When you look at an object, you're using several muscles to move both eyes to focus on it. If you have a problem with the muscles, the eyes don't work properly.
Most of us are fortunate because our eyes started to work as a team very early in infancy and have continued to work together ever since. We are able to focus each eye on whatever we look at, regardless of the direction, and our brain combines the picture or image from each eye into the mental picture we actually see in three dimensions.
About two percent of every 100 children are not as fortunate. For a variety of reasons, their eyes do not work as a team. Both eyes are not directed or focused at the same object. This condition is called "strabismus."
The child with strabismus rarely complains. In most cases, it is the appearance of the eye that first catches the parent's attention. A child should be examined by an ophthalmologist whenever the eyes appear not to be working together.
There are three basic kinds of strabismus: esotropia, exotropia and hypertropia, depending on which direction the eyes are deviated.
The most common type of strabismus is esotropia, which occurs when either one or both eyes turn in toward the nose. Some children are born with this condition. More frequently, it starts at about age 2 1/2. When esotropia occurs in these older children, eyeglasses can often help to treat the condition by correcting the child's vision for farsightedness or hyperopia. This can reduce or eliminate the crossing by changing the child's need for excessive focusing.
In some children a broad nasal bridge or an extra skin fold give the false appearance of esotropia. This condition is known as pseudoesotropia.
Exotropia is the second most common kind of strabismus. In this condition, one or both eyes turn out. It usually starts at age two or three. In the beginning, the eye may drift out only for a few seconds when the child is tired or ill. It typically occurs when the child looks far away. Closing one eye in bright sunlight when playing outside is also a common early sign.
Hypertropia is the least common type of strabismus. In this condition, one eye is higher than the other. As a result, the child often tilts or cocks his or her head to one side to get rid of the double vision that this problem frequently causes.
Strabismus in Adults
Although strabismus is much more common in children, many adults have strabismus, either since childhood or developed in adult life. A special section of the Wills Eye Pediatric and Ocular Genetics Service, called Adult Motility, is set aside for the management of this group of patients. No person is ever too old to have treatment for stabismus.
Treatment of strabismus may involve patching, eyeglasses, surgery or some combination of these therapies. Strabismus surgery is a delicate procedure performed on the muscles that attach to the outside of the eyeball. There are six muscles attached to each eyeball that move it around.
Eye muscle surgery consists of weakening or strengthening one or more of these muscles in one or both eyes, depending on the type of strabismus. This procedure is done with the child asleep under general anesthesia. Usually, the child comes to the hospital the morning of the surgery and is discharged the same day, several hours after surgery. The eyes are moderately red for a week following the procedure. Once the child leaves the hospital, there is minimal discomfort. In most cases he or she may return to his or her usual activities at home. However, it should be noted that sometimes more than one surgery is required.
Amblyopia (lazy eye) is another frequent condition, occurring in about three or four of every 100 children. When a child is born with normal eyes, he or she has the potential for good vision in both eyes, but must learn to see with each of them. If for some reason, the child prefers to use one eye more than the other, the preferred eye learns to see well but the other suffers from lack of use. It does not learn to see as well, even with glasses. The non-preferred eye is said to be lazy or have amblyopia.
One of the common causes for lazy eye is strabismus. When the child's eyes are pointed in different directions, the child has to use one eye at a time to avoid seeing double. If he or she uses one eye more than the other, the other eye becomes lazy.
Children without strabismus can also develop a lazy eye. Even though their eyes are straight, one eye is preferred more than the other. This non-preferred eye becomes lazy and does not learn to see.
Amblyopia does not bother the child because there are no symptoms. It is found only by checking the vision in each eye. This can be done fairly accurately in any child three years or older. For this reason, all children should have their vision tested by age four.
The treatment for amblyopia involves forcing the lazy eye to be used more often. Usually this is accomplished by patching the preferred, or good eye. This may have to be continued for several months until each eye sees equally well. Fortunately, it is usually successful in restoring good sight. Sometimes the patching must be continued intermittently until age nine. If the lazy eye is out of focus, eyeglasses may be required, in addition to patching the good eye, to obtain the best sight.
Eye Movement in Children
Many children enter the world with less than 100 percent of their expected visual capacity, a deficiency that is not always obvious to
parents or medical professionals. One sign of possible eye problems, however, is eye movement. Eye movements tell a lot about vision, even if a child is pre-verbal. How well a child follows faces or large objects is a clue to his or her visual abilities. Another indication of a possible disorder is unusual jiggling of a child's eye(s), called nystagmus. These eye movements can be constant or intermittent. They can be horizontal, vertical, oblique, torsional (circular) or combinations of the above. Thus, the study of eye movement can provide important information regarding sight.
Eye Movement Testing
The testing is conducted with sophisticated computer technology and video recording equipment. The specially designed tests can record eye movements in thousandths of a second and fractions of a degree that show the slightest irregularity and patterns of the jiggling.
Eye Movement Analysis
By using electro-oculograms, where small electrodes are placed on the skin around an infants eyes, eye movements of children under one year of age can be recorded. (This test is not painful or harmful to the child.) Patients are routinely videotaped for further analysis. The general behavior of the child at the time of the test is also assessed. This system has enabled our researchers to describe and document the different types of eye movements in infants — something that no other center had previously been able to do.
For more accurate recordings of adults and older children, special contact lenses containing fine hairline wires are placed on the eyes and then connected to recording devices. A computer-controlled target is directed onto a screen so that precise areas of the retina can be stimulated, even in randomly moving eyes. The special contact lenses provide horizontal, vertical or torsional recordings with a precision and range not usually available, affording measurements on eyes that cannot accurately track a target.
The vision of infants, preverbal children and certain adults is measured by a spatial frequency sweep VEP (visually evoked potential). During this test a patient watches a television screen filled with lines, the sizes of which are changed by computer. As the lines are reversed, the brain waves that are generated by vision are recorded until the widths of the lines are too small to be seen. After analyzing these waves, the computer provides highly accurate estimates of the patient's visual acuity.
Another type of VEP uses flashes instead of lines. This test is particularly helpful in determining which nystagmus patients have albinotic traits - those that occur in albino children — and whether the child has any potential for binocular vision.
Because children are always growing and developing, the Foerderer Center provides ongoing testing as the child's condition evolves. Children with eye movement disorders are monitored closely with repeat testing while they are young.
Genetic testing and counseling is available to the parents of children with eye movement disorders that are linked to heredity. The Foerderer Center staff take detailed family histories in these cases to provide for the genetics counseling and to further study family members. This information is not only helpful to parents but also to the research of
Nystagmus is a condition where the eyes make repetitive movements. The eyes can jerk exclusively in one direction, or make back and forth movements. With nystagmus, the eye can look jittery and can affect both eyes or just one eye.
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