What is Strabismus?

Strabismus, also called squint, wall eye, or crossed eyes, is a condition in which the eyes do not properly align with each other when looking at an object. Strabismus is a binocular vision problem where the two eyes fail to work together as a team. Most research suggests about 1 to 3 percent of the global population has strabismus. According to the American Association for Pediatric Ophthalmology and Strabismus, approximately 4 percent of the U.S. population has crossed eyes or some other type of strabismus.

Diagnosis of Strabismus

Routine children's eye exams are the best way to detect strabismus. Generally, the earlier strabismus is detected and treated following a child's eye exam, the more successful the outcome. Without treatment, your child may develop double vision, amblyopia, or visual symptoms that could interfere with reading and classroom learning.

It is important to note that children do not outgrow strabismus, therefore diagnosis and treatment by an eye care professional is important. Successful therapy and treatment options exist including non-surgical options for some types of strabismus.

Testing for Strabismus

It is important that a comprehensive vision assessment is done in order to properly identify strabismus (and/or Amblyopia). Some types of strabismus are visually noticeable but testing should include a full evaluation of the visual system including a binocular vision assessment that looks at how the eyes are focusing and moving. Visual acuity assessment alone is not enough. You can expect your eye care professional to take a complete history. Testing near and distance acuity is routine as is evaluating the general health and structure of the eye.

To specifically test and evaluate strabismus the doctor will likely perform a Cover Test (also called Cover-Uncover Test) as well as a Hirschberg Test (also called Hirschberg corneal reflex test).

Hirschberg Test

In the fields of optometry and ophthalmology, the Hirschberg test, also Hirschberg corneal reflex test, is a screening test that can be used to assess whether a person has strabismus (ocular misalignment). It is performed by shining a light in the person's eyes and observing where the light reflects off the corneas.

Cover Test

A cover test, or cover-uncover test, is an objective determination of the presence and amount of ocular deviation. The cover test is regarded as an essential examination in investigating strabismus. It is simple to perform, does not require great amounts of skill by the examiner, and is objective in nature. The test involves having the patient focus at either a distance or near object. A cover (often a plastic paddle) is placed over an eye for a short moment then removed while observing both eyes for movement. The misaligned eye will deviate inwards or outwards (most common) or vertically up or downwards. The process is repeated by moving the cover back and forth from the right to the left eye. Prismatic lenses may be used to neutralize eye movement, which allows the clinician to measure the amount of eye movement.

What is Squint?

Squint, when referring to an abnormality of binocular vision, is another term for strabismus, and also referred to as crossed eye(s) and sometimes lazy eye. Squint and strabismus refer to a condition in which the eyes do not properly align with each other when looking at an object. This term is more common in Europe than in the US.

What Causes Strabismus?

Most eye turns develop gradually and is due to a disorder of binocular vision. New strabismus, especially strabismus that develops later in life, could be the sign of a neurological disorder. If you experience an eye turn that develops overnight or suddenly, especially with other symptoms such as headache or vision loss, you should seek immediate evaluation by an optometrist or ophthalmologist.

A very small percentage of children develop an eye turn very soon after birth (in the first 3-6 months). Usually, this type of turn is inward. The cause of this turn is a problem with the way the eye muscles are controlled by the brain.

More commonly children develop a turned eye after the first year of life. The majority are due to high amounts of hyperopia (far-sightedness, long-sightedness). Simply stated, the child has a focusing problem. leading to the eye to over-focus and cross. When a child focuses on an object, in order to see it clearly, there is also a stimulation of the muscles around the eye that pulls the eyes in. This type of an eye turn is usually helped with eyeglass lenses.

People that develop a strabismus at a young age eventually learn to adapt by leaving one eye in constantly and “turning it off” (suppression). Suppression removed the doubled image, but can be problematic. The eye turn is more likely to be noticed as the child becomes more involved in close work (ages 2-4 years). An eye turn may also be triggered by illness or fatigue. There can also be an inherited risk (someone in the family also has an eye turn).

Some individuals have an excessive stimulation of the inner muscles that pull the eyes inwards when the eyes try to focus. This can occur even with relatively low degrees of refractive correction (glasses prescription). As stated above, if a person develops strabismus at a young age, eventually the brain learns to leave one eye in and “turns it off” to reduce the effort of coping with double vision.

Rarely is strabismus due to an eye muscle problem; however, there are a small number of strabismus conditions (such as Duane's syndrome or Moebius syndrome) that affect the eye muscle themselves. Additional risks for strabismus include premature birth or low birth weight, eye conditions such as amblyopia (lazy eye), retinopathy prematurity, pediatric cataract, or severe cornea scarring, and genetic or neurological conditions such as Downs syndrome, muscular dystrophy, or cerebral palsy.

Types of Strabismus

If you have strabismus, one eye looks directly at the object you are viewing, while the other eye is misaligned.

Types of Strabismus include:


Esotropia is where one eye looks directly at the object being viewed and the other is misaligned inward toward the nose. This is commonly referred to as crossed eyes or cross-eyed.


Exotropia is where one eye looks directly at the object being viewed and the other is misaligned outward away from the nose. This is also called wall-eyed.


Hypertropia is where one eye looks directly at the object being viewed and the other is misaligned vertically upward.


Hypotropia is where one eye looks directly at the object being viewed and the other is misaligned vertically downward.

Constant Strabismus

Constant Strabismus is when the misalignment of the eyes occurs at all times, at all distances of focus.

Intermittent Strabismus

Intermittent Strabismus is only evident intermittently (not all the time). Sometimes intermittent strabismus is missed in an eye exam if the patient doesn't report symptoms or if the eyes are aligned during the visit and the clinician cannot induce the eye turn.

Unilateral Strabismus

Unilateral Strabismus is when the misalignment always affects the same eye.

Alternating Strabismus

Alternating Strabismus is when the two eyes may take turns being misaligned.

Convergence Insufficiency and Strabismus

Convergence Insufficiency (CI) is a specific type of intermittent exotropia in which the eyes usually align properly when viewing an object in the distance, but the eyes fail to achieve or maintain proper alignment when looking at close objects, such as when reading.

Signs and Symptoms of Strabismus

The primary sign of strabismus is a visible misalignment of the eyes, with one eye turning in (esotropia), out (exotropia), up (hypertropia), or down (hypotropia). The eye may turn significantly which is called a "wide-angle" or "large-angle" strabismus. The eye may only turn a small amount which is called a "small-angle" or "micro" strabismus. Some eye turns can be so small, or only happen in certain circumstances, that they are difficult to detect. It is best to consult an eye care professional near you (such as an optometrist or ophthalmologist) if you see or suspect an eye turn, even if you don't see it all the time.

Symptoms of Strabismus

It may seem surprising, but sometimes there are no symptoms associated with strabismus. This has been found to be true mostly for people with large angle, constant strabismus. However, if left untreated early in life, this type of strabismus can often lead to amblyopia.

Those with strabismus who do suffer from symptoms may experience the following:

  • headaches
  • eyestrain
  • inability to read comfortably
  • fatigue when reading
  • blurred vision
  • diplopia (double vision)
  • lack of 3D vision (depth perception)

People with strabismus, especially those with very noticeable eye turns, have reported that the eye turn makes them self-conscious. Because of this, there is usually a strong desire for cosmetic alignment of the eyes. While this is possible through strabismus surgery, it may result in other alignment or focusing issues. A review of all treatment options with your optometrist or ophthalmologist is recommended.

Strabismus Treatments

Treatment depends on the type of strabismus and the underlying cause. Treatment options may include vision therapy, eyeglasses, prism glasses, eye patching, and possibly strabismus surgery.

Optometric Vision Therapy for Strabismus

Your doctor of optometry might prescribe a structured program of visual activities to improve eye coordination, eye focusing, and other binocular vision skills. Optometric vision therapy trains the eyes and brain to work together more effectively. Vision therapy can help problems with eye movement, eye focusing, and eye teaming and reinforce the eye-brain connection. Treatment can occur in your doctor of optometry's office as well as at home.

Strabismus Surgery

Eye muscle surgery, or strabismus surgery, can change the length or position of the muscles around the eyes so they appear straight. According to the American Optometric Association, often, people who have eye muscle surgery may also need vision therapy to improve eye coordination and to keep the eyes from becoming misaligned again.

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