Strabismus is a deviation or misalignment of the eyes where the eyes turn inward, outward, upward, or downward. The turning of the eye in strabismus is called tropia. Esotropia is an eye that turns inward. Exotropia indicates an outward turning of the eyes. Hypertropia means upward turning. Hypotropia describes a down turned eye. Strabismus is a common condition affecting about 1 out of every 25 people. It usually begins in childhood but may occur at any age.
Different Forms of Strabismus
There are six muscles attached to the outside of each eye controlling eye movements. In order to align the eyes on an object these muscles must work together with the same muscles of the opposite eye. As long as the twelve eye muscles of both eyes are balanced, the eyes focus together, but if the eyes muscles do not work together, one or both eyes are turned and strabismus results.
Normal depth perception takes place when both eyes are focused on the same object allowing the brain to fuse the pictures from each eye into a single three-dimensional image. If the muscles are not working together and the eye wanders, the brain will perceive the images as two separate images, thus double vision can result in an adult. Children learn to suppress one of these images, therefore they lose depth of field and depth of perception, and vision is also lost in that eye. This results in amblyopia or a "lazy eye."
Causes & Symptoms
Amblyopia occurs in approximately one-half of the children with strabismus. If this is diagnosed at a young age, vigorous therapy, including possible glasses and patching of the good eye can strengthen the weaker one. Strabismus can be treated with glasses, eye drops, or surgery. The cause of strabismus in childhood is not known. There are family histories that suggest that children often inherit factors from their parents. Frequently, there is no family history. It appears to affect males and females equally. There are certain medical conditions, systemic medical problems, or local eye problems that can cause strabismus and amblyopia.
The primary symptom of strabismus is an eye that is not straight. Sometimes the youngster will squint one eye in bright sunlight. Children may be fussy or irritable when trying to use their eyes. Some children will turn or tilt the head in a specific direction in order to use the eyes together. Sometimes fatigue or illness will worsen strabismus and because of this parents get the false impression that the child may "outgrow" misalignment of the eyes. If the child has a suspected turning of the eye, an immediate examination by an ophthalmologist is necessary to determine the cause and to begin treatment.
Detection and Diagnosis
The American Academy of Ophthalmology suggests the child should be examined by the family doctor, pediatrician, or ophthalmologist during infancy, then at age two to three, and again at age five in order to uncover any potential eye problems, particularly if a relative has a history of strabismus or amblyopia. It is often difficult to determine the difference between eyes that appear to be crossed "pseudostrabismus" and true strabismus. It is never too early to have a child's eyes examined. Early prevention and treatment is the best form of therapy.
Treatment of strabismus is directed at the cause. The two main types of strabismus are esotropia, where the eye turns in and exotropia, where the eye turns out. Esotropia is the most common type of strabismus seen in infants. Infants born with esotropia do not learn to use their eyes together. Usually these conditions are amenable to surgical treatment consisting of moving one or more eye muscles to re-align the eyes. Some cases of esotropia that develop after the age of one and-a-half require treatment with eyeglasses only. These children's eyes cross as they begin to focus on objects. These children are somewhat farsighted. The glasses prescribed to correct farsightedness reduce their need to over focus and helps the eyes focus correctly. In these cases, until the underlying muscle imbalance is improved, glasses must be worn all the time. Many children outgrow their need for glasses as they mature because their farsightedness lessens with growth.
Another common type of strabismus is exotropia, which is an out-turning of the eye. This accounts for around 25% of all cases of strabismus and is characterized by an outturned eye. This most commonly occurs when a child focuses on distant objects. When exotropia occurs intermittently, parents often note the child squints one eye in bright sunlight and has an out-turning of one eye when daydreaming, ill, or tired. Small misalignments require nothing more that periodic observation by an ophthalmologist but large amounts usually do need to be corrected by surgery. The strabismus surgery is relatively safe and effective. (The eyeball is not removed from the socket to perform the surgery.) Parents should understand that more than one operation may be required to straighten the eye and both eyes may require surgery in order to correct strabismus because younger infants often develop some degree of depth perception once the two eyes are straightened. As the child gets older, the chance of developing normal sight decreases. The most effective treatment for strabismus occurs when the child is younger. It becomes more difficult to treat strabismus as the child grows older. Only cosmetic straightening of the eye becomes possible. There is no known prevention of strabismus, but misaligned eyes can be straightened and a loss of sight from ambylopia is preventable if treatment is begun early.
A new form of treatment for strabismus is with "botulism toxin." The toxin is injected in minute quantities into the muscle. Due to its neurotoxic effect it apparently realigns the muscles. This is still experimental but appears promising.