Visual Field Testing
Chat Highlights
October 4, 2006

On Wednesday, October 4, 2006, Dr. Michael James Pro, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Visual Field Testing."

Moderator:   Welcome to chat, Dr. Pro.  Thank you for joining us again. Our topic tonight is visual field testing.  When is a visual field (VF) test performed?

Dr. Pro:   A VF field is a routine test, usually performed at the first visit of a patient sent for a glaucoma examination.

P:   Should people with a family history of glaucoma have VF tests if their eyes are "healthy"?  The only test I have had consisted of a technician holding out her fingers and moving them into my field of view.  How accurate is such a test and is it even valid?

Dr. Pro:  The finger-moving test is of use only in extreme cases.  It is best used in patients with severe loss, such as in retinal detachment.  I often order a VF test as a baseline for a person sent to my office with a family history of glaucoma.  But that is not always the case.  Often it is best to look at the optic nerve first.  If the nerve is healthy, the patient can often be safely observed.

P:  How do you define "safely observed"?  Is once a year sufficient?

Dr. Pro:  There is no exact period to observe a glaucoma suspect. It depends on factors such as IOP, corneal thickness, and optic nerve appearance.  But we were talking about people with positive family histories and presumably not much else.  Those people I usually see every year or two.

P:  How often should a glaucoma patient take a VF test?

Dr. Pro:  That depends.  For someone with the diagnosis of glaucoma and stable IOP, I test every six months to a year (mostly yearly).

P:  What is the purpose of the VF test?

Dr. Pro:  The VF test is used to help determine the functional loss caused by damage to the optic nerve.

P:  Are the results of a VF test subjective or objective?

Dr. Pro:  VF tests are both.  They are highly dependent upon the patient's performance during the test.  The various manufacturers, however, have software that attempts to make the results more objective and comparable to past tests.

P:  Patient responses in a VF test can vary, depending on how alert the patients are, how tired they are, how nervous, etc.  That can adversely affect the results. Isn't there also subjectivity in the interpretation of the results?

Dr. Pro:  Sure, it can be hard to separate so much fluctuation and it is easy to get lost in a VF!  Newer software tries to eliminate that by calculating change probability. When in doubt, it is useful to look at an area where there is a defect.  Then I look at the raw score for points at that location and compare that data to data on previous tests.

P:  Will we one day be able to take a VF test that doesn't require the interaction of the patient?

Dr. Pro:  There are new technologies like multifocal visual-evoked potential.  That is like an electrode scan of the brain responses to projected images. We had that where I trained in NYC.  But it took half a day!  The print-out was tough to interpret, but the exciting thing was that VF defects could be demonstrated.  So it is a truly objective test.

P:  What's the difference between SITA Standard and SITA Fast?  Do they give equally accurate results?

[Editor's note: SITA (Swedish Interactive Testing Algorithm) Standard and SITA Fast programs use "intelligent" analyses of the patient's responses and age-normed statistical data to reduce the testing time required to map the VF.]

Dr. Pro:  The SITA Fast uses the same testing strategy, but fewer test points.  It is theoretically as accurate, but I prefer SITA Standard because the literature uses this strategy, and it gives more test points.  Sorry, but I don't know the comparative sensitivity and specificity of the two strategies.

P:  Is SWAP (Short Wavelength Automated Perimetry) more sensitive than the standard, achromatic (white-on-white) perimetry?

Dr. Pro:  Two different SWAP longitudinal studies identified glaucomatous defects up to five years earlier than achromatic perimetry.  SWAP attempts to detect the earliest functional evidence of glaucomatous optic neuropathy.

Moderator:  Is SWAP the blue-yellow test?

Dr. Pro:  Yes. The Standard achromatic is more common.

P:  Can the lights or dots be projected too fast or too small and affect the patient's performance?

Dr. Pro:  Not too fast, as the testing strategy (software) controls that.  The size of the spot (stimulus) can be adjusted.  By the way, you can make the machine pause by holding down the button.

P:  Should glaucoma patients with a strong family history of glaucoma insist on a SWAP visual field test?  Do most ophthalmologists have either or both available in their office?

Dr. Pro:  That's a point of controversy in glaucoma.  SWAP theoretically detects change earlier, but it's harder and causes the patient more stress.  That's why many glaucoma specialists don't order many of those tests.  Also, it is more difficult for older patients with cataracts because the yellow cataract does not transmit the blue light as well to the retina.

Moderator:  My doctor ordered the SITA Standard after I had several blue-on-yellow tests.  That kind of upset me.  The perimetrist told me SITA Standard was better.

P:  Does SWAP require my doctor to buy a new machine or new software?

Dr. Pro:  New software.

P:  What are the MD and PSD numbers?

Dr. Pro:  MD equals mean deviation.  It refers to the total points below an expected number. PSD is pattern standard deviation.  PSD flags areas that deviate from normal.

P:  Many patients look at the grayscale map of the visual field and judge visual field health by that.  But isn't it really the numbers that count?  What are the positive numbers in the upper graph and the negative numbers in the lower graphs?

Dr. Pro:  The positive numbers in the upper graph are the raw numeric scores, point for point.  The negative points are how much each point is below an expected result, based on age.  The grayscale is just a graphic that is useful to illustrate to the doctor and patient where the problem lies.

P:  How important is it for the technician to provide the correct refraction?  Should the center point look in focus?

Dr. Pro:  Yes, the refraction is important.  The center point should be clear.  The center point is projected, so it may look fuzzy, but refraction is important.

P:  Once glaucoma has been identified and baselines are established through imaging, observation of the optic nerve, and VF tests, will progression show up first through imaging or VF testing?

Dr. Pro:  Most early studies found that damage showed up on imaging first, then came the field loss.  This may be less true with the newer test strategies and technologies (SWAP, FDT).

P:  How was that determined?

Dr. Pro:  The early studies (1970's) demonstrated that with stereo disc photos.

P:  Should a visual field test be done when a patient's pupils are dilated?

Dr. Pro:  Since a reading add (refraction) is used, it should not matter much.  Some patients complain that they have difficulty when their pupils are dilated, so I usually don't dilate the pupils.

P:  Is a "small" area of the field a statistical concept that would produce a mean defect of less than 10 decibels?

Dr. Pro:  If you mean the size of the scotoma (defect), then that is basically true.  To have a mean deviation greater than -10, the defect is usually large (or a really dense cataract).

P:  I have pseudoexfoliation glaucoma and have had perfect visual field tests during the six years of my diagnosis.  The last test showed a suspected blind spot.  Another doctor thought she saw changes in the optic nerve.  Two weeks later, my regular doctor saw no change in the optic nerve.  She thinks the scotoma may be either a testing error or that my longer eyelashes from taking Travatan may have interfered with my ability to see some areas.  How much do you trust one VF test that shows a new defect in vision?

Dr. Pro:  I do not trust one VF test to show change.  Studies have shown that fields can return to "normal" after three abnormal tests.  Nevertheless, when I find a new defect and think that it is an error, I repeat the test.  If it comes up more than once, the antenna must go up.

Moderator:  Thank you, Dr. Pro, for your time and great answers.

Dr. Pro:  Goodnight, everyone.

On October 18, Dr. Wilson discussed "Hypotony" in the Chat room. Click here for highlights of that meeting.

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