Aniseikonia is defined as a difference in the shape and/or size of images presented to the visual cortex by two eyes. The brain is unable to fuse two images, resulting in an extra ghost image or diplopia. Aniseikonia is often produced due to significant amount of anisometropia, especially when it is corrected by spectacles instead of contact lenses. It is difficult to detect aniseikonia based on history and clinical examination. Aniseikonia precludes fusion of images when the degree is large. Despite this, it is rare that a patient volunteer a difference in image size and/or shape between the two eyes. Aniseikonia is usually considered clinically significant when the image size difference is greater than 4 percent, but many patients experience distortions in spatial perception and/or uncomfortable binocular vision with differences as small as 2 percent.
The term ‘aniseikonia’ was introduced by Lancaster in 1938 to refer to a difference between two eyes in the perceived size of an object. The word aniseikonia originates from the Greek words aniso (unequal) and eikon (image).
Emmetropia is the condition where the eye has no refractive error and requires no correction for distance vision. Refractive power of the eye is determined predominantly by variables like power of the cornea, power of the lens, and axial length of the eyeball. In emmetropia, these three components of refractive power combine to produce normal refraction to the eye. In an emmetropic eye, rays of light parallel to the optical axis focuses on the retina. The far point in emmetropia (point conjugate to retina in non- accommodating state) is optical infinity, which is 6 meters. Ametropia (refractive error) results when cornea and lens inadequately focus the light rays.
The term ametropia (refractive error) describes any condition where light is poorly focused on light sensitive layer of eye, resulting in blurred vision. This is a common eye problem and includes conditions such as myopia (near-sightedness), hypermetropia (far-sightedness), astigmatism, and presbyopia (age-related diminution of vision).
Optical aniseikonia denotes aniseikonia due to a physically measured difference in the sizes of the retinal images that typically arises in uncorrected axial anisometropia or in corrected refractive anisometropia. Little if any difference in the size of the image occurs with corrected axial anisometropia or with uncorrected refractive anisometropia. Aniseikonia may occur in people for whom the images in the two eyes are equal in size, in which case it must be due to non-optical causes. This condition is referred as neural aniseikonia.
Aniseikonia is the cause of curable ocular discomfort suffered by small but not insignificant number of people.
Most commonly aniseikonia occur following eye surgery. An eye with significant refractive error as in aphakia (when operated for cataract) or refractive surgery, the refractive error is minimised in operated eye. But the other eye still requires a strong corrective lens for clear vision. Similarly, when both eyes are operated for cataract surgery but intraocular lens (IOL) with wrong power (pseudophakia) is used in one or both eyes, also produces aniseikonia.
Agarwal Sunita, Agarwal Athiya, Apple David J, Buratto Lucio, Aliό Jorge L, Pandey Suresh K, Agarwal Amar. Textbook of Ophthalmology Vol 1. Jaypee Brothers Medical Publishers (P) Ltd 2002. P 184- 185.
Lancaster Walter B. Aniseikonia. Arch Ophthalmol 1938; 20(6): 907- 912.
De Wit GC. Evaluation of a new direct-comparison anisikonia test. Binocul Vis Strabismus Q 2003; 18: 87- 94; discussion 94.
Corliss DA, Rutstein RP, Than TP, Hopkins KB, Edwards C. Aniseikonia testing in an adult population using a new computerised test, ‘the Aniseikonia Inspector’. Binocul Vis Strabismus Q 2005; 20: 205- 215; discussion 216.
Bagshaw J. Vertical deviations of anisometropia. Transactions of first international orthoptic congress. Kimpton: London 1968: 277- 286.
Tolerance for the disease varies amongst individual patient. Some patients apparently are able to tolerate rather large aniseikonia and others suffer severe symptoms with even smaller degree. It is when difference in size of the image or meridional distortions approaches tolerance levels that the symptoms manifest. Meridional distortions are poorly tolerated, especially when they are oblique.
When the variation in magnification or meridional distortion between two eyes is disproportionately high, it may produce symptoms such as
Aniseikonia may occur naturally or is produced secondary to correction of refractive error. Up to 7% of aniseikonia between two eyes is usually tolerated well, and it corresponds to about 3 dioptres (D) of anisometropia. The measuring unit for refractive error is dioptre (D), which is defined as the reciprocal of the focal length in meters.
II. Anatomical or retinal
Retinal factors may cause light projected on the retina by a perceived image to appear larger (macropsia) or smaller (micropsia), since variable number of photoreceptors are stimulated. Causes of retinal aniseikonia include retinal tears, detachment, macular hole, retinoschisis, epiretinal membranes or macular oedema.
III. Central or cortical
Diagnosis depends upon clinical symptoms and retinoscopic examination in patients with defective visual acuity.
Clinical aniseikonia may be defined as the amount of aniseikonia that is necessary to correct to eliminate symptoms. It usually occurs when the difference in image size between two eyes approaches 0.75%. The oblique meridional aniseikonia causes a rotational deviation between the fused images of vertical lines in two eyes. This is termed as declination. Declination becomes clinically significant when it approaches 0.3˚.
Clinical types of aniseikonia
Aniseikonia may be either symmetrical or asymmetrical.
In symmetrical aniseikonia, one image is larger than the other, either in all dimensions or in one meridian only. This difference in meridian may be oblique.
In asymmetrical aniseikonia, the image is distorted in some degree. This may be
Tests for aniseikonia:
An eikonometer is an instrument used to detect and measure aniseikonia. There are two basic types of eikonometer, the direct eikonometer and the space eikonometer.
Clinically, a simple printed direct comparison aniseikonia test and a computerised test is available to analyse aniseikonia.
Management should be carried out under medical supervision.
Principal factors which influence optical correction are aniseikonia and amblyopia. Anisometropia or difference in the refractive error of the two eyes is the most common cause of aniseikonia. Corrective guidelines are
- Aniseikonia is present. Contact lenses reduce the difference in the size of the image e. g. as in aphakia. Binocular single vision may be restored in favourable cases.
- There is no improvement of visual acuity in anisometropic amblyopia in children with a difference in refraction of 4 D or above.
- Trial contact lenses show a better binocular function as compared to spectacles.
Medical therapy includes
This is the preferred mode of treatment.
Surgical therapy includes
Retinal reasons for aniseikonia are treated according to the cause.