting for acuity. More importantly, in an exotrope a plus lens relaxes accommodation, reduces accommodative convergence and tends to increase the deviation. Prescribing it would be counterproductive; if any lens is used it should be an overminus lens (typically −2.00 to −3.00 D), which stimulates accommodation and convergence. Overminus has reasonable short-term evidence for improving control in intermittent exotropia, but it must be monitored for induced myopia and used with clear goals and a review date.
- Part-time occlusion of the dominant eye is used to improve control rather than acuity in this situation; the evidence is modest, and it works best as a defined trial of a few weeks with an objective endpoint such as an increase in the time the deviation remains controlled, not indefinite patching.
- Observation with documented control scores (for example a home control scale) is acceptable if the mother is reliable, because many children remain stable for years.
- Orthoptic exercises have a limited role in young children but convergence exercises can help older cooperative patients.
I would treat any intervention as a time-limited trial, review in 6–8 weeks with repeat control assessment and post-occlusion angle measurement, and proceed to surgery if control is deteriorating, the angle is increasing, or stereoacuity begins to fall. I would counsel that surgery aims to restore constant control and does not guarantee improved stereoacuity that is already good.