Aniseikonia

Excerpt

The word aniseikonia derives from the Greek words "an," "is," and "eikon," which mean "not," "equal," and "image," respectively. Aniseikonia occurs when there is a difference in an image's perceived size or shape and is often caused by anisometropia, which is a difference in refractive error. When aniseikonia is caused by anisometropia, it is called optical aniseikonia. Aniseikonia can also be caused by changes in the shape and location of the macula and is then called retinal aniseikonia.

Aniseikonia at a young age can result in amblyopia. Aniseikonia at a later age can cause asthenopia, headache, diplopia, dizziness, nervousness, imbalance, nausea, spectacle intolerance, ocular suppression, and distorted space perception. It is thought that over 0.75% of aniseikonia can start to cause symptoms, that at 1 to 3% definite symptoms are present, and that more than 5% of aniseikonia is incompatible with binocular vision.

In many cases, optical aniseikonia is predicted by calculating the difference in spectacle magnification of the right and left spectacle lens. In clinical practice, the rule of thumb that every 0.25 diopter of anisometropia causes about 0.25% to 0.5% of aniseikonia is often used. However, studies have shown that these rules of thumb significantly overestimate aniseikonia and direct measurements are much more accurate.

Aniseikonia can be subdivided into symmetrical and asymmetrical aniseikonia. Symmetrical aniseikonia further subdivides into spherical (overall difference in magnification) and cylindrical (a difference in magnification in one orientation). Asymmetrical aniseikonia is also called distortion and occurs when perceived image size differences are unequal in different parts of the visual field.

In the case of prismatic distortion, the difference in image size increases in one direction. In the case of pincushion distortion, the corners of one image are stretched more outward than the other. In the case of barrel distortion, the corners of the image appear squeezed inward. In the case of oblique distortion, one image is rotated relative to the other. Aniseikonia is most accurately measured with an eikonometer, but most clinics do not have one. Many other techniques of measuring aniseikonia exist, some of which require minimal equipment and can be done in a regular eye clinic. All these tests employ different methods to dissociate the two ocular images and have developed a method to compare the shape and size difference of the images perceived by the two eyes.

Aniseikonia only occurs when the eyes are being used together and does not occur if vision is suppressed or reduced, as may be the case in alternating strabismus and unilateral amblyopia or ocular disease. Anisometropia also does not cause aniseikonia in specific situations described by Knapp's law. According to Knapp's law, axial anisometropia does not cause aniseikonia when the refractive difference between the eyes is solely due to a difference in the axial length (distance from the cornea to the retina) and when the correcting lens is placed at the anterior focal point of the eye which is about 16 mm in front of the cornea. However, clinical applications of Knapp's law are limited as anisometropia is usually not purely axial. The retina is usually stretched in the eye with the increased axial length (a cause of retinal aniseikonia), and placing a correcting lens at 17 mm in front of the eye is impractical.

Aniseikonia is treated with the surgical or medical correction of the underlying cause, optically with refractive surgery, clear lens exchange, secondary intraocular lens (IOL) placement, contact lenses, or via spectacle correction with iseikonic lenses. When this is not possible, occlusion of one eye may be the only alternative.