Lazy eye, or amblyopia, is an eye condition in which disuse causes reduced vision in an otherwise healthy eye. The affected eye is called the lazy eye. This vision defect occurs in 2–3% of American children. If not corrected before age eight, amblyopia will cause significant loss of stereoscopic vision, the ability to perceive three-dimensional depth.
In some children, one eye functions better than the other. When a child begins to depend on the stronger eye, the weaker eye can become progressively weaker. Eventually, the weaker eye grows "lazy" from disuse. If left untreated beyond the early child-development years (from birth to seven years old), vision in the affected eye will be underdeveloped due to lack of use.
The impairment of vision in the lazy eye occurs in three phases. In the first (suspension) phase, the brain turns the weaker eye on and off. In the second (suppression) phase, the brain turns off the lazy eye indefinitely. At this point, the eye still has usable vision and can function well if the other eye is covered. In the last (amblyopia) phase, which occurs after age seven, the eye loses all the sensitivity that is essential for good vision because it has not been used for so long.
Lazy eye is a visual problem with potentially serious consequences. If left untreated, the affected child may have permanent loss of vision in the lazy eye. Because of loss of vision in one eye, these children cannot see three-dimensional images very well—all images appear flat. They also have problems with depth perception. This has serious consequences in their future ability to work in professions that require good vision in both eyes. Affected children also have increased risk of blindness should something happen to the good eye.
Causes & symptoms
The following are probable causes of lazy eye:
- Strabismus, or misalignment of a child's eyes (crossed eyes). This is the most frequent cause of lazy eye. Approximately half of all children with crossed eyes will develop a lazy eye. In children with crossed eyes, the images do not coordinate, thus confusing the brain. Therefore, the brain will suppress the image that comes from one eye and predominantly use the image from the stronger eye.
- Anisometropia (unequal refractive power). In this case, there is difference in image quality between the two eyes because one eye is severely nearsighted or farsighted. In other words, one eye focuses better than the other. The brain will mostly use the clearer image from the good eye. The other eye will become underdeveloped due to neglect.
- Congenital cataract. The lazy eye can not see well because its lens is already cloudy at birth.
- Ptosis (drooping eyelid). Vision in the lazy eye is blocked or impaired by the drooping eyelid.
- Corneal scarring. The image quality of the affected eye is poor due to scarring in the cornea.
The following are risk factors for amblyopia:
- rubella (German measles) or other infections in the mother during pregnancy
- premature birth
- other family members with vision problems in childhood
Lazy eye may not present obvious symptoms. For this reason, it is important for small children to have regular eye examinations.
Diagnosis of amblyopia is often made during visual screening during routine infant check-ups and in the preschool years (aged three to five). Premature babies need to have more frequent eye exams during early childhood to prevent this and other vision problems. A new photoscreening instrument that has been recently introduced appears to significantly increase the accuracy of diagnosis of these eye problems.
The following alternative methods may complement conventional treatment of lazy eye. However, they are not replacements for conventional treatments. Because their effectiveness is not proven, parents should consult their child's ophthalmologist about the appropriate use of these methods (if any) in their child's overall eye treatment program.
Orthoptics (eye exercises)
Eye exercises can be helpful. Orthoptic exercises are designed to help the eyes move together and assist the fusing of the two images seen by the eyes. It can help correct faulty vision habit due to misalignment of the eyes and can teach the child to use both eyes effectively and comfortably. This form of therapy can be used before or after eye-realignment surgery to improve results.
One study shows that acupuncture treatment may be effective in treating anisometropia, a condition in which one eye focuses much better than the other. Acupuncture can reduce the differences in refractive powers between the eyes so that both eyes can have similar image quality. This helps reduce the amblyopia problem. However, its long-term effectiveness remains unknown.
In order to treat lazy eye, the doctor has to identify and treat underlying causes. Depending on these underlying causes, the doctor may recommend surgical or nonsurgical treatments, as discussed below.
Refractive error correction
If both eyes need vision correction, children are given prescription glasses for better focus and to prevent misalignment of the eyes.
Forcing the use of the lazy eye
In many children with amblyopia, only one eye has a focusing problem or weak muscles. In order to force the affected eye to work, the doctor will cover the strong eye with a patch for most of the day for at least several weeks. Sometimes, this treatment requires as long as a year. The eye patch forces the lazy eye to work and thus, strengthens its vision and its muscles. This is the most common method used to treat lazy eye. To prevent the strong eye from becoming weaken due to disuse, the
Another way to force the lazy eye to work harder is to use eye drops or ointment to blur the vision in the strong eye so that the child has to use the lazy eye to see. This method is not often used because it is associated with more adverse effects.
If the problem is caused by imbalances of the eye muscles and is not treatable with nonsurgical methods, the eye muscles can be realigned surgically to help the eyes coordinate better. Sometimes more than one surgery is required for the correction. Eye patch, glasses, or orthoptic exercises may be necessary following surgery to help the child use both eyes effectively. Long-term follow-up of surgical treatment indicates that it is highly effective in correcting the problem.
In patients whose amblyopia is caused by a congenital cataract in one eye, the cloudy lens is surgically removed and replaced by an intraocular lens. However, after surgery—even with eye glasses or contact lenses—this eye will still have poorer image quality than the good eye. Thus, the risk for amblyopia remains high. Therefore, nonsurgical treatment for lazy eye is often started after cataract surgery.
For a child whose vision is affected by a drooping eyelid, ptosis surgery is needed.
With early diagnosis and treatment, children with amblyopia are expected to restore the sight in the lazy eye. However, if left untreated, the weak eye never develops adequate vision and the person may become functionally blind in that eye.
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Rebecca J. Frey, PhD