Visual Field Testing
Chat Highlights
October 22, 2003

Norma Devine, Editor

On Wednesday, October 22, 2003, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Visual Field Testing."

Moderator:  Tonight's topic is visual field testing.  Any questions for Dr. Wilson on this ever-popular topic?

P:  What is the main use of visual field testing?  How important is it?

Dr. Rick Wilson:  Visual field testing is the most sensitive and accurate test we have for following patients with moderate to severe glaucoma.

P:  What are the different types of visual field tests?

Dr. Rick Wilson:  There are at least five different types.  The best visual field test at present is the SITA Standard (SS) Humphrey.

P:  How do the tests differ?

Dr. Rick Wilson:  For patients with very early damage that is not detected by the usual white-on-white perimetry, short-wavelength-automated perimetry (SWAP) is best.  SWAP uses a blue spot on a yellow background.  It is said to pick up early damage one to two years before the white-on-white perimetry will.  The tests differ by the algorithm that the computer uses to determine the threshold for any spot on the retina.  An algorithm that is more sensitive takes more time, but the longer the visual field test, the less reliable the patient's response.  The SITA-FAST (SF) test is very good at getting a lot of information in a short time, which is why it is a good visual field test. 

P:  Is SITA the white-on-white test?

Dr. Rick Wilson:  Yes.

P:  How are screening tests for glaucoma different from the visual field tests?

Dr. Rick Wilson:  Screening tests may only show a bright light and a dim light. The test will record whether the patient was able to see either of the lights.  Clearly, that is not as much information as a test that actually can find how sensitive each spot on the retina is.

P:  Are confrontation visual field tests used any more? How about tangent screen exams?

Dr. Rick Wilson:  Confrontation visual fields are used commonly in the doctor's office to screen for large visual field defects, especially neurologic defects.  Tangent screen examinations are no longer used.

P:  I am aphakic and have always worn my contacts for my visual field tests.  But my astigmatism has changed in my glaucoma eye, and now I see a bit clearer with over-contact glasses.  Or just plain old aphakic bifocals which, unfortunately, don't give me any peripheral vision.  I'm wondering what I should use for my next visual field test, as I know consistency is important.  The glasses only give me distance, not mid-range, vision. 

Dr. Rick Wilson:  Aphakic patients do much better on the visual field machine with contact lenses.  It is much more physiologic (natural).  If you feel glasses will improve your vision, you can wear your contacts and the perimetrist will put a lens in front of your eye for the middle distance that the inside of the bowl represents.

P:  Thank you!  I never would have thought to ask, and sometimes they get busy.  

P:  Can the test tell if you are unsure, and guessing when you see the light? 

Dr. Rick Wilson:  Yes, it can.

P:  How often should a patient with mild-to-moderate glaucoma have the SITA test?

Dr. Rick Wilson:  Usually, once a year is enough, unless there has been some change.  Then, perhaps, every six months, until the doctor is sure there is no further change.

P:  Is the Goldmann visual field test still used in the U.S. and Canada?

Dr. Rick Wilson:  Yes, it is.  It is a kinetic perimeter, which means the light comes from the side until the patient sees it.  In static perimetry, the light is not moved, but made brighter and brighter until the patient sees it.

P:  Do you have any suggestions for patients taking visual field tests?  

Dr. Rick Wilson:  One of the common problems patients have taking visual field tests is that they are sleepy.  I recommend that my patients drink some coffee or tea before the test to make sure they are most alert.  On the other hand, if the patient is nervous, I think deep breathing and self-reassurance help. 

P:  With the Humphrey test using the half-power lens, I noticed it wasn't sharply in focus even at center.  Other than the wrong lens, what could be happening? 

Dr. Rick Wilson:  I suspect that your lens wasn't focusing the light accurately for you.  If the light was just dim, then the computer had recognized you were seeing all the lights and only gave you dim ones.

P:  How are long-term fluctuations, suggesting improvement or worsening of glaucomatous defects, determined? 

Dr. Rick Wilson:  Long-term fluctuations are the variations in the threshold at one point on the retina from exam to exam. There should be some fluctuation even in normal patients.  In patients with advanced glaucoma, the fluctuation can be much worse. Therefore, the more fluctuation there is, the more likely the patient is to have serious disease. 

P:  Can the visual field test take cataracts into account?  

Dr. Rick Wilson:  Yes.  The visual field test has a statistical computation that removes the generalized decrease in sensitivity across the whole visual field.  Since the cataract covers the whole lens, it will depress sensitivity everywhere.  By subtracting out the generalized decrease in sensitivity, the computer can do a moderately good job of adjusting the visual field for the cataract.

P:  What do the abbreviations MD and PSD mean on the printouts of visual field tests?  

Dr. Rick Wilson:  MD is "mean deviation," and stands for the average depression at each point, compared to age-related normals.   PSD is "pattern standard deviation," which is a measurement of the degree to which the shape of the patient's measured field departs from a normal, age-matched reference field.

End of highlights for October 22, 2003.

On October 29, Dr. Wilson discussed "Refractive Errors and Glaucoma" in the Chat room. Click here for highlights of that meeting.

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