‘Ann-EYE-soh-met-ROPE-ee-ah’ – Anisometropia = a significant difference in refractive state of one eye compared to the other.
By far, in most cases, people’s eyes have roughly the same optical (refractive) properties right and left. So, as an example, if you’re near sighted by -2.00 diopters (seeing clearly at 50cm) in one eye, the other might be in the range of -2.50 to -1.50 or so, with some minor differences in astigmatism. When the optics (refraction) of each eye is very close to the other, this is called isometropia (EYE-soh-met-ROPE-ee-ah).
Our brains are designed to seek aligned (fused) images simultaneously, and this requires the following elements:
- Alignment on target (bi-foveation, both eyes centred on target, with contours overlapping as one fused whole)
- convergence for near targets
- divergence for distant targets
- Equal clarity of the visual signal (image is clear/focused) right and left
- Accommodation / focusing of the lens by the ciliary body.
- Roughly equivalent optics right and left eyes to maintain balanced focusing effort, image clarity, and image size.
Consider: Minus (-) power lenses minimize images, reduce the perceived size of the target. Plus (+) power lenses magnify images making them appear larger than they are. As a basic example of anisometropia, we can think of the person with the following glasses prescription:
RE/OD: -2.00DS – nearsighted by 2D, glasses reduce image size.
LE/OS: +2.00DS – farsighted by 2D, glasses enlarge image size.
(DS = diopters sphere, so no astigmatism correction). Without glasses, it would be very hard to meet the requirements set out above for fused single clear imaging. With glasses on, we now have better clarity, but the size differential is enhanced so that the right eye (RE/OD) image is perceived to be notably smaller than the left eye (LE/OS). Faced with an image size disparity, the brain does what it can to maintain overlapping image contours to produce a 3D percept, but it cannot ever properly and fully align the RE/LE images. The end result is a loss of depth perception, added ocular strain, and loss of motor coordination. In this case, the difference is 4D between the eyes. Anisometropia is defined as significant and problematic when the difference is at least 2D between the eyes, and this in any meridian (so, if there’s astigmatism, the same rules apply).
Standard ophthalmic lenses, like the kind you might buy at a discount store or online, will only enhance the appearance of the magnification (‘mag’) differences. Bad or insufficient optics will save money at the outset, but this is false economy given all the attending problems the individual will have to manage if magnification is not properly balance.
Better options for managing ‘aniso’ follow:
- Contact Lenses (reduce mag differences to nearly 0).
- ‘Balanced’ or Shaw Lenses (see shawlens.com)
Both of these options can go a long way to minimize mag differences. In doing so, performance and comfort are sure to improve. This is another good reason why therapists of all stripes should be at least somewhat familiar with concepts around refraction, prescriptions, and glasses. Learn more here.
You can learn a lot more about eyes and vision at VisionMechanic.net, so feel free to go over and have a look. You’ll be especially interested in spending time with us if you’re a parent, a teacher, therapist or doctor working with reading, developmental, and learning disorders, or even brain injuries – we’ve got a load of good advice we all should have been taught in school.