What is Strabismus?
Strabismus, commonly known as squint, is a visual defect in which the eyes are misaligned and point in different directions. One eye may look straight ahead, while the other eye may turn inward, outward, upward or downward.
You may find that the misalignment is a constant feature, or it may come and go. The turned eye may straighten at times and the straight eye may turn.
Strabismus is common among children. About 4% of all children in the United States have strabismus. It can also occur later in life.
In occurs equally in males and females. Strabismus may run in families. However, many people with strabismus have no relatives with the problem.
How do the Eyes Work Together?
With normal vision, both eyes aim at the same spot. The brain then fuses the two pictures into a single three-dimensional image. This three-dimensional image gives us depth perception.
When one eye turns, the brain receives two different pictures, which do not exactly overlap. In a young child, the brain learns to ignore the image of the misaligned eye and sees only the image from the straight or better eye. However, the child is likely to lose depth perception.
Adults who develop strabismus often have double vision because the brain is already trained to receive images from both eyes and cannot ignore the image from the turned eye.
Good vision develops during childhood when both eyes are normally aligned. Strabismus may cause reduced vision, or amblyopia, in the weaker eye. The brain will recognize the image of the better-seeing eye and ignore the image of the weaker or amblyopic eye. This occurs in approximately half the children who have strabismus.
Amblyopia can be treated by patching the "good" eye to strengthen and improve vision in the weaker eye. If amblyopia is detected in the first few years of life, treatment is usually successful. If treatment is delayed, amblyopia can become permanent. As a rule, the earlier it is treated, the better the chances of saving vision.
What Causes Strabismus?
The exact cause of strabismus is not fully understood. Six eye muscles, controlling the eye movement, are attached to the outside of each eye. In each eye, two muscles function to move the eye right or left. The other four muscles move it up or down or at an angle.
To line up and focus both eyes on a single target, all the muscles in each eye must be balanced and working together. In order for the eyes to move together, the muscles in both eyes must coordinated.
The brain controls the eye muscles. Strabismus is especially common among children with disorders that affect the brain such as:
- Cerebral palsy
- Down syndrome
- Hydrocephalus, and
- Brain tumors
A cataract or eye injury that affects the vision can also cause strabismus.
What are the Symptoms of Strabismus?
he main symptom is an eye that is not straight. Sometimes children will squint with one eye in bright sunlight or tilt their head to use their eyes together.
How is Strabismus Diagnosed?
Strabismus can be diagnosed during an eye examination. It is recommended that all children have their vision checked by their pediatrician, family doctor or ophthalmologist on or before their fourth birthday. If there is a family history of strabismus or amblyopia, an ophthalmologist can check the vision even before the age of three.
An infant's eyes may seem to be crossed. Young children often have a wide, flat nose and a fold of skin at the inner eyelid that can make the eyes appear crossed. This appearance of strabismus may reduce as the child grows. A child will not outgrow true strabismus.
An ophthalmologist can usually tell the difference between true and false strabismus.
The Most Common Types of Strabismus
Esotropia Esotropia, where the eye turns inward, is the most common type of strabismus found in infants. Young children with esotropia do not use their eyes together. In most cases, early surgery can align the eyes.
During surgery, the tension of the eye muscles in one or both the eyes is adjusted. The tight inner muscles may be removed from the wall of the eye and placed further back on the eye. This adjustment weakens the pull and allows the eyes to move outward. Sometimes the outer muscles are tightened by shortening the muscle length, to allow the eyes to move outward.
Accommodative esotropia is a common form of esotropia that occurs in far-sighted children two years of age or older. When a child is young, he/she can focus the eyes to adjust for the farsightedness, but the focusing effort (accommodation) needed to see clearly causes the eyes to cross.
Glasses reduce this focusing effort and can help straighten the eyes. Sometimes bifocals are needed for close work. Eye drops, ointments, or special lenses called prisms can also be used to straighten the eyes.
Exotropia or an outward-turning eye is another common type of strabismus. This occurs most often when a child is focusing on distant objects. The exotropia may occur only from time to time, particularly when a child is daydreaming, ill or tired. Parents often notice that the child squints one eye in bright sunlight.
Although glasses, exercises or prisms may reduce or help control the outward-turning eye in some children, surgery is often needed.
How is Strabismus Diagnosed?
The eyeball is never removed from the socket during any kind of eye surgery. The ophthalmologist makes a small incision in the tissue covering the eye to reach the eye muscles. During surgery certain muscles are repositioned, depending on which direction the eye is turning. Surgery may be needed for one or both the eyes.
While performing strabismus surgery on children, a general anesthetic is administered. Local anesthesia is an option for adults. Recovery time is rapid and patients are usually able to resume their normal activities within a few days. As with any surgery, eye muscle surgery has certain risks, such as infection, bleeding, excessive scarring, and some rare complications that can lead to loss of vision.
After surgery, glasses or prisms may be useful to improve vision. Further surgery may be needed later to keep the eyes straight. For children with constant strabismus, early surgery offers the best chance for the eyes to work well together. In general, it is easier for children to undergo such surgery before school age.
Strabismus surgery is a safe and effective treatment for eye misalignment. It is not, however, a substitute for glasses or amblyopia therapy.
BotoxTM, a new drug approved by the US Food and Drug Administration for limited use, can be an alternative to eye muscle surgery for some individuals. An injection of this drug into an eye muscle temporarily relaxes the muscle, allowing the opposite muscle to tighten and straighten the eyes.
Although the effects of the drug wear off after several weeks, sometimes the misalignment may be permanently corrected.