Understanding Strabismus (Crossed Eyes): Causes, Treatments, and Solutions
What strabismus actually is
Strabismus is an eye that turns inward, outward, up, or down while the other eye looks straight ahead. It is common — studies place it in roughly 2 to 4 percent of the population — and it affects adults as well as children.
Each eye is moved by six muscles on the outside of the eyeball, with a seventh inside that handles focusing. Occasionally an eye deviates because one of those muscles or the nerve supplying it is damaged. Far more often, every muscle works perfectly well, and the problem is that the brain is not coordinating all twelve outer muscles to aim both eyes at the same point at the same moment.
That distinction determines what any treatment can accomplish. Surgery repositions muscles. It does not teach the brain to use them together. Both things can be necessary; they are not the same thing.
Two separate problems
Straightening an eye cosmetically does not, by itself, restore the two-eyed seeing that suppression took away. Those are separate outcomes, and a treatment can achieve one without the other.
The appearance problem.
An eye that visibly turns affects how people respond to you — eye contact, photographs, job interviews, a child’s confidence at school. This is real and it’s usually why people seek treatment.
The seeing problem
When the eyes point in different directions, the brain receives two irreconcilable images. Rather than tolerate double vision, it suppresses the input from one eye. Over time that eye can lose acuity even though it’s structurally healthy — a condition called amblyopia.
If you’re not sure whether you’re dealing with strabismus, amblyopia, or both, see Strabismus vs. Amblyopia.
Types of strabismus
The direction of the turn:
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Esotropia — the eye turns inward, toward the nose
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Exotropia — the eye turns outward, toward the ear
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Hypertropia — the eye sits higher than the other
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Hypotropia — the eye sits lower than the other
How it behaves:
- Constant or intermittent — some eye turns appear only when the person is tired, unwell, ill, or concentrating hard
- Alternating or unilateral — either eye may turn, or it is always the same one
- Congenital or acquired — present in infancy, or developing later, including in adulthood after illness or injury
These distinctions matter more than the label “strabismus” on its own. An intermittent exotropia that appears at the end of the day and a large constant esotropia present since infancy are different problems with different outlooks, and any figure quoted without reference to which one you have is close to meaningless.
What causes strabismus
The honest answer is that in most cases nobody knows. What’s understood:
- It runs in families. A parent or sibling with an eye turn raises the odds.
- It travels with other developmental differences. Some children with strabismus also have delayed speech or difficulty with eye-hand and whole-body coordination, suggesting a broader coordination issue rather than an isolated eye problem.
- Uncorrected farsightedness can produce accommodative esotropia, where the focusing effort required to see clearly pulls the eyes inward. This form often responds to glasses alone.
- Trauma, stroke, and neurological conditions can cause eye turns at any age, including in adults who never had one before.
- Rarely, a damaged eye muscle or nerve is the direct cause.
Symptoms
An intermittent turn that only appears when a child is tired is still strabismus. Parents are often told it will be outgrown; sometimes it is, and often it isn’t.
1 —An eye that visibly drifts, especially when tired, sick, or concentrating
2 —Double vision, or vision that doubles only sometimes
3 —Head tilting or turning to see more comfortably
4 —Closing or covering one eye in bright light or while reading
5 —Eye strain, headaches, or fatigue after reading or screen work
6 —Losing place while reading; skipping lines
7 —Difficulty judging distance — catching a ball, pouring, stairs, night driving
8 —Avoiding near work, frustration with homework, poor depth perception
How strabismus is diagnosed
A routine eye exam checks how clearly each eye sees on its own. Diagnosing strabismus means measuring how the two work as a pair — and those measurements also set the baseline against which any claim of improvement has to be judged.
A full evaluation covers five things:
1. Your history.
What you’re noticing and when, whether the turn comes and goes, family
history of eye turns, and any previous treatment or surgery. This shapes which tests matter
most.
2. Sight and prescription.
How clearly each eye sees on its own, and what refractive correction it needs. This step can end the investigation: uncorrected farsightedness causes accommodative esotropia, which glasses alone may resolve entirely.
3. Alignment.
The direction and size of the turn, measured in prism diopters — using cover
testing, the corneal light reflex, and prisms — plus whether it is constant or intermittent, and
whether it alternates between eyes.
4. Binocular vision and depth perception.
Whether the eyes can fuse an image at all, and how much stereopsis is present. This is the measurement that matters most for what treatment can achieve, and the one a standard eye exam does not include.
5. Eye health.
A dilated examination of the retina, optic nerve, and internal structures, to rule
out an underlying medical cause.
Treatment options
Glasses
For accommodative esotropia — an inward turn driven by uncorrected farsightedness — the correct prescription can eliminate the turn entirely. This is the simplest scenario and should always be ruled in or out first. Prism lenses can also relieve double vision by redirecting light, though they compensate for the turn rather than changing it.
Patching
Covering the stronger eye forces the suppressed eye to work, which treats the amblyopia that often accompanies strabismus. Patching improves the vision in the weaker eye. It does not straighten the turn, and it doesn’t restore two-eyed depth perception — for that, the eyes have to learn to work together.
Optometric vision therapy
A structured program of doctor-supervised exercises that trains the brain to aim both eyes at the same target and fuse what they see into a single three-dimensional image. Programs typically run weekly in-office with daily home practice over several months. Vision therapy addresses the coordination problem directly. It is not appropriate for every form of strabismus — large constant deviations, paralytic strabismus, and some congenital forms may need surgery, therapy, or both.
Surgery
An ophthalmologist repositions one or more eye muscles — recessing (weakening) or resecting (strengthening) them — to bring the eye into alignment. It’s performed under general anesthesia, usually as day surgery, with a recovery period of a few weeks. Surgery is often the right answer, particularly for large or constant deviations. What it does well is change where the eye points. What it doesn’t do is teach the brain to use both eyes together, which is why some patients see the turn return over months or years, and why some surgeons refer patients for therapy before or after the procedure.
Read
What “success” actually means — and why the number depends on the question
This is where most outcome figures fall apart, including ones you’ll find on other websites.
“Success” in strabismus research means at least three different things:
A study reporting 90% success and one reporting 30% may be describing the same patients — measured differently. When you compare treatments, make sure you’re comparing the same definition.
- Motor alignment. The eye is straight, or within a small margin — commonly under 10 prism diopters. This is what surgical outcome studies usually measure, and it’s the easiest target to hit.
- Functional or sensory success. The two eyes fuse into one image and produce real depth perception. This is a higher bar, and far fewer treatments of any kind clear it.
- Behavioral and quality-of-life success. The person reads without losing their place, drives at night without dread, holds eye contact without thinking about it. This is what patients actually came for, and it’s the least often measured.
How we define success, and what happened to 75 patients
Dr. David Cook took up exactly this problem in a 39-page paper published in Vision Development & Rehabilitation, the official journal of the College of Optometrists in Vision Development.
The paper works through the criteria the field has used for decades — Flom’s criteria, the revision Flom himself later offered, and a further modification Dr. Cook proposed with Dr. Bruce Wick — and argues for moving the definition of success away from “cure” measured against strict clinical thresholds, and toward behavioural success measured alongside a qualityof-life questionnaire. Put plainly: not is the eye straight to within 10 prism diopters, but can this person use both eyes together in their actual life, and do they say so themselves?
To demonstrate it, he presents a case series of 75 of his own patients with esotropia and exotropia, ages five to fifty-five, treating the two conditions differently and paying particular attention to patients whose binocular vision, while not textbook-normal, is genuinely useful to them.
The journal’s editor-in-chief described it as a paper that levels the playing field between surgical and non-surgical outcomes. That is the right way to read it. It is not an argument that therapy beats surgery. It is an argument that both should be judged by what the patient actually gets.
What this is, and what it isn’t. It is a case series from a single practice: patients evaluated as candidates for therapy, who then completed a programme. It is not a randomised controlled trial, and it cannot predict your result. What it does is set out, in full and in public, a standard of success and what became possible for a specific group of people measured against it — so you can judge the claim rather than take it on trust.
What we don’t claim
Vision therapy is not right for every eye turn. Large constant deviations, paralytic strabismus, and some congenital presentations may need surgery, sometimes alongside therapy. Patients with no remaining capacity to use the two eyes together face a harder road than those with some fusion intact. Adults generally progress more slowly than children. We say this because the evaluation exists to establish which situation you are in, and a page that promised otherwise would not be worth reading.
What surgery achieves, measured fairly
Published surgical series report motor alignment success — a residual deviation under 10 prism diopters — in roughly 70 to 90 percent of cases at one year, varying with the type and size of the eye turn. One recent series of pediatric horizontal strabismus surgery reported 73.6% overall at one year, with better results for smaller deviations. A large series of intermittent exotropia reported 92.2%.
Two things belong alongside those figures.
Results can drift. Alignment at one year is not alignment at ten. One series of secondary sensory strabismus reported 62.5% success at short-term follow-up falling to 42.1% long-term. Repeat procedures are common enough that they should form part of any conversation about what surgery involves.
Alignment is not fusion. The figures above measure where the eye points. Studies that measure whether the two eyes work together — genuine fusion and depth perception — report substantially lower rates, as the criteria study described earlier shows.
None of this is an argument against surgery. It is an argument for knowing which outcome you are being quoted, and for asking your surgeon what the plan is for the seeing problem as well as the appearance problem. Many patients do best with both approaches, in sequence.
Check Out Our Resources
Dr. Cook’s Publications:
- Authored books VISUAL FITNESS and WHEN YOUR CHILD STRUGGLES.
- Published articles in top optometric journals.
- His article “Eyesight, infinity and the human heart” was voted “Best Non-Technical Article” by the Association of Optometric Editors.
When Your Child Struggles
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