Amblyopia

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.^[1] It results in decreased vision in an eye that typically appears normal in other aspects.^[1] Amblyopia is the most common cause of decreased vision in a single eye among children and younger adults.^[1]

The cause of amblyopia can be any condition that interferes with focusing during early childhood.^[1][6] This can occur from poor alignment of the eyes (strabismic), an eye being irregularly shaped such that focusing is difficult, one eye being more nearsighted or farsighted than the other (refractive), or clouding of the lens of an eye (deprivational).^[1] After the underlying cause is addressed, vision is not restored right away, as the mechanism also involves the brain.^[6][7] Amblyopia can be difficult to detect, so vision testing is recommended for all children around the ages of four to five.^[2]

Early detection improves treatment success.

Amblyopia begins by the age of five.[2] In adults, the disorder is estimated to affect 1–5% of the population.^[5] While treatment improves vision, it does not typically restore it to normal in the affected eye.^[2] Amblyopia was first described in the 1600s.^[9] The condition may make people ineligible to be pilots or police officers.^[2] The word amblyopia is from Greek ἀμβλύς amblys, meaning "blunt", and ὤψ ōps, meaning "sight".^[10]

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 stereo vision, 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.^[11]

Amblyopia is characterized by several functional abnormalities in spatial vision, including reductions in

These deficits are usually specific to the amblyopic eye. Subclinical deficits of the "better" eye have also been demonstrated.[14]

People with amblyopia also have problems of

• Strabismic: by strabismus (misaligned eyes)
• Refractive: by
  farsightedness, or astigmatism
  ), or by significant amount of equal refractive error in both eyes
• Deprivational: by deprivation of vision early in life by vision-obstructing disorders such as congenital cataract^[16]

Strabismus

Strabismus, sometimes also incorrectly called lazy eye, is a condition in which the eyes are misaligned.^[17] Strabismus usually results in normal vision in the preferred sighting (or "fellow") eye (the eye that the person prefers to use), but may cause abnormal vision in the deviating or strabismic eye due to the difference between the images projecting to the brain from the two eyes.^[18] Adult-onset strabismus usually causes double vision (diplopia), since the two eyes are not fixed on the same object. Children's brains are more neuroplastic, so can more easily adapt by suppressing images from one of the eyes, eliminating the double vision. This plastic response of the brain interrupts the brain's normal development, resulting in the amblyopia.^{[citation needed]} Recent evidence points to a cause of infantile strabismus lying with the input to the visual cortex.^[19]

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

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.^[22]

Refractive

Refractive amblyopia may result from anisometropia (unequal refractive error between the two eyes).^[23][24] Anisometropia exists when there is a difference in the power between the two eyes. The eye which provides the brain with a clearer image typically becomes the dominant eye. The image in the other eye is blurred, which results in abnormal development of one half of the visual system. Refractive amblyopia is usually less severe than strabismic amblyopia and is commonly missed by primary care physicians because of its less dramatic appearance and lack of obvious physical manifestation, such as with strabismus.^[25] Given that the refractive correction of anisometropia by means of spectacles typically leads to different image magnification for the two eyes, which may in turn prevent binocular vision, a refractive correction using contact lenses is to be considered. Also pediatric refractive surgery is a treatment option, in particular if conventional approaches have failed due to aniseikonia or lack of compliance or both.^[26]

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 corneal haziness.^[27] These opacities prevent adequate visual input from reaching the eye, and disrupt 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. Occlusion amblyopia may be a complication of a hemangioma that blocks some or all of the eye. Other possible causes of deprivation and occlusion amblyopia include obstruction in the vitreous and aphakia.^[28] Deprivation amblyopia accounts for less than 3% of all individuals affected by amblyopia.^[28]

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).^[29] 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.

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.^[31] In young children, visual acuity is difficult to measure and can be estimated by observing the reactions of the patient when one eye is covered, including observing the patient's ability to follow objects with one eye.

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.^[32] Binocular retinal birefringence scanning may be able to identify, already in very young children, amblyopia that is associated with strabismus, microstrabismus, or reduced fixation accuracy. Diagnosis and treatment of amblyopia as early as possible is necessary to keep the vision loss to a minimum. Screening for amblyopia is recommended in all people between three and five years of age.^[33]

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

Deprivation amblyopia is treated by removing the opacity as soon as possible followed by patching or penalizing the good eye to encourage the use of the amblyopic eye.[22] The earlier the treatment is initiated, the easier and faster the treatment is and the less psychologically damaging. Also, the chance of achieving 20/20 vision is greater if treatment is initiated early.^[39]

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.^[40]

Evidence for vision therapy is unclear as of 2011.^[41]

Older age

Treatment of individuals age 9 through to adulthood is possible through applied perceptual learning.^[12][42] Tentative evidence shows that perceptual training may be beneficial in adults.^[43][44]

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.[47]

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.[54] Another study of 2013^[55] suggests that playing a version of the popular game Tetris that is modified such that each eye sees separate components of the game may also help to treat this condition in adults.^[56] Furthermore, the effects of this kind of therapy may be further enhanced by noninvasive brain stimulation^[51] as shown by a recent study using anodal tDCS.^[57]

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.^[28] However, good outcomes from occlusion treatment for sensory deprivation amblyopia likely rely on compliance with the treatment.

See also

• Functional visual loss

References

Further reading

• Birch EE (March 2013). "Amblyopia and binocular vision". Progress in Retinal and Eye Research (review). 33: 67–84.
  PMID 23201436
  .
• Daw, Nigel W. (2014). Visual Development (Third ed.). Springer.
  ISBN 978-1461490586
  .

• Chapter What is Amblyopia? pp. 123–145,
  doi:10.1007/978-1-4614-9059-3_8
  ,
• Chapter Treatment of Amblyopia pp. 167–180,
  doi:10.1007/978-1-4614-9059-3_10
  .

• Stewart CE, Moseley MJ, Fielder AR (September 2011). "Amblyopia therapy: an update". Strabismus. 19 (3): 91–8.
  S2CID 38988992
  .
• Sengpiel F (September 2014). "Plasticity of the visual cortex and treatment of amblyopia". Current Biology (review). 24 (18): R936–R940.
  PMID 25247373
  .
• Hamm LM, Black J, Dai S, Thompson B (2014). "Global processing in amblyopia: a review". Frontiers in Psychology (review). 5: 583.
  PMID 24987383
  .

External links

Wikimedia Commons has media related to Amblyopia.

• National Eye Institute (NEI) Resource Guide
• Lazy Eye Site from the
  UK
• Look After Your Eyes - patient information on Amblyopia or 'lazy eye' by College of Optometrists