View source for Medicine:Strabismus

{{short description|Eyes not aligning when looking at something}}
{{Infobox medical condition (new)
| name            = Strabismus
| synonyms        = Heterotropia, crossed eyes, squint<ref name=Ox2017/>
| image           = StrabismusVL2.jpg
| caption         = A person with exotropia, an outward deviated eye
| field           = [[Medicine:Ophthalmology|Ophthalmology]], [[Medicine:Optometry|optometry]]
| pronounce       = {{IPAc-en|s|t|r|ə|ˈ|b|ɪ|z|m|ə|s}}<ref name=Ox2017>{{cite web|title=Strabismus noun - Definition, pictures, pronunciation and usage notes {{!}} Oxford Advanced Learner's Dictionary|url=https://www.oxfordlearnersdictionaries.com/definition/english/strabismus|website=www.oxfordlearnersdictionaries.com|access-date=1 August 2017|language=en|url-status=live|archive-url=https://web.archive.org/web/20170801035735/https://www.oxfordlearnersdictionaries.com/definition/english/strabismus|archive-date=August 1, 2017|df=mdy-all}}</ref>
| symptoms        = Nonaligned eyes<ref name=NEI2016Def/>
| complications   = [[Medicine:Amblyopia|Amblyopia]], [[Medicine:Diplopia|double vision]]<ref name=Gun2015/>
| onset           =
| duration        =
| types           = Esotropia (eyes crossed); exotropia (eyes diverge); hypertropia (eyes vertically misaligned)<ref name=Gun2015/>
| causes          = Muscle dysfunction, [[Medicine:Farsightedness|farsightedness]], problems in the brain, trauma, infections<ref name=Gun2015/>
| risks           = Premature birth, [[Medicine:Cerebral palsy|cerebral palsy]], family history<ref name=Gun2015/>
| diagnosis       = Observing light reflected from the [[Biology:Pupil|pupil]]<ref name=Gun2015/>
| differential    = Cranial nerve disease,<ref name=Gun2015/> [[Medicine:Convergence insufficiency|convergence insufficiency]]
| prevention      =
| treatment       = [[Engineering:Glasses|Glasses]], surgery<ref name=Gun2015/>
| medication      =
| prognosis       =
| frequency       = ~2% (children)<ref name=Gun2015/>
| deaths          =
|alt=}}
<!-- Definition and symptoms -->
'''Strabismus''' is a [[Medicine:Vision disorder|vision disorder]] in which the eyes do not properly align with each other when looking at an object.<ref name=NEI2016Def>{{cite web|title=Visual Processing: Strabismus|date=16 June 2010|url=https://www.nei.nih.gov/faqs/visual-processing-strabismus|website=National Eye Institute|publisher=National Institutes of Health|access-date=2 October 2016|url-status=live|archive-url=https://web.archive.org/web/20161005125745/https://www.nei.nih.gov/faqs/visual-processing-strabismus|archive-date=October 5, 2016|df=mdy-all}}</ref> The eye that is pointed at an object can alternate.<ref name=Gun2015/> The condition may be present occasionally or constantly.<ref name=Gun2015/> If present during a large part of childhood, it may result in [[Medicine:Amblyopia|amblyopia]], or lazy eyes, and loss of [[Depth perception|depth perception]].<ref name=Gun2015/> If onset is during adulthood, it is more likely to result in [[Medicine:Diplopia|double vision]].<ref name=Gun2015/>

<!-- Cause and diagnosis -->
Strabismus can occur due to muscle dysfunction, [[Medicine:Farsightedness|farsightedness]], problems in the brain, trauma, or infections.<ref name=Gun2015/> Risk factors include premature birth, [[Medicine:Cerebral palsy|cerebral palsy]], and a family history of the condition.<ref name=Gun2015/> Types include esotropia, where the eyes are crossed ("cross eyed"); exotropia, where the eyes diverge ("lazy eyed" or "wall eyed"); and hypertropia or hypotropia where they are vertically misaligned.<ref name=Gun2015/> They can also be classified by whether the problem is present in all directions a person looks (comitant) or varies by direction (incomitant).<ref name=Gun2015/> Diagnosis may be made by observing the light reflecting from the person's eyes and finding that it is not centered on the [[Biology:Pupil|pupil]].<ref name=Gun2015/> This is known as the Hirschberg reflex.  Another condition that produces similar symptoms is a cranial nerve disease.<ref name=Gun2015>{{cite journal | vauthors = Gunton KB, Wasserman BN, DeBenedictis C | title = Strabismus | journal = Primary Care | volume = 42 | issue = 3 | pages = 393–407 | date = September 2015 | pmid = 26319345 | doi = 10.1016/j.pop.2015.05.006 }}</ref>

<!-- Treatment, epidemiology, and culture -->
Treatment depends on the type of strabismus and the underlying cause.<ref name=Gun2015/> This may include the use of [[Engineering:Glasses|glasses]] and possibly surgery.<ref name=Gun2015/> Some types benefit from early surgery.<ref name=Gun2015/> Strabismus occurs in about 2% of children.<ref name=Gun2015/> The term comes from the [[Social:Ancient Greek|Ancient Greek]] word {{wikt-lang|grc|στραβισμός}} (''{{grc-transl|στραβισμός}}''), meaning 'a squinting'.<ref>{{cite web |title=strabismus (n.) |url=http://www.etymonline.com/index.php?term=strabismus |website=Online Etymology Dictionary |first=Douglas R. |last=Harper |access-date=2016-10-02 |url-status=live |archive-url=https://web.archive.org/web/20161212180859/http://www.etymonline.com/index.php?term=strabismus |archive-date=2016-12-12 |df=mdy-all}}</ref> Other terms for the condition include "squint" and "cast of the eye".<ref name=Bro1993>{{cite book |last = Brown |first = Lesley |title = The New shorter Oxford English dictionary on historical principles |publisher = Clarendon |location = Oxford |year = 1993 |pages = Strabismus |isbn = 978-0-19-861271-1 |url-access = registration |url = https://archive.org/details/newshorteroxford00lesl }}</ref><ref>{{cite web |url = http://www.oxforddictionaries.com/definition/english/strabismus |title = strabismus |website = English: Oxford Living Dictionaries |publisher = Oxford University Press |date = 2016 |access-date = 6 April 2016 |url-status = dead |archive-url = https://web.archive.org/web/20160421162831/http://www.oxforddictionaries.com/definition/english/strabismus |archive-date = April 21, 2016 |df = mdy-all }}</ref><ref>{{Cite web|url=http://www.dictionary.com/browse/squint|title=the definition of squint|website=Dictionary.com|access-date=2016-07-20}}</ref>

== Signs and symptoms ==
{{multiple image
| width     = 115
| image1    = Autostereogram normal vergence.svg
| alt1      = Aligned vergence; how one ideally views objects
| caption1  = Aligned&nbsp;[[Vergence|vergence]]
| image2    = Autostereogram cross-eyed vergence.svg
| alt2      = Esotropia
| caption2  = Esotropia
| image3    = Autostereogram wall-eyed vergence.svg
| alt3      = Exotropia
| caption3  = Exotropia
}}

When observing a person with strabismus, the misalignment of the eyes may be quite apparent. A person with a constant eye turn of significant magnitude is very easy to notice. However, a small magnitude or intermittent strabismus can easily be missed upon casual observation. In any case, an [[Medicine:Eye care professional|eye care professional]] can conduct various tests, such as cover testing, to determine the full extent of the strabismus.{{cn|date=July 2022}}

Symptoms of strabismus include [[Medicine:Diplopia|double vision]] and eye strain. To avoid double vision, the brain may adapt by [[Biology:Suppression (eye)|ignoring one eye]]. In this case, often no noticeable symptoms are seen other than a minor loss of depth perception. This deficit may not be noticeable in someone who has had strabismus since birth or early childhood, as they have likely learned to judge depth and distances using monocular cues.{{Citation needed|date=November 2016}} However, a constant unilateral strabismus causing constant suppression is a risk for [[Medicine:Amblyopia|amblyopia]] in children. Small-angle and intermittent strabismus are more likely to cause disruptive visual symptoms. In addition to headaches and eye strain, symptoms may include an inability to read comfortably, fatigue when reading, and unstable or "jittery" vision.

=== Psychosocial effects ===
[[File:Fayum-40.jpg|thumb|upright|Fayum mummy portrait, c. 150–200 AD]]
[[File:Inghirami Raphael.jpg|thumb|upright|Tommaso Inghirami]]
[[File:Norma Shearer 1926.jpg|thumb|upright|Norma Shearer]]
[[File:Sartre_1967_crop.jpg|thumb|upright|[[Biography:Jean-Paul Sartre|Jean-Paul Sartre]]]]
[[File:Marty Feldman 1972.JPG|thumb|upright|Marty Feldman]]
People of all ages who have noticeable strabismus may experience psychosocial difficulties.<ref name="satterfield-et-al-1993">{{cite journal | vauthors = Satterfield D, Keltner JL, Morrison TL | title = Psychosocial aspects of strabismus study | journal = Archives of Ophthalmology | volume = 111 | issue = 8 | pages = 1100–5 | date = August 1993 | pmid = 8166786 | doi = 10.1001/archopht.1993.01090080096024 }}</ref><ref name="olitsky-et-al-1999">{{cite journal | vauthors = Olitsky SE, Sudesh S, Graziano A, Hamblen J, Brooks SE, Shaha SH | title = The negative psychosocial impact of strabismus in adults | journal = Journal of AAPOS | volume = 3 | issue = 4 | pages = 209–11 | date = August 1999 | pmid = 10477222 | doi = 10.1016/S1091-8531(99)70004-2 }}</ref><ref name="uretmen-et-al-2003">{{cite journal | vauthors = Uretmen O, Egrilmez S, Kose S, Pamukçu K, Akkin C, Palamar M | title = Negative social bias against children with strabismus | journal = Acta Ophthalmologica Scandinavica | volume = 81 | issue = 2 | pages = 138–42 | date = April 2003 | pmid = 12752051 | doi = 10.1034/j.1600-0420.2003.00024.x | doi-access = free }}</ref> Attention has also been drawn to potential socioeconomic impact resulting from cases of detectable strabismus. A socioeconomic consideration exists as well in the context of decisions regarding strabismus treatment,<ref name="satterfield-et-al-1993" /><ref name="olitsky-et-al-1999" /><ref name="uretmen-et-al-2003" /> including efforts to re-establish binocular vision and the possibility of [[Biology:Stereopsis recovery|stereopsis recovery]].<ref>See peer discussion in: {{cite journal | vauthors = Mets MB, Beauchamp C, Haldi BA | title = Binocularity following surgical correction of strabismus in adults | journal = Transactions of the American Ophthalmological Society | volume = 101 | pages = 201–5; discussion 205–7 | year = 2003 | issue = 5 | pmid = 14971578 | pmc = 1358989 | doi = 10.1016/j.jaapos.2004.07.003 }}</ref>

One study has shown that strabismic children commonly exhibit behaviors marked by higher degrees of inhibition, anxiety, and emotional distress, often leading to outright emotional disorders. These disorders are often related to a negative perception of the child by peers. This is due not only to an altered aesthetic appearance but also because of the inherent symbolic nature of the [[Biology:Eye|eye]] and [[Philosophy:Gaze|gaze]] and the vitally important role they play in an individual's life as social components. For some, these issues improved dramatically following strabismus surgery.<ref>{{cite journal | vauthors = Bernfeld A | title = [Psychological repercussions of strabismus in children] | language = fr | journal = Journal Français d'Ophtalmologie | volume = 5 | issue = 8–9 | pages = 523–30 | year = 1982 | pmid = 7142664 | trans-title = Psychological repercussions of strabismus in children }}</ref> Notably, strabismus interferes with normal [[Biology:Eye contact|eye contact]], often causing embarrassment, [[Social:Anger|anger]], and feelings of awkwardness, thereby affecting social communication in a fundamental way, with a possible negative effect on self esteem.<ref>{{cite web |url=http://www.allaboutvision.com/conditions/strabismus.htm |title=Strabismus |website=All About Vision |publisher=Access Media Group |url-status=live |archive-url=https://web.archive.org/web/20140916142449/http://www.allaboutvision.com/conditions/strabismus.htm |archive-date=September 16, 2014 |df=mdy-all }}</ref><ref>Estes, K. J., Parrish, R. K., Sinacore, J., Mumby, P. B., & McDonnell, J. F. (2020). Effects of corrective strabismus surgery on social anxiety and self-consciousness in adults. Journal of AAPOS : the official publication of the American Association for Pediatric Ophthalmology and Strabismus, 24(5), 280.e1–280.e4. https://doi.org/10.1016/j.jaapos.2020.05.017</ref>

Children with strabismus, particularly those with exotropia, an outward turn, may be more likely to develop a mental health disorder than normal-sighted children. Researchers have theorized that esotropia (an inward turn) was not found to be linked to a higher propensity for mental illness due to the age range of the participants, as well as the shorter follow-up time period; esotropic children were monitored to a mean age of 15.8&nbsp;years, compared with 20.3&nbsp;years for the exotropic group.<ref>{{cite journal | vauthors = Tonge BJ, Lipton GL, Crawford G | title = Psychological and educational correlates of strabismus in school children | journal = The Australian and New Zealand Journal of Psychiatry | volume = 18 | issue = 1 | pages = 71–7 | date = March 1984 | pmid = 6590030 | doi = 10.3109/00048678409161038 | s2cid = 42734067 }}</ref><ref>{{cite journal | vauthors = Mohney BG, McKenzie JA, Capo JA, Nusz KJ, Mrazek D, Diehl NN | title = Mental illness in young adults who had strabismus as children | journal = Pediatrics | volume = 122 | issue = 5 | pages = 1033–8 | date = November 2008 | pmid = 18977984 | pmc = 2762944 | doi = 10.1542/peds.2007-3484 }}</ref>

Investigations have highlighted the impact that strabismus may typically have on quality of life.<ref>{{cite journal | vauthors = Beauchamp GR, Felius J, Stager DR, Beauchamp CL | title = The utility of strabismus in adults | journal = Transactions of the American Ophthalmological Society | volume = 103 | pages = 164–71; discussion 171–2 | date = December 2005 | pmid = 17057800 | pmc = 1447571 }}</ref> Studies in which subjects were shown images of strabismic and non-strabismic persons showed a strong negative bias towards those visibly displaying the condition, clearly demonstrating the potential for future socioeconomic implications with regard to employability, as well as other psychosocial effects related to an individual's overall happiness.<ref>{{cite journal | vauthors = Mojon-Azzi SM, Mojon DS | title = Strabismus and employment: the opinion of headhunters | journal = Acta Ophthalmologica | volume = 87 | issue = 7 | pages = 784–8 | date = November 2009 | pmid = 18976309 | doi = 10.1111/j.1755-3768.2008.01352.x | doi-access = free }}</ref><ref>{{cite journal | vauthors = Mojon-Azzi SM, Mojon DS | title = Opinion of headhunters about the ability of strabismic subjects to obtain employment | journal = Ophthalmologica. Journal International d'Ophtalmologie. International Journal of Ophthalmology. Zeitschrift für Augenheilkunde | volume = 221 | issue = 6 | pages = 430–3 | date = October 2007 | pmid = 17947833 | doi = 10.1159/000107506 | s2cid = 29398388 | url = https://boris.unibe.ch/28257/ }}</ref>

Adult and child observers perceived a right heterotropia as more disturbing than a left heterotropia, and child observers perceived an esotropia as "worse" than an exotropia.<ref>{{cite journal | vauthors = Mojon-Azzi SM, Kunz A, Mojon DS | title = The perception of strabismus by children and adults | journal = Graefe's Archive for Clinical and Experimental Ophthalmology = Albrecht von Graefes Archiv für Klinische und Experimentelle Ophthalmologie | volume = 249 | issue = 5 | pages = 753–7 | date = May 2011 | pmid = 21063886 | doi = 10.1007/s00417-010-1555-y | s2cid = 10989351 | url = http://doc.rero.ch/record/312008/files/417_2010_Article_1555.pdf }}</ref> Successful surgical correction of strabismus, for adult as well as children, has been shown to have a significantly positive effect on psychological well-being.<ref>{{cite journal | vauthors = Burke JP, Leach CM, Davis H | title = Psychosocial implications of strabismus surgery in adults | journal = Journal of Pediatric Ophthalmology and Strabismus | volume = 34 | issue = 3 | pages = 159–64 | date = May 1997 | doi = 10.3928/0191-3913-19970501-06 | pmid = 9168420 }}</ref><ref>{{cite journal | vauthors = Durnian JM, Noonan CP, Marsh IB | title = The psychosocial effects of adult strabismus: a review | journal = The British Journal of Ophthalmology | volume = 95 | issue = 4 | pages = 450–3 | date = April 2011 | pmid = 20852320 | doi = 10.1136/bjo.2010.188425 | s2cid = 206870079 | url = https://hal.archives-ouvertes.fr/hal-00577963/file/PEER_stage2_10.1136%252Fbjo.2010.188425.pdf }}</ref>

Very little research exists regarding [[Social:Coping (psychology)|coping]] strategies employed by adult strabismics. One study categorized coping methods into three subcategories: avoidance (refraining from participation in an activity), distraction (deflecting attention from the condition), and adjustment (approaching an activity differently). The authors of the study suggested that individuals with strabismus may benefit from psychosocial support such as interpersonal skills training.<ref>{{cite journal | vauthors = Jackson S, Gleeson K |title=Living and coping with strabismus as an adult |journal=European Medical Journal Ophthalmology |date=August 2013 |volume=1 |pages=15–22 |url=http://emjreviews.com/therapeutic-area/ophthalmology/living-coping-strabismus-adult/ |url-status=live |archive-url=https://web.archive.org/web/20170621025330/http://emjreviews.com/therapeutic-area/ophthalmology/living-coping-strabismus-adult/ |archive-date=June 21, 2017 |df=mdy-all }}</ref> No studies have evaluated whether psychosocial interventions have had any benefits on individuals undergoing strabismus surgery.<ref name="MacKenzie">{{cite journal | vauthors = MacKenzie K, Hancox J, McBain H, Ezra DG, Adams G, Newman S | title = Psychosocial interventions for improving quality of life outcomes in adults undergoing strabismus surgery | journal = The Cochrane Database of Systematic Reviews | volume = 2016 | issue = 5 | pages = CD010092 | date = May 2016 | pmid = 27171652 | doi = 10.1002/14651858.CD010092.pub4 | pmc = 9250695 | url = http://openaccess.city.ac.uk/19311/1/MacKenzie_et_al-2016-.pdf }}</ref>

== Pathophysiology ==
The [[Medicine:Extraocular muscles|extraocular muscles]] control the position of the eyes.  Thus, a problem with the muscles or the nerves controlling them can cause paralytic strabismus.  The extraocular muscles are controlled by cranial nerves [[Biology:Oculomotor nerve|III]], [[Biology:Trochlear nerve|IV]], and [[Biology:Abducens nerve|VI]]. An impairment of cranial nerve III causes the associated eye to deviate down and out and may or may not affect the size of the pupil.  Impairment of cranial nerve IV, which can be congenital, causes the associated eye to drift up and perhaps slightly inward. Sixth nerve palsy causes the associated eye to deviate inward and has many causes due to the relatively long path of the nerve.  [[Medicine:Intracranial pressure|Increased cranial pressure]] can compress the nerve as it runs between the clivus and brain stem.<ref name=vaughan />{{Page needed|date=May 2017}}

Evidence indicates a cause for strabismus may lie with the input provided to the [[Biology:Visual cortex|visual cortex]].<ref>{{cite journal | vauthors = Tychsen L | title = The cause of infantile strabismus lies upstairs in the cerebral cortex, not downstairs in the brainstem | journal = Archives of Ophthalmology | volume = 130 | issue = 8 | pages = 1060–1 | date = August 2012 | pmid = 22893080 | doi = 10.1001/archophthalmol.2012.1481 }}</ref>

Amblyopia may also cause strabismus.  If a great difference in clarity occurs between the images from the right and left eyes, input may be insufficient to correctly reposition the eyes.  Other causes of a visual difference between right and left eyes, such as asymmetrical cataracts, refractive error, or other eye disease, can also cause or worsen strabismus.<ref name=vaughan>{{cite book |first1=Emmett T. |last1=Cunningham |first2=Paul |last2=Riordan-Eva | name-list-style = vanc |title = Vaughan & Asbury's general ophthalmology. |publisher = McGraw-Hill Medical |isbn = 978-0-07-163420-5 |edition = 18th |date=2011-05-17 }}</ref>{{Page needed|date=May 2017}}
[[File:Esotropia.jpg|alt=A child with accommodative esotropia affecting the right eye. The left eye points forwards while the right eye points inward towards the nose.|thumb|216x216px|A child with accommodative esotropia affecting the right eye]]

== Diagnosis ==
During an [[Medicine:Eye examination|eye examination]], a test such as [[Medicine:Cover test|cover test]]ing or the [[Medicine:Hirschberg test|Hirschberg test]] is used in the diagnosis and measurement of strabismus and its impact on vision. [[Biology:Retinal birefringence scanning|Retinal birefringence scanning]] can be used for [[Medicine:Screening|screening]] of young children for eye misalignment. A Cochrane review to examine different types of diagnosis test found only one study. This study used a photoscreener which was found to have high specificity (accurate in identifying those without the condition) but low sensitivity (inaccurate in identifying those with the condition).<ref>{{Cite journal|last1=Hull|first1=Sarah|last2=Tailor|first2=Vijay|last3=Balduzzi|first3=Sara|last4=Rahi|first4=Jugnoo|last5=Schmucker|first5=Christine|last6=Virgili|first6=Gianni|last7=Dahlmann-Noor|first7=Annegret|date=2017-11-06|title=Tests for detecting strabismus in children aged 1 to 6 years in the community|journal=Cochrane Database of Systematic Reviews|volume=2017|issue=11 |pages=CD011221|doi=10.1002/14651858.cd011221.pub2|issn=1465-1858|pmc=6486041|pmid=29105728}}</ref>

Several classifications are made when diagnosing strabismus.

=== Latency ===
Strabismus can be manifest (''-tropia'') or latent (''-phoria'').  A manifest deviation, or heterotropia (which may be ''eso-'', ''exo-'', ''hyper-'', ''hypo-'', ''cyclotropia'' or a combination of these), is present while the person views a target binocularly, with no occlusion of either eye.  The person is unable to align the gaze of each eye to achieve fusion.  A latent deviation, or heterophoria (''eso-'', ''exo-'', ''hyper-'', ''hypo-'', ''cyclophoria'' or a combination of these), is only present after binocular vision has been interrupted, typically by covering one eye.  This type of person can typically maintain fusion despite the misalignment that occurs when the positioning system is relaxed.  Intermittent strabismus is a combination of both of these types, where the person can achieve fusion, but occasionally or frequently falters to the point of a manifest deviation.

=== Onset ===
Strabismus may also be classified based on time of onset, either congenital, acquired, or secondary to another pathological process. Many infants are born with their eyes slightly misaligned, and this is typically outgrown by six to 12 months of age.<ref name="nield">{{Cite journal |last1 = Nield |first1 = Linda S. |last2 = Mangano |first2 = Linn M. | name-list-style = vanc |title = Strabismus: What to Tell Parents and When to Consider Surgery |journal = Consultant |volume = 49 |issue = 4 |date = April 2009 |url = http://www.consultantlive.com/display/article/10162/1399809 |url-status = live |archive-url = https://web.archive.org/web/20090414021159/http://www.consultantlive.com/display/article/10162/1399809 |archive-date = April 14, 2009 |df = mdy-all }}</ref>  Acquired and secondary strabismus develop later. The onset of accommodative esotropia, an overconvergence of the eyes due to the effort of [[Biology:Accommodation (eye)|accommodation]], is mostly in early childhood. Acquired non-accommodative strabismus and secondary strabismus are developed after normal binocular vision has developed. In adults with previously normal alignment, the onset of strabismus usually results in [[Medicine:Diplopia|double vision]].

Any disease that causes vision loss may also cause strabismus,<ref name=medlineplus>{{cite web |title=Strabismus |website=MedlinePlus Encyclopedia |publisher=US National Library of Medicine, National Institutes of Health |url=https://www.nlm.nih.gov/medlineplus/ency/article/001004.htm |access-date=5 April 2013 |url-status=live |archive-url=https://web.archive.org/web/20130327143335/http://www.nlm.nih.gov/medlineplus/ency/article/001004.htm |archive-date=March 27, 2013 |df=mdy-all }}</ref> but it can also result from any severe and/or traumatic injury to the affected eye. Sensory strabismus is strabismus due to vision loss or impairment, leading to horizontal, vertical or torsional misalignment or to a combination thereof, with the eye with poorer vision drifting slightly over time. Most often, the outcome is horizontal misalignment. Its direction depends on the person's age at which the damage occurs: people whose vision is lost or impaired at birth are more likely to develop esotropia, whereas people with acquired vision loss or impairment mostly develop exotropia.<ref name="Rosenbaum et al.">{{cite book |first1=Arthur L. |last1=Rosenbaum |first2=Alvina Pauline |last2=Santiago | name-list-style = vanc |title=Clinical Strabismus Management: Principles and Surgical Techniques |url=https://books.google.com/books?id=6jqOihYJvCoC&pg=PA194 |year=1999 |publisher=David Hunter |isbn=978-0-7216-7673-9 |pages=193–194 |access-date=21 June 2016 |via=Google Books |url-status=live |archive-url=https://web.archive.org/web/20160506182341/https://books.google.com/books?id=6jqOihYJvCoC&pg=PA194 |archive-date=May 6, 2016 |df=mdy-all }}</ref><ref>{{cite journal | vauthors = Havertape SA, Cruz OA, Chu FC | title = Sensory strabismus--eso or exo? | journal = Journal of Pediatric Ophthalmology and Strabismus | volume = 38 | issue = 6 | pages = 327–30; quiz 354–5 | date = November 2001 | doi = 10.3928/0191-3913-20011101-05 | pmid = 11759769 }}</ref><ref>{{cite journal |first1=Susan A. |last1=Havertape |first2=Oscar A. |last2=Cruz |s2cid=71251248 | name-list-style = vanc |title = Sensory Strabismus: When Does it Happen and Which Way Do They Turn? |journal =  American Orthoptic Journal|date = January 2001 |volume = 51 |issue = 1 |pages = 36–38 |doi = 10.3368/aoj.51.1.36 }}</ref> In the extreme, complete [[Medicine:Blindness|blindness]] in one eye generally leads to the blind eye reverting to an anatomical position of rest.<ref>{{cite encyclopedia |first1=Daniel M. |last1=Albert |first2=Edward S. |last2=Perkins |first3=David M. |last3=Gamm | name-list-style = vanc |date=24 March 2017 |title=Eye disease |url=https://www.britannica.com/science/eye-disease |at=[https://www.britannica.com/science/eye-disease#toc64990 Strabismus (squint)] |encyclopedia=Encyclopædia Britannica |url-status=live |archive-url=https://web.archive.org/web/20170316004915/https://www.britannica.com/science/eye-disease |archive-date=March 16, 2017 |df=mdy-all }}</ref>

Although many possible causes of strabismus are known, among them severe and/or traumatic injuries to the affected eye, in many cases no specific cause can be identified. This last is typically the case when strabismus is present since [[Medicine:Infantile esotropia|early childhood]].<ref>{{cite book |first1=Melvin L. |last1=Rubin |first2=Lawrence A. |last2=Winograd | name-list-style = vanc |section-url=http://www.triadpublishing.com/eyecarereports/strabismus-book.shtml |section=Crossed Eyes (Strabismus): Did you really understand what your eye doctor told you? |title=Taking Care of Your Eyes: A Collection of the Patient Education Handouts Used by America's Leading Eye Doctors |date=2003 |publisher=Triad Communications |isbn=978-0-937404-61-4 |url-access=registration |url=https://archive.org/details/takingcareofyour00melv }}</ref>

Results of a U.S. cohort study indicate that the incidence of adult-onset strabismus increases with age, especially after the sixth decade of life, and peaks in the eighth decade of life, and that the lifetime risk of being diagnosed with adult-onset strabismus is approximately 4%.<ref>{{cite journal | vauthors = Martinez-Thompson JM, Diehl NN, Holmes JM, Mohney BG | title = Incidence, types, and lifetime risk of adult-onset strabismus | journal = Ophthalmology | volume = 121 | issue = 4 | pages = 877–82 | date = April 2014 | pmid = 24321142 | pmc = 4321874 | doi = 10.1016/j.ophtha.2013.10.030 }}</ref>

=== Laterality ===

Strabismus may be classified as unilateral if the one eye consistently deviates, or alternating if either of the eyes can be seen to deviate. Alternation of the strabismus may occur spontaneously, with or without subjective awareness of the alternation. Alternation may also be triggered by various tests during an eye exam.<ref name=meei>{{cite book |first1=Neil J. |last1=Friedman |first2=Peter K. |last2=Kaiser |first3=Roberto |last3=Pineda | name-list-style = vanc |title = The Massachusetts Eye and Ear Infirmary illustrated manual of ophthalmology |year = 2009 |publisher = Saunders/Elsevier |isbn = 978-1-4377-0908-7 |edition = 3rd }}</ref>{{Page needed|date=May 2017}} Unilateral strabismus has been observed to result from a severe or traumatic injury to the affected eye.<ref name="Rosenbaum et al." />

=== Direction and latency ===
Horizontal deviations are classified into two varieties, using prefixes: ''eso-'' describes inward or convergent deviations towards the midline, while ''exo-'' describes outward or divergent misalignment. Vertical deviations are also classified into two varieties, using prefixes: ''hyper-'' is the term for an eye whose gaze is directed higher than the fellow eye, while ''hypo-'' refers to an eye whose gaze is directed lower. Finally, the prefix ''cyclo-'' refers to torsional strabismus, which occurs when the eyes rotate around the anterior–posterior axis to become misaligned and is quite rare.

These five directional prefixes are combined with ''-tropia'' (if manifest) or ''-phoria'' (if latent) to describe various types of strabismus. For example, a constant left hypertropia exists when a person's left eye is always aimed higher than the right. A person with an intermittent right esotropia has a right eye that occasionally drifts toward the person's nose, but at other times is able to align with the gaze of the left eye. A person with a mild exophoria can maintain fusion during normal circumstances, but when the system is disrupted, the relaxed posture of the eyes is slightly divergent.

=== Other considerations ===
Strabismus can be further classified as follows:
* Paretic strabismus is due to paralysis of one or several [[Medicine:Extraocular muscles|extraocular muscles]].
* Nonparetic strabismus is not due to paralysis of extraocular muscles.
* Comitant (or ''concomitant'') strabismus is a deviation that is the same magnitude regardless of gaze position.
* Noncomitant (or ''incomitant'') strabismus has a magnitude that varies as the person shifts his or her gaze up, down, or to the sides.

Nonparetic strabismus is generally concomitant.<ref>{{cite web |url=http://medical-dictionary.thefreedictionary.com/concomitant+strabismus |title=concomitant strabimus |website=TheFreeDictionary |publisher=Farlex }}</ref> Most types of infant and childhood strabismus are comitant.<ref name="Wright-etal-2003-p155">{{cite book |first1 = Kenneth Weston |last1 = Wright |first2 = Peter H. |last2 = Spiegel | name-list-style = vanc |title = Pediatric Ophthalmology and Strabismus |url = https://books.google.com/books?id=ydim6J7sFS4C&pg=PA155 |date = January 2003 |publisher = Springer Science & Business Media |isbn = 978-0-387-95478-3 |page = 155 |via = Google Books |url-status = live |archive-url = https://web.archive.org/web/20160507063427/https://books.google.com/books?id=ydim6J7sFS4C&pg=PA155 |archive-date = May 7, 2016 |df = mdy-all }}</ref> Paretic strabismus can be either comitant or noncomitant. Incomitant strabismus is almost always caused by a limitation of ocular rotations that is due to a restriction of extraocular eye movement (ocular restriction) or due to extraocular muscle paresis.<ref name="Wright-etal-2003-p155" /> Incomitant strabismus cannot be fully corrected by [[Physics:Prism (optics)|prism]] glasses, because the eyes would require different degrees of prismatic correction dependent on the direction of the gaze.<ref>{{cite web|url=http://one.aao.org/clinical-statement/adult-strabismus-surgery--january-2007 |title=Adult Strabismus Surgery – 2013 |date=April 2013 |website=ONE Network |publisher=American Association of Ophthalmology |access-date=6 September 2014 |archive-date=September 7, 2014 |archive-url=https://web.archive.org/web/20140907005412/http://one.aao.org/clinical-statement/adult-strabismus-surgery--january-2007 |url-status=dead }}</ref> Incomitant strabismus of the eso- or exo-type are classified as "alphabet patterns": they are denoted as A- or V- or more rarely λ-, Y- or X-pattern depending on the extent of convergence or divergence when the gaze moves upward or downward. These letters of the alphabet denote ocular motility pattern that have a similarity to the respective letter: in the A-pattern there is (relatively speaking) more convergence when the gaze is directed upwards and more divergence when it is directed downwards, in the V-pattern it is the contrary, in the  λ-, Y- and X-patterns there is little or no strabismus in the middle position but relatively more divergence in one or both of the upward and downward positions, depending on the "shape" of the letter.<ref>{{cite web |first1=James L. |last1=Plotnik |first2=Michael J. |last2=Bartiss | name-list-style = vanc |date=13 October 2015 |url=http://emedicine.medscape.com/article/1199714-overview |title=A-Pattern Esotropia and Exotropia |website=Medscape |access-date=8 September 2014 |url-status=live |archive-url=https://web.archive.org/web/20140908181215/http://emedicine.medscape.com/article/1199714-overview |archive-date=September 8, 2014 |df=mdy-all }}</ref>

Types of incomitant strabismus include: [[Medicine:Duane syndrome|Duane syndrome]], [[Medicine:Horizontal gaze palsy|horizontal gaze palsy]], and [[Medicine:Congenital fibrosis of the extraocular muscles|congenital fibrosis of the extraocular muscles]].<ref>{{cite journal | vauthors = Engle EC | title = Genetic basis of congenital strabismus | journal = Archives of Ophthalmology | volume = 125 | issue = 2 | pages = 189–95 | date = February 2007 | pmid = 17296894 | doi = 10.1001/archopht.125.2.189 | df = mdy-all | doi-access =  }}</ref>

When the misalignment of the eyes is large and obvious, the strabismus is called large-angle, referring to the angle of deviation between the lines of sight of the eyes. Less severe eye turns are called small-angle strabismus. The degree of strabismus can vary based on whether the person is viewing a distant or near target.

Strabismus that sets in after eye alignment had been surgically corrected is called ''consecutive strabismus''.

===Differential diagnosis===
[[Medicine:Pseudostrabismus|Pseudostrabismus]] is the false appearance of strabismus. It generally occurs in infants and toddlers whose bridge of the nose is wide and flat, causing the appearance of esotropia due to less [[Biology:Sclera|sclera]] being visible nasally. With age, the bridge of the child's nose narrows and the [[Biology:Epicanthic fold|folds in the corner of the eyes]] become less prominent.

[[Medicine:Retinoblastoma|Retinoblastoma]] may also result in abnormal light reflection from the eye.[[File:US Navy 090707-F-7923S-026 Navy Capt. Kristen Zeller and Hospital Corpsman 3rd Class Michael Peterson perform surgery to correct strabismus, or cross-eye, on an eight-month-old Nicaraguan infant.jpg|thumb|Strabismus correction surgery]]

== Management ==
{{Main|Medicine:Management of strabismus}}
Strabismus is usually treated with a combination of eyeglasses, vision therapy, and surgery, depending on the underlying reason for the misalignment. As with other binocular vision disorders, the primary goal is comfortable, single, clear, normal binocular vision at all distances and directions of gaze.<ref>{{cite journal|vauthors=Eskridge JB|date=October 1993|title=Persistent diplopia associated with strabismus surgery|journal=Optometry and Vision Science|volume=70|issue=10|pages=849–53|doi=10.1097/00006324-199310000-00013|pmid=8247489}}</ref>

===Glasses===
In cases of accommodative esotropia, the eyes turn inward due to the effort of focusing far-sighted eyes, and the treatment of this type of strabismus necessarily involves refractive correction, which is usually done via corrective glasses or contact lenses, and in these cases surgical alignment is considered only if such correction does not resolve the eye turn.{{cn|date=January 2024}}

In case of strong anisometropia, contact lenses may be preferable to spectacles because they avoid the problem of visual disparities due to size differences (aniseikonia) which is otherwise caused by spectacles in which the refractive power is very different for the two eyes. In a few cases of strabismic children with anisometropic amblyopia, a balancing of the refractive error eyes via [[Physics:Refractive surgery#Children|refractive surgery]] has been performed before strabismus surgery was undertaken.<ref>{{cite journal | vauthors = Astle WF, Rahmat J, Ingram AD, Huang PT | title = Laser-assisted subepithelial keratectomy for anisometropic amblyopia in children: outcomes at 1 year | journal = Journal of Cataract and Refractive Surgery | volume = 33 | issue = 12 | pages = 2028–34 | date = December 2007 | pmid = 18053899 | doi = 10.1016/j.jcrs.2007.07.024 | s2cid = 1886316 }}</ref>

Early treatment of strabismus when the person is a baby may reduce the chance of developing amblyopia and depth perception problems. However, a review of randomized controlled trials concluded that the use of corrective glasses to prevent strabismus is not supported by existing research.<ref>{{Cite journal|last1=Jones-Jordan|first1=Lisa|last2=Wang|first2=Xue|last3=Scherer|first3=Roberta W.|last4=Mutti|first4=Donald O.|date=2 April 2020|title=Spectacle correction versus no spectacles for prevention of strabismus in hyperopic children|journal=The Cochrane Database of Systematic Reviews|volume=2020|issue=4 |pages=CD007738|doi=10.1002/14651858.CD007738.pub3|issn=1469-493X|pmc=7117860|pmid=32240551}}</ref>  Amblyopia has long been considered to remain permanent if not treated within a critical period, namely before the age of about seven years;<ref name="nield" /> 

Glasses affect the position by changing the person's reaction to focusing. Prisms change the way light, and therefore images, strike the eye, simulating a change in the eye position.<ref name=medlineplus/>

===Surgery===
Strabismus surgery does not remove the need for a child to wear glasses. Currently it is unknown whether there are any differences for completing strabismus surgery before or after amblyopia therapy in children.<ref name="Korah">{{cite journal | vauthors = Korah S, Philip S, Jasper S, Antonio-Santos A, Braganza A | title = Strabismus surgery before versus after completion of amblyopia therapy in children | journal = The Cochrane Database of Systematic Reviews | volume = 10 | issue = 10 | pages = CD009272 | date = October 2014 | pmid = 25315969 | pmc = 4438561 | doi = 10.1002/14651858.CD009272.pub2 }}</ref>

Strabismus surgery attempts to align the eyes by shortening, lengthening, or changing the position of one or more of the extraocular eye muscles. The procedure can typically be performed in about an hour, and requires about six to eight weeks for recovery. Adjustable [[Medicine:Surgical suture|suture]]s may be used to permit refinement of the eye alignment in the early postoperative period.<ref name="leffler">{{cite journal | vauthors = Parikh RK, Leffler CT | title = Loop suture technique for optional adjustment in strabismus surgery | journal = Middle East African Journal of Ophthalmology | volume = 20 | issue = 3 | pages = 225–8 | date = July 2013 | pmid = 24014986 | pmc = 3757632 | doi = 10.4103/0974-9233.114797 | df = mdy-all | doi-access = free }}</ref> It is unclear if there are differences between adjustable versus non-adjustable sutures as it has not been sufficiently studied.<ref>{{cite journal | vauthors = Hassan S, Haridas A, Sundaram V | title = Adjustable versus non-adjustable sutures for strabismus | journal = The Cochrane Database of Systematic Reviews | volume = 2018 | pages = CD004240 | date = March 2018 | issue = 3 | pmid = 29527670 | pmc = 6494216 | doi = 10.1002/14651858.CD004240.pub4 | editor-last = Cochrane Eyes and Vision Group }}</ref> An alternative to the classical procedure is minimally invasive strabismus surgery (MISS) that uses smaller incisions than usual.

===Medication===
Medication is used for strabismus in certain circumstances. In 1989, the US [[Organization:Food and Drug Administration|FDA]] approved [[Medicine:Botulinum toxin therapy of strabismus|botulinum toxin therapy]] for strabismus in people over 12 years old.<ref>{{cite web |publisher = United States Department of Health and Human Services |date = April 30, 2009 |title = Re: Docket No. FDA-2008-P-0061 |website = [[Organization:Food and Drug Administration|Food and Drug Administration]] |url = https://www.fda.gov/downloads/Drugs/DrugSafety/PostmarketDrugSafetyInformationforPatientsandProviders/DrugSafetyInformationforHeathcareProfessionals/UCM143989.pdf |access-date = 6 April 2014 |url-status = live |archive-url = https://web.archive.org/web/20150924162556/https://www.fda.gov/downloads/Drugs/DrugSafety/PostmarketDrugSafetyInformationforPatientsandProviders/DrugSafetyInformationforHeathcareProfessionals/UCM143989.pdf |archive-date = September 24, 2015 |df = mdy-all }}</ref><ref>{{cite journal | vauthors = Kowal L, Wong E, Yahalom C | title = Botulinum toxin in the treatment of strabismus. A review of its use and effects | journal = Disability and Rehabilitation | volume = 29 | issue = 23 | pages = 1823–31 | date = December 2007 | pmid = 18033607 | doi = 10.1080/09638280701568189 | s2cid = 19053824 }}</ref> Most commonly used in adults, the technique is also used for treating children, in particular children affected by infantile esotropia.<ref>{{cite journal | vauthors = Thouvenin D, Lesage-Beaudon C, Arné JL | title = [Botulinum injection in infantile strabismus. Results and incidence on secondary surgery in a long-term survey of 74 cases treated before 36 months of age] | language = fr | journal = Journal Français d'Ophtalmologie | volume = 31 | issue = 1 | pages = 42–50 | date = January 2008 | pmid = 18401298 | doi = 10.1016/S0181-5512(08)70329-2 | url = http://www.em-consulte.com/article/138308/alertePM | url-status = live | df = mdy-all | trans-title = Botulinum injection in infantile strabismus. Results and incidence on secondary surgery in a long-term survey of 74 cases treated before 36 months of age | archive-url = https://web.archive.org/web/20171105194745/http://www.em-consulte.com/article/138308/alertePM | archive-date = November 5, 2017 }}</ref><ref>{{cite journal | vauthors = de Alba Campomanes AG, Binenbaum G, Campomanes Eguiarte G | title = Comparison of botulinum toxin with surgery as primary treatment for infantile esotropia | journal = Journal of AAPOS | volume = 14 | issue = 2 | pages = 111–6 | date = April 2010 | pmid = 20451851 | doi = 10.1016/j.jaapos.2009.12.162 }}</ref><ref>{{cite journal | vauthors = Gursoy H, Basmak H, Sahin A, Yildirim N, Aydin Y, Colak E | title = Long-term follow-up of bilateral botulinum toxin injections versus bilateral recessions of the medial rectus muscles for treatment of infantile esotropia | journal = Journal of AAPOS | volume = 16 | issue = 3 | pages = 269–73 | date = June 2012 | pmid = 22681945 | doi = 10.1016/j.jaapos.2012.01.010 }}</ref> The toxin is injected in the stronger muscle, causing temporary and partial paralysis. The treatment may need to be repeated three to four months later once the paralysis wears off. Common side effects are double vision, droopy eyelid, overcorrection, and no effect. The side effects typically resolve also within three to four months. Botulinum toxin therapy has been reported to be similarly successful as strabismus surgery for people with binocular vision and less successful than surgery for those who have no binocular vision.<ref>{{Cite journal|last1=Rowe|first1=Fiona J.|last2=Noonan|first2=Carmel P.|editor1-first=Fiona J|editor1-last=Rowe|date=2012-02-15|title=Botulinum toxin for the treatment of strabismus|journal=The Cochrane Database of Systematic Reviews|issue=2|pages=CD006499|doi=10.1002/14651858.CD006499.pub3|issn=1469-493X|pmid=22336817}}</ref>

== Prognosis ==
When strabismus is congenital or develops in infancy, it can cause [[Medicine:Amblyopia|amblyopia]], in which the brain ignores input from the deviated eye. Even with therapy for amblyopia, [[Stereoblindness|stereoblindness]] may occur. The appearance of strabismus may also be a [[Philosophy:Beauty|cosmetic]] problem. One study reported 85% of adult with strabismus "reported that they had problems with work, school, and sports because of their strabismus." The same study also reported 70% said strabismus "had a negative effect on their [[Philosophy:Self-image|self-image]]."<ref>{{cite web|title=Treatment for "lazy eye" is more than cosmetic |website=Scribe/Alum Notes |publisher=Wayne State University |date=Spring 2001 |url=http://www.med.wayne.edu/Scribe/scribe00-01/scribesp01/baker-strabismus.htm |archive-date=June 26, 2015 |archive-url=https://web.archive.org/web/20150626120545/http://www.med.wayne.edu/Scribe/scribe00-01/scribesp01/baker-strabismus.htm |url-status=dead }}</ref>{{Unreliable medical source|date=May 2017|sure=y}} A second operation is sometimes required to straighten the eyes.<ref name=vaughan />{{Page needed|date=May 2017}}

==Other animals==
[[File:Siam Strabismus.jpg|thumb|A Balinese cat exhibiting esotropia]]

Siamese cats and related breeds are prone to having crossed eyes. Research suggests this is a behavioral compensation for developmental abnormalities in the routing of nerves in the [[Biology:Optic chiasm|optic chiasm]].<ref name="BlakeCrawford1974">{{cite journal|last1=Blake|first1=Randolph|last2=Crawford|first2=M.L.J.|title=Development of strabismus in Siamese cats|journal=Brain Research|volume=77|issue=3|year=1974|pages=492–496|issn=0006-8993|doi=10.1016/0006-8993(74)90637-4|pmid=4851345}}</ref>

Strabismus may also occur in dogs, most often due to imbalanced muscle tone of the muscles surrounding the eye. Some breeds such as the Shar Pei are genetically predisposed to the condition. Treatment may involve surgery or therapy to strengthen the muscles.<ref>{{cite web |url=https://www.petmd.com/dog/conditions/eyes/c_multi_orbital_diseases |title=Diseases of the orbit of the eye in dogs |publisher=PetMD |date=September 23, 2008 |access-date=January 3, 2021}}</ref><!-- Could use a better source -->
{{Clear}}

== References ==
{{Reflist}}

== Further reading ==
* {{cite journal |first1 = Sean P. |last1 = Donahue |first2 = Edward G. |last2 = Buckley |first3 = Stephen P. |last3 = Christiansen |first4 = Oscar A. |last4 = Cruz |first5 = Linda R. |last5 = Dagi | name-list-style = vanc |title = Difficult problems: strabismus |journal = Journal of American Association for Pediatric Ophthalmology and Strabismus |volume = 18 |issue = 4 |pages = e41 |date = August 2014 |doi = 10.1016/j.jaapos.2014.07.132 }}

== External links ==
* [https://www.aao.org/interactive-tool/complex-strabismus-simulator AAO Complex strabismus simulator]
* [https://somapp.ucdmc.ucdavis.edu/eyerelease/Interface/TopFrame.htm UC Davis strabismus simulator]

{{Medical resources
 |DiseasesDB = 29577
 |ICD10 = {{ICD10|H49}}, {{ICD10|H50}}
 |ICD9 = {{ICD9|378}}
 |OMIM = 185100
 |MedlinePlus = 001004
 |MeshID = D013285}}
{{Eye pathology|pathways|state=collapsed}}

[[Category:Wikipedia medicine articles ready to translate]]

{{Sourceattribution|Strabismus}}