Aniseikonia is the difference between image size or shape perceived by both eyes. Most often caused by an asymmetric prescription of the two eyes, called optical aniseikonia. The unbalanced prescription can be caused by changes in the cornea, crystalline lens, and axial length of the eye. Higher prescription plus lenses may cause a pincushion distortion while higher minus lenses may cause a barrel distortion. Pincushion distortion causes the corners of the image to distort out, while barrel distortion the corners distort inwards. Other factors such as the index of refraction of the lens material, thickness and base curve all play a part in aniseikonia.
Figure 1: Distortion from optical lens https://www.vision-doctor.com/en/optical-errors/distortion.html
Aniseikonia can also be caused by pathological changes in the retina, called retinal aniseikonia. Swelling of the macula can be caused by multiple factors but most commonly diabetic macular edema, and wet age-related macular degeneration. Macular distortion can be caused by epiretinal membranes, vitreomacular traction, diabetic retinopathy, and others.
The clinical significance of aniseikonia is that the brain must interpret two different sized signals from each eye and attempt to combine it into a single uniform image. If the brain cannot adapt, it may lead to varying symptoms such as:
Poor adaptation to glasses
Development of amblyopia in younger children
There is no definite value that will cause clinical symptoms, each person will tolerate the imbalance differently. Generally, image size differences above 1-3% can lead to the symptoms noted above. Optical aniseikonia can be calculated by comparing the spectacle magnification between the two lenses. Direct measurement can be obtained by a clinician in office with several different diagnostic tools to be precise.
Figure 2: Awaya aniseikonia test https://optohellas.com/product/ophthalmology/vision-tests/o-vision-awaya-aniseikonia-test/
Depending on the source of the aniseikonia, it may be treated with spectacle correction, contact lens correction, aniseikonic lenses, corneal refractive surgery, and lens refractive surgery. The goal of these treatments is to decrease the difference in image size between the two eyes. Yearly eye exams and early pediatric eye exams are the key to identifying cases of anisometropia and aniseikonia.
Stokkermans TJ, Day SH. Aniseikonia. [Updated 2023 Mar 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK585108/
Wang GJ. Principles and designs of the aniseikonic lenses for near vision. Optom Vis Sci. 2006;83(12):931-936. doi:10.1097/OPX.0b013e31802e7f38
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