Amblyopia, also calledlazy 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]
Amblyopia can be difficult to detect, sovision testing is recommended for all children around the ages of four to five[2] as early detection improves treatment success.[2]Glasses may be all the treatment needed for some children.[2][4] If this is not sufficient, treatments which encourage or force the child to use the weaker eye are used.[1] This is done by either using apatch or puttingatropine in the stronger eye.[1][8] Without treatment, amblyopia typically persists.[1] Treatment in adulthood is usually much less effective.[1]
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 wordamblyopia is fromGreek ἀμβλύςamblys, meaning "blunt", and ὤψōps, meaning "eye".[10]
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 poorstereo vision, since it requires both eyes. They further may have, on the affected eye, poor pattern recognition, poorvisual acuity, and low sensitivity tocontrast andmotion.[11]
Amblyopia is characterized by several functional abnormalities in spatial vision, including reductions invisual acuity,contrast sensitivity function, andvernier 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 abnormalbinocular summation.[12] Also, central vision in amblyopes is morecrowded than central vision in normal observers.[13]
These deficits are usually specific to the amblyopic eye. Subclinical deficits of the "better" eye have also been demonstrated.[14]
Amblyopia, often referred to as "lazy eye," can be categorized into three types:[citation needed]
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.
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.
Refractive Amblyopia: This type is caused by a significant difference in uncorrected refractive errors, such as nearsightedness or farsightedness, between the two eyes.
Strabismus, sometimes also incorrectly called lazy eye, is a condition in which the eyes are misaligned.[16] 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.[17]Adult-onset strabismus usually causes double vision (diplopia), since the two eyes are not fixed on the same object.Children's brains are moreneuroplastic, so can more easily adapt bysuppressing 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 thevisual cortex.[18]
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 moresaccades per line than persons with normal stereo vision, and to have a reducedreading speed, especially when reading a text with smallfont size.[19][20][non-primary source needed]
Strabismic amblyopia is treated by clarifying the visual image with glasses, or encouraging use of the amblyopic eye with aneyepatch over the dominant eye orpharmacologic penalization of the better eye. Penalization usually consists of applyingatropine drops to temporarily paralyze the accommodation reflex, leading to the blurring of vision in the good eye. It also dilates thepupil. 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.[21]
Refractive amblyopia may result fromanisometropia (unequal refractive error between the two eyes).[22][23] Anisometropia exists when there is a difference in thepower 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 byprimary care physicians because of its less dramatic appearance and lack of obvious physical manifestation, such as with strabismus.[24] 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 usingcontact lenses is often considered.Pediatric refractive surgery is also a treatment option, in particular if conventional approaches have failed due toaniseikonia or lack of compliance or both.[25]
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.[citation needed]
Deprivation amblyopia (amblyopia ex anopsia) results when the ocular media becomeopaque, such as is the case withcongenital cataract orcorneal haziness.[26] 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 theeyelid (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 ahemangioma that blocks some or all of the eye. Other possible causes of deprivation and occlusion amblyopia include obstruction in the vitreous andaphakia.[27] Deprivation amblyopia accounts for less than 3% of all individuals affected by amblyopia.[27]
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).[28]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 andTorsten Wiesel won theNobel Prize in Physiology or Medicine in 1981 for their work in showing the extent of the damage toocular 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.[29]
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. onSnellen chart) when the eye optics are maximally corrected.[30] 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.[citation needed]
Stereotests like theLang 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.[31] Binocularretinal 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.[32]
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 instillingtopicalatropine in the good eye, or both.[16]: 130 [33] Atropine appears to result in similar outcomes to patching.[34][35] If there is overpatching or overpenalizing the good eye when treating amblyopia, "reverse amblyopia" can result.[21][36] 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.[37]
Deprivation amblyopia is treated by patching the good eye to encourage the use of the amblyopic eye.[21] The earlier 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.[38]
One of theGerman 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.[39]
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.[40]
Treatment of individuals age 9 through to adulthood is possible through appliedperceptual learning.[12][42][43][44][45] Tentative evidence shows that perceptual training may be beneficial in adults.[46][47]
Amblyopia occurs in 2–5% of the population in Western countries.[which?][30][48] In the UK, 90% of visual health appointments in the child concern amblyopia.[49]
Depending on the chosen criterion for diagnosis, 1–4% of the children have amblyopia.[50]
A 2009 study,[51] widely reported in the popular press,[52] suggested that repetitivetranscranial magnetic stimulation may temporarily improve contrast sensitivity and spatial resolution in the affected eye of adults with amblyopia. This approach is still under development,[53] and the results await verification by other researchers. Comparable results may be achieved using different types of brain stimulation,[54] such as anodaltranscranial direct current stimulation[55] and theta burstrTMS.[56]
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.[57] Another study of 2013[58] suggests that playing a version of the popular gameTetris that is modified such that each eye sees separate components of the game may also help to treat this condition in adults.[59] Furthermore, the effects of this kind of therapy may be further enhanced by noninvasive brain stimulation[54] as shown by a recent study using anodaltDCS.[60]
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.[27] However, good outcomes from occlusion treatment for sensory deprivation amblyopia likely rely on compliance with the treatment.[citation needed]
^Ferri FF (2010).Ferri's differential diagnosis: a practical guide to the differential diagnosis of symptoms, signs, and clinical disorders (2nd ed.). Philadelphia, PA: Elsevier/Mosby. p. Chapter A.ISBN978-0-323-07699-9.
^"Amblyopia (Lazy Eye)".National Eye Institute. 2 July 2019. Retrieved31 January 2020.Putting special eye drops in the stronger eye. A once-a-day drop of the drug atropine can temporarily blur near vision, which forces the brain to use the other eye. For some kids, this treatment works as well as an eye patch, and some parents find it easier to use (for example, because young children may try to pull off eye patches).
^Bianchi PE, Ricciardelli G, Bianchi A, Arbanini A, Fazzi E (2016)."Chapter 2: Visual Development in Childhood". In Fazzi E, Bianchi PE (eds.).Visual Impairments and Developmental Disorders: From diagnosis to rehabilitation Mariani Foundation Paediatric Neurology. John Libbey Eurotext. p. 26.ISBN978-2-7420-1482-8.Archived from the original on 8 September 2017.
^Tychsen L (August 2012). "The cause of infantile strabismus lies upstairs in the cerebral cortex, not downstairs in the brainstem".Archives of Ophthalmology.130 (8):1060–1.doi:10.1001/archophthalmol.2012.1481.PMID22893080.
^Kanonidou E, Proudlock FA, Gottlob I (July 2010). "Reading strategies in mild to moderate strabismic amblyopia: an eye movement investigation".Investigative Ophthalmology & Visual Science.51 (7):3502–8.doi:10.1167/iovs.09-4236.PMID20207968.
^Rutstein RF, Corliss D (April 1999). "Relationship between Anisometropia, Amblyopia, and Binocularity".Optometry & Vision Science.76 (4):229–33.doi:10.1097/00006324-199904000-00026.PMID10333185.
^Weakley DR (January 2001). "The association between nonstrabismic anisometropia, amblyopia, and subnormal binocularity".Ophthalmology.108 (1):163–71.doi:10.1016/s0161-6420(00)00425-5.PMID11150283.
^Astle WF, Rahmat J, Ingram AD, Huang PT (December 2007). "Laser-assisted subepithelial keratectomy for anisometropic amblyopia in children: outcomes at 1 year".Journal of Cataract and Refractive Surgery.33 (12):2028–34.doi:10.1016/j.jcrs.2007.07.024.PMID18053899.S2CID1886316.
^Angell LK, Robb RM, Berson FG (December 1981). "Visual prognosis in patients with ruptures in Descemet's membrane due to forceps injuries".Archives of Ophthalmology.99 (12):2137–9.doi:10.1001/archopht.1981.03930021013004.PMID7305711.
^Grossman DC, Curry SJ, Owens DK, Barry MJ, Davidson KW, Doubeni CA, et al. (September 2017). "Vision Screening in Children Aged 6 Months to 5 Years: US Preventive Services Task Force Recommendation Statement".JAMA.318 (9):836–844.doi:10.1001/jama.2017.11260.PMID28873168.S2CID205084845.
^Coats DK and Paysse EA. Overview of amblyopia UpToDate. Last updated: 25 Sep 2014
^Hess RF, Thompson B (February 2013). "New insights into amblyopia: binocular therapy and noninvasive brain stimulation".Journal of AAPOS.17 (1):89–93.doi:10.1016/j.jaapos.2012.10.018.PMID23352385.
^Spiegel DP, Byblow WD, Hess RF, Thompson B (October 2013). "Anodal transcranial direct current stimulation transiently improves contrast sensitivity and normalizes visual cortex activation in individuals with amblyopia".Neurorehabilitation and Neural Repair.27 (8):760–9.doi:10.1177/1545968313491006.PMID23774122.S2CID38437179.
^Clavagnier S, Thompson B, Hess RF (November 2013). "Long lasting effects of daily theta burst rTMS sessions in the human amblyopic cortex".Brain Stimulation.6 (6):860–7.doi:10.1016/j.brs.2013.04.002.PMID23664756.S2CID36592293.
