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Definition: Amblyopia from Black's Medical Dictionary, 43rd Edition

Defective vision for which no recognisable cause exists in any part of the eye. The most important form is that associated with SQUINT, or gross difference in refraction between the two eyes. It has been estimated that in Britain around 5 per cent of young adults have amblyopia due to this cause.

Summary Article: Amblyopia
From Encyclopedia of Global Health

Amblyopia, or “lazy eye,” is an ocular disorder characterized by dominance of one eye’s input to the brain over the other. It occurs in about 3 percent of the population. If left untreated, amblyopia can cause permanent visual impairment in one eye, leading to lifelong consequences. However, early detection and treatment of amblyopia through the use of eye patches or eye drops can help strengthen the weaker eye and lessen or cure the disorder.

Amblyopia is most commonly diagnosed as a two-line difference of visual acuity between both eyes. Several more comprehensive testing methods exist for determining the causes, classification, and extent of the amblyopia. Classification depends on the symptoms leading up to development. Under normal conditions, each eye’s retina picks up light-induced signals that travel through the optic nerve, combine, and reseparate into left and right visual hemispheres at the optic chiasm, and travel to the contralateral (opposite) sides of the visual cortex. Binocular interactions between the signals from both eyes result in integration of neural signals from both sides of the brain to form a single perceived image.

Anisometropic amblyopia occurs when the unfocused eye’s image is inhibited to favor the image from the focused eye. Strabismus is characterized by dysfunction of ocular muscles and uncoordinated eye movement and fixation. Since both eyes send very different images to the brain, intraocular image conflict occurs. In strabismic amblyopia, the patient strongly favors movement and fixation using just one eye and neglects these functions in the other eye.

Visual deprivation can be caused by a variety of conditions such as cataract, high refractive error (nearsightedness or farsightedness), corneal opacities, ptosis (drooping of the eyelid), or other disorders that result in understimulation of the retina. Such problems may lead to amblyopia and are especially detrimental during the earliest years of life, when the retina and visual pathways are rapidly developing and setting the course for all future visual function. This explains why the earlier amblyopia is diagnosed and treated, the better the hope for recovery. If a cataract or other ocular disorder is congenital or present from birth, immediate treatment is necessary to prevent permanent debilitation in the afflicted eye.

Studies have shown that early detection and treatment of amblyopia can result in surprising levels of improvement in the weak eye’s function. Previously, it was thought that the damage caused by amblyopia became permanent after age 7. New studies by the National Institutes of Health (NIH) have shown that the possibility for at least some improvement remains well into the teenage and adult years. Of course, since the visual system loses its neuroplasticity (ability to develop and adapt to new conditions) with age, the earlier this neuro-ophthalmological disorder is dealt with the better. With amblyopia prevalent in about 3 percent of the population, detection and treatment by 5 years of age reduced the prevalence to 2 percent, while even earlier measures taken by 3 years of age reduced the prevalence to 1 percent.

The two major ways of treating amblyopia are use of eye patches (occlusion) and atropine drops (penalization). Measures taken to correct the specific cause of amblyopia, such as refractive correction or surgery, can also significantly improve binocularity. Occlusion involves covering up one eye with a patch and forcing use and improvement of the weaker eye. While occlusion is straightforward, the treatment has a low compliance rate, especially among young children, because eye patches are uncomfortable and can cause difficulty and embarrassment in situations such as school. Penalization involves using atropine drops to blur the image in the dominant eye, forcing use of the weak eye. Previously, it was believed that patients ought to be treated for long daily intervals. However, new NIH studies reveal that occlusion or penalization for as little as two hours each day can still be as beneficial as treatment for several hours. In light of low compliance rates, these results are good news for amblyopic children worldwide.

Amblyopia is the leading cause of monocular vision loss in adults. Because they only have one functional eye, amblyopic patients are at greater risk for blindness. Negative consequences of untreated amblyopia can include reduced depth perception, reduced visual-motor skills, and psychosocial impacts such as low self-esteem, damaged interpersonal communication, and lower employability. Globally, amblyopia deserves attention because of its lifelong consequences on patients’ lives.

  • Cataract; Eye Care; Eye Diseases (General); Ophthalmologist; Ophthalmology.

  • Lorraine Steefel, “Amblyopia,” in The Gale Encyclopedia of Medicine, 3rd ed. (Thomson Gale, 2006).
  • A. Webber; J. Wood, “Amblyopia: Prevalence, Natural History, Functional Effects and Treatment,” Clinical and Experimental Optometry (v.88, 2005).
  • Kimberly G. Yen, “Amblyopia,” (cited October 2006).
  • Nakul Shekhawat
    Vanderbilt University
    Copyright © 2008 by SAGE Publications, Inc.

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