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{{Infobox medical condition (new)
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{{Infobox medical condition (new)
| name = Aniseikonia
+
| name = Anisometropia
| image =
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|
| caption =
+
| synonyms =
|
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pronounce = {{IPAc-en|æ|n|ˌ|aɪ|s|ə|m|ɪ|ˈ|t|r|oʊ|p|i|ə}} {{respell|ann|EYE|sə|mi|TROH|pee|ə}}|
| pronounce =
+
| symptoms = one eye is [[myopia]] other eye is [[hyperopia]]
| field =
+
| complications = [[Amblyopia]]
| synonyms =
 
| symptoms = objects different sizes in each eye
 
| complications =
 
 
| onset =
 
| onset =
 
| duration =
 
| duration =
 
| types =
 
| types =
| causes = [[Cataract surgery]], [[refractive surgery]]
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| causes =
 
| risks =
 
| risks =
 
| diagnosis =
 
| diagnosis =
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| deaths =
 
| deaths =
 
}}
 
}}
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'''Anisometropia''' is when two [[human eye|eye]]s have unequal [[refractive power]].<ref>{{cite web |title=Anisometropia - American Association for Pediatric Ophthalmology and Strabismus |url=https://aapos.org/glossary/anisometropia |website=aapos.org |accessdate=10 February 2020 |language=en}}</ref> Generally a difference in power of two [[diopters]] or more is the accepted threshold to label the condition anisometropia.
   
'''Aniseikonia''' is an ocular condition where there is a significant difference in the [[magnification|perceived size]] of images. It can occur as an overall difference between the two eyes, or as a difference in a particular meridian.<ref name = Berens>{{Citation|last = Berens |first = Conrad | last2 = Loutfallah | first2 = Michael | year = 1938 | title = Aniseikonia: A Study of 836 Patients Examined with the Ophthalmo-Eikonometer | periodical = Trans Am Ophthalmol Soc. | series = | volume = 36 | pages = 234–67 | url = http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1315746&blobtype=pdf | pmc=1315746 | pmid=16693153}}</ref>
+
In certain types of anisometropia, the visual cortex of the brain will not use both eyes together (binocular vision), and will instead suppress the central vision of one of the eyes. If this occurs often enough during the first 10 years of life while the visual cortex is developing, it can result in [[amblyopia]], a condition where even when correcting the refractive error properly, the person's vision in the affected eye is still not correctable to 20/20.
   
==Symptoms==
+
The name is from four [[Greek language|Greek]] components: ''an-'' "not," ''iso-'' "same," ''metr-'' "measure," ''ops'' "eye."
When this [[magnification]] difference becomes excessive the effect can cause [[diplopia]], [[Suppression (eye)|suppression]], [[disorientation]], [[eyestrain]], [[headache]], and [[dizziness]] and [[balance disorder]]s.{{Citation needed|date=January 2015}}
 
 
==Causes==
 
Retinal image size is determined by many factors. The size and position of the object being viewed affects the characteristics of the light entering the system. [[Corrective lens]]es affect these characteristics and are used commonly to correct [[refractive error]]. The optics of the eye including its refractive power and axial length also play a major role in retinal image size.
 
 
Aniseikonia can occur naturally or be induced by the correction of a [[refractive error]], usually [[anisometropia]] (having significantly different refractive errors between each eye) or [[antimetropia]] (being [[myopia|myopic]] (nearsighted) in one eye and [[hyperopia|hyperopic]] (farsighted) in the other.) Meridional aniseikonia occurs when these refractive differences only occur in one meridian (see [[astigmatism]]). [[Refractive surgery]] can cause aniseikonia in much the same way that it is caused by [[corrective lens|glasses and contacts]].{{Citation needed|date=January 2015}}
 
 
One cause of significant anisometropia and subsequent aniseikonia has been [[aphakia]]. Aphakic patients do not have a [[lens (anatomy)|crystalline lens]]. The crystalline lens is often removed because of opacities called [[cataract]]s. The absence of this lens left the patient highly hyperopic (farsighted) in that eye. For some patients the removal was only performed on one eye, resulting in the anisometropia / aniseikonia. Today, this is rarely a problem because when the lens is removed in [[cataract surgery]], an [[intraocular lens]], or IOL is left in its place.{{Citation needed|date=January 2015}}
 
   
  +
An estimated 6% of subjects aged 6 to 18 have anisometropia.
 
==Diagnosis==
 
==Diagnosis==
A way to demonstrate aniseikonia is to hold a near target (ex. pen or finger) approximately 6 inches directly in front of one eye. The person then closes one eye, and then the other. The person should notice that the target appears larger to the eye that it is directly in front of. When this object is viewed with both eyes, it is seen with a small amount of aniseikonia. The principles behind this demonstration are relative distance magnification (closer objects appear larger) and asymmetrical convergence (the target is not an equal distance from each eye).{{Citation needed|date=January 2015}}
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{{Empty section|date=April 2018}}
  +
== Treatment ==
   
==Treatment==
+
=== Spectacle correction ===
Treatment is done by changing the optical magnification properties of the auxiliary optics ([[corrective lens]]es). The optical magnification properties of spectacle lenses can be adjusted by changing parameters like the [[base curve]], [[vertex distance]], and [[center thickness]]. Contact lenses may also provide a better optical magnification to reduce the difference in image size. <!-- IS The following sentence RIGHT??? This makes sense because a contact lens can be thought of as a very thin spectacle lens with very little vertex distance. --> The difference in magnification can also be eliminated by a combination of contact lenses and glasses (creating a weak telescope system). The optimum design solution will depend on different parameters like cost, cosmetic implications, and if the patient can tolerate wearing a contact lens.{{Citation needed|date=January 2015}}
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For those with large degrees of anisometropia, spectacle correction may cause the person to experience a difference in image magnification between the two eyes ([[aniseikonia]]) which could also prevent the development of good binocular vision. This can make it very difficult to wear glasses without symptoms such as headaches and eyestrain. However, the earlier the condition is treated, the easier it is to adjust to glasses.
   
Note however that before the optics can be designed, first the aniseikonia should be measured. When the image disparity is astigmatic (cylindrical) and not uniform, images can appear wider, taller, or diagonally different. When the disparity appears to vary across the visual field (field-dependent aniseikonia), as may be the case with an [[epiretinal membrane]] or [[retinal detachment]], the aniseikonia cannot fully be corrected with traditional optical techniques like standard corrective lenses. However, partial correction often improves the patient's vision comfort significantly. Little is known yet about the possibilities of using surgical intervention to correct aniseikonia.{{Citation needed|date=January 2011}}<!-- Is the previous sentence accurate / up to date? -->
+
It is possible for spectacle lenses to be made which can adjust the image sizes presented to the eye to be approximately equal. These are called iseikonic lenses. In practice though, this is rarely ever done.
   
==Etymology==
+
The formula for iseikonic lenses (without cylinder) is:
Gr. "an" = "not", + "is(o)" = "equal," + "eikōn" = "image"{{Citation needed|date=January 2015}}
 
   
==See also==
+
: <math> \textrm{Magnification} = \frac{1}{(1-(\frac{t}{n})P)}\cdot \frac{1}{(1-hF)} </math>
*[[Adelbert Ames, Jr.]] (Dartmouth Eye Institute, research in the 1930s and 1940s on aniseikonia)
 
*[[macropsia]], [[micropsia]]
 
   
==References==
+
where:
{{reflist}}
+
t = center thickness (in meters)
  +
n = refractive index
  +
P = front base curve
  +
h = [[vertex distance]] (in meters)
  +
F = back vertex power (essentially, the prescription for the lens)
   
==Further reading==
+
If the difference between the eyes is up to 3 diopters, iseikonic lenses can compensate. At a difference of 3 diopters the lenses would however be very visibly different - one lens would need to be at least 3mm thicker and have a base curve increased by 7.5 spheres.
*{{Citation
 
| last = Bannon
 
| first = Robert E.
 
| last2 = Neumueller
 
| first2 = Julius
 
| last3 = Boeder
 
| first3 = Paul
 
| last4 = Burian
 
| first4 = Hermann M.
 
| title = Aniseikonia and space perception: After 50 years
 
| journal = American Journal of Optometry & Archives of American Academy of Optometry
 
| volume = 47
 
| issue = 6
 
| pages = 423–441
 
| origyear =
 
|date=June 1970
 
| doi=10.1097/00006324-197006000-00001
 
}}
 
*{{Citation
 
| last = Bisno
 
| first = David C.
 
| title = Eyes in the Storm—President Hopkins' Dilemma: The Dartmouth Eye Institute
 
| place = Norwich, Vermont
 
| publisher = Norwich Book Press
 
| year = 1994
 
| pages = 288 }}
 
   
== External links ==
+
=== Contact lenses ===
{{Medical resources
+
The usual recommendation for those needing iseikonic correction is to wear [[contact lens]]es. The effect of vertex distance is removed and the effect of center thickness is also almost removed, meaning there is minimal and likely unnoticeable image size difference. This is a good solution for those who can tolerate contact lenses.
| DiseasesDB = 29646
 
| ICD10 = {{ICD10|H|52|3|h|49}}
 
| ICD9 = {{ICD9|367.32}}
 
| ICDO =
 
| OMIM =
 
| MedlinePlus =
 
| eMedicineSubj =
 
| eMedicineTopic =
 
| MeshID = D000839
 
}}
 
   
  +
=== Refractive surgery ===
  +
Refractive surgery causes only minimal size differences, similar to contact lenses. In a study performed on 53 children who had [[amblyopia]] due to anisometropia, surgical correction of the anisometropia followed by strabismus surgery if required led to improved visual acuity and even to stereopsis in many of the children<ref>{{cite journal|author1=William F. Astle|author2=Jamalia Rahmat|author3=April D. Ingram|author4=Peter T. Huang|title=Laser-assisted subepithelial keratectomy for anisometropic amblyopia in children: Outcomes at 1 year|journal=Journal of Cataract & Refractive Surgery|volume=33|issue=12|date=December 2007|pages=2028–2034|doi=10.1016/j.jcrs.2007.07.024|pmid=18053899}}</ref> (''see:'' [[Refractive surgery#Children]]).
   
<!-- The following is taken mostly from Optical Diagnostics' [http://www.opticaldiagnostics.com/products/ai/aniseikonia.html aniseikonia webpage] The site is commercial, trying to sell software. But the information is not false. Other references need to be found to support this information and it can be re-included into this article
+
== Epidemiology ==
  +
A determination of the prevalence of anisometropia has several difficulties. First of all, the measurement of refractive error may vary from one measurement to the next. Secondly, different criteria have been employed to define anisometropia, and the boundary between anisometropia and isometropia depend on their definition.<ref name="pmid23773832">{{cite journal |vauthors=Barrett BT, Bradley A, Candy TR |title=The relationship between anisometropia and amblyopia |journal=Progress in Retinal and Eye Research |volume=36 |issue= |pages=120–58 | date=September 2013 |pmid=23773832 |pmc=3773531 |doi=10.1016/j.preteyeres.2013.05.001 |url= |issn=}}</ref>
   
Aniseikonia is a binocular condition in which the two eyes perceive images of different size. These unequal images can be caused by a difference in:
+
Several studies have found that anisometropia occurs more frequently and tends to be more severe for persons with high [[ametropia]], and that this is particularly true for myopes. Anisometropia follows a U-shape distribution according to age: it is frequent in infants aged only a few weeks, is more rare in young children, comparatively more frequent in teenagers and young adults, and more prevalent after [[presbyopia]] sets in, progressively increasing into old age.<ref name="pmid23773832"/>
   
* optical magnification (i.e. different retinal image sizes)
+
One study estimated that 6% of those between the ages of 6 and 18 have anisometropia.<ref>Czepita D, Goslawski W, Mojsa A. "Occurrence of anisometropia among students ranging from 6 to 18 years of age." ''Klin Oczna.'' 2005;107(4-6):297-9. Polish. {{PMID|16118943}}.</ref>
* retinal receptor distribution (i.e. a different sampling of the retinal images)
 
* cortical processing (i.e. different processing of the sampled retinal images)
 
   
Patients at risk of aniseikonia
+
Notwithstanding research performed on the biomechanical, structural and optical characteristics of anisometropic eyes, the underlying reasons for anisometropia are still poorly understood.<ref name="pmid24939167">{{cite journal |vauthors=Vincent SJ, Collins MJ, Read SA, Carney LG |title=Myopic anisometropia: ocular characteristics and aetiological considerations |journal=Clinical & Experimental Optometry |volume=97 |issue=4 |pages=291–307 |year=2014 |pmid=24939167 |doi=10.1111/cxo.12171 |type=Review|url=https://eprints.qut.edu.au/72894/1/Vincent_CXO_Review_2014_accepted.pdf }}</ref>
   
Aniseikonia is often associated with unequal refractive errors between the eyes (anisometropia). However, there are several other patient groups at risk. For example, research has shown that appr. 40% of the patients that underwent cataract surgery/surgeries and who had an intra-ocular lens(es) implanted, have complaints referable to aniseikonia. This also makes one wonder how much aniseikonia is induced with refractive surgery such as LASIK.
+
Anisometropic persons who have [[strabismus]] are mostly far-sighted, and almost all of these have (or have had) [[esotropia]].<ref>"When strabismus is present in an anisometropic individual, it is almost always of the convergent type and is generally found in anisohyperopes but not anisomyopes." {{cite journal |vauthors=Barrett BT, Bradley A, Candy TR |title=The relationship between anisometropia and amblyopia |journal=Progress in Retinal and Eye Research |volume=36 |issue= |pages=120–58 | date=September 2013 |pmid=23773832 |pmc=3773531 |doi=10.1016/j.preteyeres.2013.05.001 |url= |issn=}}</ref> However, there are indications that anisometropia influences the long-term outcome of a surgical correction of an inward squint, and vice versa. More specifically, for patients with esotropia who undergo strabismus surgery, anisometropia may be one of the risk factors for developing consecutive exotropia<ref name="pmid23641958">{{cite journal |vauthors=Yurdakul NS, Ugurlu S |title=Analysis of risk factors for consecutive exotropia and review of the literature |journal=Journal of Pediatric Ophthalmology and Strabismus |volume=50 |issue=5 |pages=268–73 |year=2013 |pmid=23641958 |doi=10.3928/01913913-20130430-01 |url= |issn=}}</ref> and poor binocular function may be a risk factor for anisometropia to develop or increase.<ref>{{cite journal |vauthors=Fujikado T, Morimoto T, Shimojyo H |title=Development of anisometropia in patients after surgery for esotropia |journal=Japanese Journal of Ophthalmology |volume=54 |issue=6 |pages=589–93 |date=November 2010 |pmid=21191721 |doi=10.1007/s10384-010-0868-z |url=}}</ref>
Another group of patients at risk are patients who have a retinal condition or who underwent retinal surgery. For example with an epiretinal membrane (macular pucker) or after a retinal detachment surgery. The aniseikonia in these patients may be complicated because the aniseikonia is field-dependent (variable over the retina), but fortunately also these patients often can get more comfortable binocular vision by optically correcting the aniseikonia.
 
 
Symptoms
 
 
Most aniseikonic symptoms are quite general (a-specific), for example: headaches, asthenopia (ocular fatigue, burning, tearing, pain, pulling, etc.), light sensitivity, reading difficulty, nausea, and double images (diplopia). This is one of the reasons why aniseikonia is sometimes overseen by the treating eye care provider. Only, if the aniseikonia is severe, the patient could also actually see an image size difference by closing one eye at a time. However, symptoms usually occur already with much less aniseikonia.
 
 
Clinically significant aniseikonia values
 
 
Aniseikonia of 3% or more is generally considered clinically significant, but sensitive individuals may have symptoms with less aniseikonia.
 
 
Testing for aniseikonia
 
 
Testing for aniseikonia is important. In older books sometimes rules of thumb are given based on retinal images size differences alone. However, research has shown that even in anisometropia the retinal receptor distribution may be different between the eyes, making aniseikonia management based on calculated retinal image sizes inaccurate.
 
Testing for aniseikonia can be done using a space eikonometric method (based on space distortions accompanying the aniseikonia) or a direct comparison method. The space eikonometric method is sometimes still used in research, but it is less suited for clinical purposes (and commercially unavailable). There are two commercially available aniseikonia tests: the New Aniseikonia Test1 (NAT, 1982) and the Aniseikonia Inspector2 (2003-2007). The first is a booklet and the second is software. The two tests are based on the same principle, but the Aniseikonia Inspector has several advanges due to the interactive possibilities when using a computer. Also, the Aniseikonia Inspector contains a module to calculate aniseikonia correcting prescriptions.
 
 
Correcting aniseikonia
 
 
Aniseikonia can be corrected by changing the optical magnification properties of the auxiliary optics (glasses, contact lenses). For example, if the curvature or thickness of a spectacle lens is changed (without changing its refractive power, so there will still be a good visual acuity), the optical magnification will change. Also, the distance between the spectacle lens and the eye (vertex distance) affects the optical magnification. Therefore, contact lenses will in general give a different aniseikonia than glasses. It might even be possible to correct aniseikonia by fitting a contact lens together with a spectacle lens, creating a weak telescope system. What solution is best depends on the amount of aniseikonia, type of aniseikonia (optical-induced or retinally-induced), the refraction, and the importance of cosmetics.
 
 
-->
 
   
  +
==References==
  +
{{Reflist}}
  +
== External links ==
  +
{{Medical resources
  +
| ICD10 = {{ICD10|H|52|3|h|49}}
  +
| ICD9 = {{ICD9|367.31}}
  +
}}
 
{{Eye pathology}}
 
{{Eye pathology}}
 
[[Category:Disorders of ocular muscles, binocular movement, accommodation and refraction]]
 
[[Category:Disorders of ocular muscles, binocular movement, accommodation and refraction]]

Latest revision as of 19:05, March 19, 2020

Template:Infobox medical condition (new) Anisometropia is when two eyes have unequal refractive power.[1] Generally a difference in power of two diopters or more is the accepted threshold to label the condition anisometropia.

In certain types of anisometropia, the visual cortex of the brain will not use both eyes together (binocular vision), and will instead suppress the central vision of one of the eyes. If this occurs often enough during the first 10 years of life while the visual cortex is developing, it can result in amblyopia, a condition where even when correcting the refractive error properly, the person's vision in the affected eye is still not correctable to 20/20.

The name is from four Greek components: an- "not," iso- "same," metr- "measure," ops "eye."

An estimated 6% of subjects aged 6 to 18 have anisometropia.

DiagnosisEdit

Template:Empty section

Treatment Edit

Spectacle correction Edit

For those with large degrees of anisometropia, spectacle correction may cause the person to experience a difference in image magnification between the two eyes (aniseikonia) which could also prevent the development of good binocular vision. This can make it very difficult to wear glasses without symptoms such as headaches and eyestrain. However, the earlier the condition is treated, the easier it is to adjust to glasses.

It is possible for spectacle lenses to be made which can adjust the image sizes presented to the eye to be approximately equal. These are called iseikonic lenses. In practice though, this is rarely ever done.

The formula for iseikonic lenses (without cylinder) is:

$ \textrm{Magnification} = \frac{1}{(1-(\frac{t}{n})P)}\cdot \frac{1}{(1-hF)} $

where: t = center thickness (in meters) n = refractive index P = front base curve h = vertex distance (in meters) F = back vertex power (essentially, the prescription for the lens)

If the difference between the eyes is up to 3 diopters, iseikonic lenses can compensate. At a difference of 3 diopters the lenses would however be very visibly different - one lens would need to be at least 3mm thicker and have a base curve increased by 7.5 spheres.

Contact lenses Edit

The usual recommendation for those needing iseikonic correction is to wear contact lenses. The effect of vertex distance is removed and the effect of center thickness is also almost removed, meaning there is minimal and likely unnoticeable image size difference. This is a good solution for those who can tolerate contact lenses.

Refractive surgery Edit

Refractive surgery causes only minimal size differences, similar to contact lenses. In a study performed on 53 children who had amblyopia due to anisometropia, surgical correction of the anisometropia followed by strabismus surgery if required led to improved visual acuity and even to stereopsis in many of the children[2] (see: Refractive surgery#Children).

Epidemiology Edit

A determination of the prevalence of anisometropia has several difficulties. First of all, the measurement of refractive error may vary from one measurement to the next. Secondly, different criteria have been employed to define anisometropia, and the boundary between anisometropia and isometropia depend on their definition.[3]

Several studies have found that anisometropia occurs more frequently and tends to be more severe for persons with high ametropia, and that this is particularly true for myopes. Anisometropia follows a U-shape distribution according to age: it is frequent in infants aged only a few weeks, is more rare in young children, comparatively more frequent in teenagers and young adults, and more prevalent after presbyopia sets in, progressively increasing into old age.[3]

One study estimated that 6% of those between the ages of 6 and 18 have anisometropia.[4]

Notwithstanding research performed on the biomechanical, structural and optical characteristics of anisometropic eyes, the underlying reasons for anisometropia are still poorly understood.[5]

Anisometropic persons who have strabismus are mostly far-sighted, and almost all of these have (or have had) esotropia.[6] However, there are indications that anisometropia influences the long-term outcome of a surgical correction of an inward squint, and vice versa. More specifically, for patients with esotropia who undergo strabismus surgery, anisometropia may be one of the risk factors for developing consecutive exotropia[7] and poor binocular function may be a risk factor for anisometropia to develop or increase.[8]

ReferencesEdit

  1. Anisometropia - American Association for Pediatric Ophthalmology and Strabismus. URL accessed on 10 February 2020.
  2. (December 2007) Laser-assisted subepithelial keratectomy for anisometropic amblyopia in children: Outcomes at 1 year. Journal of Cataract & Refractive Surgery 33 (12): 2028–2034.
  3. 3.0 3.1 (September 2013) The relationship between anisometropia and amblyopia. Progress in Retinal and Eye Research 36: 120–58.
  4. Czepita D, Goslawski W, Mojsa A. "Occurrence of anisometropia among students ranging from 6 to 18 years of age." Klin Oczna. 2005;107(4-6):297-9. Polish. PMID 16118943.
  5. (2014). Myopic anisometropia: ocular characteristics and aetiological considerations. Clinical & Experimental Optometry 97 (4): 291–307.
  6. "When strabismus is present in an anisometropic individual, it is almost always of the convergent type and is generally found in anisohyperopes but not anisomyopes." (September 2013) The relationship between anisometropia and amblyopia. Progress in Retinal and Eye Research 36: 120–58.
  7. (2013). Analysis of risk factors for consecutive exotropia and review of the literature. Journal of Pediatric Ophthalmology and Strabismus 50 (5): 268–73.
  8. (November 2010) Development of anisometropia in patients after surgery for esotropia. Japanese Journal of Ophthalmology 54 (6): 589–93.

External links Edit

Template:Medical resources

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