Aniseikonia
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Aniseikonia is the difference in image size or shape perceived between the eyes from unequal magnification.[1] The two major categories are optical aniseikonia (most commonly from anisometropia) and retinal aniseikonia (from macular pathology). Optically induced aniseikonia due to differences between the eyes occurs in approximately 1% to 3.5% of the population.[1] Symptoms include diplopia, headache, dizziness, spatial distortion, and asthenopia. Aniseikonia remains underrecognized in clinical practice, particularly the retinally induced form, despite its significant impact on binocular vision and quality of life.[1]
Disease Entity
Disease
Aniseikonia is a binocular vision disorder in which the two eyes perceive images of different sizes and/or shapes when viewing the same object.[2] It can be classified as overall (uniform magnification difference in all meridians) or meridional (magnification difference in one or more specific meridians).[3] When the magnification variance between the two eyes is disproportionately high, binocular fusion is disrupted and symptoms arise.
Etiology
Aniseikonia is broadly divided into two etiologic categories: optical (refractive) and retinal.
Optical Aniseikonia
Optical aniseikonia results from differences in the optical properties of the two eyes. This may be due to:
Anisometropia: Spectacle correction of anisometropia ≥1 D induces differential magnification. As a general rule, each diopter of anisometropia corrected by spectacles produces approximately 1% of aniseikonia.[4] Spectacle correction of ≥3 D of anisometropia may produce clinically significant aniseikonia (>3%).[5] Anisometropia may occur due to antimetropia (one eye hyperopic and the other myopic), simple anisometropia, or aphakia.
Pseudophakia: Cataract surgery and intraocular lens (IOL) implantation, particularly when performed unilaterally, can induce anisometropia and secondary aniseikonia. In anisometropic patients undergoing cataract surgery with a target of emmetropia, postoperative aniseikonia of 4% or more may be induced, particularly in axial anisometropia.[6] Aniseikonia increases after the first cataract surgery but typically returns to near baseline after the second eye is treated.[7]
Refractive surgery: Corneal refractive procedures performed asymmetrically between the eyes can induce aniseikonia.[8]
The distinction between axial and refractive anisometropia is clinically important: axial anisometropia corrected with spectacles produces less aniseikonia than refractive anisometropia corrected with spectacles (Knapp's rule), whereas the reverse is true for contact lens correction.[9] However, nonoptical factors such as differential retinal stretching in myopic eyes also contribute to aniseikonia beyond what Knapp's rule would predict.[9]
Retinal Aniseikonia
Retinal aniseikonia results from mechanical distortion and displacement of retinal photoreceptors.[1] When photoreceptors are compressed together (e.g., by an epiretinal membrane), a fixed visual angle stimulates more photoreceptors than normal, causing the brain to perceive the image as larger (macropsia). Conversely, when photoreceptors are stretched apart, fewer receptors are stimulated, resulting in a smaller perceived image (micropsia).[10]
Retinal conditions associated with aniseikonia include:
Epiretinal membrane (ERM): The most common cause of retinal aniseikonia. Approximately 68% of ERM patients exhibit macropsia, with a mean aniseikonia of approximately 6%.[11][12] Notably, vitrectomy improves visual acuity but often does not resolve aniseikonia in ERM patients, as the aniseikonia is associated with inner nuclear layer thickness.[12]
Macular hole: Approximately 55% of patients present with micropsia. Vitrectomy for macular hole does improve aniseikonia, unlike in ERM.[13]
Rhegmatogenous retinal detachment (RRD): About half of patients with successfully repaired RRD have aniseikonia. Macula-off RRD tends to cause micropsia, while macula-on RRD tends to cause macropsia (often from secondary ERM formation).[14]
Cystoid macular edema (CME): Including CME from branch/central retinal vein occlusion and diabetic macular edema, typically causes micropsia.[11]
Central serous chorioretinopathy
Age-related macular degeneration[15]
An important feature of retinal aniseikonia is that it may be field-dependent—the degree of aniseikonia varies with visual field angle—unlike optical aniseikonia, which is constant across the visual field. This means retinal aniseikonia cannot be fully corrected with conventional optics.[2]
Epidemiology
Optically induced aniseikonia occurs in approximately 1% to 3.5% of the general population.[1] Among patients with retinal disorders undergoing vitrectomy, 59% present with measurable aniseikonia preoperatively.[11] Retinal aniseikonia is increasingly recognized in the aging population due to the rising incidence of ERM, macular holes, and retinal detachments.[1] Anisometropia itself is becoming more prevalent in populations with high rates of myopia, particularly in East and South-East Asia, which may in turn increase rates of optically induced aniseikonia.[16]
Symptoms
Aniseikonia can induce:
Diplopia: The diplopia caused by aniseikonia is characteristically not relieved by prism therapy, which can be an important diagnostic clue.[15]
Asthenopia (eye strain and visual fatigue): Induced aniseikonia increases orbicularis oculi muscle activity, an objective measure of visual fatigue, at levels as low as 3%.[17]
Spatial distortion: Patients may perceive tilting or warping of surfaces. Meridional aniseikonia specifically causes a trapezoidal distortion of rectangular shapes.[3]
Headache, dizziness, and disorientation
Impaired stereopsis and suppression: Stereoacuity is degraded with increasing aniseikonia.[4]
Tolerance
Individual tolerance to aniseikonia varies substantially. While 2% to 3% is often cited as a general threshold (corresponding roughly to 2–3 diopters of anisometropia), there is marked inter-individual variation. In one study, 19% of individuals lost stereopsis at ≤1% aniseikonia, while 54% tolerated >10% without impaired stereoacuity.[4]
Diagnosis
Clinical Evaluation
Aniseikonia should be suspected in patients with binocular vision complaints—especially diplopia unresponsive to prism—in the setting of anisometropia, recent cataract/refractive surgery, or known macular pathology. Key diagnostic considerations include:
History of recent change in refractive correction or ocular surgery
Monocular testing for macropsia or micropsia (e.g., using Amsler grid)
Assessment of stereopsis, which may be degraded
Measurement
Several instruments are available to quantify aniseikonia:
Space Eikonometer: The historical gold standard. Measures aniseikonia accurately in both meridians, but the instrument is no longer commercially manufactured and is rarely available.[18]
New Aniseikonia Test (NAT): A hand-held direct comparison test using red/green anaglyphs. Practical and portable but tends to underestimate aniseikonia.[19]
Aniseikonia Inspector (AI3): A computer-based software program using red-green anaglyphs with adaptive psychophysical protocols. It allows measurement of both overall and meridional aniseikonia and can also be used to design iseikonic lenses. While it also underestimates aniseikonia compared to the Space Eikonometer, it is the most widely used clinical tool currently available.[18][20]
Clinical estimation: In the absence of formal testing, aniseikonia can be estimated from the dioptric difference between eyes (~1% per diopter of spectacle-corrected anisometropia), though this applies only to optical aniseikonia and does not account for retinal or neural factors.[21]
Despite the limitations of current tests, they remain clinically useful to identify whether aniseikonia exists, determine which eye has the larger perceived image, and guide management decisions.[22]
Management
Treatment varies based on the etiology of aniseikonia.
Optical Aniseikonia
Contact lenses: Moving the correcting lens from the spectacle plane to the corneal surface minimizes magnification differences, making contact lenses a first-line option for anisometropia-induced aniseikonia. Contact lenses are particularly effective in axial anisometropia and alleviate both aniseikonia and prismatic effects.[16]
Iseikonic (size) lenses: Custom spectacle lenses designed to equalize image magnification between the two eyes. The shape factor (base curve and center thickness) and power factor of the spectacle lens can be independently modified to achieve a target magnification. Modern high-index and aspheric lens designs allow higher levels of aniseikonic correction in cosmetically acceptable frames.[21] The Aniseikonia Inspector software can assist in designing iseikonic prescriptions.[22]
Corneal refractive surgery: Procedures such as LASIK or PRK can reduce or eliminate anisometropia, thereby correcting the source of optical aniseikonia.
IOL-based strategies: In cataract surgery, IOL power selection can be optimized to minimize postoperative aniseikonia, particularly in patients with pre-existing anisometropia. Prediction models using pseudophakic eye models and matrix algebra have been developed for this purpose.[23] Toric IOLs combined with spherocylindrical spectacle overcorrection can be used to match meridional magnification between eyes.[24]
Retinal Aniseikonia
Management of retinal aniseikonia is more challenging because the aniseikonia may be field-dependent and therefore not fully correctable with conventional optics.[2]
Treatment of the underlying retinal pathology: Vitrectomy for macular hole improves aniseikonia in most patients.[13] However, vitrectomy for ERM generally improves visual acuity but not aniseikonia.[12] Aniseikonia may persist or even worsen after surgical repair of retinal detachments.[1]
Partial iseikonic correction: Even when aniseikonia is irregular and field-dependent, correcting 5% to 10% of the overall aniseikonia with iseikonic lenses may improve visual comfort, particularly for reading.[2][22]
Bangerter filters: Degrading the image in one eye with a Bangerter filter can relieve diplopia and binocular discomfort when other methods are insufficient. This was the most frequently used treatment modality in one case series of retinally induced aniseikonia.[15]
Occlusion: Monocular occlusion is a last resort for intractable cases.
Additional Resources
Porter D, Turbert D. Eyestrain (Symptom). American Academy of Ophthalmology. EyeSmart/Eye health. https://www.aao.org/eye-health/symptoms/eyestrain-2. Accessed March 20, 2023.
Vimont C, Barber LG. Eye Strain: How to Prevent Tired Eyes. American Academy of Ophthalmology. EyeSmart/Eye health. https://www.aao.org/eye-health/diseases/what-is-eye-strain. Accessed March 20, 2023.
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Rutstein RP, Currie DC. Topical review: retinally induced aniseikonia. Optom Vis Sci. 2019;96(10):739-747.
- ↑ 2.0 2.1 2.2 2.3 de Wit GC, Muraki CS. Field-dependent aniseikonia associated with an epiretinal membrane: a case study. Ophthalmology. 2006;113(1):12-17.
- ↑ 3.0 3.1 Velez FG, Pineles SL, Rosello N, Nguyen A, Guyton DL. Meridional aniseikonia—causes, symptoms, and therapies. J AAPOS. 2021;25(1):34.e1-34.e5.
- ↑ 4.0 4.1 4.2 Krarup T, Nisted I, Kjaerbo H, et al. Measuring aniseikonia tolerance range for stereoacuity—a tool for the refractive surgeon. Acta Ophthalmol. 2021;99(1):e114-e119.
- ↑ Cavuoto KM, Chang MY, Heidary G, et al. Effectiveness of laser refractive surgery to address anisometropic amblyogenic refractive error in children. Ophthalmology. 2022;129(11):e146-e163.
- ↑ Gobin L, Rozema JJ, Tassignon MJ. Predicting refractive aniseikonia after cataract surgery in anisometropia. J Cataract Refract Surg. 2008;34(8):1353-1361.
- ↑ Rutstein RP, Fullard RJ, Wilson JA, Gordon A. Aniseikonia induced by cataract surgery and its effect on binocular vision. Optom Vis Sci. 2015;92(2):201-207.
- ↑ Enoch JM. Management of aniseikonia after intraocular lens implantation or refractive surgery. J Refract Surg. 1997;13(1):79-82.
- ↑ 9.0 9.1 Bradley A, Rabin J, Freeman RD. Nonoptical determinants of aniseikonia. Invest Ophthalmol Vis Sci. 1983;24(4):507-512.
- ↑ Benegas NM, Egbert J, Engel WK, Kushner BJ. Diplopia secondary to aniseikonia associated with macular disease. Arch Ophthalmol. 1999;117(7):896-899.
- ↑ 11.0 11.1 11.2 Okamoto F, Sugiura Y, Okamoto Y, Hiraoka T, Oshika T. Aniseikonia in various retinal disorders. Graefes Arch Clin Exp Ophthalmol. 2017;255(6):1063-1071.
- ↑ 12.0 12.1 12.2 Okamoto F, Sugiura Y, Okamoto Y, Hiraoka T, Oshika T. Time course of changes in aniseikonia and foveal microstructure after vitrectomy for epiretinal membrane. Ophthalmology. 2014;121(11):2255-2260.
- ↑ 13.0 13.1 Okamoto F, Sugiura Y, Moriya Y, et al. Aniseikonia and foveal microstructure in patients with idiopathic macular hole. Ophthalmology. 2016;123(9):1926-1932.
- ↑ Okamoto F, Sugiura Y, Okamoto Y, Hiraoka T, Oshika T. Aniseikonia and foveal microstructure after retinal detachment surgery. Invest Ophthalmol Vis Sci. 2014;55(8):4880-4885.
- ↑ 15.0 15.1 15.2 Rutstein RP. Retinally induced aniseikonia: a case series. Optom Vis Sci. 2012;89(11):e61-e69.
- ↑ 16.0 16.1 Evans BJW, Shah R, Vlasak N. The changing natural history of anisometropia: a scoping review. Ophthalmic Physiol Opt. 2026;46(2):267-289.
- ↑ Redondo B, Vera J, Molina R, Molina-Molina A, Jiménez R. Orbicularis oculi muscle activity during computer reading under different degrees of artificially-induced aniseikonia. PeerJ. 2023;11:e16548.
- ↑ 18.0 18.1 Rutstein RP, Corliss DA, Fullard RJ. Comparison of aniseikonia as measured by the Aniseikonia Inspector and the Space Eikonometer. Optom Vis Sci. 2006;83(11):836-842.
- ↑ McCormack G, Peli E, Stone P. Differences in tests of aniseikonia. Invest Ophthalmol Vis Sci. 1992;33(6):2063-2067.
- ↑ Tan X, Chin MP, Ai G, et al. Aniseikonia tests: a review of current measurement methods. Curr Eye Res. 2026;51(6):555-566.
- ↑ 21.0 21.1 Achiron LR, Witkin N, Primo S, Broocker G. Contemporary management of aniseikonia. Surv Ophthalmol. 1997;41(4):321-330.
- ↑ 22.0 22.1 22.2 Currie D. Partial correction of irregular aniseikonia secondary to retinal traction. Optom Vis Sci. 2012;89(7):e87-e96.
- ↑ Langenbucher A, Szentmáry N, Cayless A, Wendelstein J, Hoffmann P. Prediction of ocular magnification and aniseikonia after cataract surgery. Acta Ophthalmol. 2022;100(8):e1611-e1620.
- ↑ Langenbucher A, Reese S, Huber S, Seitz B. Compensation of aniseikonia with toric intraocular lenses and spherocylindrical spectacles. Ophthalmic Physiol Opt. 2005;25(1):35-47.
