Medical condition
Aniseikonia
is an ocular condition where there is a significant difference in the
perceived size
of images. It can occur as an overall difference between the two eyes, or as a difference in a particular meridian.
[1]
If the ocular image size in both eyes are equal, the condition is known as
iseikonia
.
[2]
Symptoms and signs
[
edit
]
Up to 7% difference in image size is well tolerated.
[3]
If
magnification
difference becomes excessive the effect can cause
diplopia
,
suppression
,
disorientation
,
eyestrain
,
headache
, and
dizziness
and
balance disorders
.
[3]
Asthenopic symptoms alone may occur even if image size difference is less than 7%.
[4]
Causes
[
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]
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 lenses
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.
[
citation needed
]
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
myopic
(nearsighted) in one eye and
hyperopic
(farsighted) in the other.) Meridional aniseikonia occurs when these refractive differences only occur in one meridian (see
astigmatism
).
One cause of significant anisometropia and subsequent aniseikonia has been
aphakia
. Aphakic patients do not have a
crystalline lens
. The crystalline lens is often removed because of opacities called
cataracts
. 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
]
Retinal aniseikonia occur due to forward displacement, stretching or edema of retina.
[4]
Diagnosis
[
edit
]
A way to demonstrate aniseikonia is to hold a near target (
e.g.
, a pen or a 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
]
Treatment
[
edit
]
Optical aniseikonia due to anisometropia can be corrected by spectacles, contact lenses or refractive corneal surgeries.
[5]
Spectacle correction is done by changing the optical magnification properties of the auxiliary optics (
corrective lenses
). The optical magnification properties of spectacle lenses can be adjusted by changing parameters like the
base curve
,
vertex distance
, and
center thickness
. Magnification size matched lenses that are used to correct aniseikonia are known as iseikonic lenses.
[3]
Contact lenses may also provide less difference in retinal image size.
[4]
Wider and better field of vision is another benefit of contact lens use. 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
]
For reducing aniseikonia, similar to contact lens correction, optical image size difference will be reduced in
refractive surgeries
also.
[5]
[6]
Aniseikonia due to uniocular
aphakia
is best corrected surgically by
intraocular lens
implantation.
[4]
Similarly retinal aniseikonia is corrected by treating causative retinal disease.
[4]
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.
[
citation needed
]
Etymology
[
edit
]
The word "aniseikonia" is derived from the following Greek
morphemes
:
- "an" = "not" (as in "anæmia");
- "is(o)" = "equal" (as in "isobar");
- "eik?n" = "image" (as in "icon")
[
citation needed
]
.
See also
[
edit
]
References
[
edit
]
- ^
Berens, Conrad; Loutfallah, Michael (1938),
"Aniseikonia: A Study of 836 Patients Examined with the Ophthalmo-Eikonometer"
,
Trans Am Ophthalmol Soc.
, vol. 36, pp. 234?67,
PMC
1315746
,
PMID
16693153
- ^
"Fusion and binocularity".
Borish's clinical refraction
(2nd ed.). Butterworth Heinemann/Elsevier. 2006.
ISBN
978-0-7506-7524-6
.
- ^
a
b
c
"Aniseikonia - EyeWiki"
.
eyewiki.aao.org
.
- ^
a
b
c
d
e
Khurana, AK (September 2008). "Errors of refraction and binocular optical defects".
Theory and practice of optics and refraction
(2nd ed.). Elsevier.
ISBN
978-81-312-1132-8
.
- ^
a
b
"Patients with anisometropia and aniseikonia".
Borish's clinical refraction
(2nd ed.). Butterworth Heinemann/Elsevier. 2006.
ISBN
978-0-7506-7524-6
.
- ^
Mravicic, Ivana; Bohac, Maja; Lukacevic, Selma; Jagaric, Kruno; Maja, Merlak; Patel, Sudi (2019).
"The relationship between clinical measures of aniseikonia and stereoacuity before and after LASIK"
.
Journal of Optometry
.
13
(1): 59?68.
doi
:
10.1016/j.optom.2019.06.004
.
ISSN
1888-4296
.
PMC
6951829
.
PMID
31668775
.
Further reading
[
edit
]
- Bannon, Robert E.; Neumueller, Julius; Boeder, Paul; Burian, Hermann M. (June 1970), "Aniseikonia and space perception: After 50 years",
American Journal of Optometry & Archives of American Academy of Optometry
,
47
(6): 423?441,
doi
:
10.1097/00006324-197006000-00001
,
PMID
4912937
,
S2CID
37288776
- Bisno, David C. (1994),
Eyes in the Storm?President Hopkins' Dilemma: The Dartmouth Eye Institute
, Norwich, Vermont: Norwich Book Press, p. 288
External links
[
edit
]
- Aniseikonia Calculator
, Chadwick Optical (U.S.A.) ? a magnification calculator for iseikonic prescriptions or aniseikonia.