Amblyopia, or lazy eye, causes decreased vision in a child’s eye despite normal eye structure. Three to 5 percent of children are affected by amblyopia. When a child is young, several conditions can cause an eye to send a blurry image to the brain. Then, during visual development, a child’s brain gets used to receiving a blurry image; even if the underlying cause of vision impairment is addressed, the brain may not be able to interpret the clear visual image, resulting in reduced vision. If not treated, amblyopia causes a permanent decrease or loss of vision in the affected eye. With treatment, most children have improved vision.
Hippocrates (460—377 BCE) used the term amblyopia to describe diminished visual acuity. The treatment at that time focused on the whole person rather than the eye, because the eye problem was viewed as a symptom of a larger problem in the body. Many cultures subsequently have records of treating amblyopia by attempting to strengthen the eyes directly.
By the 1700s in France, the field of ophthalmology was growing with hospitals and specialists dedicated to the eye. Georges-Louis Leclerc, Comte de Buffon (1707—1788), a French naturalist, described occlusion (patching) therapy. Patching therapy involves applying a patch to the stronger eye in order to strengthen the weaker eye. Working on a theory similar to patching, eye drops have also been used to block the vision in the stronger eye. Many therapies have been described for amblyopia, but the use of patching remains constant throughout history.
Types and Symptoms
Several underlying causes result in amblyopia. Strabismic amblyopia is caused by a misalignment of the eyes. Strabismus (also called crossed eye or eye turn) happens when the eye muscles fail to align the eyes symmetrically. The brain receives two different images from the eyes, and to resolve the images and see a single image, the brain ignores one image. Deprivation amblyopia results when the brain is not receiving input from an eye. Cataracts, for example, can decrease vision and lead to deprivation amblyopia as can other physical causes including drooping eyelids. Refractive amblyopia results when there is a significant difference between the prescription needed to correct each eye, causing the brain to suppress the eye sending the blurrier image.
Treatment for amblyopia involves several steps and may take a year or longer to correct. The first step in treating amblyopia is to identify patients affected. Children with one eye functioning normally may not report vision difficulties. After the cause of amblyopia has been identified, appropriate treatment will be started. Correcting the misalignment of the eyes treats strabismic amblyopia. Eye muscles can be shortened or lengthened surgically. Nonsurgical treatments such as drug injections can weaken the muscles pulling the eye. Deprivation amblyopia is corrected by removing the visual block; treatments include cataract surgery, or addressing the cause of the drooping eyelid. Correcting the refractive difference between the eyes with glasses treats refractive amblyopia.
After the underlying condition causing the amblyopia has been corrected, the brain learns to interpret signals from both eyes. Patching the strong eye allows the brain to accept and interpret signals from the weaker eye, improving vision. Vision therapy may be designed to help the weaker eye improve, and unlike patching can be designed to encourage both eyes to work together.
Early diagnosis is key to preventing vision loss secondary to amblyopia. Because of the difficulty of detecting some forms of amblyopia and the finding that there is a window during which treatment is more successful, vision screening is recommended at each well child check with referrals to a pediatric eye specialist if needed. Many schools in the United States also offer vision screenings.
Treatment of amblyopia can reverse or reduce vision loss. Treatment of children younger than six years of age is often effective, with success rates cited of restoring near normal visual acuity in 50—75 percent of children. Recent literature (Scheiman et al., 2005) showed that treating older children also had benefits: up to 47 percent of children treated with glasses, patching, and eye drops for 2—6 hours per day showed more than a two-line improvement on an eye chart (a measure of improved visual acuity) after treatment.
Additional research will reveal therapies appropriate for treating amblyopia and refine the amount of treatment time patients need to achieve maximal results. As more is understood about how the brain works, treatments will continue to focus on the brain and brain plasticity and may be offered to older patients, too.
Lisa A. Rabe
See also: Cataracts; Diplopia; Eye Protection; Ptosis; Strabismus
Scheiman, Mitchell M., Richard W. Hertle, Roy W. Beck, Allison R. Edwards, Eileen Birch, Susan A. Cotter, … & Susanna M. Tamkins. (2005). Randomized trial of treatment of amblyopia in children aged 7 to 17 years. Archives of Ophthalmology, 123(4), 437—447.