Aniseikonia is a condition that causes each eye to perceive the size of objects differently. A difference in image size of about 5% is well tolerable. If there is equal image size in both the eyes, the condition is known as iseikonia.
What are the causes of Aniseikonia?
- Optical aniseikonia may occur due to inherent or acquired anisometropia (having significantly different refractive error in both eyes) of high degree.
- Retinal aniseikonia may develop due to displacement of retinal elements, stretching or oedema of the retina.
- Cortical aniseikonia implies asymmetrical simultaneous perception inspite of equal retinal image size.
What are the types of Aniseikonia?
- Optically-induced aniseikonia: this condition occurs secondary to anisometropia caused naturally, or secondary to refractive surgery, pseudophakic IOL implantation, or aphakia.
- Retinally-induced aniseikonia: compression, stretching, or damage to the retina can cause light projected on the retina by a perceived image to appear larger (macropsia) or smaller (micropsia), as a variable number of photoreceptors may be stimulated. Causes of retinally-induced aniseikonia include retinal detachment, retinal tears, retinoschisis, macular edema, macular hole, or epiretinal membranes.
Clinical features of aniseikonia
Symptoms can include headaches, double vision, disorientation or dizziness, tearing, eye pain or fatigue, sensitivity to light, visual acuity issues and trouble reading, as well as nausea.
Measurement of degree of Aniseikonia
1. Space eikonometer method: The degree of aniseikonia can be exactly measured with the help of an instrument called space eikonometer. This instrument is expensive, difficult to use and of little therapeutic value.
2. Rough estimate of degree of aniseikonia can be made by the rule of thumb’ given to the practitioner by the Dartmouth studies, which is as follows: ‘If the difference in image size associated with anisometropia is primarily of refractive origin, the aniseikonia produced will be about 1.5% per dioptre of anisometropia, but since the anisometropia may be partly axial, an estimate of 1.0% per dioptre is more useful clinically’.
- Optical aniseikonia can be corrected as given below:
- Unilateral aphakia is best corrected by IOL implantation.
- Contact lenses are a better choice than spectacles for correcting anisometropic aniseikonia.
- Refractive corneal surgery is probably the best choice at present for treating aniseikonia.
- Aniseikonic spectacles are difficult to make and expensive. These have been reported to be very effective in relieving the symptoms. However, nowadays they are sparingly used.
- Retinal aniseikonia is corrected by treating the causative disease.
- Cortical aniseikonia is very difficult to treat.
Reference: Theory and practice of optics and refraction
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