Strabismus (Crossed Eyes) Overview

Strabismus is a group of conditions in which one or both eyes are not aligned, or pointed in exactly the same direction. If one eye is not aimed at the same target as the other eye, then two images will result, causing double vision. Although strabismus usually shows up in infancy or early childhood, adults can develop strabismus as well.

Little girl with crossed eyes
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Overview

Humans have six eye muscles per eye that allow for eye movement. These muscles constantly receive nerve signals from the brain that control eye movement and allow for the eyes to coordinate movements together so that both eyes are pointed at the same target. If the information received from these nerve impulses is incorrect, the eye may turn in, out, up, or down.

Strabismus can affect one eye or both eyes. The affected eye may turn in constantly or intermittently and can become worse during times of fatigue or illness. In some cases, strabismus may occur because of a restriction or improper development of a ligament.

Using both eyes properly is important for good depth perception. If one eye is not being used, then depth perception is limited. If one eye is not pointed at the same target, double vision may result. In order to deal with seeing two images, the brain may suppress one of the images so that you will only see one image. The brain soon learns to ignore the second image.

Types

Strabismus is named according to the direction in which the eye deviates.

  • Esotropia occurs when an eye turns inward.
  • Exotropia occurs when an eye turns outward.
  • Hypertropia occurs when an eye turns upward.
  • Hypotropia occurs when an eye turns downward.

Causes

Some types of strabismus are caused by abnormal development of the binocular system in the brain. The accommodative system (focusing system) allows our eyes to change power and focus so that objects remain clear no matter the distance. The binocular/alignment system controls how our eyes work together. When we look far away, our eyes are straight. When we look at something very close, our eyes converge or turn in and our eyes increase their focusing power. When we look back to a distance, we relax our focusing power and the eyes become straight again.

Young children have huge amounts of focusing power. As a result, when a child has a very large amount of uncorrected farsightedness, the child attempts to make things clear by focusing extremely hard. In order to achieve that, they must focus a great deal to compensate for the uncorrected vision problem. When they focus this much, the binocular and focusing systems begin to get mixed signals. Usually, one eye will turn in. This is called esotropia.

The focusing system will also begin to send signals for the eye muscles to converge the eyes inward. When the eyes are forced to focus this much, abnormal signals go to the eye muscles causing one eye to turn in and become crossed. Some children may not compensate and their eyes will not turn in, but they will have a very poor vision because they are not over-focusing. Their brain chooses to keep the muscles straight, but they see a very blurred image.

Other types of strabismus, such as exotropia, also sometimes referred to as wall-eyed or a wandering eye may be caused by a large difference in the eye’s refractive status between the two eyes. When there is a difference in the refractive power between the eyes, or a difference in the ability to bring an object into focus, the weaker eye may not receive the same sensory input as the stronger eye. Over time, the brain may choose to ignore the eye with the weaker visual input (also known as ambylopia), and the eye drifts outward (exotropia).

This also occurs when a person has an eye disease that causes long-standing decreased vision. The brain does not receive good information from that eye and it may begin to wander outward. In many cases of childhood exotropia, there is no detectable cause.

Vertical deviations, where one eye may be up or one eye down, are often caused by a palsy in the fourth cranial nerve. A palsy is often caused by trauma, stroke to the muscle or nerve and more rarely, a tumor. Vertical strabismus can also be caused by neurologic problems, thyroid disease and fibrosis or scarring that is sometimes a part of eye muscle syndromes that people are born with, such as Duane’s retraction syndrome.

Temporary or False Strabismus

Parents often see intermittent eye crossing in their infant children, usually during the first six months. This is a sign of normal development and is simply a sign of children learning to use their eyes together. However, this intermittent cross is quite rare after six months of age and if it is seen after that time, parents should seek the advice of their pediatrician or a pediatric optometrist or ophthalmologist.

The term pseudo-strabismus is given to certain infants and young children that only appear to have crossed eyes. This is seen in certain ethnic groups where the bridge of the nose is flatter than average or extra skin covers the inner corner of the eyes. This extra skin is referred to as epicanthic folds. This sometimes gives the appearance of slightly crossed eyes and usually goes away as the child’s face grows.

Treatment

Strabismus is treated in a variety of ways. Your eye doctor will be able to determine the best treatment plan for you.

Eyeglasses

When a child develops accommodative esotropia as a result of the eyes over-focusing to compensate for their large amount of uncorrected farsightedness, the treatment is simply to fully correct the child’s vision problem. After appropriate eyeglass prescription is worn, the eye will no longer over-focus and the eyes often straighten out immediately. Correcting the vision problem often eliminates most of the deviation but it may still occur when converging the eyes in during reading. In this case, a bifocal lens is prescribed to give added power.

Prism

Prism is a type of lens that bends light in a certain direction. Eye doctors are able to prescribe prism to move objects over to a position that allows the wearer to be able to fuse images so double vision does not occur. This is crucial for some patients who experience double vision as a result of strabismus. In certain cases, the opposite power prism can be prescribed to enhance the cosmetic appearance of strabismus.

Vision Therapy

Vision therapy (VT) encompasses different techniques to either correct strabismus or allow the person suffering from strabismus to learn when an eye deviates so that they can learn to use their eye muscles to minimize it. Sometimes certain machines and computer software programs are used to give the person behavioral feedback to control the eye muscle deviation. Other times, specific eye muscle exercises are performed to strengthen the eye muscles. VT is very successful in exotropia and small esotropia deviations but not as successful in large amounts of esotropia.

Vision therapy is considered controversial. Some ophthalmologists do not recommend vision therapy except in the case of convergence insufficiency (a disorder most common in infants and children in which the eyes have difficulty working together when focusing on a nearby object), and some insurances do not cover vision therapy, as well. Talk to your doctor about whether vision therapy is right for you or your loved one, and discuss your coverage options with your insurance provider.

Botox

Botulinum toxin is injected directly into one of the muscles controlling eye movement. This temporarily blocks the nerve impulse and causes the muscle to be paralyzed. This muscle relaxes and the other eye muscles take up the slack to straighten the eye. Injections for strabismus was one of the first uses of Botox in health care.

Surgery

Eye muscle surgery for strabismus is very successful in most cases. Eye surgeons who specialize in eye muscle surgery use a variety of techniques that involve shortening or moving eye muscles. Adjustable sutures are used to fine-tune the eye muscle correction directly after surgery.

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  • Boyd, Kierstan. What Is Strabismus? American Academy of Ophthalmology (AAO), 14 April 2014.