Amblyopia
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Amblyopia or lazy eye is a disorder of the eyes. It is characterised by poor or blurry vision that is not correctable with glasses in an eye that is otherwise physically healthy and normal. The problem is due to either no transmission or poor transmission of the visual image to the brain for a sustained period of disfunction or disuse during early childhood. The condition will only arise at this young age because most of the visual system's development in humans is complete and "locked in" by a few years of age. Amblyopia normally only affects one eye, but it is possible to be amblyopic in both eyes if both are similarly deprived of a good, clear visual image.
Amblyopia affects 2-5% of the population. Amblyopia is a developmental problem in the brain, not an organic problem in the eye. The part of the brain corresponding to the visual system from the affected eye is not stimulated properly and develops abnormally. This has been confirmed in brain specimens.
Many children who have amblyopia, especially those who are only mildly amblyopic, are not even aware they have the condition until tested at older ages, since the vision in their stronger eye is normal. However, people who have severe amblyopia may experience associated vision disorders, most notably poor depth perception.
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Types
Amblyopia can be caused by deprivation of vision early in life, or by strabismus (misaligned eyes), vision obstructing disorders and anisometropia (different degrees of myopia or hyperopia in each eye). Since strabismus is a common cause of amblyopia, it is sometimes also referred to as "lazy eye."
Strabismic amblyopia
Strabismus (misaligned eyes) will result in normal vision in the fixating eye, but abnormal vision in the strabismic eye due to the brain discarding its information. Strabismus in an adult usually causes double vision (diplopia), since the two eyes are not fixated on the same object. Children's brains, however, are more plastic, and therefore can more easily adapt by ignoring images from one of the eyes, getting rid of the double vision. This plastic response of the brain, however, interrupts the brain's normal development, resulting in the amblyopia.
Strabismic amblyopia is best treated by treating the strabismus through the use of prescription glasses, surgery or patching.
Refractive amblyopia
If anisometropia is present, refractive amblyopia may result. Anisometropia is when there is a difference in the refraction between the two eyes. The eye with less refractive error provides the brain with a clearer image, and is favoured by the brain. Those with this condition then become more susceptible to the development of amblyopia, which may be as severe as strabismic amblyopia. Despite its severity, refractive amblyopia is usually missed by doctors because of its less dramatic appearance and lack of obvious physical manifestation, such as with strabismus.
Refractive amblyopia is diagnosed when there is a wide disparity in visual acuity between the two eyes. Refractive amblyopia is treated by correcting the refractive error early, with or without eye patching.
Form deprivation and Occlusion amblyopia
Form deprivation amblyopia (Amblyopia ex anopsia) results when the ocular media is opaque, such as is the case with cataracts or corneal scarring. These opacities prevent adequate sensory input from reaching the eye, and therefore disrupt visual development. If not treated in a timely fashion, amblyopia may persist even after the cause of the opacity is removed. Sometimes, drooping of the eyelid (ptosis) or some other problem causes the upper eyelid to physically occlude a child's vision, which may cause amblyopia quickly.
One should also be wary of creating this type of amblyopia in a 'good' eye when treating for amblyopia in the other eye – so-called reverse amblyopia.
This type of amblyopia is treated by removing the opacity as soon as possible.
Treatment
Treatment is most useful if started before age 5, and seldom started after age 10. If it is detected early enough, it may be corrected by occluding (covering) the stronger eye for periods of time to force the weak eye to develop good vision.
Research has shown that even after age 10, some children can show improvement in the affacted eye. Children from 7 to 12 who wore the patch for 2 to 6 hours per day were four times as likely to show a two line improvement on a standard 11 line eye chart than amblyopic children who weren't patched. Children 13 to 17 showed improvement as well, albeit in smaller amounts than younger children but only if they hadn't received any previous therapy. (Pediatric Eye Disease Investigator Group, 2005)
This treatment rarely works for adults, since the visual system is fully developed before adolescence. However, recent research has shown that some therapy can improve visual acuity slightly in adult amblyopes.
References
External Links
BBC article on new study that finds lazy eye can be treated in teen years also (http://news.bbc.co.uk/1/hi/health/4436439.stm)de:Amblyopie fr:Amblyopie he:עין עצלה