Curious about autorefractor

Discussion in 'Optometry Archives' started by Dick Adams, Oct 12, 2005.

  1. Dick Adams

    Dick Adams Guest

    After IOL implant, surgeon's technician autorefracted me, and
    showed me clearly the 20/15 line with a phoropter. I understood
    the correction was minus 1.0 D consisting partly of cylinder. Surgeon
    mildly chastised tech for using the autorefractor. "Just do a standard
    refraction", he said. To me he said that I must go to my referring
    optometrist for prescription. Well, that guy says he can't get me
    to better than 20/25, and cannot show me the 20/20 line clear
    enough to read. He obstinately refused to start with the numbers
    I got from the surgeon's technician. He has no autorefractor.

    I did not, until then, know about the autorefractor. On a subsequent
    visit, I asked the surgeon why my optometrist might get a different
    result from his tech who starts with the autorefractor. The question
    irritated him, and he said again I should take it up with my
    optometrist.

    Well, 20/25 is pretty good for 70+-year-old eyes, but, particularly
    for night driving, I sure like to get 20/20 or better, if it is possible.

    Can anybody guess what is going on here?

    Is there any reason why the autorefractor would not get the right
    answer for a eye which has lost its ability to focus?

    Is the business with the phoropter an obsolete song and dance
    routine when it comes to refracting us old people?
     
    Dick Adams, Oct 12, 2005
    #1
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  2. As an old school refractionist myself, I never ignore any information I
    can get. I've never owned an autorefractor, but if I had a patient walk
    in with an autorefractive result that gave 20/20 and I didn't put it in
    the refractor as a starting point, I'd be pretty dumb, wouldn't I?

    And no, subjective refracting is still THE standard of care, and is NOT
    a song and dance, unless the refractionist is unskilled.

    If you got 20/20 with an autorefractor, then you should get 20/20 with
    any kind of refraction, unless something happened, or unless the 20/20
    wasn't really 20/20.

    w.stacy, o.d. (a 61 yo IOLed optometrist with 20/15:)
     
    William Stacy, Oct 12, 2005
    #2
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  3. Dick Adams

    Dom Guest

    Although theroetically 20/20 should be the same whichever eye chart
    you're reading, it may be that you were sitting a little closer to the
    chart in the surgeon's rooms, or a little further from the chart in your
    optometrist's rooms, or both, which may account for at least part of the
    discrepancy?

    Just an idea.

    Dom
     
    Dom, Oct 12, 2005
    #3
  4. Dick Adams

    SiG Guest

    Having had IOLs recently installed, I've been testing independent ways to
    monitor changes in acuity, etc. between office visits. If you can obtain an
    eyechart, either downloaded or do-it-yourself programming, pinhole viewing can
    give a useful guide to the limits for spherical refractive correction. (I use a
    drill gage from the shop.) Both distance and lighting are important. For
    instance, with a chart taped to the garage door, I can better read a 40/40 line
    (forty ft.) than a line half its height at 20 ft (20/20). Under a clear sky in
    open shade, a 40/35 line is fully legible and in direct sunlight, 40/30. While
    the difference between 20 and 40 ft is only a 0.08D difference, sans
    accomodation it is perceptible both with corrective lenses and pinhole viewing.

    Absolute comparisons can be a dicey matter if distance and illumination levels
    are not standardized, reflective charts vs. backlit view boxes, etc.

    SiG
     
    SiG, Oct 12, 2005
    #4
  5. Dick Adams

    Dick Adams Guest

    It's a standard setup -- you look at a chart on the wall, at a standard distance,
    through the manually-operated lens-flipping gadget named phoropter. Or,
    with added magnification, you look at a chart at reading distance through the
    same contraption. All while the practitioner is flipping lenses and asking
    "better or worse??".

    Well, like "SiG" says in illumination is important. My regular eye guy, for instances, sports a wall chart
    which is brighter at the center that at the periphery. It has been that way for
    years. He does not want to hear any comments about it.
     
    Dick Adams, Oct 12, 2005
    #5
  6. I think you mean 20/40, not 40/40.

    40/35 is not standard terminology, so it is unclear what your are
    measuring and from what distance. Are you actually reading a 20 ft chart
    from 40 ft away, or what?

    But the angular subtense of any object is half the size at 40 ft than it
    is at 20 ft, a big difference in acuity, and the refractive difference
    is meaningless. So I'm not really understand what you are trying to say.

    w.stacy, o.d.
     
    William Stacy, Oct 12, 2005
    #6
  7. Dick Adams

    SiG Guest

    My understanding is that 20/20 refers to a line of 5 min. angular height viewed
    at 20 ft (characters 0.349 in. high), 20/40 one of 10 min. arc at 20 ft. (0.698
    in. chars), etc. Thus 40/40 corresponds to a line of the latter height viewed
    at 40 ft. (also 5 min. arc). While 20 ft may be a convenient distance for
    indoor measurements, it is not infinity and differs by 20ft = 6.096 meters =
    0.164D. I'd suppose that even a high degree of presbyopia might have enough
    accommodation left to render the difference inconsequential - but that isn't the
    case with my zero accommodation IOLs.

    SiG
     
    SiG, Oct 12, 2005
    #7
  8. Dick Adams

    Dan Abel Guest

     
    Dan Abel, Oct 12, 2005
    #8
  9. There are some other possibilities here.
    (1) Sometimes acuity drops slightly from one day post-op to a few weeks
    post-op. There is something called cystoid macular edema, or CME, that
    causes this drop in vision quality. If you have signs of CME, a
    non-steroidal anti-inflammatory drop (Acular) is commonly prescribed to
    help it resolve.
    (2) In an earlier thread, I talked about black box myopia, which is a
    common problem with autorefractors. When you have a normal human lens
    in your eye and look into a dark box, you measure more myopic than you
    really are. This might be an innate fear response, it might be a
    response to accomodate to see a wall before you walk into it in the
    dark. All autorefractors have a subprogram/subroutine to attempt to
    negate this black box myopia. Since you have no human lens, and you
    don't accomodate anymore...your autorefractor doesn't know that. Some
    newer autorefractors allow the doctor or tech to "toggle over" to a
    pseudoaphakic setting. Most don't.
    (3) Depending on whether your surgical wound was stitched or glued, and
    how large it is, your refraction can "bounce around" for a good 4 to 6
    weeks while the stitch dissolves and the conjunctiva seals around the
    area of insertion. Relax, and look again in about a month.
     
    doctor_my_eye, Oct 12, 2005
    #9
  10. OK then it does seem that you have an understanding of the Snellen
    acuity conventions.

    I think most of the differences you're finding between 20 ft and 40 ft
    are artifacts of measurement, since the slight dioptric differences are
    less than the normal depth of field you get with zero accommodation, so
    yes, I think inconsequential even with iols. You might have a bit of
    hyperopic residual error, which would explain why 40 ft is slightly
    better than 20, but it would not explain the same observation through a
    pinhole, where depth of field is very large and if illumination is
    equivalent, there should be no measureable difference. What size
    pinhole do you use?

    w.stacy, o.d.
     
    William Stacy, Oct 12, 2005
    #10
  11. Dick Adams

    Dan Abel Guest

    I did a little Google search on IOLs from Alcon. They don't even seem
    to sell them in increments other than .5D.
     
    Dan Abel, Oct 12, 2005
    #11
  12. Dick Adams

    SiG Guest

    Scratch the pinhole difference between 20 and 40 ft. Couldn't find any results
    in my notes to substantiate it and the sun's not going to be seen in these parts
    for several days at least to do the experiments. Both eyes are hyperopic. My
    latest countertop measurements gave -0.48±0.06 and -0.59±0.04D. Office
    refractions led to +0.25 and +0.75D prescriptions. The latter eye is the one
    capable of resolving 40/30 while the best the former can achieve via refraction
    or pinhole viewing is 20/20. It seems limited by an opacification as indicated
    by glare from overhead lighting, a dimmer image, and a blotchiness when looking
    at newsprint that makes one read word by word instead of scanning line clusters.
    The refraction has been slowly shifting from plano. Sounds like PCO exceptthat
    these effects were noted in the first weeks after surgery and, at a one-month
    checkup, Doc K. couldn't find anything with his slit lamp.

    Pinhole sizes ranged 0.040 to 0.080 in. As an aside, with ReSTOR multifocals, I
    see two sharp pinhole images due to the two image planes separated by 3D.
    Fortunately, the only other time I've noticed this 'double' vision was in the
    early weeks for the problem eye - at times one would start with 20/30 vision,
    blink hard and see a double image, and stare hard and see the 20/25 and 20/20
    lines pop into focus. In retrospect, I'd guess that pinholes in a developing
    film were involved.

    Is there a depth-of-field definition applicable to vision? Some ancient Kodak
    documentation from the darkroom takes a 0.01 in. circle-of-confusion in a print
    at 10 inches (3.4' arc) as their spec. How does one objectively define and
    measure depth-of-field for a eye 20/200 or 20/20?

    SiG
     
    SiG, Oct 13, 2005
    #12
  13. Your minus self-measurments indicate myopia, while the plus office meas.
    show hyperopia. A significant conflict there.
    I think the blur mentioned above could be due the the funky optics of
    the Restor lens itself, or you could have a defective iol, or it could
    have a smear or something on it, all assuming your eye is otherwise
    normal/healthy.
    I'm sure somewhere it's been defined and measured, but clinically, I
    just estimate it to be about +or- 0.25 D. for average pupils, 0.5 for
    small ones, and maybe 0.125 for big pupils.

    w.stacy, o.d.
     
    William Stacy, Oct 13, 2005
    #13
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