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Medical Forum / General / Vision / August 2005Acuity measurements
When certain studies say there has been a "loss of acuity of 3 or more lines", which lines (height) are they starting from and what would be the overall angle loss? > When certain studies say there has been a "loss of acuity of 3 or more > lines", which lines (height) are they starting from and what would be the > overall angle loss? The loss of lines is from whatever line the particular subject could see at the start. So if subject A could see 20/40 at the start and sees 20/80 at the end, then he has lost three lines (/50, /60, /70). Subject B starts at 20/15 and ends at 20/40, also lost three lines (/20, /25, /30). Subject C starts at 20/70 and ends at 20/400, losing /80, /100, /200. The overal angle loss is different for each case, so you can't deduce angle from lines. This question points out the problems with using Snellen charts for research. Had a logmar chart been used, the question would not need to be asked as the loss would have been expressed in logmar units lost. Dr Judy >This question points out the problems with using Snellen charts for >research. Had a logmar chart been used, the question would not need to be >asked as the loss would have been expressed in logmar units lost. > >Dr Judy > Dr Judy, what are the advantages of logmar chart compared to Snellen, and is there any website I can download and print an actual size logMAR chart? Dear John, Subject: Express visual acuity as an angle It would be easier to just express visual acuity as an angle, i.e., 5 minutes of arc for 20/20, 10 minutes of arc at 20/40, etc. Clearly expressing the number of "lines" lost has no meaning. Or just express it as an angle at greater than 6 meters. Best, Otis (Engineer) > It would be easier to just express visual acuity as > an angle, i.e., 5 minutes of arc for 20/20, 10 minutes of > arc at 20/40, etc. If you studied modern methods just a little bit, you'd know there is already an angular measurement in wide clinical use. It's called "logmar". > Clearly expressing the number of "lines" lost has no meaning. Unless you actually spend time in an eye clinic. -MT >>This question points out the problems with using Snellen charts for >>research. Had a logmar chart been used, the question would not need to be [quoted text clipped - 4 lines] > Dr Judy, what are the advantages of logmar chart compared to Snellen, and > is there any website I can download and print an actual size logMAR chart? The advantages are that the letters have been selected and designed to minimize the confounding factors present in tradional charts, to provide the same number and same difficulty of identifying letters in each line and the progression of size of letters varies with the log of the minimal angle of resolution. It's main clinical benefit is with quantification of acuity in people with reduced best corrected. With the Snellen chart there are no lines between 20/100 and 20/200 and between 20/200 and 20/400 and no lines worse than 20/400. LogMar solves that problem so is useful for those who do a lot of low vision work. It provides limited benefit for those with normal acuity. http://bjo.bmjjournals.com/cgi/content/full/87/10/1232 for discussion and pictures of LogMar chart It is more time consuming to use, takes a lot of space and is not readily available in slides for projected charts, so few clinicians use it. It is very useful for research as it is definitely allows for more precise measurement of acuity. I don't know of any downloadable charts, you could buy one but why do you want one? Dr Judy >I don't know of any downloadable charts, you could buy one but why do you >want one? > I was mainly interested if there was a downloadable one, like many snellens online, I thought I would print it out and check which line I could see, no big deal really. >>I don't know of any downloadable charts, you could buy one but why do you >>want one? > > I was mainly interested if there was a downloadable one, like many > snellens online, I thought I would print it out and check which line I > could see, no big deal really. If you are 20/20 best corrected you would read "0", which is equivalent to 20/20. The value of the chart is on the other end, when patients have reduced best corrected due to disease. It allows for finer gradations of acuity and hence can document subtle changes. Dr Judy > This question points out the problems with using Snellen charts for > research. Had a logmar chart been used, the question would not need to > be asked as the loss would have been expressed in logmar units lost. > > Dr Judy Dr. Judy Thanks. Unfortunately, it's what I had expected. I also hate readings like 20/25 minus 2. Oh, well. Don W. >> This question points out the problems with using Snellen charts for >> research. Had a logmar chart been used, the question would not need to [quoted text clipped - 6 lines] > Thanks. Unfortunately, it's what I had expected. I also hate readings > like 20/25 minus 2. Oh, well. 20/25 -2 is more precise than just 20/25. To qualify for 20/25, you need to read half the letters correctly, ie read 3 correct out of the 6. So an acuity of 20/25 means that 3, 4, 5 or 6 of the letters were read. 20/25 -2 means that exactly 4 out of the 6 were read. Dr Judy >> 20/25 -2 is more precise than just 20/25. To qualify for 20/25, you need > to read half the letters correctly, ie read 3 correct out of the 6. So > an acuity of 20/25 means that 3, 4, 5 or 6 of the letters were read. > 20/25 -2 means that exactly 4 out of the 6 were read. > > Dr Judy So if you just read only 2 characters of a given line, and wanted a restest and again only reliably read 2 characters, the result would still negate that line? Also, the standard "E" with the 1 minute of arc extensions. Do the acuity charts present the same acuity testing in the horizontal direction as in the vertical? Seems like they wouldn't. Don W > Also, the standard "E" with the 1 minute of arc extensions. Do the acuity > charts present the same acuity testing in the horizontal direction as in > the > vertical? Seems like they wouldn't. Why not? The vertical elements and spaces use the same 1 minute of arc. -MT >> Also, the standard "E" with the 1 minute of arc extensions. Do the >> acuity [quoted text clipped - 5 lines] > > -MT Well, (speculating), the standard "E" is a grid of so many pixels high, and so many wide. OK. The eye is to discern the horizontal legs of the E. At 1 minute spaces. It should see on, off, on, off, on, in the center of the E. And it must differenciate between the on's and the off's to make 20/20. And it may see these differences with some measure of uncertainty. But going horizontally, it sees, on, on, on,etc and it is easier to see the on's (half the acuity) than just one on, off. So the E forms some kind of bias as to being recognized horizontally. But this may have been thought out in making up the charts. Don W > But this may have been thought out in making up the charts. True... E is not the only letter in the chart, and some letters like "I" aren't used. -MT >>> Also, the standard "E" with the 1 minute of arc extensions. Do the >>> acuity [quoted text clipped - 14 lines] > easier to see the on's (half the acuity) than just one on, off. So the E > forms some kind of bias as to being recognized horizontally. Although an E will be difficult to tell from a B, K, F, P, R, horizontally. The one minute refers to the "critical detail". In the case of the E, the one minute is the width of any of the arms. > But this may have been thought out in making up the charts. Don't count on it, the original Snellen has lots of problems for precise measurements. Newer charts take many things into account, including the difficulty of recognizing a particular letter. Equality horizontal and vertical, crowding and letter difficulty are all take into account with the LogMar type charts. Clinically, it doesn't make much difference. Precise acuity measurement only matters if someone is just past the legal limit for driving, in office I will usually pull out a paper LogMar chart to see the patient will pass with it. Dr Judy > Don W If truly accurate acuity measurements were necessary, and I doubt that they are, some kind of two dimensional modulation transfer function (MTF) should be used. I do not know if such instrumentation exists. This is the engineer, not scientist, in me speaking. Bill > If truly accurate acuity measurements were necessary, and I doubt that > they [quoted text clipped - 3 lines] > engineer, > not scientist, in me speaking. Contrast sensitivity testing is done very much as you describe, using sine-wave gratings with variable contrast and variable spatial frequency. -MT > If truly accurate acuity measurements were necessary, and I doubt that > they [quoted text clipped - 5 lines] > > Bill Accurate acuity measurements are necessary (IMHO) to check the progression of eye diseases over time. Stuff that could change fast. There is no need to wait x weeks to find out that you've gone from 20/y to 20/z where y to z is dramatic. Otis' program, tuned up, would be a big help. > If truly accurate acuity measurements were necessary, and I doubt that > they [quoted text clipped - 3 lines] > engineer, > not scientist, in me speaking. It exists, called contrast sensitivity testing. With it, visual performance can be expressed as graphical function. Clinically, that kind of testing takes about 10 minutes to do. Not clinically useful except in research, low vision clinics and monitoring very subtle disease. Dr Judy > Bill |
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