Visual Field Interpretation
Chat Highlights
April 15, 2009

On Wednesday, April 15, 2009, 2009, Dr. Michael Pro, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Visual Field Interpretation".

Moderator: Welcome back Dr Pro. Tonight our topic is Visual Field (VF) Interpretation.
Are Electronic Health Records (EHR) changing how doctors interpret visual fields? Do you suspect more and more doctors are reading from EHR versus print outs? Does importing an image directly from the visual field machine into the EHR change the quality of the image enough to effect the doctor's ability to interpret the test?

Dr. Pro: OK, there are bunch of questions in there. I'll answer them, and then, if everyone's interested, I have prepared some information about visual field interpretation.

EHR is new technology that is definitely coming into use; it really does not change the quality of the test image and how we perform or interpret visual fields. It may improve the ability to compare old tests, and helps prevent lost exams.

Moderator: Thank you, doctor. Please proceed with the information you have prepared. I will hold sending questions until you are ready.

Dr. Pro: Feel free to break in, but I thought you should know what physicians are thinking about when we look at your visual field test; after all, it carries all sorts of useful information and is not just a sadistic test that we make our patients perform!

I use the Humphrey Visual Field which has several different testing strategies. The most statistical information is available for white on white perimetry, and I use this test style primarily. The most commonly used testing strategy is the 24-2. This tests 54 points of the central field out to 24 degrees (30 degrees nasally). Each point is six degrees apart, and in this way the central field is tested where most diseases manifest.

I start by evaluating the quality of the test. There are three measures regarding this: fixation loss is evaluated during the test and recorded as a percentage; and false positives and false negatives are also recorded. These can affect the outcome of the test. For instance, if a patient had a high ratio of false positives, the test may look better than it really should, so a “trigger happy” tester may obscure the magnitude of field loss from glaucoma.

P: What's the difference between the test with the 'white lights' and the test with the yellow background and 'purple" lights?

Dr. Pro: Great question. The test with white lights on a white background or “white on white” is the most commonly used test and is the one most rigorously analyzed in the big clinical trials, but some researchers have demonstrated that the SWAP (short wavelength Automated Perimetry) or blue on yellow may detect defects from glaucoma earlier. The blue on yellow test is less popular with patients because it is generally harder to perform and more influenced by the presence of a cataract, so I rarely perform it.

P: What is fixation loss?

Dr. Pro: It's when you look away from the fixation light or pattern, so the test is less accurate because it is testing unintended parts of the retina.

P: My doctor never uses the term “degree” with me so I have no clue what it means. I do have all my past visual fields, so can you explain it as I look at an old test?

Dr. Pro: A degree is a measure of an arc from a central point, so it is the central area of your vision. Every test is measuring a small area of your total peripheral vision, but it measures the same area each time.

P: My doctor said he will use larger lights on the test so that he can tell what I see better. Is that something you do?

Dr. Pro: I sometimes use a larger spot size if a patient cannot respond well to the smaller spot size. We do this most often for people with poorer vision.

P: How frequently should your doctor recommend visual field testing in order to identify disease progression?

Dr. Pro: There is no exact answer to this question. But because there is so much variabilityfrom one test to the next, we recommend three fields to really ascertain progression and that may mean testing up to four times in a year if needed.

P: How many degrees are considered the central vision, the part of the vision we use for reading?

Dr. Pro: Basically, we read with about the central 10 degrees or so, but testing 24 degrees catches where most glaucoma defects begin.

P: Is there a good book that you know of that could teach an interested patient more about visual fields?

Dr. Pro: Well, there is no patient level book that I know of. The text that I learned from is from Heijl and Patella and is called Essential Perimetry.

Moderator: The the Glaucoma Service Foundation website has a good article--http://willsglaucoma.org/testing/vf.html – with examples –http://willsglaucoma.org/testing/vfexamples.htm.

P: How accurate are the field tests that will show the peripheral loss of vision in my eyes? Are there any other tests that are as reliable as the field vision test to show peripheral vision loss?

Dr. Pro: The visual field test is pretty accurate and there are no other tests that can document functional peripheral vision loss. I can talk about the print-out a bit more if you would like. Some may be interested in the statistical analysis that the VF does.

To compare to age-matched controls I look at the total deviation plot. But a uniform reduction of sensitivity is the hallmark of a cataract or other media opacity (like a hazy cornea or blood in the back of the eye). Specific for glaucoma is the pattern deviation, which adjusts for overall depression in responses that may be seen from a cataract. Glaucoma typically causes visual field loss in the Bjerrum areas of the upper and lower hemifields. These areas curve around the macula and show up as an arcuate shaped depression. These two plots are statistically evaluated with probability plots which are really the most important statistical measures in the printout. Essentially, they are a series of probability plots for each tested point. It analyzes the chance that the change at each point could be due to random chance alone. If a point is colored with a light hashed box, then fewer than 5 percent of normal subjects would be expected to have such low sensitivity. If it is covered with a black box, than the percentage drops to less than 0.5, very unlikely in a normal subject.

P: So the visual field only tests the center of vision. How many degrees does each square (or defect in my case) represent?

Dr. Pro: Six degrees.

P: Do 10 degrees mean the eight test points near the center of the field test?

Dr. Pro: Yes, plus or minus, but there is a 10-2 test that tests only the 10 degree central field and tests points closer together.

P: Should a 'normal 'white on white' test followed by an abnormal 'blue on yellow' test be cause for concern?

Dr. Pro: A single abnormal blue on yellow may be due to difficulty with performing the new test and may need to be repeated.

P: On my last visual field test my false negative error rate was 33 percent. Is that good /bad?

Moderator: Can you elaborate on false positives and false negatives doctor?

Dr. Pro: False negatives are when a patient does not press the button on the visual field machine for a point that had been responded to before, so the patient is pressing less than they should and the test looks worse than it should. Conversely a false positives are when the patient presses the button when they really don't see the light and these tests are inaccurate, too.

P: So if a field test has mostly black on the test it is a bad test?

Dr. Pro: If a mostly black visual field test is accurate it is due to very decreased sensitivity to light stimuli from glaucoma or other diseases; alternatively, it could be very inaccurate (very high false negative rate).

Moderator: Thank you doctor. We are out of time for the evening. Thank you for sharing with us tonight.

Dr. Pro: You are all welcome. Goodnight, folks!

On May 6, 2009, Dr. Pro discussed "Plateau Iris Syndrome" in the Chat room. Click here for highlights of that meeting.

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