Translated from Greek aniseikonia means "unequal images". It is a binocular condition, so the image in one eye is perceived as different in size compared to the image in the other eye. Two different types of aniseikonia can be differentiated: static and dynamic aniseikonia.
Static aniseikonia or aniseikonia in short means that in a static situation where the eyes are gazing in a certain direction, the perceived (peripheral) images are different in size (see Fig.1).
Dynamic aniseikonia or (optically induced) anisophoria means that the eyes have to rotate a different amount to gaze (i.e. look with the sharpest vision) at the same point in space (see Fig. 1). This is especially difficult for eye rotations in the vertical direction.
In the remainder of this web-page we will speak of aniseikonia and anisophoria instead of static aniseikonia and dynamic aniseikonia. Aniseikonia will be the target parameter to be measured and corrected. By contrast, anisophoria can more or less been determined from the prescription and is a parameter that needs to be kept within bounds when correcting the aniseikonia.
Aniseikonia values by definition represent how much the right eye should be magnified or minified to cancel the aniseikonia. For example, a measured aniseikonia of -5% means that the image is in the right eye is perceived as approximately 5% larger than the image in the left eye and that therefore the aniseikonia is corrected by minifying the image in the right eye by 5% (or magnifying the image in the left eye by approximately 5%, or a combination of both).
With optically-induced aniseikonia (i.e. aniseikonia due to a difference in the optical magnification properties of the eyes + auxiliary optics), the aniseikonia is sometimes described by means of the aniseikonic ellipse. This ellipse describes the aniseikonia in all directions. It consists of an overall aniseikonia and a meridional aniseikonia. The overall part describes the minimum (closest to zero) aniseikonia and the meridional part describes the direction-dependent part of the aniseikonia (usually induced by the cylinder part of the prescription).
In the case of retinally-induced aniseikonia, the aniseikonia most likely varies with field angles (we call this field-dependent aniseikonia). The field angle is defined as the angle between the gaze direction and the direction of a peripheral point (see Fig. 2). With field-dependent aniseikonia, one cannot speak of the aniseikonia anymore, since the aniseikonia varies depending on from which location on the retina the aniseikonia is determined. For more information on field-dependent aniseikonia, see our article: Retinally-induced aniseikonia1.
Fig. 2 schematically shows how an eye perceives the size of an image. First objects in the outside world are imaged with a certain optical magnification (minification) on the retina. Next, the retinal receptors sample the retinal image into 'pixels'. Finally, this information is processed by the brain.
Fig. 2: Schematic presentation of the different steps to get to a perceived
image size and the visualization of a field angle a..
Aniseikonia could basically arise if
there is a difference between the eyes in any of the three steps depicted
in Fig. 2. Optically-induced aniseikonia patients
might be anisometropes, pseudophakes,
and refractive surgery patients. Retinally-induced
aniseikonia patients are those patients in which the retina may be
compressed or stretched due to an ocular condition or surgery. Due
to the compression or stretching, an image projected onto the retina will
be sampled by either a greater pr lesser number of receptors, causing the perceived image to appear
bigger or smaller (macropsia or micropsia). There may also be a more
random (local) distortion, called metamorphopsia. Retinally-induced
aniseikonia patients are for example patients with an epiretinal
membrane (macular pucker), a retinal detachment, a macular
hole, macular edema, or a retinoschisis.
We have not found any cortically-induced aniseikonia
patients in the literature.
Bannon and Triller2 reported a list of characteristic aniseikonia symptoms based on a study with 500 patients (see Table 1). Burian3 published a similar list presenting the symptoms the patients most frequently complained about. His main categories were I. Astenopic symptoms, II. Impaired fusion and/or poor stereopsis, and III. Anomalous spatial localization.
|Symptom||Percentage of patients|
|Astenopia (fatigue, burning,
ache, pain, pulling, etc)
|Vertigo and dizziness||7%|
|Distorted space perception||6%|
Aniseikonia seems to become
clinically significant at values of 3-5%4-8.
Sometimes sensitive individuals are suspected to have symptoms with less
aniseikonia9, but it is well possible that these symptoms are
caused by optically-induced anisophoria and not aniseikonia.
The topic of clinically significant aniseikonia values warrants further research. It is unclear what the contribution to aniseikonic symptoms is of aniseikonia versus anisophoria. In case of (retinally-induced) field-dependent aniseikonia, it is also unclear which field angles and field directions are causing the most symptoms.
In older optometric/ophthalmic textbooks, rules of thumb have been defined to correct aniseikonia, without actually testing for the amount of aniseikonia. These rules are based on Knapp's law, which deals with an image size difference as projected onto the retina in anisometropia (i.e., only optical effects are taken into account). Eye care professionals using these rules will base treatment on the patient's prescription and perhaps the difference in corneal curvature or eye length between the eyes. However, in the more recent literature it has been well established that even in anisometropia, the retinal receptor distribution may also play a role,10-14 and therefore these rules of thumb should not be used. Instead the aniseikonia should be measured.
There are basically two methods to test for aniseikonia: the space eikonometric method and the direct comparison method.15,16 The space eikonometric method is based on binocular space perception, while the direct comparison method is based on directly comparing perceived image sizes between the two eyes. Table 2 compares the space eikonometer with the three commercially available direct comparison tests: the New Aniseikonia Test (Awaya test), the Basic Aniseikonia Test (version 1), and the Aniseikonia Inspector (version 3).
|Space Eikonometer||'New' Aniseikonia Test (NAT)||Basic Aniseikonia Test, v.1||Aniseikonia Inspector, version 3|
|Ease of use||-||++||++||++|
|Measurement range, resolution||-, ++||++, 0||++, +||++, +|
|Test result accuracy indicator||0||0||0||+|
|Field dependency test
|Fixation disparity compensation||NA||--||--||++|
|Aniseikonia correction calculation||--||--||--||++|
Because it is the most complete product for aniseikonia management, we will now discuss the aniseikonia test of the Aniseikonia Inspector, version 3. The patient looks through red-green glasses at the computer screen, which gives a series of test images (see Fig. 3a for one such an image). For each test image the patient's task is to identify which of the two I-shaped bars is perceived as larger (a forced choice procedure). Note that because of the red-green glasses, one eye sees only the left I-bar and the other eye sees only the right I-bar. If all images of a series have been presented, the data can be plotted in a 'psychometric curve' (see Fig. 4). Using a maximum likelihood mathematical method, the aniseikonia value can be obtained from this data. One of the advantages of this forced choice method compared to a method of adjustment (such as used in the Basic Aniseikonia test and the 'New' Aniseikonia Test) is that the forced choice method also provides information about the consistency/accuracy of the measurement. This is also very useful when the patient self-administers the test and the results are analyzed elsewhere.
Figure 3: a) Example of a single aniseikonia test image, b) same aniseikonia test image as on the left, but now with an (exaggerated) vertical fixation disparity compensation (click on image to enlarge).
Figure 4: Raw aniseikonia test data. Each data point represents the result of one presentation as for example shown in Fig. 3, each with different I-bar sizes. The transition where the left I-bar is perceived as larger to where the right I-bar is perceived as larger is the patient's aniseikonia.
If a fixation disparity is present (which might occur especially in the horizontal direction), the aniseikonia test is more difficult to do because the two I-bars will not be symmetrically aligned next to each other. In software it is easy to compensate for a fixation disparity by shifting the I-bars relative to each other (see for example Fig 3b). So when a patient with a certain vertical fixation disparity looks at Fig. 3b through the red-green glasses, he will see the image as Fig. 3a with the I-bars symmetrically next to each other.
aniseikonia testing discussed above has all been 'objective'
testing. However, an eye care provider serious about aniseikonia
management should also have a set of
size lenses for
subjective testing. Size lenses are lenses without refractive power
(the image stays sharp), but with an optical magnification. They may
provide the patient with a sense of what it would be like to have a
certain aniseikonia correction.
How to use size lenses
|If an aniseikonia test showed that a patient has 4%
aniseikonia, this means that the image in the
right eye (OD) needs to be magnified by 4% to cancel the aniseikonia.
To see what the effect of a 4% aniseikonia correction is for the patient, a 4% size lens can be held in front of the right eye with the concave side towards the eye (see top-left image).
Note that aniseikonia is a binocular condition and therefore a relative measure between the two eyes. Instead of magnifying the image in the right eye, equal image sizes are also obtained by minifying the image in the left eye (OS). A size lens minifies by flipping it as in the top-right image.
The lower two images give two more alternative ways to correct a 4% aniseikonia. They also show that with two size lenses, it is possible to correct a larger amount of aniseikonia. Using both the 7% and the 6% allows you to correct appr. 13%!
Sometimes a patient suffering from aniseikonia is told that his/her problem can be solved by covering (fully occluding) one eye. Of course, no patient is happy about that 'solution', and it should only be a last resort. Aniseikonia and its accompanying aniseikonic symptoms can often be reduced by changing the magnification properties of the auxiliary optics of the patient. There are basically 4 possible optical solutions, each with its own advantages and disadvantages. Depending on the patient's case (optically or retinally induced aniseikonia, refractive errors of the eyes, amount of aniseikonia to be corrected, whether contact lenses are an option, importance of cosmetics, etc.) the best solution will be different from patient to patient and might be a combination of the different solutions.
Below 4 possible optical solutions will be discussed assuming the amount of aniseikonia has been measured. Even though it is best practice to measure the aniseikonia before designing and prescribing a specialized correction (see section on testing aniseikonia), not all eye care providers may have testing equipment. In case of optically-induced aniseikonia such as associated with anisometropia, the eye care provider could first examine if contact lenses would improve the vision comfort enough. Contact lenses fully eliminate any optically induced anisophoria (dynamic aniseikonia) and often lessen the (static) aniseikonia. Alternatively to contact lenses, the ShawTM spectacle lenses could be tried. These are specifically aimed at reducing the optically induced anisophoria and may also reduce the aniseikonia. How much and if the aniseikonia will be reduced can only be known though by measuring the aniseikonia first.
Optical solution 1 (contact lenses only)
If a patient tolerates the use of contact lenses, the first option that should be investigated (calculated after the measurement of aniseikonia) is what the remaining aniseikonia would be if all of the patient's prescription were put into contact lenses. Especially with anisometropia it has been shown that contact lenses often (but not always) give less aniseikonia than regular spectacle lenses. If this solution happens to work, then there only seems to be advantages. This solution does not compromise the visual acuity, it is cosmetically very acceptable, there is no anisophoria (contact lenses move with eye rotations, so do not induce any prism effects with eye rotations), and it is relatively low cost. To get a subjective experience of how much this solution would improve the vision comfort, the patient should look first through some trial contact lenses.
Optical solution 2 (glasses only, good VA)
This is the classic aniseikonia correction. Optical magnification changes are induced by changing the shape of the spectacle lenses (curvature, thickness) and/or the distance of the lenses to the eyes. This solution does not compromise the central visual acuity. Because of the changes in shape of the spectacle lenses, the cosmetics of the glasses might be compromised (depending on the amount of aniseikonia to be corrected). In optically-induced aniseikonia both aniseikonia and anisophoria are often reduced at the same time. In retinally-induced aniseikonia a correction of the aniseikonia may often result in the introduction of anisophoria, so a balance needs to be found between how much aniseikonia is corrected and how much anisophoria is introduced. The amount of introduced anisophoria for this solution may be less though than that of solutions 3 and 4. The cost of these spectacle lenses might be a little higher than standard spectacle lenses, because of the non-standard thickness and curvature of the lenses. To get a subjective experience of how much this solution would improve the vision comfort, the patient should look through his/her habitual glasses and a size lens.
Optical solution 3 (glasses only,
In this solution, the optical magnification change is accomplished by altering the refractive power of one of the spectacle lenses. As a consequence the image is blurred in one eye, reducing the effective visual acuity in that eye. Since the other eye is supposed to have a good visual acuity, the overall binocular visual acuity should not be affected too much. However, the blurring may cause visual discomfort (for example, if the blurred eye is strongly dominant). Also, the patient's eye care professional will need to determine if a decoupling of the central vision due to the blurring in one eye is likely to cause, for example, eye alignment problems. If the refractive power change results in an overcorrection (some plus-power added), then the glasses may become useful as monovision glasses (one eye sees sharp at far, and one eye sees sharp at near). The spectacle lenses used for this solution can be standard spectacle lenses, so the cosmetics as well as the cost are favorable compared to solution 2. The resulting amount of anisophoria will depend mainly on the final refractive power difference between the two spectacle lenses (for retinally-induced aniseikonia, this will often mean that the amount of anisophoria will increase). The anisophoria may be less or more than that of solution 4, depending on the refractive state of the eyes and the aniseikonic correction necessary. To get a subjective experience of how much (and if) this solution would approximately improve the vision comfort, the patient could be fitted with a trial frame with standard trial lenses.
Optical solution 4 (contact - glasses
This solution is similar to solution 3, but now one contact lens is added to restore the compromised visual acuity. The cosmetics will be good, as standard spectacle lenses can be used. The cost will go up some, because now both glasses and a contact lens are used at the same time. The anisophoria may be less or more than that of solution 3, depending on the refractive state of the eyes and the aniseikonic correction necessary. For an isometrope (the two eyes appr. equal refractive error), it seems that for an overcorrection of the spectacle lens (negative contact lens) solution 4 gives less anisophoria, while with an undercorrection solution 3 gives less anisophoria. To get a subjective experience of how much (and if) this solution would approximately improve the vision comfort, the patient should be fitted with a trial frame with standard trial lenses and a trial contact lens. Alternatively, the patient could also look through his/her habitual (standard) glasses together with a size lens, although the amount of induced anisophoria might be a bit under- or over-estimated in that case (most likely under-estimated).
In the case of field dependent aniseikonia (associated with retinally-induced aniseikonia), the aniseikonia cannot be fully corrected by optical means, because an optical correction is approximately field independent. Nevertheless, a partial correction often improves the vision comfort considerably. For more information on field dependent aniseikonia, see our article: Retinally-induced aniseikonia1.
If an optical correction is not possible or does not provide enough correction, a partial occlusion may still be tried. Some (field dependent) aniseikonia patients have gained some relief by occluding part of the visual field (for example, placing a (removable) dot in the center of one of the spectacle lenses or (partially) shielding the periphery in one eye, creating kind of a tunnel vision for one eye). Instead of occluding part of the visual field, another approach could be to occlude the whole visual field, but with a partial transparent foil (see an article by Silverberg et al.17).
Anisometropes: Patients with anisometropia, i.e. a difference in refractive power (prescription) between the two eyes. Fig. 1 shows that anisometropia may induce aniseikonia. The prevalence of anisometropia is 5-10% of the population above the age of twenty.18
Epiretinal membrane: Also known as preretinal membrane, cellophane maculopathy, or macular pucker. A thin membrane growing on the retina that contracts the retina. Besides a (slight) blurring of the vision, the contraction may result in (field-dependent) macropsia, i.e. an enlargement of the perceived image. This monocular macropsia means that binocularly (field-dependent) aniseikonia will be present.19-22
Macular edema: Swelling of the retina in the area of the macula (part of the retina with best visual acuity). May be associated with conditions such as diabetic retinopathy. May also give rise to micropsia23-25 and therefore aniseikonia.
Pseudophakes: Patients with pseudophakia, i.e. the presence of an intraocular lens (IOL) implant inside an eye, usually after cataract extraction. A scientific study27 found that 40% of all pseudophakes have ophthalmic complaints referable to aniseikonia. With more than 1.5 million cataract operations per year in the US alone,28 McCormack et al. stated that aniseikonia can be considered a significant health issue.29
Refractive surgery: Surgery that corrects visual acuity, with the objective of reducing or eliminating the need for glasses and contact lenses. Refractive surgery includes radial keratotomy, PRK, LASIK, and corneal implants. With the high incidence of aniseikonia in pseudophakes, an important question to be answered by researchers is what the incidence of aniseikonia is in the fast growing group of corneal refractive surgery patients.
Retinal detachment: Separation of part of the retina from its supporting tissues, caused for example by aging, trauma, inflammation, high myopia and diseases such as diabetic retinopathy, and scleritis. After (a prompt) surgical correction, the visual acuity may be good again, but field dependent aniseikonia may have developed, with the affected eye often perceiving a smaller image (micropsia).24.30,31
- de Wit GC. Retinally-induced aniseikonia. Binocular Vision & Strabismus Quarterly 2007;22:96-101
- Bannon RE, Triller W. Aniseikonia - a clinical report covering a ten year period. Am. J. of Optometry 1944;21:171-182
- Burian HM. Clinical significance of aniseikonia. Archives of Ophthalmology 1944;29:116-133
- Jiménez JR, Ponce A, del Barco LJ, Díaz JA, Pérez-Ocón F. Impact of induced aniseikonia on stereopsis with random-dot stereogram. Optom Vis Sci. 2002;79:121-125.
- Jiménez JR, Ponce A, Anera RG. Induced aniseikonia diminishes binocular contrast sensitivity and binocular summation. Optom Vis Sci. 2004;81:559-562.
- Katsumi O, Tanino T, Hirose T. Effect of aniseikonia on binocular function. Invest Ophthalmol Vis Sci. 1986;27:601-604.
- Crone RA, Leuridan OM. Tolerance for aniseikonia. I. Diplopia thresholds in the vertical and horizontal meridians of the visual field. Albrecht Von Graefes Arch Klin Exp Ophthalmol. 1973;188:1-16.
- Crone RA, Leuridan OM. Tolerance for aniseikonia. II. Determination based on the amplitude of cyclofusion. Albrecht Von Graefes Arch Klin Exp Ophthalmol. 1973;188:17-22.
- Michaels DD, Visual Optics and Refraction: A Clinical Approach, Mosby, St. Louis (1985)
- Lubkin V, Shippman S, Bennett G, Meininger D, Kramer P, Poppinga P. Aniseikonia quantification: error rate of rule of thumb estimation. Binocul Vis Strabismus Q. 1999;14:191-6.
- Romano PE, Von Noorden GL. Knapp's Law and Unilateral Axial High Myopia. Binocular Vision & Strabismus Quarterly 1999;14:215-222
- Rabin J, Bradley A, Freeman RD. On the relation between aniseikonia and axial anisometropia. Am J Optom Physiol Opt. 1983; 60:553-558.
- Rose L, Levinson A. Anisometropia and aniseikonia. Am J Optom Arch Am Acad Optom. 1972; 49:480-484.
- Kramer P, Shippman S, Bennett G, Meininger D, Lubkin V. A study of aniseikonia and Knapp's law using a projection space eikonometer. Binocul Vis Strabismus Q. 1999;14:197-201.
- Remole A, Robertson KM. Aniseikonia and Anisophoria, Runestone Publishing, Waterloo, Ontario, Canada (1996)
- Bennett A, Rabbetts R. Clinical Visual Optics, Butterworth-Heinemann, Oxford (1998)
- Silverberg M, Schuler E, Veronneau-Troutman S, Wald K, Schlossman A, Medow N. Nonsurgical management of binocular diplopia induced by macular pathology. Arch Ophthalmol. 1999;117:900-3.
- Weale RA. On the age-related prevalence of anisometropia. Ophthalmic Res. 2002;34:389-92.
- Benegas NM, Egbert J, Engel WK, Kushner BJ. Related Articles, Links Diplopia secondary to aniseikonia associated with macular disease. Arch Ophthalmol. 1999;117:896-9.
- Enoch JM, Schwartz A, Chang D, Hirose H. Related Articles, Links Aniseikonia, metamorphopsia and perceived entoptic pattern: some effects of a macular epiretinal membrane, and the subsequent spontaneous separation of the membrane. Ophthalmic Physiol Opt. 1995;15:339-43.
- de Wit GC, Muraki CS. Related Articles, Links Field-dependent aniseikonia associated with an epiretinal membrane a case study. Ophthalmology. 2006;113:58-62. Epub 2005 Dec 15.
- Ugarte M, Williamson TH. Aniseikonia associated with epiretinal membranes. Br J Ophthalmol. 2005; 89:1576-80
- Frisen L, Frisen M. Micropsia and visual acuity in macular edema. A study of the neuro-retinal basis of visual acuity. Albrecht Von Graefes Arch Klin Exp Ophthalmol. 1979;210:69-77.
- Sjostrand J, Anderson C. Micropsia and metamorphopsia in the re-attached macula following retinal detachment. Acta Ophthalmol (Copenh). 1986;64:425-32.
- Hisada H, Awaya S. Aniseikonia of central serous chorioretinopathy [in Japanese]. Nippon Ganka Gakkai Zasshi. 1992; 96:369-374.
- Jensen OM, Larsen M. Objective assessment of photoreceptor displacement and metamorphopsia: a study of macular holes. Arch Ophthalmol. 116:1303-1306 (1998)
- Kramer PW, Lubkin V, Pavlica M, Covin R. Symptomatic aniseikonia in unilateral and bilateral pseudophakia. A projection space eikonometer study. Binocul Vis Strabismus Q. 1999;14:183-90.
- Foster A, "Vision 2020: The cataract challenge," J. of Community Eye Health 2000;13: 17-19
- McCormack G, Peli E, Stone P. Differences in tests of aniseikonia. Invest Ophthalmol Vis Sci. 1992;33:2063-7.
- Curtin BJ, Linksz A, Shafer DM. Aniseikonia following retinal detachment. Am J Ophthalmol. 1959; 47:468-471.
- Ugarte M, Williamson TH. Horizontal and vertical micropsia following macula-off rhegmatogenous retinal-detachment surgical repair. Graefes Arch Clin Exp Ophthalmol. 2006;244:1545-1548
- Ogle, KN. Researches in Binocular Vision, Saunders, Philadelphia (1950)