Two-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

 

Mike Tyner wrote
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.

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.

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.

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.

(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! ???

 

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