Visual Fields
Chat Highlights
October 23, 2002

Norma Devine, Editor

On Wednesday, October 23, 2002, Dr. Elliot Werner, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Visual Fields."

Moderator:  Welcome back, Dr. Werner. Tonight's topic is about taking visual field tests. 

P:  What is the visual field? 

Dr. Elliot Werner:  The visual field is everything the eye can see in all directions, while remaining fixed.  Measurement of the extent of the field of vision has been an important part of managing glaucoma patients for 100 years.  

Moderator:  How is the visual field measured?  Are the visual field tests painful?  How long do they take?  

Dr. Elliot Werner:  The visual field is measured by shining little dots of light (stimuli) on a screen or bowl and asking patients if they see them.  It is not painful, but it is tedious and boring. A visual field test takes, on average, about 10 to 12 minutes per eye.

P:  My visual field test took 20 minutes per eye.  Why do some tests take longer than others?

Dr. Elliot Werner:  That mostly depends on how well the patient responds.  Patients who respond quickly and consistently take less time.  Patients who are slow and give inconsistent responses take longer for the machine to reach an end point.

P:  How do you determine the results of a visual field test?

Dr. Elliot Werner:  You look at the computer printout from the machine and read the patterns and numbers.  It's kind of like reading a cardiogram or an x-ray.  Interpreting the tests takes a fair amount of training and experience.

Moderator:  How do you determine how often a patient should take a visual field test?  What about blood relatives who are at risk?

Dr. Elliot Werner:  That depends on how bad the glaucoma is, and how stable the patient has been in the past.  Most vary between one and four times per year.  Blood relatives with no signs of glaucoma probably don't need visual field testing.

P:  Can a visual field test be accurate for a patient with nystagmus if the number of fixation losses, false negatives, and false positives are all within acceptable limits?

Dr. Elliot Werner:  "Accurate" is not a term that is used for visual field testing, because there is no objective way to test the accuracy of the test. "Reliable" and "repeatable" are terms used to describe the visual field.  So, to answer your question, reliable and repeatable visual fields can be obtained from patients with nystagmus.

Moderator:  How operator-dependent is the visual field test?  Does the technician need to undergo training?

Dr. Elliot Werner:  Modern computerized visual field testing is fairly operator-independent.  Some training is needed, but it is minimal.  Most certified "techs" have gone to school for about one or two years to learn ophthalmic testing.  Some are trained on the job, but that is usually less than satisfactory.

Moderator:  What is the blue-yellow field test?

Dr. Elliot Werner:  The blue-yellow field test uses a blue light on a yellow background, as opposed to the routine white light on a white background.  There is some evidence that the blue-yellow test is able to detect damage earlier than the white-white test, but not everyone believes that.

P:  How can visual field testing be done for a child who cannot tell you what he can and cannot see?

Dr. Elliot Werner:  Generally, it can't be done except with very crude techniques using eye and head movements as responses.  In general, visual field testing is almost impossible below the age of six or seven.  

P:  How reliable is the visual field test for a person with a fairly ripe cataract, and how is that test done?

Dr. Elliot Werner:  Visual field tests are not reliable in the presence of very dense cataract.  The test is usually useless in that circumstance.

P:  How can you tell when the cataract is so bad it renders the visual field test useless?

Dr. Elliot Werner:  If the cataract has reduced the visual acuity below about 20/100, the test will not be much help.

P:  How about using a size 3 spot to quantify the overall field depression caused by cataract, and then using a size 5 spot to quantify the glaucomatous field defects?

Dr. Elliot Werner:  The size 5 spot, the larger spot size, can be useful in any patient with decreased acuity. The problem is in separating the changes due to the cataract from those due to the glaucoma.  The effect of the cataract often masks other visual field changes that may be present.

P:  How many visual field tests are needed to document true progression?

Dr. Elliot Werner:  Usually about six or so in most patients.

P:  Even though I reported a sizeable blind spot to an ophthalmologist, my glaucoma was not diagnosed, primarily because my first visual field test showed no loss of vision.  The test was a Humphrey Full Field 120 Point Screening Test.  What's the use of such a test? My doctor said he suspected optic neuritis, and that's why he used it.  Can you explain this?

Dr. Elliot Werner:  For the most part, screening tests are not very useful, because the number of false negatives and false positives is very high. Too much damage can be missed with screening tests.  It is better to use full-threshold tests from the beginning.  Visual field testing is not very useful for the diagnosis of glaucoma.  The tests are the most useful for monitoring patients over time to see if they are getting worse or remaining stable.

P:  So what is the best diagnostic tool for glaucoma?

Dr. Elliot Werner:  The best single diagnostic test for glaucoma appears to be a careful examination of the optic disc.

P:  Assuming a patient has been diagnosed with glaucoma, why would there be a relatively high mean deviation (e.g., -10 dB) and a relatively low pattern standard deviation (e.g., -3 dB) on a visual field test?  

Dr. Elliot Werner:  A high MD (mean deviation) and a low PSD (pattern standard deviation) indicates a generalized loss of the visual field as opposed to a localized loss.  Both patterns can occur in glaucoma, although localized loss is more common.

P:  What percentage of visual field does a patient have to lose before it becomes a real problem as far as driving and other everyday activities?

Dr. Elliot Werner:  That depends on what part of the field is lost.  A small loss near the center can have a much more devastating effect than a much larger loss in the periphery.  It has been shown, however, that loss of as little as 10 percent of the visual field is associated with higher auto accident rates, for example.

P:  How do you measure 10 or 20 percent loss of visual field?  

Dr. Elliot Werner:  There is a chart called an Esterman grid that tells you the extent of  visual field loss in percentages.  

P:  Where did the Esterman grid chart get the data to measure the visual field loss in percentages? 

Dr. Elliot Werner:  Esterman performed many tests on many people and published his results.  He broke the field up into many small parts and assigned a value to each section, depending on how close it was to the center and whether it was superior or inferior, nasal or temporal.  A percentage is calculated on the basis of the number of sections affected by the visual field loss.  The calculation is not built into the machine; it is done by hand.  

P:  How is the distance measured in degrees?  

Dr. Elliot Werner:  Since the visual field is circular in shape, the distance from the center can be measured in degrees.  Zero degrees would be right at the center.  Ninety degrees would be way out at the side.  Most useful vision occurs within the central 45 degrees.  Legal blindness is less than twenty degrees of field remaining.

P:  If a patient has lost the entire superior side, would that be legal blindness?

Dr. Elliot Werner:  In most states, the loss has to be within 20 degrees in all directions.

P:  You've got me rather upset about the prospect of not driving an automobile.  Who should make that determination?  If I've got good vision in one eye, is that okay for driving?  (The other eye has superior loss.)

Dr. Elliot Werner:  Most states have specific laws defining the visual function needed for driving.  In Pennsylvania, it is at least 20/40 vision and at least 90 degrees total field in the better eye.

P:  I have lost most of the vision in one eye, and I hate driving at night.  I have no depth perception.

P:  I recently had a Humphrey Central 24-2 Threshold Test.  Is that a good test?  

Dr. Elliot Werner:  The 24-2, or sometimes the 30-2, are the standards for glaucoma care.  The 10-2 test is useful for people with very small remaining fields, for example, tunnel vision.

P:  Some patients complain about a "veil" or darkness while taking the test.  Is that due to a phenomenon called "retinal rivalry?"

Dr. Elliot Werner:  Retinal rivalry refers to what happens when the two eyes are not looking at the same object.  The brain often either suppresses one eye completely or rapidly alternates between the two images.

P:  Does that happen during visual field tests?

Dr. Elliot Werner:  No, because only one eye is tested at a time.  The other eye is covered.  The strange things you see during the testing are due mainly to retinal fatigue -- looking at the same thing for a long time.  Although the bowl appears to go dark and "flashy," that doesn't seem to affect the results of the test.

P:  Does retinal rivalry occur if light is leaking into the patched eye?

Dr. Elliot Werner:  If there is enough light leaking in around the occluder (patch), the occluder has not been put on properly.

P:  Does retinal fatigue have to do with regeneration of the rhodopsin cycle, or something else?  I think I get it with my computer when I sit in a dark room and read with only the one eye.

Dr. Elliot Werner:  The exact mechanism of retinal fatigue is not well understood.  It may be due to bleaching of rhodopsin, but there also appears to be a central component in the cortex of the brain.

P:  Perimetrists always seem to want to start with the right eye.  If the left eye has sustained the most damage, wouldn't it be better sometimes to start with the left eye?  

Dr. Elliot Werner:  You are right, but starting with the right eye has become an ingrained habit.  I sometimes tell my perimetrists to start with the left eye if there is a good reason to.

P:  Is there a way for people to grow out of glaucoma?

Dr. Elliot Werner:  The only glaucoma that typically gets better with age is pigmentary glaucoma. Most other types of chronic glaucoma are a life sentence.

Moderator:  Does a visual field test reveal decreased contrast sensitivity?

Dr. Elliot Werner:  No.  Contrast sensitivity testing is a different test, which uses dark and light bars on a screen.  People with significant visual field loss usually also have contrast sensitivity loss.

P:  I was told I have a "notch" on my optic nerve (or the rim, perhaps) that seems to correspond to the blind spot in my right eye, as well as the dark area on my visual field test print-out.  Can you tell me what a "notch" is?   

Dr. Elliot Werner:  A notch is a localized loss of the rim.  The rim normally looks a bit like a doughnut -- round with a round hole in the middle.  If you can imagine taking a bite out of the center of the doughnut so one area is thinner than the rest, that would be a notch.  A notch indicates localized, as opposed to generalized, optic nerve damage.

P:  What would cause a notch in a rim?  Is this typical progression of normal-tension glaucoma (NTG)?

Dr. Elliot Werner:  Glaucoma is the most common cause of a notch.  It can be either NTG or high-tension glaucoma.

Moderator:  Can you detect a defect with just one visual field test, or do you need at least two to determine a defect?

Dr. Elliot Werner:  Mostly you need more than one, because the first visual field test is subject to a lot of artifact and false positives.  Also visual field testing is highly variable:  Any change must be repeatable to be significant.

P:  My visual field test shows 0% false positive errors and  0% false negative errors for both eyes.  Does that mean I had no misses?

Dr. Elliot Werner:  Yes, that means you didn't respond incorrectly to any test target presentations.

P:  Do you know how much a visual field test usually costs?

Dr. Elliot Werner:  I'm not sure. Most are probably in the $100 to $150 range for automated tests.  The cost is high because the machines are expensive, and you have to pay someone to conduct the tests. 

P:  Is there any new test on the horizon to replace the present type of visual field test?  

Dr. Elliot Werner:  The most exciting prospect in visual field testing is something called multifocal visual evoked response.  It records the visual field directly from brain activity using electrodes on the scalp.  It does not require any response from the patient, other than remaining awake.  I predict it will replace standard perimetry in my lifetime, and I am 56-years-old.

P:  An eye doc (don't know which kind) told one of my nieces that she probably doesn't have to worry about getting glaucoma (which runs in the family) because she has dark eyes.  Sounds outrageous to me.  Please comment.

Dr. Elliot Werner:  It is outrageous and totally untrue.

End of highlights for October 23, 2002.

On October 30, Dr. Spaeth discussed "OHTS - Ocular Hypertension Treatment Study" in the Chat room. Click here for highlights of that meeting.

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