Iseikonia (eye-suh-koh-nee-ah) is a state where the left and right eyes produce equally sized images on the retina. Aniseikonia, then, is a condition where, for whatever reason, the retinas are shown imagery of distinctly different size. Aniseikonia results from a number of causes, but the end product is that the eyes see two different images. While the images share all the same features, they will be of sufficiently differing size that the brain will struggle in overlapping these to form a 3-dimensional and stable image, what vision scientists refer to as 3rd degree fusion.
Assuming the eyes are of equal geometry, one common cause of aniseikonia is anisometropia (an-eye-soh-meh-troh-pee-ah), or where each eye has a very different lens prescription. (We all have some sort of ‘prescription’, but not all people need lenses to function well.) If, for example, the right eye has a prescription of +2.00D (diopters), so farsighted, and the left’s prescription is -2.00D, nearsighted, there is a 4.00D difference between the two. This alone may not be a concern: While uncomfortable at times, a child can focus through +2D of farsightedness to see clearly; at the same time, the left eye at -2D is is nearsighted, rendering near objects clear, with blur in the distance. In the end, the child will see far with the right, and near with the left. Not ideal, but it works.
The problem arises when we put a lens on this child to ease the strain of the farsightedness (hyperopia) and sharpen the distance viewing for the nearsighted eye (myopia). A hyperopia-correcting lens is a fat ‘plus’ lens, thick in the middle, and it will magnify images. The nearsighted-correcting lens is a thin-in-the-middle ‘minus’ lens, which will minify images, making them look smaller. Putting these lenses in front of the child’s eyes will cause the right image to appear significantly larger than the left image, that is, the images appear this way to the right and left retinas. What the retina sees, the brain sees.
With different sized images, the brain now struggles to overlap them. This causes extra work for targeting and focusing and generally falls apart as visual strain, headache, poor concentration, and a generally miserable time dealing with fine visual detail, period.
Another complication: Plus lenses tend to move the eye away from the center of the lens due to prism effects. Minus lenses move the eye towards the center, for the same reason. So, as the eyes wander outward from the center of the lens, to look at something in the corner of one’s vision or for reading, for example, the lenses themselves will pull the right eye away from center, and the left lens will tend to keep the eye near the center. It’s not hard to appreciate how if the eyes are pulled in different directions, there will be ongoing visual fatigue and discomfort, not to mention the hit on visuomotor skills performance and fluidity of movement. If the prism effects are not reduced, glasses themselves can inadvertently be adding to any eye turns, or cause one where there was none to begin with. (This is where buying glasses online is often a problem: Misaligned glasses cause eyestrain due to prismatic effects.)
So what do we do? If we seek clarity of eyesight right and left where the prescriptions are very different, we risk causing other problems. Ideally, we should be able to achieve clear eyesight without affecting function, but with standard ophthalmic lenses, we have to tread lightly: The more we try to achieve the ideal single-eye prescriptions, the more of a difference in lens power we create between the eyes. Smart prescribers will pay attention to these important details.
Historically, optometrists could adapt the lenses and create unique left and right lenses, like a matched set. Various parameters in the lens manufacturing process can be modified to create lenses that minimize size differences (iseikonia) as well as the differences in strain and alignment caused by prismatic effects (isophoria). This was once done manually and required a fair bit of math and some reliable tables of values to draw from.
Nowadays, thankfully, we have lens providers such as ShawLens.com, in Toronto, Ontario, Canada. ShawLens provides a ‘Lens Tool’ that allows prescribers to enter various values from measurement and the Tool then calculates the difference in optics between using standard lenses, and when using iseikonic/isophoric lens designs.
In the right cases, such balanced lenses make a world of difference, especially in anisometropic amblyopia, significant anisometropia, or in antimetropia (where one eye is nearsighted, the other farsighted). This a generalization, and there are other situations where such lens balancing can be life-altering. In cases of anisometropic amblyopia, for example +3D in the right eye and +6D in the left, wearing balanced lenses with an ideal prescription, as well as pursuing active vision rehabilitation therapy can, combined, eliminate the need for patching and reduce the risk of eye turns.
Iseikonic lens treatment is not cheap: It will add perhaps $200-$300 to the total cost of your lenses. In the right circumstances, however, it’s just what the doctor ordered for those difficult prescriptions.
As a guideline: If there is 1D or more difference between the eyes, in either the sphere (nearsighted/farsighted) or cylinder (astigmatism) values, it can be considered anisometropia. At differences of 2D or more, balanced (aka ‘iseikonic’) lenses should be considered, especially if the individual is symptomatic for learning, reading, or behaviour concerns.