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Banish "Count Fingers"!

     
 

We are all very familiar with the visual acuity progression. First, you attempt to get the patient to read on the Snellen acuity chart.  If nothing can be seen on the acuity chart, then you have the patient count fingers.  If fingers cannot be seen, then you have the patient look for hand motion, etc.  The problem with this method is that "count fingers" is not precise.  Anyone who works for a retina practice should be aware that there is a very wide range of functional vision between "count fingers at 4 feet" and "hand motion".  Some patients who cannot read the big E on the acuity chart are still able to function well in their daily environment.  Other patients who cannot read the big E are functionally blind.  It's not that all the patients in the second group are just complainers, its that they really cannot see as well as the folks in the first group.  The problem for us is that the "count fingers" measurment does not do a good job of differentiating the two groups.

There is a more precise way to measure visual acuity if the eye cannot see any letter on the Snellen chart at 20 feet.  As described above, the usual procedure is to check "count fingers" at this point.  Many retina specialists, who deal with low vision patients often, prefer to obtain the best Snellen visual acuity in this situation by bringing the Snellen chart closer than 20 feet.

If you have a chart hanging on the wall, you can simply take the chart off the wall and hold it closer to the patient, or ask the patient to move closer to the chart.  If you have a projected Snellen chart, then you will need a handheld chart. You can buy a cardboard Snellen chart, or you can buy a Fonda Low Vision Chart, which is very similar.  Once you have a chart, you can then run some copies on your copier, and then you will have charts for each exam room.  A standard Snellen chart is about 11 inches wide and 22 inches long, so you will need to copy the chart in 2 sections since your paper is 8.5 x 11 inches.  You will not need to copy the smallest letters, because if the patient can see those then he can see the big E on the wall chart.

Procedure:  The room light will need to be as bright as possible.  Start by holding the chart 5 feet away.  The fellow eye is occluded.  Just as with a regular Snellen acuity test, have the patient read the smallest line possible.  If no letters an be seen at 5 feet, then hold the chart at 2 feet.  If nothing can be seen at 2 feet, then hold the chart at 1 foot.  There is nothing magic about the distance. Meters can be used also, for example, a one meter distance and then a half meter distance.

Once you have the distance and the smallest line read, you will then need to calculate/record the visual acuity.  There are two options, depending upon the preference of your doctor.  The first option is to simply record the VA in regular notation.  For example, the patient can read the 20/80 line at a distance of 5 feet away.  The visual acuity can be recorded as 5/80.  The problem with this method is that there is no standard distance.  The patient may see 5/80 on one visit and 3/60 on the next visit.  It is not immediately obvious which is the better visual acuity. 

The other option is to convert to 20/xx equivalent.  This is accomplished by dividing the test distance into 20, and they multiplying the result by the line read.  For example, the smallest line our patient can read is the 20/80 line at a distance of 5 feet away.  As discussed, the regular notation would be 5/80.  Divide 5 into 20 to get a result of 4.  Now multiply 80 by 4 to get 320.  The 20/xx equivalent of 5/80 is 20/320.

To avoid having to do calculations all the time, decide on 2 standard distances to use.  A good choice is 5 feet and 2 feet.  You can then record the standard results on your charts, to be read and recorded after testing.  Here are the calculations below.  For example, next to the 20/80 line on your chart you would mark, in small figures, 20/320 and 20/800.  If the 20/80 line is the smallest line read, you would record 20/320 if you are 5 feet away, 0r 20/800 if you are 2 feet away.

   
    Line

5 feet

 2 feet
            20/400 20/1600  20/4000
            20/200 20/800  20/2000
            20/100 20/400  20/1000
            20/80 20/320  20/800
            20/70 20/280  20/700
            20/60 20/240  20/600
            20/50 20/200  20/500
            20/40 20/160  20/400