Aniseikonia - A Review


What is Aniseikonia?

Aniseikonia is a term coined to describe a difference in size of the optical images of the two eyes. By optical is meant not the retinal image, but that image which reaches consciousness as a perception. However, according to anther definition, in clinical syndrome of aniseikonia, the retinal images differ in size in the right and left eyes.

How do you get aniseikonia?

Depending on their nature, the causes of aniseikonia may be classified as the following:

  • Anisometropia: The most obvious cause of aniseikonia is anisometropia and the wearing of glasses that differ in magnifying power for the two eyes. In fact, aniseikonia is most commonly encountered with spectacle corrections in anisometropia. The closer to the eye’s principal point an optical correction is located; the less is its effect on the retinal image size.
  • Asymmetrical convergence: The next most common cause of aniseikonia is asymmetrical convergence. When an object is brought near the eyes, its image increases in size. If it is in the midline, so that the eyes converge symmetrically upon it, the increase is equal in the two eyes. But if the object is far to the right or the left, it is obviously not equally near each eye, but much nearer one eye. The retinal image in that eye is, therefore, larger.
  • Unknown cause: There are certain people in which no well-known cause exists to account for the aniseikonia. In these cases also the eyes are able to compensate for small or moderate amounts of aniseikonia.

Can laser surgery (LASIK) cause Aniseikonia?

Aniseikonia can be induced by refractive laser surgery (such as LASIK), typically when refractive error is different in one eye than the other after surgery. Similarly, Aniseikonia may also occur if a myopic patient is overcorrected in one eye into hyperopia, a hyperopic patient is overcorrected in one eye into myopia, or any combination where the refractive error is significantly different after refractive surgery. In addition, temporary aniseikonia almost always occurs during the gap between refractive surgeries when a patient has surgery performed on one eye at a time.

Why does it matter if I have it?

Aniseikonia is not usually easy to determine from the symptoms because they are the general manifestations of eyestrain. Aniseikonia is considered to be clinically significant when a patient’s visual system has difficulty combining two images of different size and/or shape in a single perceived image. Aniseikonia throws a burden on the neuromuscular mechanism for binocular vision.

What can I do about it?

The treatment for aniseikonia depends on what is causing it. It can be corrected with glasses, contact lenses, additional laser eye surgery, wavefront-guided surgery or topography-guided surgery.

The most effective way to reduce or eliminate aniseikonia is to provide an iseikonic prescription. One cannot change the effective power at the cornea, because this would reduce the patient’s visual acuity. However, one can change the accompanying spectacle magnifications of the corrective lenses by manipulating the base curve, centre corneal thickness and index of refraction.

Are there any possible long-term effects if I don’t get it corrected?

The consequences of aniseikonia may be considered from two points of view:

  • How it affects seeing, and
  • What symptoms it causes.

Generally speaking, aniseikonia within the range of 1 to 5 per cent produces faulty spatial localization. Above the 5 per cent level, the predominant symptoms are those of diplopia or suppression. If left untreated, severe aniseikonic symptoms have been experienced by patients following retinal detachment surgery, 49 macular edema, and astigmatism induced by dendritic ulcer. Such symptoms may include headache, vertigo, asthenopia, stammering, and numerous other complaints.

Won’t my eyes just adjust to seeing objects differently?

In ordinary cases, patient’s eyes do get adjusted to some extent, so that binocular vision goes on satisfactorily. However, the amplitude of adjustment in aniseikonia is limited as compared to other amplitudes, viz., accommodation which compensates for certain common errors of refraction, or the amplitude of fusion which compensates for heterophoria. In case of severe or large aniseikonia, however, the task of compensating for any of the defects of binocular vision is too great for the amplitude of adjustment and, hence, the patient’s eye tend to avoid binocular vision which eventually leads to strabismus.

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