mirror astigmatic axes

[B. Janse]

Is there a tendency that the astigmatic axes of the left eye and the right
eye are mirror images?

I was wondering because they are in my eyes (right S-4.00 C-1.5 110°,
left S-5.50 C-1.5 70°).

[Dr. Leukoma]

Very common.

DrG

[Mike Tyner]

Post by B. Janse
Is there a tendency that the astigmatic axes of the left eye and the right
eye are mirror images?
I was wondering because they are in my eyes (right S-4.00 C-1.5 110°,
left S-5.50 C-1.5 70°).

Yes, so common that it's expected. And the higher the cyl, the more
symmetrical they tend to be, with less variation year-to-year.

When there's asymmetry (like both eyes x 135) we expect a history of
surgery, or trauma, or a wrong refraction.

-MT

[dumbstruck]

Post by Mike Tyner

Post by B. Janse
Is there a tendency that the astigmatic axes of the left eye and the right
eye are mirror images?

...

Post by Mike Tyner
Yes, so common that it's expected. And the higher the cyl, the more
symmetrical they tend to be, with less variation year-to-year.

Wow, does anyone have an explanation for why axes would tend to
perpendicular? I can only imagine it might be an adaptation to regain
lost data in the other eye by having the opposite kind of distortion
per each rotational plane; in fact don't the L+R cyls cancel out if
overlayed, which perhaps the brain attempts?

P.S. Does anyone feel the convention of .25 stepsize for cyl may be too
coarse? I was thinking since astig is so distracting, and since (I
think) the physical eye can't really adjust to even small cyl errors,
perhaps the stepsize should be .20 or even .10? Also my prescription
template only allows for a plus sph correction for close reading;
should it allow perhaps a tiny plus for cyl as well (maybe under .25)?

[Dr. Leukoma]

Post by dumbstruck
Wow, does anyone have an explanation for why axes would tend to
perpendicular? I can only imagine it might be an adaptation to regain
lost data in the other eye by having the opposite kind of distortion
per each rotational plane; in fact don't the L+R cyls cancel out if
overlayed, which perhaps the brain attempts?

If that was the explanation for perpendicular axes, then I would expect
that nature would be more generous. As it turns out, it is pretty
uncommon.


DrG

[Mike Tyner]

Post by dumbstruck
Wow, does anyone have an explanation for why axes would tend to
perpendicular?

"Perpendicular" is the exception, not the rule.

Post by dumbstruck
I can only imagine it might be an adaptation to regain
lost data in the other eye by having the opposite kind of distortion

Then you must think that astigmatism increases over time. To some degree, it
might, but the symmetry we were talking about is typically present at birth.

Post by dumbstruck
P.S. Does anyone feel the convention of .25 stepsize for cyl may be too
coarse? I was thinking since astig is so distracting, and since (I
think) the physical eye can't really adjust to even small cyl errors,
perhaps the stepsize should be .20 or even .10?

It'd be impractical to change a million instruments, and there's little
benefit in prescribing 0.10 if a lab's fabrication tolerances are +/- 0.125.

Post by dumbstruck
Also my prescription
template only allows for a plus sph correction for close reading;
should it allow perhaps a tiny plus for cyl as well (maybe under .25)?

Plus and minus notation are only numeric conventions. If you want to express
the value 5, it doesn't matter whether you say 3+2 or 7-2. Both equal 5.

-MT

[William Stacy]

Post by Mike Tyner
It'd be impractical to change a million instruments, and there's little
benefit in prescribing 0.10 if a lab's fabrication tolerances are +/- 0.125.

This brings up one of my pet peeves. Many software programs won't .12
steps in sphere or cyl. So if your refraction indicates a particular
paramter is more than .5 but less than .75, I want to Rx .625 because
given the .125 lab slop, I've got a better chance than otherwise. I
mean say the "real" value is .70, and I chose .5 because I'm pretty sure
.75 would be an over-correction. Then the lab makes a .375 because
that's within .125 of what I specified. Now you're off by .375 while if
you could have Rxed .625, you'd only allow the Rx to be between .50 and
.75, and the resulting real error would be no more than .2 D.

So this little bit of software and erroneous optometric thought almost
doubles the potential error.

It's the same reason I never Rx glasses in 5 degree axis steps. If I
think it's over 10 deg but under 15, I'll chose 12 or 13. At least I'm
making the lab shoot at a smaller bullseye. Some guys laugh at a half
cyl axis 13, but if you follow my logic, you might stop laughing.

Thankfully, most phoropters are still capable of .125 steps in the
sphere at least, and lensometers even are still calibrated in 1 degree
steps.

w.stacy, o.d.

[Neil Brooks]

Post by William Stacy

Post by Mike Tyner
It'd be impractical to change a million instruments, and there's little
benefit in prescribing 0.10 if a lab's fabrication tolerances are +/- 0.125.

This brings up one of my pet peeves. Many software programs won't .12
steps in sphere or cyl. So if your refraction indicates a particular
paramter is more than .5 but less than .75, I want to Rx .625 because
given the .125 lab slop, I've got a better chance than otherwise. I
mean say the "real" value is .70, and I chose .5 because I'm pretty sure
.75 would be an over-correction. Then the lab makes a .375 because
that's within .125 of what I specified. Now you're off by .375 while if
you could have Rxed .625, you'd only allow the Rx to be between .50 and
.75, and the resulting real error would be no more than .2 D.
So this little bit of software and erroneous optometric thought almost
doubles the potential error.
It's the same reason I never Rx glasses in 5 degree axis steps. If I
think it's over 10 deg but under 15, I'll chose 12 or 13. At least I'm
making the lab shoot at a smaller bullseye. Some guys laugh at a half
cyl axis 13, but if you follow my logic, you might stop laughing.
Thankfully, most phoropters are still capable of .125 steps in the
sphere at least, and lensometers even are still calibrated in 1 degree
steps.

(not an OD here, but . . . )

I totally agree with your approach . . . especially in a case like
mine (high Rx, acc. spasm).

When you start adding (multiplying?) the margins of error for vertex
distance, pantoscopic angle, axis of astigmatism, and manufacturing
tolerances . . . you can be pretty darned far off.

I was seen a couple of years ago by an OD who refracted me using trial
frames. She and I disagreed about the appropriate positioning for the
high-plus lens (I thought it should be *nearest* the eye; she thought
it mattered not). According to Weiss (Vertexometer mfgr), the
resulting delta could easily have been >= 1d of undercorrection. My
acc. mechanism is very particular about undercorrection.

Since we couldn't agree, I asked her to write the accurate vertex
distance for the plus on the Rx.

[dumbstruck]

Post by Mike Tyner
"Perpendicular" is the exception, not the rule.

...

Post by Mike Tyner
Then you must think that astigmatism increases over time. To some degree, it
might, but the symmetry we were talking about is typically present at birth.

Ok, I see that you mean mirroring in a precise L-R sense. This quirk
seems like a clue about the origin of such astigmatism, and now smells
like the original mirroring of all features in the womb.

Post by Mike Tyner
It'd be impractical to change a million instruments, and there's little
benefit in prescribing 0.10 if a lab's fabrication tolerances are +/- 0.125.

Fab tolerances range over an entire quarter diopter!? Such error
sounds alarming, and I wonder if the cheapo outlets are even worse and
whether there is a way to ensure your lab has good or better precision.

Post by Mike Tyner

Post by dumbstruck
Also my prescription
template only allows for a plus sph correction for close reading;
should it allow perhaps a tiny plus for cyl as well (maybe under .25)?

Plus and minus notation are only numeric conventions. If you want to express
the value 5, it doesn't matter whether you say 3+2 or 7-2. Both equal 5

No, I meant that besides the distant correction of sph, cyl, and ax,
the form has a further correction for reading that only allows for sph
rather than cyl. I was wondering if this is just to simplify the
construction of progressive bifocals, or whether there was absolutely
no reason to depower the cyl as well as sph.

[Mike Tyner]

Post by dumbstruck
Ok, I see that you mean mirroring in a precise L-R sense. This quirk
seems like a clue about the origin of such astigmatism, and now smells
like the original mirroring of all features in the womb.

Yeppers.

To go a step further, it helps to know that keratocytes are long tendrils
and they tend to associate in sheets, with a "grain" or directional quality.
Then each sheet in the cornea tends to alternate 90 degrees from the sheets
above and below.

Of course it isn't this simple, but statistically the keratocytes follow
this pattern more than they fall randomly.

But the bottom line is you have these two sets of sheets, presumably
following independent growth patterns. It isn't so surprising that they tend
toward a football-shaped result. What's surprising is how accurate (smooth,
uniform) they usually are.

Post by dumbstruck
Fab tolerances range over an entire quarter diopter!? Such error
sounds alarming, and I wonder if the cheapo outlets are even worse and
whether there is a way to ensure your lab has good or better precision.

You can walk into any optician and tell them you want to check glasses made
somewhere else. They may charge you a few bucks. If you want your money's
worth, don't tell them where you got the glasses until after they're
verified.

But that's overkill, most of the time. Surely you can tell between sharp and
blurry vision. Likewise you're the final authority on comfort. So if your
vision is sharp and comfortable, my advice is don't worry about it. "Cheapo"
outlets are capable of making glasses right. The most expensive pair of
glasses doesn't guarantee no mistakes. The best refractionists still have to
rewrite prescriptions sometimes, often because you told them "1" when you
meant "2". Stuff happens. Buy the guarantee. Buy fast service if you want.

Post by dumbstruck
No, I meant that besides the distant correction of sph, cyl, and ax,
the form has a further correction for reading that only allows for sph
rather than cyl.

Your "cyl" is the same at near as it is far away. There are exceptions but
they're so rare and slight that nobody composes a form that way. Once the
cyl is ground into the entire surface, the difference between far and near
is spherical. And usually the same in both eyes.

Post by dumbstruck
or whether there was absolutely
no reason to depower the cyl as well as sph.

Once the cylinder is ground into the back surface, a spherical "add" can
literally be glued onto the front. They used to do it that way, with canada
balsam.

-MT

[William Stacy]

Post by dumbstruck
No, I meant that besides the distant correction of sph, cyl, and ax,
the form has a further correction for reading that only allows for sph
rather than cyl. I was wondering if this is just to simplify the
construction of progressive bifocals, or whether there was absolutely
no reason to depower the cyl as well as sph.

No, just to simplify the Rx writing. It's an "add" to the spherical
portion of the distance Rx. The cyl is automatically duplicated in the
reading Rx

w.stacy, o.d.

[d***@msn.com]

The mirror tendency of astigmatic axes is similar to the mirror
tendency of the two sides of the face during normal fetal development.
If the right orbit is almond shaped and rises at the ear, it will be
almond shaped and raised symmetrically in most normal fetal
development. There are three major subclassifications of astigmats:
"with-the-rule", "against the rule" and "obliques". With the rule
means a minus cyl right around 180
(a football lying on its side); against-the-rule is the football
standing on a "t", and obliques are footballs falling off of the "t"
heading for the ground. The proportion of with-the-rule to
against-the-rule to oblique is something like 60-30-10.
"With-the-rule" is most common in ethnic groups whose heritage is
closer to the Equator. Hispanics, Native Americans and the like have
lots of with-the-rule astigmatism. Against-the-rules are much less
common, but tend to be easier to find passed from generation to
generation. When I see a kid with 't" football eyes, I can almost
always find a parent or grandparent with the trait.
The traditional belief is that cultures near the Equator squinted their
eyes more from working outside in the hot sun for many generations.
Hence, the eye got a "squinted"
appearance even generations after being away from the sun. Since the
football-on-end
group is so much less common but appears in family clusters, on theory
is that is anherited trait for the two-sides of the eye to grow longer
than normally during early fetal development before meeting at the top,
hence the oval result instead of a sphere.

Obliques were put on this earth for the sole purpose of giving
optometrists fits. ;) When they attempt to wear toric soft contacts,
the vector of force from the eyelid going straight down but the eyeball
being elongated at about 45 degrees allows them to spin contacts like a
rocking boat. Their glasses elongate images at oblique angles, so
doors can tilt and bend at obtuse angles. These are the people that
hate their glasses, hate their contacts, can't see well
naked-eyed...but they are still pretty happy walking around naked-eyed.
These people should be immediately charged an extra $500 for walking
into the optometrist's office, because that's how much he'll spend
making them stuff that they won't wear anyway. ;) O.K....just
kidding. I think...

[William Stacy]

Post by d***@msn.com
Obliques were put on this earth for the sole purpose of giving
optometrists fits. ;)

And as a recruitment tool for optometry schools. I was born oblique and
it's the very thing that drew me toward optometry (as in I'll fix it
myself, thank you very much).

w.stacy o.d.

(used to be -2.5 cyls ax 104 and 76, not now thanks to iols and
simultaneous astigmatic relaxing incisions)

[dumbstruck]

I can almost always find a parent or grandparent with the trait.

So cyl errors emerge at fetal stages apparently due to genetics? So we
aren't likely to find this can be preventable by the mother avoiding
alcohol, taking certain vitamins, or whatever during pregnancy?

I wonder if this asymetrical development also happens in other parts of
the body based on the same trigger, like heart or brain? Maybe could
run some statistical tests to find that certain kinds of atigs have
more or less chances of irregular heartbeats, imprisonment for
embezzling, or responsiveness to ...?!

[Mike Tyner]

Post by dumbstruck
I wonder if this asymetrical development also happens in other parts of
the body based on the same trigger, like heart or brain?

I'm not sure why you say "asymetrical." It's remarkably _symmetrical._

Post by dumbstruck
Maybe could
run some statistical tests to find that certain kinds of atigs have
more or less chances of irregular heartbeats, imprisonment for
embezzling, or responsiveness to ...?!

You'd expect just as much association with a certain shape of ears, or nose,
or fingers. Physical characteristics have never been good predictors of
behavior. Look up "phrenology" and "iridology."

Perhaps you'd be intrigued by the differences between ethnic populations.
Astigmatism is significantly more common in Hispanics.

-MT

[dumbstruck]

Post by Mike Tyner

Post by dumbstruck
I wonder if this asymetrical development also happens in other parts of
the body based on the same trigger, like heart or brain?

I'm not sure why you say "asymetrical." It's remarkably _symmetrical._

Of course I meant the cyl asymmetry within each eye, but that was
admittedly confusing in the context of this curious bilateral sym
between the eyes.

Post by Mike Tyner

Post by dumbstruck
Maybe could
run some statistical tests to find that certain kinds of atigs have
more or less chances of irregular heartbeats, imprisonment for
embezzling, or responsiveness to ...?!

You'd expect just as much association with a certain shape of ears, or nose,
or fingers. Physical characteristics have never been good predictors of
behavior. Look up "phrenology" and "iridology."

It just seems astig is very different than cosmetics, but something
evolution would ruthlessly drive out by strong mechanisms in fetal
development. The bilateral mirroring seems a smoking gun for a
particular kind of breakdown - you would never expect to see such a
pattern in a birthmark for instance. Where else does this happen,
maybe being pigeon toed, or having 2 bum kidneys? Or could it be
related to bad cross-layering of heart muscles? Maybe other branches of
science are looking at those with some ideas that could be borrowed
like gene therapy or adding vitamin x to milk or water.

Post by Mike Tyner
Perhaps you'd be intrigued by the differences between ethnic populations.
Astigmatism is significantly more common in Hispanics.

Wonder if the great star navigators like Polynesians or Scandinavians
are less liable to astig. I guess that's too recent in evolution, and
anyway I've tired myself out by all this brainstorming out of my depth
:-)

[Mike Tyner]

Post by dumbstruck
Of course I meant the cyl asymmetry within each eye, but that was
admittedly confusing in the context of this curious bilateral sym
between the eyes.

But each eye shows symmetry across the axis of astigmatism.

Post by dumbstruck
It just seems astig is very different than cosmetics, but something
evolution would ruthlessly drive out by strong mechanisms in fetal
development.

You're assuming astigmatism is detrimental. To a degree it is beneficial if
the individual lives into presbyopia. I'm fully presbyopic but I can read a
newspaper using the vertical lines and road signs using the horizontals.

-MT

[dumbstruck]

Post by Mike Tyner

Post by dumbstruck
Of course I meant the cyl asymmetry within each eye, but that was
admittedly confusing in the context of this curious bilateral sym
between the eyes.

But each eye shows symmetry across the axis of astigmatism.

This is the thanks I get for not being pendantic with a term like
"radial" symmetry (or "semi-reverse-quadrilateral" or whatever)?

Post by Mike Tyner

Post by dumbstruck
It just seems astig is very different than cosmetics, but something
evolution would ruthlessly drive out by strong mechanisms in fetal
development.

You're assuming astigmatism is detrimental. To a degree it is beneficial if
the individual lives into presbyopia. I'm fully presbyopic but I can read a
newspaper using the vertical lines and road signs using the horizontals.

And lucky is the elderly kanji reader with 45 degree astig? No, that
is too contrived, and easy situations to wear glasses. You should be a
passenger on a club sailboat when some of us astigs have to take a turn
at the helm on a night shift. Always the waves are higher than the
boat and encrusting glasses with windblasted salt spray. Without
glasses we have to avoid oncoming boat traffic with the worlds dimmest
nav lights, including tugs with a half mile of cable hooking to a barge
or two (I could swear they just duct tap a penlight to a mast as a
legal minimum). Can't make out such pinpoints of light with astig.
except maybe some of the strong shoreline nav lights. Lucky we have a
whirling compass rather than always following the stars which look
double or smeared. If we do chase a star without glasses, it could
turn out to be a cessna flying low at us or away (there is so much such
traffic on these routes that I have to suspect drug ferrying). Anyway,
that is an example of real life; still hostile to astigmatics (and
their passengers).

[Philip D Izaac]

Post by Mike Tyner
You're assuming astigmatism is detrimental. To a degree it is beneficial if
the individual lives into presbyopia. I'm fully presbyopic but I can read a
newspaper using the vertical lines and road signs using the horizontals.
-MT

How interesting. What about astigmats 0f -2.00D at 135 and 45 axis. will the
two images super impose to provide good distance and near vision.

Roland Izaac

[Mike Tyner]

Post by Philip D Izaac
How interesting. What about astigmats 0f -2.00D at 135 and 45 axis. will the
two images super impose to provide good distance and near vision.

I do think the acuity is better with both eyes open, but printed material
favors horizontal and vertical more than oblique.

-MT