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Medical Forum / General / Vision / November 2006LogMAR, Snellen equivalents, and etc.
To me, it is unfortunate that the established scales differences (the steps) get larger as the acuity declines. For example, a person with the acuity of 20/200 would like very much to know if they have achieved the improvement of 20/180 instead of waiting to the next logMAR value of 20/160. If one is receiving treatments for the 20/200, any improvement is most welcome instead of the next step that fits the log scale best. Like would you accept a fever thermometer that has the high end in a log scale? Don W. > To me, it is unfortunate that the established scales differences (the > steps) get larger as the acuity declines. For example, a person with > the acuity of 20/200 would like very much to know if they have achieved > the improvement of 20/180 instead of waiting to the next logMAR value > of 20/160. If one is receiving treatments for the 20/200, any > improvement is most welcome instead of the next step that fits the log In my personal experience (not a doctor), the difference between 20/180 and 20/200 is barely noticiable. The Snellen test is really not very accurate. There are so many variables that trying to differentiate between 20/100 and 20/200 isn't all that useful. > scale best. Like would you accept a fever thermometer that has the > high end in a log scale? Of course. The difference between 200F and 300F just has to do with how long you have been in the cremation fire. Are you an engineer?  SignatureDan Abel dabel@sonic.net Petaluma, California, USA In fact, the Snellen numbers become increasingly meaningless as you go beyond 20/100. Unlike a thermometer, where the accuracy is pretty level across the entire scale. I suppose once you exceed 1000 degrees, a similar problem with accuracy also occurs, where such a log scale might be appropriate as well. w.stacy, o.d. > To me, it is unfortunate that the established scales differences (the >steps) get larger as the acuity declines. For example, a person with [quoted text clipped - 8 lines] > > > In fact, the Snellen numbers become increasingly meaningless as you go > beyond 20/100. Unlike a thermometer, where the accuracy is pretty level [quoted text clipped - 3 lines] > > w.stacy, o.d. Well, when a major drug company enlists symptomatic patients with an acuity of at least 20/320, (Snellen equivalent, of course), and then announces the result of a 6.6 letter gain for everyone, would you still call this all meaningless?? Don W. > Well, when a major drug company enlists symptomatic patients with an > acuity of at least 20/320, (Snellen equivalent, of course), and then > announces the result of a 6.6 letter gain for everyone, would you still > call this all meaningless?? Stacy and I use it with individual patients and decimal/letter precision is absurd in our setting. But drug companies always choose units and comparisons that paint their results in the best possible light. "6.6 letters" sounds much better than "about one line." -MT > > Well, when a major drug company enlists symptomatic patients with an > > acuity of at least 20/320, (Snellen equivalent, of course), and then [quoted text clipped - 9 lines] > > -MT It would seems to me that it would not matter too much where a patient is in acuity but how well you could get him to 20/20 (with precision). Although slightly off topic, how do you refract someone that primarily has just peripheral vision? Don W. PS. The drug company, in the case, would never say "about one line". They would say, "greater than one line". > It would seems to me that it would not matter too much where a > patient is in acuity but how well you could get him to 20/20 (with > precision). And maximizing focus doesn't really require letters at all. Photographers do it all the time. > Although slightly off topic, how do you refract someone that > primarily has just peripheral vision? Retinoscopy or autorefractor would do nicely. -MT >Well, when a major drug company enlists symptomatic patients with an >acuity of at least 20/320, (Snellen equivalent, of course), and then >announces the result of a 6.6 letter gain for everyone, would you still >call this all meaningless?? > > Yes I would. the 20/320 is a completely arbitrary number and doesn't correspond to any acuity charts in common use. a 6.6 letter gain for EVERYONE is an absurdity beyond description. It's kind of like a drug company saying that people complaining of a fever that measured less than 102.543 F. took their drug and every one of them had a reduction of their fever by 0.249 deg. F. > >Well, when a major drug company enlists symptomatic patients with an > >acuity of at least 20/320, (Snellen equivalent, of course), and then [quoted text clipped - 9 lines] > fever that measured less than 102.543 F. took their drug and every one > of them had a reduction of their fever by 0.249 deg. F. Make that "expected 6.6 letter gain". Don W. About like "expected a 0.249 degree F. drop in temperature". Makes about as much sense. > > [quoted text clipped - 22 lines] > > Dear Don, Subject: Calculating intermediate letter size for the Snellen You are correct, the chart jumps from 20/200 to 20/100. But, you can always make your own using a simple ratio: 0.375 inches / 20 feet = x / distance or, for 20/40 letters: Letter Size = 0.375 ( 40 / 20 ) Letter Size = 0.75 inches. For 20/180 Letter Size = 0.375 ( 180 / 20 ) Letter Size = 3.36 inches. Best, Otis > To me, it is unfortunate that the established scales differences (the > steps) get larger as the acuity declines. For example, a person with [quoted text clipped - 6 lines] > > Don W. > You are correct, the chart jumps from 20/200 to 20/100. > [quoted text clipped - 17 lines] > > Otis Otis, There are probably many versions of the Snellen charts, but the one that I think is preferred is the one that uses the "Snellen Equivalents". In those charts the LogMAR's vary by 0.1. And those charts are preferred (as I understand it) on groups because they can do averages, standard deviations, etc, more "appropriately". In your equation, I just use a fixed letter size and vary the distance. Then of course, compute the arctan and ratio that to 0.08333 degrees. And that gives the denomenator in the 20/something. One paper I read used this technique and claimed "infinite resolution" using this technique. I enjoyed that. Don W. |
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