Amblyopia, also called lazy eye, is a disorder of sight in which the brain fails to fully process input from one eye and over time favors the other eye.[ It results in decreased vision in an eye that typically appears normal in other aspects.][ Amblyopia is the most common cause of decreased vision in a single eye among children and younger adults.]
The cause of amblyopia can be any condition that interferes with focusing during early childhood.[ After the underlying cause is addressed, vision is not restored right away, as the mechanism also involves the brain.]
Amblyopia can be difficult to detect, so eye examination is recommended for all children around the ages of four to five[ Eye glasses may be all the treatment needed for some children.][ This is done by either using a eyepatch or putting atropine in the stronger eye.][ Treatment in adulthood is usually much less effective.][
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Amblyopia begins by the age of five.[ In adults, the disorder is estimated to affect 1–5% of the population.][ Amblyopia was first described in the 1600s.][ The word amblyopia is from Ancient Greek ἀμβλύς amblys, meaning "blunt", and ὤψ ōps, meaning "eye".]
Signs and symptoms
Many people with amblyopia, especially those who only have a mild form, are not aware they have the condition until tested at older ages, since the vision in their stronger eye is normal. People with amblyopia typically have poor stereopsis, since it requires both eyes. They further may have, on the affected eye, poor pattern recognition, poor visual acuity, and low sensitivity to contrast and motion.
Amblyopia is characterized by several functional abnormalities in spatial vision, including reductions in visual acuity, contrast sensitivity function, and vernier acuity, as well as spatial distortion, abnormal spatial interactions, and impaired contour detection. In addition, individuals with amblyopia have binocular abnormalities such as impaired stereoacuity (stereoscopic acuity) and abnormal binocular summation.
These deficits are usually specific to the amblyopic eye. Subclinical deficits of the "better" eye have also been demonstrated.
People with amblyopia also have problems of binocular vision such as limited stereoscopic depth perception and usually have difficulty seeing the three-dimensional images in hidden stereoscopic displays such as .
Cause
Amblyopia, often referred to as "lazy eye," can be categorized into three types:
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Deprivation Amblyopia: This occurs when something obstructs light from entering the eye, preventing proper vision development. A congenital cataract is an example of such a condition.
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Strabismic Amblyopia: The most common form, this type arises when the eyes are not properly aligned, leading to double vision. To avoid confusion, the brain compensates by ignoring the visual input from the misaligned eye.
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Refractive Amblyopia: This type is caused by a significant difference in uncorrected refractive errors, such as nearsightedness or farsightedness, between the two eyes.
Strabismus
Strabismus, sometimes also incorrectly called lazy eye, is a condition in which the eyes are misaligned.
Those with strabismic amblyopia tend to show ocular motion deficits when reading, even when they use the nonamblyopic eye. In particular, they tend to make more per line than persons with normal stereo vision, and to have a reduced reading speed, especially when reading a text with small font size.
Strabismic amblyopia is treated by clarifying the visual image with glasses, or encouraging use of the amblyopic eye with an eyepatch over the dominant eye or pharmacologic penalization of the better eye. Penalization usually consists of applying atropine drops to temporarily paralyze the accommodation reflex, leading to the blurring of vision in the good eye. It also dilates the pupil. This helps to prevent the bullying and teasing associated with wearing a patch, although sometimes application of the eye drops is challenging. The ocular alignment itself may be treated with surgical or nonsurgical methods, depending on the type and severity of the strabismus.
Refractive
Refractive amblyopia may result from anisometropia (unequal refractive error between the two eyes).
Frequently, amblyopia is associated with a combination of anisometropia and strabismus. In some cases, the vision between the eyes can differ to the point where one eye has twice average vision while the other eye is completely blind.
Deprivation and occlusion
Deprivation amblyopia (amblyopia ex anopsia) results when the ocular media become opaque, such as is the case with congenital cataract or haziness.
Pathophysiology
Amblyopia is a developmental problem in the brain, not any intrinsic, organic neurological problem in the eyeball (although organic problems can lead to amblyopia which can continue to exist after the organic problem has resolved by medical intervention).[
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The part of the brain receiving images from the affected eye is not stimulated properly and does not develop to its full visual potential. This has been confirmed by direct brain examination. David H. Hubel and Torsten Wiesel won the Nobel Prize in Physiology or Medicine in 1981 for their work in showing the extent of the damage to ocular dominance columns produced in kittens by sufficient visual deprivation during the so-called "critical period". The maximum "critical period" in humans is from birth to two years old.
Diagnosis
Amblyopia is diagnosed by identifying low visual acuity in one or both eyes, out of proportion to the structural abnormality of the eye and excluding other visual disorders as causes for the lowered visual acuity. It can be defined as an interocular difference of two lines or more in acuity (e.g. on Snellen chart) when the eye optics are maximally corrected.
Stereotests like the Lang stereotest are not reliable exclusion tests for amblyopia. A person who passes the Lang stereotest test is unlikely to have strabismic amblyopia, but could nonetheless have refractive or deprivational amblyopia.
Treatment
Treatment of strabismic or anisometropic amblyopia consists of correcting the optical deficit (wearing the necessary spectacle prescription) and often forcing use of the amblyopic eye, by patching the good eye, or instilling topical atropine in the good eye, or both.[Coats DK and Paysse EA. Overview of amblyopia UpToDate. Last updated: 25 Sep 2014] Atropine appears to result in similar outcomes to patching.[ Amblyopia NEI Health Information ] Eye patching is usually done on a part-time schedule of about 4–6 hours a day. Treatment is continued as long as vision improves. It is not worthwhile continuing to patch for more than 6 months if no improvement continues.
Deprivation amblyopia is treated by patching the good eye to encourage the use of the amblyopic eye.
One of the German public health insurance providers, Barmer, has changed its policy to cover, as of 1 April 2014, the cost of software for amblyopic children whose condition did not improve through patching. The app offers dedicated eye exercises that the patient performs while wearing an eyepatch.
Treatment for amblyopia depends on the child's age, severity of the vision loss and the prescribing doctor's preference. Treatment options include, vision therapy, total or partial occlusion therapy, prescription eyeglasses or any combination. Common types of occlusion therapy include the use of eye-patches or optical blurring. Optical blurring can include the use of pharmacological agents (eye drops) or visual degradation using either power to blur the image or foils placed over the lens.
Evidence for vision therapy is unclear as of 2011.
Older age
Treatment of individuals age 9 through to adulthood is possible through applied perceptual learning.[
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Volume 8, Issue 4, Page 260: Neural Vision Perceptual Learning as an Effective Treatment of Amblyopia
Epidemiology
Amblyopia occurs in 2–5% of the population in Western countries.
Depending on the chosen criterion for diagnosis, 1–4% of the children have amblyopia.
Research
A 2009 study,
A 2013 study concluded that converging evidence indicates decorrelated binocular experience plays a pivotal role in the genesis of amblyopia and the associated residual deficits.
A 2014 Cochrane review sought to determine the effectiveness of occlusion treatment on patients with sensory deprivation amblyopia, but no trials were found eligible to be included in the review.
See also
Further reading
- * Chapter What is Amblyopia? pp. 123–145, ,
- * Chapter Treatment of Amblyopia pp. 167–180, .
External links
