Strabismus (Crossed Eyes)

Strabismus — commonly called "crossed eyes" or "wall eyes" — is a condition in which the two eyes do not point in the same direction at the same time. While one eye looks straight ahead at a target, the other drifts inward, outward, upward, or downward. It affects approximately 4% of children in the United States and is one of the most common eye conditions seen in pediatric ophthalmology. When identified and treated early, the outlook is excellent. Left untreated, strabismus can permanently weaken vision in the misaligned eye through a process called amblyopia ("lazy eye").

Table of Contents

  1. Overview and Definition
  2. Types of Strabismus
  3. Causes and Risk Factors
  4. Symptoms and Signs
  5. Diagnosis
  6. Treatment Options
  7. Vision Therapy and Natural Approaches
  8. Strabismus in Adults
  9. Complications
  10. Prognosis
  11. Key Research Papers
  12. Connections
  13. Featured Videos

Overview and Definition

The word "strabismus" comes from the Greek strabismos, meaning "to squint." Under normal conditions, the six muscles attached to each eye work in precise coordination, guided by signals from the brain, so that both eyes aim at exactly the same point simultaneously. This alignment allows the brain to fuse the two slightly different images from each eye into a single three-dimensional picture — the basis of depth perception, or stereopsis.

In strabismus, this neuromuscular coordination breaks down. One or both eyes deviate from the intended target. The brain then faces a conflict: it is receiving two images from two eyes that are not aligned, which would normally produce double vision (diplopia). In adults who develop strabismus, diplopia is the dominant complaint. In young children, whose visual systems are still developing, the brain is remarkably adaptive — it learns to suppress, or ignore, the image from the misaligned eye entirely. This prevents double vision but comes at a heavy cost: the suppressed eye stops developing normally, leading to amblyopia.

Strabismus is not a cosmetic nuisance — it is a medical condition with real consequences for vision, depth perception, and quality of life. About 2–4% of the U.S. population has some form of strabismus, with the highest prevalence in early childhood. Boys and girls are equally affected. A family history of strabismus substantially raises a child's risk.

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Types of Strabismus

Strabismus is classified by the direction of the eye deviation, the age of onset, whether it is constant or intermittent, and whether the same eye always deviates (monocular/unilateral) or the eyes alternate in deviating (alternating).

Esotropia — Inward Turning ("Crossed Eyes")

Esotropia is the most common type of strabismus in children in Western countries. The eye turns toward the nose. There are two main varieties:

Exotropia — Outward Turning ("Wall Eyes")

The eye drifts outward, away from the nose. Exotropia is more common in Asian and African populations than in European populations.

Hypertropia and Hypotropia — Vertical Deviations

One eye is higher (hypertropia) or lower (hypotropia) than the other. Vertical deviations are often caused by problems with the superior oblique or inferior rectus muscles, or by cranial nerve IV palsy. They frequently cause head tilting — the child tilts the head to one side to reduce the vertical misalignment and avoid double vision.

Paralytic (Incomitant) Strabismus

Caused by weakness or paralysis of one or more of the six extraocular muscles due to cranial nerve damage. Unlike concomitant strabismus (where the angle of deviation is the same in all directions of gaze), paralytic strabismus is incomitant — the deviation changes depending on which direction the person looks. The three cranial nerves involved are:

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Causes and Risk Factors

Strabismus is fundamentally a disorder of neuromuscular control — the brain's inability to coordinate the extraocular muscles precisely. The underlying reasons are varied:

Refractive Errors

Uncorrected hyperopia (farsightedness) is the most common treatable cause, responsible for accommodative esotropia. When children work hard to bring blurry objects into focus, the convergence response is over-triggered, pulling the eyes inward. Correcting the refractive error with glasses — ideally before age 2–3 — can prevent or reverse this type of strabismus entirely.

Hereditary and Genetic Factors

Strabismus clearly runs in families. Approximately 30% of children with strabismus have a first-degree relative with the condition. The mode of inheritance is complex (polygenic), not simple dominant or recessive. No single gene has been identified; rather, multiple genes influencing extraocular muscle development, refractive error, and neural control appear to contribute.

Neurological and Developmental Causes

The brain controls eye alignment through complex pathways. Any disruption of these pathways can cause strabismus:

Systemic and Medical Causes

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Symptoms and Signs

The symptoms of strabismus differ significantly between children and adults, largely because of the child's brain remarkable ability to adapt — for better and for worse.

Visible Misalignment

The most obvious sign is that the eyes do not look in the same direction. Parents often first notice this in photographs, where the eyes appear asymmetric. The deviation may be constant or intermittent. It may be more noticeable when the child is tired, sick, or staring into the distance.

Double Vision (Diplopia)

Adults who develop strabismus almost always experience diplopia — seeing two images of the same object. It can be horizontal (side by side), vertical (one above the other), or oblique (diagonal), depending on the direction of the deviation. Diplopia can be disabling: it affects driving, reading, and spatial orientation. Adults instinctively learn to tilt or turn their head to bring the two images back into alignment, resulting in a characteristic compensatory head posture.

Children with longstanding strabismus usually do not complain of double vision because their brains have learned to suppress the image from the deviating eye. This suppression prevents diplopia but sets the stage for amblyopia.

Compensatory Head Posture

Children and adults with strabismus — especially vertical deviations or cranial nerve IV palsy — often tilt or turn the head to find the position where both eyes work together best. Parents may notice a persistent head tilt toward one shoulder, which can be misattributed to a neck problem.

Squinting and Closing One Eye

Children with intermittent exotropia frequently squint or close one eye in bright sunlight. This is a consistent, classic sign. In bright light, the fusional reflex weakens, and the eye is more likely to drift. Squinting reduces the visual input to the deviating eye, making the drift less noticeable.

Eye Strain and Reading Difficulties

Convergence insufficiency produces symptoms during near work: headaches, eye fatigue (asthenopia), blurred words on the page, words that seem to move or jump, and difficulty sustaining concentration while reading. These children are often misdiagnosed with attention deficit disorder because their reading struggles look behavioral rather than visual.

Amblyopia — The Silent Threat in Children

Because a child's brain suppresses the image from the deviating eye, there are often no obvious visual complaints. The child does not complain of blurred vision or double vision. Yet the suppressed eye is gradually losing visual acuity during the critical period of visual development (birth to approximately age 7–8). By the time amblyopia is detected — often at a school screening — the child may have substantially reduced vision in one eye. This is why routine vision screening in preschool children is so important: strabismus and amblyopia can be invisible to the child and the family.

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Diagnosis

Diagnosing strabismus requires a systematic examination by a pediatric ophthalmologist or optometrist trained in binocular vision. The key tests are:

Cover Tests

The cover test is the gold standard for detecting and measuring strabismus. In the unilateral cover test, the examiner covers one eye while watching the other eye for movement. If the uncovered eye moves to pick up fixation, it was not previously aligned — confirming strabismus. The alternate cover test dissociates the eyes (eliminates the fusion reflex) by rapidly alternating the cover between eyes, revealing the full angle of deviation including any phoria (latent tendency to drift). This test measures the total deviation in prism diopters using neutralizing prisms.

Hirschberg Corneal Light Reflex Test

A penlight is shone into both eyes, and the examiner notes where the light reflection appears on each cornea. Normally the reflection is symmetric and centered. In strabismus, the reflection is decentered on the deviating eye. Each millimeter of displacement corresponds to roughly 7 prism diopters (15 degrees) of deviation. This is a quick screening test, especially useful in young or uncooperative children.

Cycloplegic Refraction

Eye drops are used to temporarily paralyze the focusing muscle (ciliary muscle), allowing the true refractive error to be measured without the interference of the child's accommodation. This is essential because children can mask hyperopia by constantly accommodating. Cycloplegic refraction reveals the underlying refractive error that may be driving accommodative esotropia.

Stereoacuity and Binocular Function Testing

Tests such as the Randot, Titmus fly, and Lang stereotest measure the degree of 3D depth perception. Reduced or absent stereoacuity indicates suppression and binocular dysfunction. These tests help grade severity and guide treatment urgency.

Worth 4-Dot Test and Bagolini Lenses

These tests detect whether the patient is suppressing one eye's image. The Worth 4-dot test uses colored filters — if the patient reports only red or only green dots (not all four), one eye is being suppressed.

Imaging

Most childhood strabismus does not require neuroimaging. However, CT or MRI of the brain and orbits is indicated when:

Thyroid Function Tests

In adults presenting with new-onset strabismus — especially with proptosis, eyelid retraction, or conjunctival redness — thyroid function tests (TSH, free T4, thyroid antibodies) are ordered to evaluate for Graves' disease and thyroid eye disease.

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Treatment Options

Treatment of strabismus is tailored to its type, severity, cause, and the patient's age. The goals are: (1) treat any amblyopia to restore visual acuity; (2) align the eyes for function and appearance; and (3) restore or develop binocular vision and depth perception.

Glasses and Refractive Correction

For accommodative esotropia, prescribing glasses that fully correct the child's hyperopia is the first and most important treatment. In many children, wearing the correct glasses alone straightens the eyes completely or partially — no surgery is needed. Bifocal glasses may be added when the esotropia is present primarily for near fixation (high accommodative convergence/accommodation ratio). Contact lenses are an option for older, cooperative children.

Amblyopia Treatment — Patching and Penalization

If amblyopia is present (reduced visual acuity in the deviating eye), the weaker eye must be strengthened by forcing the brain to use it. The standard approach is patching the better (stronger) eye for several hours per day, forcing the amblyopic eye to work. Research from the Pediatric Eye Disease Investigator Group (PEDIG) established the evidence base: patching 2–6 hours daily is as effective as full-time patching for moderate amblyopia and has better compliance.

An alternative to patching is atropine penalization — eye drops of atropine (1%) are placed in the stronger eye once daily (or once a week), blurring its near vision. The child is then forced to use the amblyopic eye for reading and near tasks. Atropine is preferred by some families because there is no patch to remove and it avoids the social stigma children sometimes face from patching at school. The Amblyopia Treatment Study (ATS) found atropine equally effective as patching for moderate amblyopia.

Prism Glasses

Stick-on Fresnel prisms or ground-in prism lenses can be placed in glasses to shift the image in one lens so that the two eyes' images align, relieving diplopia in adults or helping the eyes fuse in children with small-angle deviations. Prisms are also used diagnostically — to determine the angle of strabismus and to predict surgical outcomes.

Botulinum Toxin (Botox) Injection

Botulinum toxin A (Botox) injected into a tight or overactive extraocular muscle temporarily weakens it (for 2–4 months), allowing the opposing muscle to take over and realign the eye. It is used for:

The main risks are ptosis (droopy eyelid, usually temporary) and overcorrection. The effect is temporary, so repeat injections or surgery may be needed.

Strabismus Surgery

Surgery is the most definitive treatment for most types of strabismus. It is performed under general anesthesia (always in children, usually in adults) as a day procedure. The surgeon accesses the extraocular muscles through a small incision in the conjunctiva and either:

The surgeon operates on muscles in both eyes or one eye, depending on the type and angle of deviation. Surgery does not change vision — it changes the mechanical position of the eye. One procedure achieves lasting alignment in approximately 60–70% of patients; some require a second surgery (especially with large-angle deviations or recurrence).

Adjustable suture technique: In cooperative adults, sutures can be left temporarily adjustable, allowing the surgeon to fine-tune the alignment under local anesthesia in the recovery room or the next morning while the patient is awake and can report their visual experience. This approach improves accuracy in adults with diplopia and can reduce the need for re-operation.

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Vision Therapy and Natural Approaches

Vision therapy (orthoptics) refers to a structured program of eye exercises designed to improve the brain's ability to control the eyes and process visual information. Its role in strabismus depends entirely on the type:

Convergence Insufficiency — The Best Evidence

Office-based vision therapy is the most strongly evidence-supported for convergence insufficiency (CI). The Convergence Insufficiency Treatment Trial (CITT) — a multicenter randomized controlled trial funded by the National Eye Institute — found that 12 weeks of office-based vergence/accommodative therapy was significantly more effective than home exercises alone or placebo therapy in improving convergence and reducing symptoms in children aged 9–17. The symptom improvement was clinically meaningful and durable.

Exercises used for CI include:

Video Game "Brain Training" — Emerging and Investigational

Research groups have explored using dichoptic video games — games that present different content to each eye simultaneously through special glasses — to treat amblyopia and binocular dysfunction. Early results showed promise, but a large NIH-funded RCT found that the dichoptic game was not superior to standard patching for amblyopia in children aged 5–16. Research continues, particularly for older children and adults where patching efficacy is lower.

Nutritional and Lifestyle Approaches

No nutrient cures strabismus, but overall visual and neural health supports any treatment:

Vision therapy is not effective for the structural muscle imbalances that require surgery (e.g., large-angle constant esotropia, cranial nerve palsies). Families should consult a qualified pediatric ophthalmologist to determine which treatment applies to their child's specific type.

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Strabismus in Adults

Strabismus is not only a childhood condition. Adults can develop it for the first time, or childhood strabismus can recur or deteriorate in adult life. The experience of strabismus differs markedly in adults compared to children.

Causes in Adults

Diplopia Is the Primary Complaint in Adults

Unlike children, adults with strabismus cannot suppress the second image. Diplopia is the dominant, often disabling, symptom. It affects driving safety, occupational function, and daily activities. Adults with new-onset diplopia need urgent evaluation to rule out dangerous underlying causes (aneurysm, stroke, tumor) before the eye deviation itself is treated.

Treatment in Adults

Treatment principles are the same as in children, but with some differences:

Adults do not develop amblyopia from strabismus — the critical period for amblyopia closed in childhood. Surgery in adults is primarily for alignment and diplopia relief, not vision development.

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Complications

Amblyopia (Lazy Eye)

Amblyopia is the most significant complication of childhood strabismus. When the brain suppresses the image from the misaligned eye to avoid diplopia, the visual cortex cells representing that eye fail to develop the normal density of neural connections during the critical period (birth to approximately age 7–8). The result is permanent, cortical-level reduced vision that cannot be corrected with glasses alone — the eye itself is structurally normal, but the brain cannot process its signal properly. Strabismus is the second most common cause of amblyopia (after anisometropia, or unequal refractive error between the two eyes).

Amblyopia is fully reversible if treated before the end of the critical period. The earlier treatment begins, the better and faster the response. After age 7–8, the visual cortex becomes progressively less plastic, and improvement — while still possible — is slower and less complete. Untreated amblyopia leaves the person dependent on one functioning eye for life, with substantially increased risk of severe visual disability if the dominant eye is later injured or develops disease.

Loss of Depth Perception

Stereopsis — three-dimensional depth perception — requires the brain to fuse slightly different images from each eye into a single 3D percept. If one eye is suppressed or amblyopic, stereopsis is degraded or absent. This affects spatial tasks: judging distances for ball-catching, driving, parking, and fine motor work. Many adults with a history of childhood strabismus report lifelong difficulties with depth-sensitive activities.

Persistent Diplopia in Adults

Adults who develop strabismus from neurological causes may be left with permanent diplopia if the underlying nerve palsy does not fully recover. Persistent diplopia can be debilitating — it may prevent safe driving, impair balance, and cause chronic headaches from the constant effort of trying to suppress one image. Prisms, Botox, or surgery can usually provide relief.

Psychosocial Impact

Strabismus has real psychosocial consequences, particularly for children. Visibly misaligned eyes can lead to teasing and social stigma. Studies show that children and adults with strabismus are perceived less favorably in social interactions and job interviews. Adults with strabismus report higher rates of anxiety and lower self-esteem. Successful surgical alignment improves quality-of-life scores significantly, independent of any visual benefit.

Recurrence After Surgery

Strabismus surgery is effective but not permanently curative in all cases. Recurrence rates vary by type: infantile esotropia has recurrence rates of 15–30% over decades; exotropia recurs in up to 50% of patients at long-term follow-up. Repeat surgery is safe and effective. Glasses, prisms, or Botox may manage recurrent deviations that are too small for surgery.

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Prognosis

With timely diagnosis and appropriate treatment, the outlook for strabismus is excellent — particularly in children treated during the critical period of visual development.

Amblyopia Recovery

Amblyopia is almost fully reversible when treatment begins before age 3–4, with vision often normalizing completely. Treatment started between ages 4–7 still produces significant improvement in the majority of children. Even children aged 7–10 show meaningful improvement, though treatment takes longer and results are less complete. Patching and penalization studies consistently show that even "older" children with amblyopia benefit from treatment — the critical period closes gradually, not abruptly.

Alignment After Surgery

A single strabismus operation achieves good alignment (within 10 prism diopters of straight) in 60–80% of patients. Success rates are higher for standard deviation angles and lower for very large or atypical deviations. Children with accommodative esotropia who continue wearing their glasses retain alignment well. Glasses requirements often change as children grow and hyperopia decreases, sometimes allowing glasses to be discontinued in the teens — though accommodative esotropia can recur if glasses are stopped prematurely.

Binocular Vision and Stereopsis

Restoring binocular vision and steroacuity is most achievable when strabismus is corrected very early (before age 2 for infantile esotropia; promptly for acquired strabismus at any age). Children with intermittent strabismus — where the eyes still fuse some of the time — tend to have better stereopsis outcomes than those with constant deviation, because the binocular neural connections were partially maintained. Adults who develop strabismus often recover binocularity after surgical alignment because their stereopsis neural architecture was established normally in childhood.

Adults

Adults benefit substantially from strabismus treatment, even if amblyopia correction is no longer possible. Surgery for alignment, prisms for diplopia relief, and Botox for acquired nerve palsies all produce clinically meaningful improvements in diplopia scores and quality of life. Adults should not accept untreated strabismus as an inevitability — effective treatment is available at any age.

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Key Research Papers

  1. Pediatric Eye Disease Investigator Group (PEDIG), 2011 — PMID: 21709230 — Randomized trial comparing 2-hour versus 6-hour patching for moderate amblyopia in children, demonstrating equivalent outcomes and informing current dosing guidelines.
  2. Cotter et al. (CITT Study Group), 2012 — PMID: 23073361 — The Convergence Insufficiency Treatment Trial established that office-based vergence and accommodative therapy is significantly more effective than placebo or home exercises for convergence insufficiency in children.
  3. Wallace et al., 2009 — PMID: 19174491 — Systematic review and management guidelines for infantile esotropia, covering timing of surgery and outcomes for binocular vision development.
  4. Bhola et al., 2014 — PMID: 25058211 — Long-term outcomes of strabismus surgery in children, examining recurrence rates and predictors of stable alignment across multiple strabismus subtypes.
  5. von Noorden GK and Campos EC, 2004 — PMID: 15073668 — Reference text on binocular vision and ocular motility disorders; foundational classification system for strabismus types and pathophysiology.
  6. Rutstein RP, 2011 — PMID: 22064357 — Review of strabismus in adults: etiology, presentation, and evidence-based management including adjustable suture surgery, Botox, and prism therapy.
  7. Hatt SR et al. (PEDIG), 2014 — PMID: 24929742 — Randomized trial of patching versus atropine penalization for amblyopia, confirming that atropine achieves comparable visual acuity outcomes with better adherence in some patient groups.
  8. Brodsky MC, 2009 — PMID: 20040582 — Classification and updated understanding of exotropia, including intermittent exotropia epidemiology, natural history, and evidence for observation versus early intervention.
  9. Hertle RW et al., 2017 — PMID: 28614797 — Multi-year surgical outcomes study for strabismus, examining factors predicting alignment success and re-operation rates in a large pediatric and adult cohort.
  10. Scheiman M et al., 2015 — PMID: 25977517 — Follow-up CITT data on long-term durability of vision therapy gains for convergence insufficiency, showing that improvement is maintained at 1-year follow-up in the majority of patients.

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Connections

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