mirror astigmatic axes

Discussion in 'Optometry Archives' started by B. Janse, May 28, 2005.

  1. B. Janse

    B. Janse Guest

    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°).
    B. Janse, May 28, 2005
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  2. B. Janse

    Dr. Leukoma Guest

    Very common.

    Dr. Leukoma, May 29, 2005
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  3. B. Janse

    dumbstruck Guest

    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)?
    dumbstruck, Jun 10, 2005
  4. B. Janse

    Dr. Leukoma Guest

    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

    Dr. Leukoma, Jun 10, 2005
  5. 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

    w.stacy, o.d.
    William Stacy, Jun 10, 2005
  6. B. Janse

    Neil Brooks Guest

    (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.
    Neil Brooks, Jun 10, 2005
  7. 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
    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...
    doctor_my_eye, Jun 10, 2005
  8. B. Janse

    dumbstruck Guest

    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.
    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.

    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.
    dumbstruck, Jun 10, 2005
  9. 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)
    William Stacy, Jun 10, 2005
  10. 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.
    William Stacy, Jun 10, 2005
  11. B. Janse

    dumbstruck Guest

    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 ...?!
    dumbstruck, Jun 12, 2005
  12. B. Janse

    dumbstruck Guest

    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.
    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.
    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
    dumbstruck, Jun 12, 2005
  13. 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
    Philip D Izaac, Jun 13, 2005
  14. B. Janse

    dumbstruck Guest

    Of course I meant the cyl asymmetry within each eye, but that was
    This is the thanks I get for not being pendantic with a term like
    "radial" symmetry (or "semi-reverse-quadrilateral" or whatever)?
    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).
    dumbstruck, Jun 13, 2005
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