Aniseikonia  Symptoms, Causes , Diagnosis and Unknown Facts
Aniseikonia is a disorder of structure in a human, specific symptoms or that affects a specific location and is not simply a direct result of physical injury.

What is Aniseikonia or Anisekonia Symptoms ?

Aniseikonia   symptoms 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 symptoms is often produced due to significant amount of anisometropia symptoms, especially when it is corrected by spectacles instead of contact lenses.  It is difficult to detect aniseikonia symptoms based on history and clinical examination. Aniseikonia symptoms 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 symptoms 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.

Aniseikonia  Symptoms

  • 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  Symptoms  Causes

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.

Aniseikonia  Symptoms Diagnosis

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.