Hidden drift or visible turn?
Most eyes drift a little. What matters is whether the brain is still holding them straight, and in children, whether a turn is getting in the way of normal sight developing.
Two eyes, one picture
Part 01 · 01Both eyes must point at the same spot so the brain can merge two images into one. Many people have a natural tendency for the eyes to drift. Usually the brain holds them straight without you ever noticing.
The difference between a harmless drift and a problem is whether the brain is still in control.
Phoria or tropia
Part 01 · 02Held straight by the brain
The brain holds the eyes straight using fusion. The drift only shows when fusion is broken: one eye is covered, or you are tired or unwell.
The eye is actually turned
Either all the time (constant) or on and off (intermittent). The brain is no longer holding the eyes together.
The cover test
Covering one eye at a time shows whether an eye moves to pick up the target, and in which direction.
Which way the eye drifts
Part 01 · 03Exo = drifts outward · Eso = drifts inward. In the drawing the covered eye is shown under the dark shield, as in a cover test.
Why a turn in a child matters
Part 01 · 04In young children the visual system is still developing. If one eye is turned much of the time, the brain can learn to ignore it. That eye may never see as sharply (amblyopia, or “lazy eye”), and the two eyes may not learn to judge depth together (3D or stereo vision).
A child whose eye turns in or out, even only some of the time, should be assessed promptly. Treatment works best while the visual system is still developing, so earlier is better.
Book a children’s binocular vision assessmentCompensation matters more than size
For optometristsA phoria is a latent deviation compensated by fusional vergence. A tropia is a manifest deviation that fusion is not controlling.
Compensated phoria
Fusion holds it. Often asymptomatic, even when large.
Decompensating phoria
Vergence reserves running low. Symptoms appear.
Intermittent tropia
Breaks down with fatigue, illness or sustained near work.
Constant tropia
Fusion no longer controls the deviation.
The clinical question: is it compensated? A small, poorly compensated phoria can cause more symptoms than a large, well-compensated one, and can break down into an intermittent tropia. In children, a constant or frequent tropia carries amblyopia and stereo risk: treat or refer rather than watch.
Key takeaways
Part 01- 01Most eyes drift a little; fusion usually holds them straight without symptoms.
- 02A tropia is a turn the brain is no longer controlling, all or some of the time.
- 03Any turned eye in a child needs prompt assessment because of the risk of amblyopia.
Book a binocular vision assessment
An eye that wanders, or reading that tires quickly? We’ll measure how the eyes hold together.
General health information only, not personal advice. Everyone’s eyes are different. An optometrist or doctor should examine you before any treatment is decided.