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Medical Forum / General / Vision / October 2005Curious about autorefractor
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 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:) > After IOL implant, surgeon's technician autorefracted me, and > showed me clearly the 20/15 line with a phoropter. I understood [quoted text clipped - 25 lines] > -- > Dick Adams 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 > After IOL implant, surgeon's technician autorefracted me, and > showed me clearly the 20/15 line with a phoropter. I understood [quoted text clipped - 25 lines] > -- > Dick Adams > 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? 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 news:MPG.1db6d356c39061a9989682@netnews.worldnet.att.net 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. 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 > 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). I think you mean 20/40, not 40/40. > Under a clear sky in >open shade, a 40/35 line is fully legible and in direct sunlight, 40/30. 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? > 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. > > 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. > > 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). > > > I think you mean 20/40, not 40/40. > > Under a clear sky in open shade, a 40/35 line is fully legible > >and in direct sunlight, 40/30. [quoted text clipped - 11 lines] > > w.stacy, o.d. 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 > > 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. > My understanding is that 20/20 refers to a line of 5 min. angular height > viewed [quoted text clipped - 8 lines] > the > case with my zero accommodation IOLs. Zero accommodation is not the same as not having *any* ability to focus. William Stacy, myself and yourself all have IOLs. Our eyes are like "box cameras", because a box camera has no way to focus. A box camera is usually in focus from about four feet to infinity. In addition, even if you asked for your IOL to be set for infinity, the margin of error is much greater than your calculation. > In addition, even if you asked for your IOL to be set for infinity, the > margin of error is much greater than your calculation. I did a little Google search on IOLs from Alcon. They don't even seem to sell them in increments other than .5D. 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. > 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 > 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 except that 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 > 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. Your minus self-measurments indicate myopia, while the plus office meas. show hyperopia. A significant conflict there. > The latter eye is the one >capable of resolving 40/30 while the best the former can achieve via refraction [quoted text clipped - 8 lines] >see two sharp pinhole images due to the two image planes separated by 3D. > 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. >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? > > 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. 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. > After IOL implant, surgeon's technician autorefracted me, and > showed me clearly the 20/15 line with a phoropter. I understood [quoted text clipped - 25 lines] > -- > Dick Adams |
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