It’s a term that describes an ocular condition caused by a defect in the lens of the eye.
The term “Aniseikonia” comes from the greek wors “an” (not), “is” (equal), and “eikōn” (icon or image). The literal meaning being “unequal image.”
It refers to a condition featuring a defect of the binocular vision that results in the retinal image of an object in one eye getting perceived as different in size from the other eye.
In simpler terms, one eye sees an object as more prominent while the other sees it smaller.
Some doctors acknowledge two types of Aniseikonia:
- Static Aniseikonia, the most common form of Aniseikonia; in these cases, when the eyes are gazing in a specific direction, the peripheral images they perceive appear different in size.
- Dynamic Aniseikonia means that eyes need to rotate a different amount for the person to perceive or gaze a specific point in space.
One factor related to the development of this ocular condition is the eyes having a remarkable difference in their respective refractive error.
A Refractive Error refers to an eye disorder in which the eye fails to bring parallel light to focus on the retina. In some of these cases, Aniseikonia is an optically induced condition.
A medical expert who specializes in this field explains that if someone were to use glasses, even a slight difference in the potency of the lens could cause each eye to perceive the size of an image differently.
Another potential cause of Aniseikonia is facial asymmetry, which occurs when one side of the face doesn’t match the other.
In this case, this ocular condition may appear only in specific meridians.
For example, if patients have one eye set in a lower position relative to their other eye, they may perceive a disparity in how large an image appears.
Ocular surgery can also cause Aniseikonia. One of the procedures that may be related to this condition is the cataract removal in one eye.
Others may be surgical treatments meant to repair a torn or partially detached retina or fixing a case of swelling in the macular area (near the center of the retina).
Some causes could be related to much simpler factors such as accidents or physical trauma.
More often than not, the signs of Aniseikonia are the same from case to case, with some little variation depending on the cause of the issue as well as the circumstances or physicality of the patient.
Amblyopia, also known as “Lazy Eye,” or problems with the binocular vision.
It may occur due to patients having used lens in their infant age in an attempt to correct Anisometropia, but end up creating Dynamic Aniseikonia.
Other symptoms of Aniseikonia are similar to Asthenopia or Eye Strain, a condition characterized by dimness of vision, ghost images, and pain or discomfort in the eyes.
The difference is that while the typical signs of Asthenopia get alleviated by using some corrective lens, in the case of Aniseikonia, the symptoms persist even if the patient use such lenses.
Another sign of this ocular condition is related to some degree of intolerance to the correction of a refractive error through ophthalmic lenses in cases of astigmatism.
More often than not, the brain has to work harder to fuse these images to suppress the issue, which results in other signs such as headaches and lack of depth perception.
Medical experts have several ways to test the amount of disparity and the direction of the eyes.
Doctors typically use methods based on the perception of binocular space and a comparison of the size of the image as perceived by both eyes.
A comparison of visual perception, which involves having the patient gaze at an object while the eyecare specialist induces Diplopia (double vision) to quantify the difference in how each eye perceives size.
The Brecher Test, a simple diagnostic procedure in which the medical expert uses two penlights, a Maddox rod, and some handheld afocal magnifiers.
A Maddox rod is a commonly used handheld instrument that consists of several red-colored glasses set together in a disk often made of metal.
The patient has to cover one eye with the Maddox rod, while the eyecare specialist uses the penlights to make two red lines which the patient will have to determine their location relative to the light source.
Turville Test refers to a method in which the eyecare specialist measures the ocular disparity in the eyes by using a test chart containing two vertical rows of test letters and a mirror.
Upon the mirror, a broad black septum segregates the letters so that only one column is visible to each eye.
The New Aniseikonia Test (NAT), known by some people as the Ayawa Test thanks to the medical expert who developed it.
NAT uses a direct comparison approach to diagnose Aniseikonia. The patient will have to look at pairs of adjacent half-moon targets (one half is red, and the other is green) of a calibrated size difference.
Eyecare specialists will then use a software called Aniseikonia Inspector™ to separate the right and left eye images and determine the degree of Aniseikonia.
Some doctors consider this diagnostic procedure as a test that is easy to administer and interpret.
Other medical experts like to use the Eikonometer, which is a medical device designed to detect Aniseikonia and test stereoscopic vision.
This device projects an image with polarized light. The eyecare specialist gives the patient glasses with optical filters called “Polarizers.”
During this test, the left eye perceives half of the image and the right one sees the other half.
Miles Test, which is a diagnostic procedure pretty similar to the Brecher Test. In this case, the patients must place two Maddox rods over their eyes horizontally. The purpose is to determine vision disparity.
Space Eikonometer, a medical device developed in 1940. Some medical experts used to consider it the “gold standard” in measuring a patient’s Aniseikonia.
However, while it was a relatively useful tool despite being difficult to adjust, this device is no longer in production. Very few people have such equipment.
There is no universal consensus when it comes to dealing with cases of Aniseikonia. People can find several opinions and views on the best way to correct this ocular condition.
Depending on the patient’s case, the best solution will vary and sometimes medical experts may need to use a combination of multiple methods.
Contact lenses can help to eliminate Dynamic Aniseikonia and lessen Static Aniseikonia.
If the patient can tolerate the use of contact lenses, the eyecare specialist will prescribe a formula fitting of their degree and type of Aniseikonia.
This solution doesn’t compromise the visual acuity, it is cosmetically acceptable, and it’s not very expensive.
The classic method to correct Aniseikonia is optical magnification through the use of glasses. This option, just like the contact lenses, doesn’t affect visual acuity.
Other means to deal with this ocular condition involve toric lenses, doublet lenses, and fused bifocal lenses.