I’ve had dry eye for a few years and have posted quite a bit about my problems focussing on images on a computer screen for hours on end. What I didn’t know is that there is a scientifically accepted reason why some patients can’t adjust properly to glasses but can see significantly better with contact lenses.
The condition is known as Aniseikonia.
“Aniseikonia is an ocular condition where there is a significant difference in the perceived size of images. It can occur as an overall difference between the two eyes, or as a difference in a particular meridian “
There are 2 kinds of aniseikonia, static and dynamic. Static is observable when we focus on an object while dynamic is observed when we try to move our eyes to focus on a moving object or parse text, for instance. See here for more information and some explanatory graphics.
For whatever reason I see a smaller image in my left eye versus my right eye, despite the optical correction being greater in my right eye by about -1.5 diopters of sphere and -0.75 of cylinder. This is a relatively small difference (my father has a -4.5 diopters SPH difference between right and left eyes) but the focussing problems described are similar to the problems I have. Amazingly, most of my problems go away when wearing contact lenses and looking at a screen without PWM. The difference is staggering. I can only surmise that the screen flicker and fusional problems (difficulty “fusing” an image from both eyes into a coherent single image) are what leads to the irritating and persistent breakdown of vision that I’ve experienced for a few years.
Aniseikonia isn’t a new phenomenon. It’s been appreciated for decades. A dramatic asymmetry of optical correction required for right and left eyes is common in my family, as is amblyopia.
It’s odd but no optician has suggested to any of us that isekonic lenses may help? In my case it’s marginal, in my father’s not so. Isekonic lenses correct the degree of magnification for each eye such that the size of the image in each eye is uniform.
“Treatment is done by changing the optical magnification properties of the auxiliary optics (corrective lenses). The optical magnification properties of spectacle lenses can be adjusted by changing parameters like the base curve, vertex distance, and center thickness. Contact lenses may also provide a better optical magnification to reduce the difference in image size. The difference in magnification can also be eliminated by a combination of contact lenses and glasses (creating a weak telescope system). The optimum design solution will depend on different parameters like cost, cosmetic implications, and if the patient can tolerate wearing a contact lens.”
A difference of around 5% or more between the size of images in each eye is enough to create fusional problems and reading difficulties in some patients. It’s covered in many optometry textbooks available on googlebooks and I can only conclude it’s taught in colleges. Yet I’ve seen 3 opticians and an ophthalmologist to treat fusional problems while wearing glasses and I’ve never heard about it nor heard a suggestion to get isekonic lenses made. It’s a puzzler. Nor has the suggestion been made to my father and he has little depth perception because he suppresses the image from the weaker eye. Would isekonic glasses have helped? Perhaps, it was never suggested or tried.
There’s a belief, perhaps, that patients can adapt to pretty much any lens based on the famous experiments where people became accustomed to living and working with glasses that inverted an image through the horizontal plane (turning it upside down) within 4 days. Yes, you got that right, people can adapt, mostly, to seeing upside-down after only 4 days. However, there’s no evidence as far as I can see that this means that people can commonly adapt to say 5-10% size difference in images. When patients can’t adapt to new spectacles, there are steps the optician should go through to help resolve the problem. I wonder how often this happens or whether they just hide :).
Obvious signs that someone can’t adjust are complaints about ghosted images, closing one eye to focus, double vision, headaches, poor depth perception etc. It’s tough to estimate but I reckon I see a 15% difference between image sizes in left versus right eyes. It’s that noticeable and that’s really distracting.
Optical designer Peter Shaw has a video on youtube about the phenomenon. Opticians selling his lenses (which require some elaborate measurement to adjust for frame height, size, tilt etc. ) claim to have treated amblyopia successfully with the Shaw Lens. When the image size is homogenised in all directions, correcting for lateral and vertical distortions, the brain stops suppressing images and the person can…. SEE.
I can well believe this because my fusional issues started when I had a lot of computer work and had a big change in eyeglass prescription. The jump was about 50% of SPH and CYL in both eyes. That’s created greater asymmetry and I started to have serious problems fusing images when reading.
Shaw’s video is here.
His company website has a detailed description of lens design and the technical issues to overcome here. I love my contacts but I plan to check out a set of Shaw Lenses as soon as I can find an optician who’ll order them 🙂 I don’t anticipate that being an easy task.