Strabismus is a condition in which a person's eyes are not properly aligned with each other. One eye is either constantly or occasionally turned inward, outward, upward, or downward. Someone looking at a person with strabismus may notice that their eyes are pointed in different directions. If one or both eyes are turned inward, the person is said to have crossed eyes; if either eye is turned outward, they are said to be wall-eyed.
Ophthalmologists (doctors who specialize in treating eye disorders) classify strabismus as congenital, acquired, or secondary. Congenital strabismus is present at birth and affects about 1 percent of infants. Acquired strabismus develops in later life from injury to the eye, the brain, or such diseases as diabetes, while secondary strabismus results from another eye disorder, usually cataracts.
Strabismus is an eye disorder in which the person's eyes do not focus on a single point at the same time. The lack of coordination of the eyes
results from eye muscles that do not work together properly. One or both eyes may turn outward, inward, upward, or downward. Some people may have two different subtypes of strabismus, one in each eye. Strabismus may be constant or it may come and go. In children, strabismus may vary over the course of a few hours as well as from day to day.
Although strabismus is not a life-threatening condition, it can have serious consequences for a child's vision if it is not corrected. The reason is that unlike an adult's fully developed brain, a child's brain is still learning to coordinate the visual perceptions from both eyes. If the two eyes are focusing in different directions, the brain may start to ignore the input from one eye. This condition is called amblyopia, or lazy eye. Although the affected eye is completely normal in terms of its basic structures, it can lose its ability to see over time. Uncorrected amblyopia resulting from strabismus is the leading cause of blindness in one eye in American adults over the age of twenty.
Some children have a condition called pseudostrabismus or false strabismus because their facial features make them look cross-eyed or walleyed. Children with eyes set unusually close together, a wide flat nose, or an extra fold of skin near the inner eye may look as if they have strabismus. False strabismus does not affect vision and usually goes away as the child grows older and his or her face lengthens.
According to the American Academy of Ophthalmology, about four in every one hundred adults in the United States have strabismus. About 2 percent of children have strabismus, half of them being born with the condition. Although strabismus can develop at any age, the age group most commonly affected is children between three and six years of age.
Strabismus appears to run in some families, although no specific genes have yet been identified that are linked to strabismus. It is also not known whether strabismus itself is genetic or whether it is the underlying causes of the disorder (weakness of the eye muscles or cranial nerves controlling the eye muscles) that are inherited. Asian Americans appear to be at a slightly higher risk of strabismus than people of other races or ethnic backgrounds. As far as is known, males and females are equally affected.
Other risk factors for strabismus include: farsightedness or hyperopia; premature birth; and developmental delays.
Abraham Lincoln's Strabismus
Abraham Lincoln (1809–1865) was ridiculed by many of his contemporaries for his “homely” and “misshapen” face. The left side of Lincoln's face was noticeably smaller than the right side, a difference that can be clearly seen in photographs taken of him during his presidency. Some medical historians think that this difference was caused by a hereditary developmental defect—Lincoln had a first cousin with strabismus—while others think that the facial problem was caused by trauma. Lincoln had been kicked in the face by a horse when he was nine years old.
Several people who knew Lincoln reported that his left eye sometimes moved upward independently of his right eye—a clear indication of strabismus. Lincoln himself noted that he saw double on occasion; however, he was not particularly troubled by this symptom. In 2007 a retired ophthalmologist and history buff made a laser scan of two facial masks that had been made of Lincoln during his lifetime. The three-dimensional scan allowed the doctor to analyze the differences between the muscles in Lincoln's left and right eyes, and to confirm the diagnosis of adult strabismus.
Strabismus in children can be caused by weakness of the eye muscles or defects in the baby's developing nervous system. Children with cerebral palsy, Down syndrome, Edwards syndrome, or whose mother had rubella during pregnancy have a significant risk of congenital strabismus. In adults, acquired strabismus may result from a concussion, brain tumor, stroke, traumatic brain injury that affects the cranial nerve controlling the eye muscles, loss of vision in one eye, or diabetes.
Symptoms of strabismus in a child may include:
- Eyes focusing in different directions when the child becomes interested in playing with a nearby object.
- The child's eyes roll or wander for no apparent reason.
- The child does not seem to have normal depth perception.
- The child covers one eye with the hand in order to focus the other eye.
The symptoms of strabismus in adults are slightly different from those in children because the visual system in the adult's brain is completely developed. Amblyopia is therefore unusual in adults with acquired strabismus. Common symptoms in adults include:
- Seeing double. This is the most annoying symptom for most adults with acquired strabismus because it interferes significantly with reading, driving, and many other everyday tasks.
- Having problems with depth perception.
- Recurrent headaches or feelings of eyestrain.
- Squinting or tilting the head to one side in order to focus the affected eye.
- Eyes that appear crossed or walleyed to other people. This can be a cause of considerable social embarrassment; one study found that 70 percent of adults with strabismus had difficulties in school, work, or family life because of the impact of strabismus on their appearance.
Early diagnosis of strabismus in children is critical in order to prevent amblyopia and later loss of vision. The eyes of newborns cannot always focus directly on an object because the muscles of the eye are still developing. A baby should, however, be able to focus both eyes in parallel by the age of three to four months. If parents notice that the infant still looks cross-eyed or walleyed at four months, they should have the child examined by the child's primary care doctor or by an ophthalmologist.
The most common tests used to diagnose strabismus are the cover test and the Hirschsprung test. In the cover test, the examiner covers each of the patient's eyes in turn with a handheld shield while asking the patient to focus on a small object at a distance. If a person has strabismus, the unaffected eye will not change focus when uncovered, while the strabismic eye will move or change its focus. In the Hirschsprung test, the examiner shines a small flashlight or penlight on the patient's eyes. The examiner can see the light reflected on the front surface of the pupil. If the eyes are properly aligned, the reflection will be in the same spot on each eye. If the person has strabismus, the reflection from the light will appear on different areas of each eye.
The ophthalmologist may also perform other examinations to rule out the possibility that a brain tumor is causing the strabismus.
Treatment of strabismus in children may consist of one or more of the following measures:
- Patch therapy and vision therapy. In patch therapy, the child's stronger eye is covered with a patch, forcing the weaker eye to function at a higher level. Vision therapy consists of exercises performed in the eye doctor's office under supervision to change the way the eyes process visual information. It is not concerned with strengthening the eye muscles.
- Medications injected into the eye muscles to weaken overactive muscles. Interestingly, the drug most often used for this purpose—botox—is derived from the same organism that produces the deadly toxin of botulism food poisoning.
- Corrective lenses. These are intended to improve the focusing of the patient's eyes and redirect the line of sight.
- Surgery. Surgery to realign the eye muscles may be done if corrective lenses and patch therapy are not successful. It is not usually performed on children younger than four years of age.
Adults with strabismus may be treated with corrective lenses, botox injections in the eye muscles, or surgery on the eye muscles. Surgery is usually done to correct double vision, the most troublesome symptom of strabismus in adults.
When strabismus in children is detected and treated early, the prognosis is good. The prognosis for strabismus in adults depends on the cause (trauma, brain tumor, other diseases) and its severity.
Congenital strabismus can be prevented in some cases by immunization against rubella in women planning a pregnancy. Acquired strabismus can be prevented in some cases by observing safety measures to lower the risk of head injuries in sports or the workplace and by avoiding
improperly canned foods (a common source of botulism) or poisonous shellfish.
As of 2008, the National Institutes of Health (NIH) was conducting about 29 different clinical trials related to strabismus. Most are studies comparing the effectiveness of different types of treatment for the condition, but three of the trials are exploring genetic factors that may be linked to strabismus.
SEE ALSO Cataracts; Cerebral palsy; Concussion; Diabetes; Edwards syndrome; Hyperopia; Rubella; Stroke
WORDS TO KNOW
Amblyopia: Dimness of sight in one eye without any change in the structure of the eye. It is also known as lazy eye.
Botulism: A rare but potentially fatal paralytic illness caused by a bacterial toxin in contaminated food.
Congenital: Present at birth.
Pseudostrabismus: A condition in which a child may seem to have strabismus because of certain facial features that change as the child's face matures.
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National Eye Institute (NEI). Facts about Amblyopia. Available online at http://www.nei.nih.gov/health/amblyopia/amblyopia_guide.asp (accessed July 4, 2008).
Prevent Blindness America. Strabismus. Available online at http://www.preventblindness.org/children/strabismusFAQ.html (accessed July 4, 2008).
Research to Prevent Blindness (RPB). Animation: Strabismus. Available online at http://www.rpbusa.org/rpb/eye_info/AmblyopiaStrabismus/ (accessed July 4, 2008). Clicking on the animation sidebar opens a series of three animations on the characteristics, symptoms, and treatments of strabismus. Each animation takes about a minute to play.