Aniseikonia

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.

References

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Khurana AK. Theory and Practice of Optics and Refraction Second Edition. Reed Elsevier India Private Limited 2008. P 86- 88.

Khurana AK. Comprehensive Ophthalmology Sixth Edition. Jaypee Brothers Medical Publishers (P) Ltd. 2015. P 18- 19.

Coats David K, Olitsky Scott E. Strabismus surgery and its complications. Springer- Verlag Berlin Heidelberg 2007. P 293.

Howard Ian P, Rogers Brian J. Binocular Vision and Stereopsis- Oxford Psychology Series No. 29. Oxford University Press 1995. P 62- 68.

Lens AI, Nemeth Sheila Coyne, Ledford Janice K. Ocular Anatomy and Physiology Second Edition. SLACK Incorporated 2008. P 163.

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http://eyewiki.aao.org/Aniseikonia

http://jamanetwork.com/journals/jamaophthalmology/article-abstract/613956

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

• Headache.
• Asthenopia (eye strain).
• Ocular discomfort or fatigue.
• Blurring of vision.
• Difficulty in reading.
• Photophobia.
• Disturbances in binocular vision.
• Amblyopia in children at an early stage of life.
• Diplopia.
• Disorientation.
• Disturbance in depth perception.
• Dizziness.

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.

Causes includes

I. Optical

• Inherent: This is due to defect in the diopteric system of the eye and is usually related to anisometropia.
• Acquired: This is determined by the correcting lenses and it depends upon the lenses worn, their power, position, thickness and form.

II. Anatomical or retinal

• Displacement of retinal elements: Displacement of retinal elements towards nodal point in one eye.
• Separation of neuroepithelial elements: Separation of neuroepithelial elements of retina may produce aniseikonia.
• Streching of retina.
• Retinal oedema.

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

• Asymmetrical simultaneous perception by cerebral cortex: The asymmetrical simultaneous perception in spite of equal size of images formed on the retina leads to aniseikonia.

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.

Symmetrical aniseikonia

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.

• Overall aniseikonia: In this type, the size of one ocular image is symmetrically different than the other image. The image may be magnified or minified symmetrically in both meridia.
• Meridional or cylindrical aniseikonia: Here the size of retinal image of one eye is symmetrically larger or smaller than that of other in one meridian only. The involved meridian may be vertical, horizontal or oblique.
• Compound aniseikonia: There is combination of overall and meridional differences.

Asymmetrical aniseikonia

In asymmetrical aniseikonia, the image is distorted in some degree. This may be

• Prismatic type: In this type, image difference increases progressively in one direction.
• Pincushion type: In this type, there is progressive increase in all directions from the visual axis, as is seen with high plus correction in aphakia.
• Barrel type: In this type, image distortion decreases progressively in both directions, as is seen in correction with high minus lenses.
• Oblique type: In this type, the size of image is same, but there occurs an oblique distortion of shape.

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.

• Direct comparison eikonometer: Direct comparison eikonometer presents dichoptic (separate and independent field by each eye) stimuli simultaneously in a stereoscope. The stimuli may be two identical patterns which appear side by side or they may be concentric displays. The difference in size of the two images that appear equal in size indicates the magnitude of anisikonia.
• Space eikonometer: Space eikonometer involves the psychophysical determination of distortions in stereoscopic vision induced by aniseikonia. This method can be used only in subjects with stereoscopic vision.

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.

Medical therapy

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

• Anisometropia should be fully corrected in the presence of amblyopia.
• Anisometropia of 3 D or more at one year of age should be corrected as it may produce amblyopia in 60% of cases.
• Degree of anisometropia between 1.50 D to 3 D that do not decline on follow-up should also be corrected.
• Older children and adults may not be able to tolerate full correction binocularly. Therefore, reduced optical correction of the more ametropic eye may be required even with compromised visual acuity.
• Large differences in refraction may require contact lenses, and are prescribed if

- 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.

• Patients with marked anisometropic myopia may show hypophoria in primary position with a head tilt to the side of more myopic eye to maintain a comfortable binocular single vision (the ‘heavy eye’ phenomenon).

Medical therapy includes

• Spectacles: The corrective glasses may be tolerated up to a maximum difference of 4 D in two eyes. A difference more than 4 D produces diplopia. In children younger than 12 years of age, where best visual correction is required in both the eyes, contact lenses are preferred. In adults, best correction may be prescribed which does not result in ocular discomfort.
• Contact lenses: Contact lenses are advised for higher degrees of anisometropia. These are more useful in young children with high anisometropia, who may become amblyopic in more ametropic eye.

Surgical therapy

This is the preferred mode of treatment.

Surgical therapy includes

• Intraocular lens implantation: Intraocular lens may be implanted for uniocular aphakia.
• Refractive corneal surgery: Refractive corneal surgery (refractive laser) may be done for unilateral myopia, astigmatism and hypermetropia. Photorefractive surgery, both photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK), is increasingly being used.
• Removal of crystalline lens: Crystalline lens may be removed for unilateral high myopia.
• Phakic IOL: Intraocular lens may be implanted without removing the crystalline lens.
• Clear lens extraction with the implantation of an IOL: Clear lens extraction with the implantation of an IOL, preferably foldable IOL or a piggyback IOL, may be done. Piggyback IOL means two IOLs are placed in the eye, one on top of the other. This is done if the biometry is +40 D and one does not have a lens of that power to implant.

Retinal reasons for aniseikonia are treated according to the cause.