Blue-on-Yellow Visual Field Testing
Chat Highlights
June 29, 2005

On Wednesday, June 29, 2005, Dr. Rick Wilson a glaucoma specialist at Wills, and the glaucoma chat group discussed "Blue-on-Yellow Visual Field Testing."

Moderator:  Welcome to chat, Dr. Wilson. Tonight our topic is "Blue-on-Yellow Visual Fields."  We glaucoma patients soon learn that visual field measurement is an essential tool in glaucoma diagnosis and management.  But it seems that more of us are familiar with white-on-white perimetry than with blue-on-yellow perimetry, called SWAP (short wavelength automated perimetry). Which test takes longer?

Dr. Rick Wilson:  The SWAP test usually takes longer, as patients are not used to it.  They always tell me it is harder to take than the white-on-white test.

Moderator:  Approximately how long does SWAP take for each eye?

Dr. Rick Wilson:  The SWAP should take just a few minutes longer than the white-on-white or Standard Automated Perimetry (SAP).

P:  Do you know why the blue test stimuli that are presented as dots on a yellow background bowl in SWAP are larger in diameter than the stimuli on white-on-white perimetry?

Dr. Rick Wilson:  The blue light, 440 nanometers, falls at the peak for blue cones, but a larger target area is needed to get reliable results.

P:  What is it about the blue-yellow test that makes it hard to take?

Dr. Rick Wilson:  According to my patients, the target light is more subtle, and many of the patients are already used to the white stimulus.

P:  Is there more inter-test variability with white-on-white or with blue-on-yellow?

Dr. Rick Wilson:  Since the SWAP is more sensitive to earlier injury, there is more noise and more inter-test variability.

P:  What are the advantages of SWAP over white-on-white perimetry?

Dr. Rick Wilson:  The retinal ganglion cells that perceive blue-yellow color are injured earlier in the disease in many people with glaucoma.  Therefore, testing for just those ganglion cells, rather than all the different kinds of ganglion cells, should give a more sensitive test, and it seems to do that.  SWAP is supposed to pick up early glaucoma damage one to three years before white-on-white perimetry.

P:  Is SWAP or white-on-white perimetry more affected by cataracts and other media opacification?

Dr. Rick Wilson:  SWAP seems to be more affected by cataracts, but they cause a decrease in sensitivity for all types of perimetry.

P:  Do you think SWAP will become more commonly used than white-on-white perimetry?

Dr. Rick Wilson:  Not in the near future, due to the variability in results.

P:  Is it more difficult to interpret the SWAP grayscale?

Dr. Rick Wilson:  Yes, very much more difficult, and the SWAP grayscale was meaningless until recently.  Most of us never look at the grayscale with SWAP.

P:  Is it easier or more difficult to monitor progressive field loss from glaucoma with SWAP or with white-on-white perimetry?

Dr. Rick Wilson:  SWAP is more difficult, but more sensitive.

P:  Is SWAP available in most ophthalmologists’ offices?  I never had one.

Dr. Rick Wilson:  I would think it is in all specialists' offices, but only in some of the generalists' offices.

P:  Can the usual visual field machine accommodate SWAP, or must the doctor buy an expensive new machine?

Dr. Rick Wilson:  Same machine, different filter for the light, and different software to run the machine.

P:  If you already know you have glaucoma, is there any advantage to having a SWAP test?

Dr. Rick Wilson:  If you have visual field loss with a white-on-white visual field test, the SWAP would not be additive.  If you have glaucoma diagnosed by IOP (intraocular pressure) and optic nerve changes, but no visual field loss, a SWAP would be the best way to follow you.

P:  I felt fortunate to have an AccuMap visual field (AccuMap Objective Perimetry System) at my doctor's office.  Do you think that easier test will become more commonplace?

Dr. Rick Wilson:  Yes, I think it will.

P:  What is the AccuMap field test?

Dr. Rick Wilson:  It is a new computerized test for topography (surface map ) of the area around the nerve.  The AccuMap maps the visual field, rather than the posterior pole of the eye.  It does that by recording the electrical stimuli, which are produced when the part of the brain that interprets what the eye sees records seeing a light stimulus.  That feature makes the AccuMap objective, as opposed to subjective, as the SWAP and SITA tests are.

P:  Is the AccuMap being used by many eye doctors?

Dr. Rick Wilson:  Not yet. The technology had some bugs, but it is now becoming much more useful.

P:  Which visual field testing methods do you, personally, prefer to use, and on what type of patient?

Dr. Rick Wilson:  I use a SWAP on glaucoma suspects and those with early glaucoma who don't have white-on-white visual field defects.  I use a SITA (Swedish Interactive Thresholding Algorithm) standard test on everyone else, on all my white-on-white visual fields.

SITA is just a testing strategy for the Humphrey perimeter.  It uses a more sophisticated algorithm than the 30-2 that used to be used.  It is said to have the same sensitivity and accuracy, but takes less time to accomplish.

P:  How closely do the areas of loss shown on the results of a visual field test match the cupping damage seen by ophthalmologists on examination, or by computerized tests like the HRT (Heidelberg Retinal Tomography) or OCT (Ocular Coherence Tomography)?

Dr. Rick Wilson:  They should match it exactly.  Glaucoma specialists should be able to predict the appearance of the optic nerve from the visual field and vice versa.

P:  If you are tested with SWAP once, should you always have SWAP, or is it okay to switch back and forth?

Dr. Rick Wilson:  Since you are testing the field differently, you cannot change back and forth.

P:  I remember having had a red and white visual field test.  What is that one called?

Dr. Rick Wilson:  Usually, testing a visual field with a red test target is used in cases of neurologic problems like strokes, tumors, etc. It can also be used to test for retinal toxicity to certain medications.

P:  I recall seeing field test printouts that looked like geographical maps, with sets of circular or oval rings.  What are those?

Dr. Rick Wilson:  The digital information obtained from the perimeter can be plotted in a variety of ways, one of which is in an elevation, the higher levels being the most sensitive.

Moderator:  Thank you, Dr. Rick.

Dr. Rick Wilson:  You're welcome. Have a great 4th of July.  Greet Dr. Myers for me next week.  I'll be attending a meeting of all the world's glaucoma societies in Vienna, so I may have some news to bring back for you the next time we meet here.

Moderator:  Have a safe trip.  We will appreciate having Dr. Jonathan Myers with us next week.

On July 6, Dr. Myers discussed "Understanding Eye Drops" in the Chat room. Click here for highlights of that meeting.

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