November 19, 2007
MEMORANDUM FOR RSB STAFF
FROM: Mike Merrick
SUBJECT: Determining Visual Efficiency
I've recently heard some discussion that made me believe that some RSB staff are not clear on how to perform a visual efficiency calculation. In order to make certain that we are doing this correctly (and all doing it the same), I have written the following instructions for determining eligibility through the use of visual efficiency.
First, you must understand that there are three objective measures of eye condition used to determine eligibility: 1) visual acuity, 2) visual field, and 3) visual efficiency.
Visual acuity (usually expressed as 20/100, 20/200, etc.) is a measurement of what a person can see compared to a person with normal vision at 20 feet (therefore the 20/ included in each set of numbers). So, someone with 20/100 vision sees at 20 feet what a person with normal vision could see at 100 feet (or at 20 feet it looks like it’s 100 feet away, however you want to think about it). These measurements are the standard of visual examination, and they give them to you for both eyes (OD: 20/100, OS: 20/400)(by the way, OD is right eye, OS is left eye, OU is both eyes). By our standards of eligibility, you must be 20/200 in the BEST eye WITH BEST correction if you have a disease or defect of the visual system that is not progressive (i.e., someone who has an eye injury which is stable and not getting any worse, say as the result of an accident). OR, you can be 20/70 in the BEST eye WITH BEST correction if you have a progressive eye condition, one that is expected to get worse, such as RP, glaucoma or diabetic retinopathy.
Visual field is the number of degrees of useful vision expressed from the point of fixation. That is, if you can see objects within 5 degrees either side of the point you are staring at on this screen right now, you have five degrees of visual field. Of course, most people have much larger fields than that, going up to 90 degrees in some directions. By our eligibility criteria, your visual field (best eye, of course) must be less than 20 degrees in order to be considered eligible. And there are no established lists of progressive or non-progressive diseases. You are dependent on the eye care professional to determine the nature of the disease or defect as being progressive or non-progressive, although some are commonly known as progressive, such as RP, glaucoma or diabetic retinopathy).
There is a third method of determining visual eligibility: visual efficiency. Visual efficiency is a way of combining visual acuity and visual fields. Back in the late 1980’s , RSB made a decision that too many people were ineligible for services because their acuity was not low enough, or their visual field was not small enough, but they still had vision problems that required our help. In many cases they would eventually need help that we would be better off providing at an earlier date, rather than waiting until they reached legal blindness (20/200 in the best eye). So they came up with the visual efficiency method that takes into account the visual acuity in each eye, the visual field of each eye, assigns a value to each, then puts the results into a formula that determines a percentage of visual efficiency that is applied to our eligibility standards (20 percent for non-progressive conditions, 64 percent for progressive conditions). Sounds complicated, but it’s not.
First, you must have a 'corrected' visual acuity for each eye. Let’s say 20/60 OD, 20/80 OS. If you open the file located at: http://dss.mo.gov/fsd/rsb/manual/vrman/forms/visual_acuity.xls
you will find a listing of visual acuity readings, with a percent figure by each one. For our example, 20/60 equals 70 percent, 20/80 equals 58 percent. Write down the results for each eye’s visual acuity percentage.
Next you must have a percentage of the visual fields. To determine visual fields, you must get the results of a test (either a Goldmann field or the SSA Kinetic test) that gives you the degrees of field at each of eight meridians (the eight lines that radiate out from the point of fixation at 0 degrees, 45, 90, 135, 180, 225, 270 and 315 (think of it like a compass, North, South, East, West, Northeast, Southeast, Southwest, Northwest). You total the degrees of field from these eight meridians and come up with a number of from 0-500 degrees (normal visual field, if you total all of the eight meridians is ...500!). You then look at the file located at: http://dss.mo.gov/fsd/rsb/manual/vrman/forms/visual_field_percent.xls
and find the number of degrees of total field (in our case, let’s say 235 degrees OD and 210 degrees OS). In the chart, 235 degrees equals 47 percent, and 210 degrees equals 42 percent. Write down those two results.
Now, we have four numbers:
Visual acuity percentages of 70 percent right eye, 58 percent left eye.
Visual field percentages of 47 percent right eye, 42 percent left eye.
We now open the file located at: http://dss.mo.gov/fsd/rsb/manual/vrman/forms/visual_efficiency.xls
Enter the four numbers we have in the correct blocks for visual acuity efficiency (left and right eye) and visual field efficiency (left and right eye), and you will end up with 33 percent efficiency in the right eye, 24 percent efficiency in the left eye and a visual efficiency result (for both eyes) of 30.7 percent. If you look at our standards for eligibility, (20 percent non-progressive condition, 64 percent progressive condition), you'll see that this person would be eligible if they had a progressive eye condition.
By the way, all these Excel spreadsheets are on the policy manual website. If you look at the Table of Contents page and click on "Forms" at the bottom, you'll find these files as well as the narrative header for eligibility (which has all the various criteria for eligibility on it).
For those of you who would like to perform the calculations yourself, it is relatively simple. First you must determine the visual efficiency of each eye. To do that, you multiply the percentage of visual acuity efficiency times the percentage of visual field efficiency for each eye. From our example above, that would be:
Right Eye: .70 X .47 = .329 (33 percent)
Left Eye: .58 X .42 = .244 (24 percent)
Next, we multiply three times the visual efficiency of the better eye (33 percent) plus the visual efficiency in the worse eye (24 percent), then divide the result by four. From our example, that would look like this:
3( percentVE of better eye) + VE in worse eye divided by 4 = binocular visual efficiency:
or,
3(33) + 24 / 4 = 30.7 percent
Please let me know if you have any questions.