Editing Strabismus (section)

== Diagnosis ==
{{more citations needed section|date=November 2020}}
During an [[eye examination]], a test such as [[cover test]]ing or the [[Hirschberg test]] is used in the diagnosis and measurement of strabismus and its impact on vision. [[Retinal birefringence scanning]] can be used for [[screening (medicine)|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=November 6, 2017|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. |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 [[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 [[diplopia|double vision]].[[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]]

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=April 5, 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  }}</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 [[vision impairment|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 |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=June 21, 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  }}</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 |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 [[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 |date=March 24, 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  }}</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 [[Infantile esotropia|early childhood]].<ref>{{cite book |first1=Melvin L. |last1=Rubin |first2=Lawrence A. |last2=Winograd |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 |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: [[Esotropia|''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: [[Hypertropia|''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 [[Cyclotropia|''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 [[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 |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 [[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=September 6, 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 [[lambda|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 |date=October 13, 2015 |url=http://emedicine.medscape.com/article/1199714-overview |title=A-Pattern Esotropia and Exotropia |website=Medscape |access-date=September 8, 2014 |url-status=live |archive-url=https://web.archive.org/web/20140908181215/http://emedicine.medscape.com/article/1199714-overview |archive-date=September 8, 2014  }}</ref>

Types of incomitant strabismus include: [[Duane syndrome]], [[horizontal gaze palsy]], and [[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===
[[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 [[sclera]] being visible nasally. With age, the bridge of the child's nose narrows and the [[Epicanthic fold|folds in the corner of the eyes]] become less prominent.

[[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]]