Chat Highlights
Visual Field Testing I
August 2, 2000

Norma Devine, Editor

On Wednesday, August 2, 2000, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Visual Field Testing in Glaucoma." 

Moderator:  Good evening, Dr. Wilson.

Dr. Wilson:  Hello, team.

Moderator:  Welcome everyone.  Tonight's topic is Visual Field Testing.  

Dr. Wilson:   Let me start by saying that visual field testing is very subjective.  The variability is significant, and the more field damage there is, the greater is the variability.  

P:  Is "scotoma" the term for a visual field loss?  Is the Humphrey Visual Field test accurate for children, especially a child with nystagmus?

Dr. Wilson:  A scotoma is an area of vision loss surrounded by a seeing area. The visual field test depends upon the child's concentration and cooperation, so it can be accurate or not. Nystagmus, or the rapid flicker of the eyes back and forth, makes visual field testing impossible.

P:  How do you know when the visual field is really getting worse?

Dr. Wilson:  The computer tests how much fluctuation there is in the answers of the patient.  Usually, if the change in visual field is more than 2 1/2 times the fluctuation, then the loss is considered real.

Moderator:  When we look at a visual field printout, we usually see the picture first, with it's dark areas, but when you look, aren't you looking more at the numbers?  For you, what is the significant information on the visual field test?

Dr. Wilson:  The grayscale helps me pick out the areas of the number scale to focus on; otherwise, it is hard to pick out the wheat from the chaff.

P:  Is subjective clinical impression based on looking at grayscales probably the most widely used means of determining progression?

Dr. Wilson:  No.  What I spoke about above was a change in the actual numbers of  2 1/2 times the fluctuation. The gray scales can be very misleading.  For example, a change from 20 to 19 might result in a change of gray, but a change of 25 to 20 might not.

P:  If six experienced clinicians were asked to determine progression by clinical impression in the same series of field tests, do you think their impressions would all be different?

Dr. Wilson:  Yes, the impressions would probably be different but, hopefully, not dramatically different.

P:  Dr. Wilson, how often should visual field tests be done after trabs in both eyes?  Are the tests done less often once the IOPs (intraocular pressures) are low enough?

Dr. Wilson:  If the IOP is low-normal, the visual field tests can be done yearly, unless the field loss is great; then the tests are done more often.  

P:  If someone has had trabeculectomies and the eye pressure is low (less than 10), why would that person need frequent visual field tests? It seems to me that more surgery and medications are not an option.  So what would you do if visual field testing showed field loss?

Dr. Wilson:  If the IOP is less than 10, one would probably not do anything with the information except encourage exercise, the use of vitamin E and aspirin, as well as stopping smoking and losing weight,  if those are an issue.  That is, try to increase the body's resistance to further loss.

P:   How much aspirin would a man in his sixties take?  

Dr. Wilson:  One baby aspirin MAY be helpful in people with advanced damage or low-tension glaucoma, according to one school of thought.

P:  I take a baby aspirin once a day.  Is that all right?

Dr. Wilson:  Yes. So do I,  and most doctors over 50.

P:  How much aspirin and Vitamin E would you recommend for a 14-year-old boy who is large (6', 200 pounds) for his age? 

Dr. Wilson:  I would not use aspirin in a 14-year-old.  The cause of his glaucoma should not be increased platelet adhesiveness.  I would use Vitamin E,  400 I. U.,  along with a multivitamin with iron and probably 500 I. U. of vitamin C, as well as a diet high in vegetables, whole grains and fruits. That is what I use,  except men don't want the iron because of their hearts.  I must clarify that vitamin E for neuro-protection is not universally accepted. The evidence for help well outweighs the evidence for harm, so I recommend Vitamin E.

P:  Dr. Wilson, is a 10-degree angle loss in the nasal superior quadrant considered a large field loss?

Dr. Wilson:  That depends on if  it is right on the horizontal line that crosses the whole field and how dense it is. If the line is closer to the center of the visual field, then the importance is increased.

P:   If  the 10-degree angle loss is right on the horizontal line and just above it, is that a bad place to have a loss?

Dr. Wilson:   It is a common place to have early loss. 

P:  In a visual field test, is a difference in pupil diameter between the right and left eyes (5.0 vs. 5.9) significant?  

Dr. Wilson:  It is only when the pupil size drops below 3 mm that the pupil size matters much.

P:  Should patients check the visual field printouts to see that the lens, pupil size, etc. are the same for all tests?  In fact, should patients learn to interpret those tests or just leave it to the doctors? 

Dr. Wilson:  Probably leave it to their doctors. I recommend to my Mom to spend her time picking out the best doctor she can find, rather than double-checking every move he or she makes.  I like my patients to be well educated about their problems, because they understand the obstacles we both are up against and don't have unrealistic expectations.

P:  On one of my visual field printouts, there are numbers in parentheses below the regular line. There are two intersecting lines with rows of numbers radiating outwards, up and down.  In mine, the two lines of numbers contain 5 (30, 31, 0, 25, 22.)  In parentheses below the 30 and 31 are (30) and (28),  respectively. What does that mean?  

Dr. Wilson:  The cross hairs point out the center of your vision.  The numbers radiating out from there are points six degrees apart on the x and y axis.  The numbers in parentheses indicate that a point has been tested twice.  The difference between up to 10 pairs of points determines the short- term fluctuation that is so important in determining whether a change is real or just fluctuation.

P:   Is it correct to think that the higher the number, the less intense the light that can be detected? 

Dr. Wilson:   The higher the number, the more sensitive the retina is at that point.

P:   Will you describe long-term fluctuation? 

Dr. Wilson:   Long-term fluctuation is the fluctuation between tests separated by a week or two at the same point. This fluctuation is greater the more damage one has.

P:  Is normal-tension glaucoma (NTG) associated with a specific pattern of visual field loss?

Dr. Wilson:  Yes, in NTG one often gets a dense, small scotoma close to the center of the visual field.

P:   Is a different pattern, say loss in an outer lower corner, diagnostic to rule out NTG?

Dr. Wilson:   No, one cannot rule out NTG, but a characteristic visual field can make one suspect NTG.

P:  On a visual field test, what do MD, P, and PSD mean?  

Dr. Wilson:  "MD"  stands for Mean Deviation, i.e., the average difference of the points in the patient's visual field from an average "normal" group. "P" equals Probability that the change seen could happen by chance. "PSD" stands for  Pattern Standard Deviation, which subtracts out the global decrease in sensitivity caused by a small pupil or a cataract, leaving what is suspected to be due to pathology like glaucoma.

P:  When a visual field gets worse from one time to the next, does it usually end up with a larger dark area from the time before, or many smaller dark areas?  I don't know whether to expect one large black area or many small dots around the printout.

Dr. Wilson:  The loss can be very moth-eaten or a large dark area.

P:  In a repeated test, wouldn't the area need to be larger or deeper to suggest progression?

Dr. Wilson:  Yes, it would.  

P:  When you refer to "normal" in comparing results, do you take into consideration 'normal' for the individual's age?

Dr. Wilson:  There are usually three groupings of numbers on a composite page. One is the actual sensitivity of the patient, another is what would be normal for that patient's age,  and the third is the difference between the two.

P:  When I have -30, -29, -33, etc., I'm in pretty bad shape there, right?

Dr. Wilson:  If you are looking at the difference chart, those are bad numbers. 

P:   The IOPs in my nearly half-blind eye are 3 to 4 (after a trabeculectomy).  The IOPs in the other eye run 14 to 17, and my doctor wants the IOP  to be no more than 16. I'm keeping my fingers crossed on that one. I have intraocular lens implants in both eyes. On the vision chart,  my vision is 20/20 (through pin holes) in my worst eye, using my 'distance' driving glasses.  So much for 20-20 vision!

Dr. Wilson:  That shows why one can't rely on symptoms.  Eyes with tiny visual fields can have 20/20 vision. If the loss happens slowly enough,  people don't realize what is happening to them.

Moderator:  Next week we will be discussing the same topic.

Dr. Wilson:  I'll look forward to next week.  Have a good week, all.

Moderator:  Thanks, Dr. Rick.

End of highlights for August 2nd chat.

On August 9th, Dr. Rick Wilson and the group spent more time discussing "Visual Field Testing". Click here for highlights of that meeting.

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