Understanding Visual Fields
Chat Highlights
January 30, 2002

Norma Devine, Editor

On Wednesday, January 30, 2002, Dr. Elliot Werner, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Understanding Visual Fields."

Moderator:  Welcome, Dr. Werner.  Nice to have you back.  The topic tonight is understanding visual field testing.

Dr. Elliot Werner:  Hello, everyone.  Nice to be back.  I'm ready when you are. 

Moderator:  Doctor, is there a typical pattern of loss of visual field in a glaucoma patient?  If so, where is it?

Dr. Elliot Werner:  Typically, glaucoma patients develop loss of visual field in the paracentral area.  That is, between about 5 degrees and 20 degrees from the center.

P:  What area of the visual field is involved first?

Dr. Elliot Werner:  Most often that's superiorly, above the center, and more often on the nasal, or inside, part.  

P:  On the printout of the visual field test, one section shows black and gray boxes.  I was told three contiguous boxes are needed to determine a defect.  Is that right? 

Dr. Elliot Werner:  Not really.  Any area of loss that is consistently reproducible is a defect.

P:  Should someone with good visual fields for ten years, but high intraocular pressure, worry about glaucoma or do something about it?

Dr. Elliot Werner:  I wouldn't worry a lot.  Developing glaucoma damage is a risk, but the risk is fairly low.

P:  I am very sensitive to noise of any kind and become easily distracted when I am taking visual field tests.  My local glaucoma doctor knows this and tells the technician not to rattle papers, etc.  I am the only patient in the room.  But at large glaucoma hospitals I've seen two patients, back to back, in the room, with a technician preparing one of them.  I find it extremely difficult to concentrate under those circumstances.  Do other patients feel as I do?

Dr. Elliot Werner:  Yes.  Your complaint is common. We have only one perimeter per room in our office and do not test two patients in the same room at the same time.

P:  When does it make sense to go to the blue/yellow visual field testing?  Isn't that a more sensitive test than the white/white test?

Dr. Elliot Werner:  Blue/yellow testing makes sense when standard white/white perimetry is normal, but there is other evidence of glaucoma damage -- from the appearance of the optic nerve.  Early defects can often be discovered that would not show up on white/white testing.

P:  Dr. Werner, does the vision loss from a cataract show up differently on the field than the loss from the glaucoma?  In other words, is it easy to tell the difference in a person who has both?

Dr. Elliot Werner:  Yes, the visual field defects are quite different and can usually be distinguished, but it requires clinical examination as well.

P:  I have asked for a visual field test using first a size III and then a size V spot to quantify the glaucomatous field defects and the depression of visual field caused by cataract.  My glaucoma specialist readily agreed to the test.

Dr. Elliot Werner:  Size V is usually used in patients with poor vision who cannot see the size III spot.  Size V does not really distinguish between cataract and glaucoma.

P:  Yes, my understanding is that the size V stimulus is used for highly damaged areas.  According to the Humphrey manual, defects that might be recorded as absolute when the size III stimulus is used may be found to be relative defects when testing is done with size V stimulus.

P:  I have never heard of different sizes.  What does that mean?  Does that refer to the light the patient stares at?  (The light in the center of the bowl.)  

Dr. Elliot Werner:  No, it refers to the spot that is flashed all around the bowl to test the field.  Different sizes are available for different testing situations.

[Editor's note: "Spot" refers to the small lights, called "stimuli."  According to "The Field Analyzer Primer" by Humphrey systems, the Field Analyzer uses projected stimuli which can be varied in intensity over a range of more than 5.1 log units (51 decibels) between 0.08 and 10,000 asb (apostilb)].

P:  I have been advised to start with a different eye at every testing, but my left eye is much worse than my right.  Should I not always start with the worst eye?

Dr. Elliot Werner:  There is a fatigue effect with perimetry, so it is a good idea to alternate eyes.

P:  I have a visual field test tomorrow.  Any tips?  

Dr. Elliot Werner:  Don't worry.  Just look straight ahead and push the button when you see the light.  The machine will accurately measure your visual field if you do that.

P:  You can hold the button down to stop the test if you need to rest.  

P:  When I hold the button down, that stops the test momentarily.  When I asked why we weren't told that sooner, the reply was probably because that would take longer to test people.

Dr. Elliot Werner:  That is probably true, but I, for example, did not know you can stop the test by pushing the button, so I suspect other people also don't know that.

P:  Is visual field testing used for other types of vision loss too, or just glaucoma?  I have a friend with a pigmentary lesion who is taking visual field tests. 

Dr. Elliot Werner:  Visual field testing is used extensively in all forms of eye disease, as well as neurologic disease, such as strokes or brain tumors.  It is an extremely valuable diagnostic tool and can often be used to distinguish different types of vison loss.

P:  Some of the newer visual field set-ups have a lens placed in front of the eye.  The  technician or physician keeps re-adjusting this lens during the test.  They also tell me how well I'm doing.  I find that distracting.

Dr. Elliot Werner:  Ask them not to do that.

P:  The last visual field test I took was with a new glaucoma specialist.  I was so anxious to have a good test, I was exhausted and dizzy afterward.  Did I overdo my intensity in trying to see as many lights as possible?

Dr. Elliot Werner:  Yes.  There are no right or wrong answers.  Just relax, breathe normally and push the button when you're sure you see the light.  If you're not sure, it's best not to push.

P:  I think visual field testing is probably the most stressful medical test I have ever had.

P:  Please define "fixation."

Dr. Elliot Werner:  Fixation is the ability of the patient to fixate on or look at the central spot (light), rather than move the eye around.  

P:  After looking at the results of my visual field tests, my doctor says "that eye is at fixation." What does that mean?

Dr. Elliot Werner:  Not sure.  I've never heard that phrase.  It might refer to the visual field defect being very close to the center where fixation takes place.

P:  Dr. Werner, lately the vision in my left eye has been blurred, and the blur never goes away completely.  I am concerned that my next visual field test of my left eye is going to show much worse results than really exist.  How can the glaucoma specialist determine if there is progression when comparing it to my previous visual test when I did not have any blurring?

Dr. Elliot Werner:  Progression is determined by comparing the recent fields to the older ones.  If your vision is getting progressively more blurred, this could be a sign of something serious and needs to be evaluated.

P:  The blurring in my left eye is not getting progressively worse. It went from no blurring to blurring, which is pretty constant.  I think it is a side effect of the med (Alphagan).  I do not use that in my right eye and I don't have blurring in my right eye.  If the blurring is due to the med,  what might you advise?

Dr. Elliot Werner:  You need to be examined to determine the cause of the blurring.  Tell your doctor about the blurring and what you think might be causing it and let him or her figure it out.

P:  I have been told I am an excellent "test taker."  Does that mean I cannot fool the machine?

Dr. Elliot Werner:  No.  It means you fixate well and give consistent, reliable results without any artifact.

P:  I have many vitreous floaters that travel around in clouds in both eyes.  Sometimes they get thick right in the line of vision.  I look left or right and then center to clear them.  When I do that during a visual field test, they think I am cheating.  I tell them what I am doing, but I wonder if they believe me.

Dr. Elliot Werner:  Floaters usually have very little effect on the results of the field test.  It is best to look at the center light all the time and try to ignore the floaters.  They won't interfere.

P:  I'm writing this again because I'm not sure if you understood.  I need to clear the floaters from the center of vision to be able to see better for the test.  The floaters are so thick that they block print that I am looking at.  This requires looking away from the center light for a second.  I must do that or the test will be corrupted.  

Dr. Elliot Werner:  Despite what you may perceive, the test will not be corrupted.  The machine can detect your eye movements and eye movements have a much greater deleterious effect on the results of the test than the floater.  If you keep looking straight ahead, the machine will accurately measure your field even if you think you're not seeing.

P:  I don't know the speed of the processors involved, but it seemed at my last test that I was actually faster than the computer.  I saw lights, pushed the button immediately and it didn't register properly. The tech told me to slow down.  What can you add to this?

Dr. Elliot Werner:  The machine "knows" the normal reaction times of human beings.  If you push the button too fast, the machine makes note of that and records it as a false positive response. 

[Editor's note:  The Humphrey Field Analyzer uses a stimulus duration of .02 seconds.] 

P:  Isn't it true that if you don't keep your eye on the spot in the middle of the bowl the test can take much longer?

Dr. Elliot Werner:  Yes, and the test stops while your eyes are closed.  These machines are very smart.

P:  Before my very first visual field test, the technician patched my eye and gave me a few minutes to get used to it.  But that has not happened since then.  Should patients be given a minute or two to adjust to the patch?  

Dr. Elliot Werner:  It probably doesn't make much difference.

P:  I have been told to push the button if I think I see the light.  There are many times, especially as the test goes on and I am tired, that I click, probably because I think it is about time and I am not sure.  I'm afraid I'd really have a poor test if I only indicated when I was sure.

Dr. Elliot Werner:  Generally, patients who push the button because they think they should do not give reliable results.  It is best to be sure you see the light even if some time goes by and you don't see anything.  Patients need to receive detailed and easily understood instructions. 

P:  I have been told by the tech that if my eye gets tired to hold the button down for a break.  I do it often, too.

P:  Several patients here have droopy eyelids (ptosis) from surgery.  Do you ever tape up such eyelids during a visual field test?

Dr. Elliot Werner:  Yes.  Droopy eyelids can really interfere with the test and should be taped if they cause a problem.

P:  I always had my one eyelid taped, but during my last visual field test it was not taped because I couldn't blink, which is a problem.

Dr. Elliot Werner:  It's a problem that is very difficult to solve.  Ask if they can switch to the SITA Fast strategy to reduce the test time so blinking might be less of a problem.

P:  Does the test start with brightest light at each spot and then progressively dim to the point that the patient can no longer detect the light at each individual spot?

Dr. Elliot Werner:  No.  The machine presents dim and bright lights in a random sequence to come to the brightness that the patient is just able to perceive -- called the threshold.

P:  Would you please explain about testing the blind spot at the beginning of the test and sometimes during the test?

Dr. Elliot Werner:  The blind spot is tested several times during the test to be sure the patient is not moving his or her eye around too much.  The blind spot should be in the same place all the time and the machine "knows" that.

P:  I have been told my " blind spot " is four times normal size.  Is that because of glaucoma? 

Dr. Elliot Werner:  It could be, but there are other causes of an enlarged blind spot.  If your doctor has examined you and told you it is due to your glaucoma, that is probably true. 

P:  I have refractive amblyopia in my right eye.  Vision is +5.00 cyl -2.75. My vision can only be corrected to around 20/30 and always stays blurred.  When I concentrate on one area like reading, my vision gets "noisy" like fine TV snow.  I have my first visual field  tomorrow.  I am really worried about sustaining the intensity of using that eye.

Dr. Elliot Werner:  Don't worry.  If the snow is significant, the machine will measure that, too. 

P:  Would amblyopia like I described have any influence on whether ocular hypertension should be treated before glaucomatous changes occur?

Dr. Elliot Werner:  Probably not, but amblyopia makes it more difficult to detect visual field loss, so the doctor might make a judgment based on the optic nerve appearance only.

Moderator:  Why would amblyopia make it more difficult to detect visual field loss? 

Dr. Elliot Werner:  Because the eye and visual system do not respond normally to visual stimuli.

P:  The visual field test has gotten shorter over the years.  Any chance that it may get shorter still? 

Dr. Elliot Werner:  Yes.  In fact, in a few years we may have objective visual field testing that does not require any response from the patient.  There are some very exciting techniques in development right now.

P:  Could you please explain frequency doubling perimetry (FDP)?

Dr. Elliot Werner:  FDP uses a different target, which consists of black and white stripes rather than a white light spot.  There is some evidence that it is more sensitive for early defects, but seems less able to detect progression.  

P:  How much training do perimetrists have?

Dr. Elliot Werner:  That varies a lot, depending on who trained them.  There are not specific standards and no specific degree is required.  

P:  How long should a normal test take?

Dr. Elliot Werner:  A normal test with the new SITA program takes between 5 and 8 minutes per eye.

Moderator:  Thanks Doctor.  We really appreciate your giving of your time and knowledge to join us.

Dr. Elliot Werner:  Happy to do it.  I'll see you again the end of  February. 

Note:  For more information about visual field testing go to http://www.willsglaucoma.org/testing/vf.html.     

End of highlights for January 30, 2002.

On February 6, Dr. Wilson discussed "Leading a Healthy Lifestyle" in the Chat room. Click here for highlights of that meeting.

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