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Medical Forum / General / Vision / April 2005Two-mirror device for strabismus correction
Has anyone heard of read of anything similar to this? Any comments? My friend with strabismus is a former chief engineer, and I have a PhD in engineering, which I hope will lend credence to what I'm about to say. Strabismus is always associated with three or fewer rotations - inward/outward turning, up/down turning, and torsion of the eye. Every mathematician knows that the result of two rotations applied successively is a single rotation along an inclined axis. So the same is true of three. Also, two reflections always equals one rotation. The angle between the mirrors is half the rotation angle. So the rotation of light rays associated with strabismus can be corrected either by a prism or by a pair of plane mirrors. The correction of strabismus using a prism is well known; correction using a pair of plane mirrors is less well known. My friend has a 35 degree in-turning and 35 degree torsion. Through simple vector calculus (dot and cross products) I showed that this equals a single 52 degree rotation with axis (0.69, 0.69, -0.2) where x is horizontal, y is vertical and z away from the eye. I've built him a two mirror hand held correction device (images attached are from video). The angle between the mirrors is 52/2 = 26 degrees. By rotating the two dowels the mounting can be used for in-turning and out-turning eyes with a small amount of up/down turning, and for any degree of rotation from -75 to +75 degrees. The mounting, field of view and mirror shapes could all be improved. In theory, the field of view can be expanded to be as good as that of normal spectacles. It works; and doesn't cost much. If I may be so bold, I suggest that two-mirror devices are a viable alternative to prisms for other sufferers when the resultant rotation from strabismus is large. Looking forward, I see versions of this that are not hand-held, and versions that simultaneously correct for long/short sight by using one plane and one curved mirror. Making a curved mirror is not formidable, it's just a lens with a metal coating on one side. A two mirror system is like a periscope and is getting inpractical for everyday use if prisms solve that particular problem. I have played around with this kind of system. A none strabismic person can see normally with their eyes in a variety of different positions with no obvious binocular problems. There is a name for this phenonema which escapes me. Similarly you can place identical photos on the floor and move them around quite a bit with your feet and yet still get binocular vision providing you dont rotate them. where your system would be interesting is if you could vary the system so that around a small degree of variation the system continually created movement of the strabismic eye and therefore encouraged moves towards normal use of the eyes. So that eventually the eye was able to see normally without the mirrors. I have attempted to point out to experts here that eye movements are made via large movements which are essentially matched by each eye and also smaller movements which are only created via feedback from each eye and its required relative position needed to produce accurate binocularity. Most here seem to think this commonsense observation is impossible because they believe that eye movements are yolked together. Therefore via that reasoning strabismus must necessarily be a major neurological fault. However strabismus can be seen to vary with stress etc etc etc. Sounds interesting! Andrew . Where such a device defi oh by the way. You might want to look at the drawings for a 'myopter' which allows none strabismic eyes pointing directly ahead to be able to read books etc. Its a head mounted thing and looks like a small welding eye guard:-) It uses a prism and a two way mirror to split the incoming images and then enable recombination at a different angle. I am not an optical expert but if you use mirrors you may get strange effects because the strabismic eye is not in the same plane/axis/what not as the target? I found that felt very peculiar myself for bigger angles. The myopter solves that problem. This thing is made by a guy who advertises on the web. http://www.myopia.org/myopterpaper.htm Andrew Hi Andrew, In regard to the the use of a dual mirror device and the question of binocularity, I built a similar device (although for a different purpose), for which I had to take into account a little-known phenomenon called Panum's fusional area. To quote from my paper: "...Panum's fusional area, which in stereopsis allows the image to be pulled apart by some 2 degrees before being broken up into two separate images. The images are actually pulled apart on the retina, but a supra-retinal function maintains perception of a single image". If your are interested, you can read it on my Web site at http://aarcolin4.net Regards, Rich > Has anyone heard of read of anything similar to this? Any comments? Your attached video didn't come through on my news server. It's a neat idea. Very large deviations are either exotropes, usually intermittent, or large-angle esotropes who are best corrected surgically. When strab is corrected by prism, we still find the problem of what to do to break the suppression and make the wearer actually _use_ the corrected eye. Now if you could figure a compact way to provide _torsional_ correction (around the z axis) your market will be small but very, very interested. Most of those problems are acquired, so they already have healthy binocularity. > plane and one curved mirror. Making a curved mirror is not formidable, > it's just a lens with a metal coating on one side. Like Hubble? :) -MT > Has anyone heard of read of anything similar to this? Any comments? > [quoted text clipped - 40 lines] > plane and one curved mirror. Making a curved mirror is not formidable, > it's just a lens with a metal coating on one side. >>When strab is corrected by prism, we still find the problem of what to do to break the suppression and make the wearer actually _use_ the corrected eye. I have wondered if a way to solve this problem is to have some kind of device similar to one of those virtual reality game helmets. Most of the technology is already available off the shelf. The suppressing eye could get opportunities to see when the seeing eye sees only a blank image. Since kids like computer games and it seems best to catch this in childhood it seems like an idea worth pursuing. Andrew Thanks mike and andrew, great feedback. I forgot to give you the hyperlink to images. It's http://freepages.misc.rootsweb.com/~hallsofjamaica/Strab01.jpg and 02 and 03. This is just a first version, remember. I have hopes of increasing the field of view in version 2. I've also bought a Dick Smith magnifier that looks remarkably like the Myopter, in the hope that I can attach mirrors to it. The Myopter is a great looking system, with beamsplitter and 3 mirrors. Two of them could be realigned on skew axes for it to work with strabismus with Z-torsion. That's not impossible. > A two mirror system is like a periscope and is getting impractical for everyday use if prisms solve that particular problem. Yes, and no. I've figured out that a prism can be used to solve my problem, but it would have to be aligned at a very peculiar angle because of the eye torsion, not something easy to do with normal spectacles. Further, the prism would have to have a diopter of 125 to 130, which is well beyond the range normally prescribed. > if you could vary the system so that around a small degree of variation the system continually created movement of the strabismic eye and therefore encouraged moves towards normal use of the eyes. So that eventually the eye was able to see normally without the mirrors. I hadn't though of that, for the reason that my friend with strabismus got it at age 60, and the affected eye is totally fixed in place, can't move at all. He's now 80 and would like to avoid surgery. Yes, the variation could easily be done, by rotating the two dowels holding the mirrors, continuously from 75 or so degrees of divergence all the way down to zero divergence. The only drawback of that is that the mirrors would have to be more nearly circular and this would reduce (perhaps halve) the total field of view. > we still find the problem of what to do to break the suppression and make the wearer actually _use_ the corrected eye In this case the eye with strabismus has better vision than the other eye. > Now if you could figure a compact way to provide _torsional_ correction (around the z axis) your market will be small but very, very interested. Most of those problems are acquired, so they already have healthy binocularity. The torsional correction around the z axis is exactly what I set out to fix. For my friend I'm correcting his 35 degree torsion. I can't yet see any limit on how big a torsion can be corrected, 90 degree looks possible. looking at picture two i thought you might be a Kiwi. Thats got to be rimu! ??? |
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