Aniseikonia

Aniseikonia is a binocular condition in which the two eyes perceive images of different size. These unequal images can be caused by a difference in:
 * optical magnification (i.e. different retinal image sizes)
 * retinal receptor distribution (i.e. a different sampling of the retinal images)
 * cortical processing (i.e. different processing of the sampled retinal images)

Patients at risk of aniseikonia
Aniseikonia is often associated with unequal refractive errors between the eyes (anisometropia). However, there are several other patient groups at risk. For example, research has shown that appr. 40% of the patients that underwent cataract surgery/surgeries and who had an intra-ocular lens(es) implanted, have complaints referable to aniseikonia. This also makes one wonder how much aniseikonia is induced with refractive surgery such as LASIK. Another group of patients at risk are patients who have a retinal condition or who underwent retinal surgery. For example with an epiretinal membrane (macular pucker) or after a retinal detachment surgery. The aniseikonia in these patients may be complicated because the aniseikonia is field-dependent (variable over the retina), but fortunately also these patients often can get more comfortable binocular vision by optically correcting the aniseikonia.

Symptoms
Most aniseikonic symptoms are quite general (a-specific), for example: headaches, asthenopia (ocular fatigue, burning, tearing, pain, pulling, etc.), light sensitivity, reading difficulty, nausea, and double images (diplopia). This is one of the reasons why aniseikonia is sometimes overseen by the treating eye care provider. Only, if the aniseikonia is severe, the patient could also actually see an image size difference by closing one eye at a time. However, symptoms usually occur already with much less aniseikonia.

Clinically significant aniseikonia values
Aniseikonia of 3% or more is generally considered clinically significant, but sensitive individuals may have symptoms with less aniseikonia.

Testing for aniseikonia
Testing for aniseikonia is important. In older books sometimes rules of thumb are given based on retinal images size differences alone. However, research has shown that even in anisometropia the retinal receptor distribution may be different between the eyes, making aniseikonia management based on calculated retinal image sizes inaccurate. Testing for aniseikonia can be done using a space eikonometric method (based on space distortions accompanying the aniseikonia) or a direct comparison method. The space eikonometric method is sometimes still used in research, but it is less suited for clinical purposes (and commercially unavailable). There are two commerically available aniseikonia tests: the New Aniseikonia Test1 (NAT, 1982) and the Aniseikonia Inspector2 (2003-2007). The first is a booklet and the second is software. The two tests are based on the same principle, but the Aniseikonia Inspector has several advanges due to the interactive possibilities when using a computer. Also, the Aniseikonia Inspector contains a module to calculate aniseikonia correcting prescriptions.

Correcting aniseikonia
Aniseikonia can be corrected by changing the optical magnification properties of the auxilary optics (glasses, contact lenses). For example, if the curvature or thickness of a spectacle lens is changed (without changing its refractive power, so there will still be a good visual acuity), the optical magnification will change. Also, the distance between the spectacle lens and the eye (vertex distance) affects the optical magnification. Therefore, contact lenses will in general give a different aniseikonia than glasses. It might even be possible to correct aniseikonia by fitting a contact lens together with a spectacle lens, creating a weak telescope system. What solution is best depends on the amount of aniseikonia, type of aniseikonia (optical-induced or retinally-induced), the refraction, and the importance of cosmetics. For more information on aniseikonia and possible correction options, see Optical Diagnostics' aniseikonia webpage3.