Crossed eyes, or strabismus, is a condition in which both eyes do not look at the same place at the same time. It usually occurs in people who have poor eye muscle control or are very farsighted.
Six muscles attach to each eye to control how it moves. The muscles receive signals from the brain that direct their movements. Normally, the eyes work together so they both point at the same place. When problems develop with eye movement control, an eye may turn in, out, up or down. The eye turning may occur all the time or may appear only when the person is tired, ill, or has done a lot of reading or close work. In some cases, the same eye may turn each time. In other cases, the eyes may alternate turning.
Proper eye alignment is important to avoid seeing double, for good depth perception, and to prevent the development of poor vision in the turned eye. When the eyes are misaligned, the brain receives two different images. At first, this may create double vision and confusion. But over time the brain will learn to ignore the image from the turned eye. Untreated eye turning can lead to permanently reduced vision in one eye. This condition is called amblyopia or lazy eye.
Some babies' eyes may appear to be misaligned, but they are actually both aiming at the same object. This is a condition called pseudostrabismus or false strabismus. The appearance of crossed eyes may be due to extra skin that covers the inner corner of the eyes or a wide bridge of the nose. Usually, the appearance of crossed eyes will go away as the baby's face begins to grow.
Strabismus usually develops in infants and young children, most often by age 3. But older children and adults can also develop the condition.
People often believe that a child with strabismus will outgrow the condition. However, this is not true. In fact, strabismus may get worse without treatment. An optometrist should examine any child older than 4 months whose eyes do not appear to be straight all the time.
Strabismus is classified by the direction the eye turns:
- Inward turning is called esotropia
- Outward turning is called exotropia
- Upward turning is called hypertropia
- Downward turning is called hypotropia.
Other classifications of strabismus include:
- The frequency with which it occurs—either constant or intermittent
- Whether it always involves the same eye—unilateral
- If the turning eye is sometimes the right eye and other times the left eye—alternating.
Treatment for strabismus may include eyeglasses, prisms, vision therapy, or eye muscle surgery. If detected and treated early, strabismus can often be corrected with excellent results.
What causes strabismus?
Strabismus can be caused by problems with the eye muscles, the nerves that transmit information to the muscles, or the control center in the brain that directs eye movements. It can also develop due to other general health conditions or eye injuries.
Risk factors for developing strabismus include:
- Family history. People with parents or siblings who have strabismus are more likely to develop it.
- Refractive error. People who have a significant amount of uncorrected farsightedness (hyperopia) may develop strabismus because of the additional eye focusing they must do to keep objects clear.
- Medical conditions. People with conditions such as Down syndrome and cerebral palsy or who have suffered a stroke or head injury are at a higher risk for developing strabismus.
Many types of strabismus can develop in children or adults, but the two most common forms are:
- Accommodative esotropia often occurs because of uncorrected farsightedness (hyperopia). The eye's focusing system is linked to the system that controls where the eyes point. So people who are farsighted are focusing extra hard to keep images clear. This may cause the eyes to turn inward. Symptoms of accommodative esotropia may include seeing double, closing or covering one eye when doing close work, and tilting or turning the head.
- Intermittent exotropia may develop when a person cannot coordinate both eyes together. The eyes may point beyond the object being viewed. People with intermittent exotropia may experience headaches, difficulty reading and eye strain. They also may close one eye when viewing at distance or in bright sunlight.
How is strabismus diagnosed?
An optometrist can diagnose strabismus through a comprehensive eye exam. Testing for strabismus, with special emphasis on how the eyes focus and move, may include: :
- Patient history. An optometrist will ask the patient or parent about any current symptoms. In addition, the optometrist will note any general health problems, medications or environmental factors that may be contributing to the symptoms.
- Visual Acuity. An optometrist will measure visual acuity to assess how much vision is being affected. For the test, you will be asked to read letters on reading charts that are near and at a distance. Visual acuity is written as a fraction, such as 20/40. The top number is the standard distance at which testing is done (20 feet). The bottom number is the smallest letter size you were able to read at the 20-foot distance. A person with 20/40 visual acuity would have to get within 20 feet of a letter that should be seen clearly at 40 feet. "Normal" distance visual acuity is 20/20. Your eye doctor has other methods of measuring vision in young children or patients who cannot speak or comprehend the visual acuity test.
- Refraction. An optometrist can conduct a refraction to determine the appropriate lens power you need to compensate for any refractive error (nearsightedness, farsightedness or astigmatism). Using an instrument called a phoropter, the optometrist places a series of lenses in front of your eyes and measures how they focus light using a handheld lighted instrument called a retinoscope. Or the doctor may use an automated or handheld instrument that evaluates the refractive power of the eye without the patient needing to answer any questions.
- Alignment and focusing testing. Your optometrist needs to assess how well your eyes focus, move and work together. In order to obtain a clear, single image of what you are viewing, your eyes must effectively change focus, move and work in unison. This testing will look for problems that keep your eyes from focusing effectively or make it difficult to use both eyes together.
- Examination of eye health. Using various testing procedures, your optometrist will observe the internal and external structures of your eyes to rule out any eye disease that may be contributing to strabismus. This testing will determine how the eyes respond under normal seeing conditions. For patients who can't respond verbally or when some of the eyes focusing power may be hidden, your optometrist may use eye drops. The eye drops temporarily keep the eyes from changing focus during testing.
Using the information obtained from these tests, along with results of other tests, your optometrist can determine if you have strabismus. Once testing is complete, your optometrist can discuss treatment options.
How is strabismus treated?
People with strabismus have several treatment options to improve eye alignment and coordination. They include:
- Eyeglasses or contact lenses. This may be the only treatment needed for some patients.
- Prism lenses. These special lenses have a prescription for prism power in them. The prisms alter the light entering the eye and reduce how much turning the eye must do to view objects. Sometimes the prisms can eliminate the eye turning.
- Vision therapy. Your optometrist might prescribe a structured program of visual activities to improve eye coordination and eye focusing. Vision therapy trains the eyes and brain to work together more effectively. These eye exercises can help problems with eye movement, eye focusing and eye teaming and reinforce the eye-brain connection. Treatment can occur in your optometrist's office as well as at home.
- Eye muscle surgery. Surgery can change the length or position of the muscles around the eye so they appear straight. Often, people who have eye muscle surgery will also need vision therapy to improve eye coordination and to keep the eyes from becoming misaligned again.