Chat Highlights
Visual Field Testing II
August 9, 2000

Norma Devine, Editor

On Wednesday, August 9, 2000, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Visual Field Testing in Glaucoma." 

Dr. Wilson:   Hello, everyone. 

Moderator:  Hello, Doctor Rick.  We have a unique visitor, Dr. Pupil.  He  is a glaucoma doctor and a glaucoma patient.

Dr. Wilson:  Welcome, Dr.  Pupil.  Feel free to chime in with the answers. 

Dr.Pupil:  Thank you.

Moderator:  The topic tonight is again about visual field testing.

P:  Dr. Wilson, I was recently diagnosed with glaucoma.  Do you have any suggestions for how patients  should take the visual field test?   Things we should and should not do?  I know I shouldn't move my eyes around and look down,  but I felt I would fixate and see nothing if I kept staring at the light. 

Dr. Wilson:  You have to keep looking at the fixation light, otherwise the test is not testing your side vision.  Signal the operator to stop if you can't hold the fixation any more.  

P:  Yes,  I kept moving my eyes around to look for that little green light.

Dr. Wilson:   How much visual field loss do you have?

P:  I don't know.  My doctor said I did well on the test.  I feel I cheated though, moving my eyes around and down.

P:  If  your doctor said you did well on the test, you must not have lost fixation too much. Remember to blink often and relax. There is a learning curve with the tests.

P:  Okay, thanks.   I felt like I was going to be hypnotized.  The assistant kept having me tell her to stop and then we'd rest some before continuing.

Moderator:   Your doctor said your optic nerve and visual field looked good, right?  

P:  Yes, whatever that means.  

Dr. Wilson:   Then, in all likelihood, the effect on your life will be the tremendous nuisance of having to take medications and see the doctor regularly.  Most of visual loss due to glaucoma occurs before the patient sees the doctor.  Few people need surgery.  Our abilities are rapidly advancing, so that is reassuring, as well.

Moderator:   Dr. Rick, can you explain loss of fixation for the group, please?

Dr. Wilson:   Loss of fixation has two meanings.  In test taking, it means the test taker is not looking at the fixation dot on the back of the bowl.  Loss of fixation also means that the center part of vision, the part that sees fine detail and color,  is lost to glaucoma. 

P:  I hate to be obtuse, but when you say "center" is that literally the middle part of your visual field?

Dr. Wilson:   Yes. 

P:  My 12-year-old was diagnosed a few weeks ago with juvenile open angle glaucoma.  His first VF (visual field test) is next week.  They say it takes an hour.  Any words of wisdom on having a child sit through this?

Dr. Wilson:  It should not take an hour.  A Sita Fast test would give perfectly adequate information in about five to seven minutes per eye. 

P:  My 12-year-old has had VF tests for years. It has been no big deal to him.

Dr. Pupil:   It's almost like playing a video game.

P:  My son also said it was like a video game.

P:  That's good to hear.

P:  What are the five areas which are accounted for in the Glaucoma Hemifield test?

Dr. Wilson:  The Glaucoma Hemifield basically looks at the same area above and below the horizontal meridian and compares them for sensitivity. They should be fairly symmetric. 

Moderator:  Can a droopy eyelid cause an artifact on the visual field test?

Dr. Wilson:  Absolutely.  It can cause a constriction across the top of the visual field. 

P:  What is the percentage of false positives or negatives allowable before the results are compromised on a VF  test?

Dr. Wilson:  It should probably be less than 15%, but the less, the better.

P:  Please explain false negatives and false positives.  

Dr. Pupil:  False positives come from pressing the button when no target light is being presented ("trigger-happy").  False negatives come from not pressing the button when the light should  have been seen (fatigue or inattention).    

Moderator:  Doctor Pupil,  do you find the visual field tests easy or tough?

Dr. Pupil:  I have always found them physically easy, and psychologically tough.  The mind often wanders and usually starts thinking about glaucoma, blindness, etc.

P:  Yes, very tough psychologically.

Moderator:   Yes, very tough mentally.

P:  They are tiring,  I just had one today, as a matter of fact.  My eyes are still kind of dilated.

P:  Is it okay to use reversing drops after the VF test?

Dr. Wilson:  Reveyes would be fine to use.  I wouldn't use Pilocarpine.

Dr. Pupil:  It's helpful to remember that it is normal to miss half the lights. 

Moderator:  Should we stare and blink often, or stare and try not to blink, or blink normally?

Dr. Wilson:  You need to blink frequently to keep your cornea moist and seeing well, so you can see those small lights.

P:   Come on, guys.  The visual field test is the easiest thing we have to contend with.

P:  Dr. Wilson, my 20-year-old son is a glaucoma suspect.  His IOPs  vary between 17/18 and 19/20.  He uses no drops.  His visual field test last December was perfect; his HRT in July was perfect, and so was his optic nerve.  How often should he have a VF test if his intraocular pressures are not continually rising?

Dr. Wilson:  Glaucoma is almost always picked up earliest on a thorough exam of the optic nerve by a good examiner. The visual field  test could be every one to two years.  

P:  I don't understand.  Why would they think your son has glaucoma if  his IOP and optic nerve are normal and there's no loss of visual field?  

Dr. Wilson:   I think they meant suspect due to family history.

P:  Yes, my son is a glaucoma suspect because of my glaucoma history, the young age at which I was diagnosed, and last year there was a sudden rise in his IOP from 13/14 to 19/20.  I feared that he had my type glaucoma, but he is only a suspect.

Dr. Pupil:  Dr. Rick, how do you like the Frequency Doubling VF test?  What role does it play?

Dr. Wilson:  We've been using it in screening.  In an inner-city setting, it is almost worthless.  In a clinic with trained patients, it agrees fairly well with regular Humphrey testing. 

P:  Doctors, am I correct in thinking that on the printout of a visual field test, a negative (-) number on the little "maps" of mean deviation (MD), and pattern deviation  (PD) means you see that many db less than "normal" for your age?

Dr. Pupil:  That is correct.  You are compared to an age-matched set of "normal" patients.

P:  So, if it is a positive number, you see better at that spot than "normal"?

Dr. Pupil:  "Better than normal" is unusual (by definition), and often represents being trigger happy, with high false positives.

Moderator:  What kind of education does the technician doing the VF receive?  

Dr. Wilson:  The computer has reduced dramatically the expertise needed to do a visual field.  The technician still needs to have a basic knowledge of what a field is, what the possible artifacts are, know how to prevent them, and be able to troubleshoot the machine.

Dr. Pupil:  In my experience,  the technician is a very valuable part of the VF test:  giving encouragement, monitoring the patient, etc.  "Techs" are better at telling me if a patient is "reliable" than is the computer.

P:  Dr. Wilson, I wear glasses designed for good distance vision, which I use for driving. I believe I'm asked to wear them during my VF test.  Doesn't that make it harder for me to pick up the light flashes, since everything looks farther away with these glasses? This just occurred to me.

Dr. Pupil:  Good question.  Your vision should be corrected with lenses and then adjusted for the distance of the test object.

Dr. Wilson:  Only if you are young should you use distance glasses during visual field testing. If you need reading glasses, the amount of extra power you need for the closer distance should be added to the distance glasses.

P:  Does Wills have a site explaining how to read a VF  test printout?

Moderator:   Yes, the URL for an article on VF testing by Dr. Jeff Henderer is: http://willsglaucoma.org/testing/vf.html.  

P:  Doctors, what does it mean if the number for Mean Deviation is negative and the number for Pattern Deviation is positive?

Dr. Pupil:  Mean Deviation is a measure of the average loss across the whole field, for example, -2dB.  Pattern Deviation is a measure of  how much variation is in the field.  If it is uniform, Pattern Deviation is near zero.

P:  So one should be negative and the other positive?

Dr. Pupil:  If there is a lot of variation (i.e., some areas white and some black), the pattern deviation is larger (and always positive).

P:  Thank you!  That clears up a question I've had for a long time.

Dr. Pupil:  A cataract might cause a large Mean Deviation, but usually affects the field fairly uniformly, and the Pattern Deviation remains low.

P:  I've had cataract surgery in both eyes and I have a "distance"  lens implant in the right eye and a  "reading" distance lens in the left eye.  My "driving" glasses balance this out. But I read without glasses.  I'm an "old lady" who had nuclear cataracts and was getting more and more near-sighted before surgery.  Should I be taking the VF test without glasses? 

Dr. Wilson:  Each eye will have a separate refraction for the distance of  the bowl from the patient's eye.  Be sure to inform technicians what your distance prescription is so they can calculate the near refraction.

P:  How important is an accurate refraction in obtaining an accurate VF?

Dr. Wilson:  Moderately important.  The refraction can't be too far off. 

P:  Can the diameter of one pupil be larger than the other?  One of mine is 6.3 mm and the other is 6 mm.  Is that possible?

Dr. Wilson:   It happens all the time. 

Dr. Pupil:  A small difference is usually nothing to be concerned about.

P:  What is the fovea, please ?

Dr. Wilson:   It is the center part of the retina; the very center of your vision.

P:  So does "fovea 34 db" mean that's the average a person should see?

Dr. Wilson:   It varies with age, but 34 would not be unusual for a middle-aged person.  I have to run gang, have a good week everyone.

End of highlights for August 9th chat.

On August 16th, Dr. Rick Wilson discussed "HRT - Heidelberg Retina Tomographs" in the Chat room. Click here for highlights of that meeting.

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