What is Strabismus?
Strabismus (or crossed eyes) is the misalignment of the eyes when looking at an object. When an eye is misaligned, two different pictures are sent to the brain, causing double vision. The person may learn to suppress one of their eyes in order to eliminate the double vision. However, depth perception is lost by suppressing one eye. Long-term suppression of one eye can lead to amblyopia, partial blindness of the eye.
What is Amblyopia?
Amblyopia, commonly called lazy eye, refers to the improper development or significant loss of vision in one or both eyes. It occurs when the brain does not acknowledge the images seen by the amblyopic eye(s).
Amblyopia occurs when an individual cannot use binocular vision (both eyes working together) due to one of three reasons:
1. Strabismus - The most common cause of amblyopia is strabismus, a misalignment of the eyes. To prevent the double vision caused by strabismus, the brain ignores information from one eye.
2. High Prescription in one eye - Refractive amblyopia occurs when the brain favors one eye due to extreme nearsightedness, farsightedness, or astigmatism in the other eye.
3. High Prescription in both eyes - Called deprivation amblyopia, this type of lazy eye is caused when an infant's vision is obstructed and hindered from normal development. Congenital cataracts typically cause this type of amblyopia, and require surgery for treatment.
Who is at Risk?
A condition associated with eye development, amblyopia usually begins in infancy or early childhood. For this reason, amblyopia can be difficult to detect. Eye care professionals recommend children have an eye exam at six months, three years, and before starting school to diagnose amblyopia early.
Signs and Symptoms of Amblyopia
The primary symptom of amblyopia is the loss of vision in one or both eyes. Since amblyopia does not have many outward symptoms and is often present in infants and young children, it can be difficult to spot.
In some cases, a misalignment of the eyes will be apparent. To test infants at home, a parent can try covering one of the child's eyes at a time while observing behavior. If the infant consistently fusses or cries when one eye is covered, this might indicate a vision problem. Since amblyopia most commonly affects only one eye, children will also consistently bump into objects on the affected side. If in both eyes, children will experience clumsiness, poor motor development, and the need to touch everyday things more.
Diagnosis and Treatment
An eye care provider will diagnose amblyopia with visual acuity and binocular vision tests. Treatment will focus on strengthening the amblyopic eye(s) and retraining the brain to use the weaker eye(s) with eye patches, glasses, vision therapy, and sometimes strabismus surgery.
Treatment is most effective at a young age, but developments in eye care have successfully treated older patients. If left untreated, amblyopia leads to problems with depth perception, blindness in one eye, and if the stronger eye becomes injured, serious problems with visual acuity.
Insightful observations about amblyopia and slow reading are shared by two Ph.D. researchers, Eileen Birch and Krista Kelly. Here are some key passages from their commentary:
“Using natural, binocular silent reading of age-appropriate paragraphs of text, we recently reported that children with amblyopia read slowly compared with controls and non-amblyopic strabismic children, regardless of amblyopia type (i.e., strabismic or anisometropic). Unlike earlier reading studies that focused on strabismus, our study clearly identified that amblyopia alone is sufficient to impair reading. Importantly, comprehension did not differ significantly between amblyopic children and controls, indicating that amblyopic children did not read slowly because they had dyslexia or a learning disability.”
“Unlike dyslexic readers, amblyopic children in our study read more slowly because they made more forward saccades during reading. It is likely that the increased number of forward saccades during reading reflects oculomotor control system deficits. Amblyopia is associated with substantial fixation instability … Taken together, slower saccade initiation, saccade amplitude variability, and increased frequency of secondary saccades can be expected to significantly slow reading speed in amblyopic children.”
“Saccadic eye movements clearly play a key role in efficient reading, a key component of academic achievement. There is a solid evidence base demonstrating slower initiation of saccades and more variable saccade amplitudes in amblyopic individuals. Even though it usually results in poor vision in just one eye, amblyopia can reduce reading speed in natural, binocular reading by 20 and negatively affect academic performance. We have also found that amblyopic children are slower at transferring answers from a standardized test to a Scantron answer sheet compared with visually normal controls. Longer Scantron completion times may exacerbate the problem of slow reading when amblyopic children are administered standardized tests that are time- sensitive, limiting the child’s academic success. The child’s academic achievement, in turn, is a major contributor to the child’s developing self-perception, which has been shown to be affected by amblyopia. Positive self-perception has an important influence on well-being and quality of life.”
Birch and Kelly conclude: “While currently there is no evidence that training eye movements will help amblyopic children read faster, parents and educators can work together to implement accommodations (e.g., extra time) to help amblyopic students succeed in their daily school tasks, and improve their performance on the timed, stan- dardized tests that are critical for promotion and admission to magnet schools, TAG programs, high schools, and colleges.”