Strabismus is a disorder in which the eyes do not line up in the same direction. People with strabismus cannot look at the same object or place with both eyes at the same time. The condition is often referred to as “crossed eyes.”
Strabismus usually develops in infants and children by the time they are 3 years old. The condition occurs when the muscles of each eye do not work together to point to the same place.
The brain will then receive two images, which can cause double vision or confusion. It can be a result of problems with the eye muscles themselves, the nerves that transfer information to the eye muscles, or the part of the brain that controls eye movements.
Over time, the brain will learn to ignore one of the images. If strabismus is not treated, it can lead to a permanent reduction of vision in one eye. This is usually called amblyopia or lazy eye. However, if treated early, strabismus can often be corrected with very good results.
According to the National Institutes of Health (NIH), in most children the true cause of strabismus is not known.
The condition can also develop in adults. In adults, strabismus may be caused by:
- stroke (the leading cause of strabismus in adults)
- trauma that damages the brain, nerves, or muscles of the eye
- neurological problems
- Grave’s disease (a thyroid disorder affecting the eyes)
There are several different types of strabismus. The types are usually described by the direction of the eye misalignment. The most common types of strabismus are:
- estotropia: inward turning of the eyes, or “cross-eyed”
- exotropia: outward turning of the eyes, or “wall-eyed”
- hypertropia: when the abnormal eye is higher than the normal fixating eye
- hypotropia: when the abnormal eye is lower than the normal fixating eye
Some types of strabismus have special patterns and unique names such as Brown syndrome (a mechanical problem with the eyes causing difficulty with looking up and inward), and Duane syndrome (abnormal eye movements causing difficulty with rotating eyes outward or inward). The eye-turning symptoms may be present all the time, or may occur only when the patient is tired or sick.
According to the American Association for Pediatric Ophthalmology and Strabismus, an estimated four percent of people in the United States have strabismus (AAPOS). It can develop in otherwise healthy children. However, risk factors include:
- family history: children with parents or siblings who also have strabismus are more likely to have it
- other medical disorders that affect the brain, including:
- Down syndrome
- cerebral palsy
- brain tumors
- farsightedness: people who have farsightedness may develop strabismus because they need to focus more to keep objects clear
Any child older than 4 months whose eyes do not seem to be straight all the time should be examined by a doctor for strabismus.
Strabismus is diagnosed by an eye exam. The eye exam is comprehensive, but the doctor will pay particular attention to how the eyes focus and move. According to the American Optometric Association (AOA), an eye exam typically includes the following tests or procedures:
An eye doctor will ask the patient about any symptoms he or she is experiencing and about general health problems, medications, and environmental factors that may contribute to the patient’s health.
Visual acuity measures the clearness of a person’s vision. A patient will usually be asked to read letters on a chart from a distance of 20 feet. Visual acuity is written as a fraction. 20/20 is considered normal. The top number is the distance at which the testing is done (20 feet), and the bottom number is the smallest size of the letter that the patient is able to read from 20 feet away.
In other words, if a person has 20/40 visual acuity, it means they need to be 20 feet away from the letter to see it, when they actually should be able to see when standing 40 feet away.
An eye doctor will use a refraction test to find out what type of lens a patient needs to correct a refractive error. This test requires the patient to look through lenses of different strengths, one eye at a time, at an eye chart 20 feet away. Refractive errors include nearsightedness, farsightedness, or astigmatism.
Alignment and Focusing Testing
An eye doctor will measure how well the eyes focus and move together.
Examination of Eye Structure and Health
An eye doctor will look at the inside and outside structure of the eye.
An eye doctor should be able to diagnose strabismus based on the results of the comprehensive eye exam. If strabismus is diagnosed, an eye doctor will normally recommend a treatment plan.
Strabismus treatment aims to improve the alignment of the eyes so they can work better together. Treatment may involve:
- eye glasses: to correct farsightedness
- eye exercises and vision therapy: to help train the eyes and brain to work together; these exercises may take place at home or in a doctor’s office
- prism lenses: special lenses with a prescription for prism power; they alter the light that enters the eye to reduce the amount of turning the eye has to do to look at objects
- eye patch: if the patient has lazy eye, an eye patch will be placed over the good eye, forcing the weak eye to work harder
- surgery: to change the length or position of the eye muscles
With treatment, the prognosis is typically very good and the eyes become straight. However, vision problems may still remain, even after surgery. In general, the younger the child is when treatment is started or surgery is performed, the better the outcome.
If strabismus is not treated, it can lead to a permanent reduction of vision in one eye.