Visual Field Defects
Chat Highlights
February 4, 2004

Norma Devine, Editor

On Wednesday, February 4, 2004, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Visual Field Defects."

Moderator:  Tonight the topic concerns visual field (VF) defects.   Most of us know what a visual field test is, but for newcomers will you please explain what is tested? 

Dr. Rick Wilson:  A visual field test tests the entire range of a patient's vision, side to side and up and down.  It is usually done by having the patient stare at a light in a bowl, while other lights are flashed in the periphery. 

P:  What is a visual field defect?

Dr. Rick Wilson:  The end point at each point of the visual field is a light of a size and brightness that the person taking the test sees 50% of the time.  A visual field (VF) defect is an area where the eye of the person taking the test is less sensitive than normal.

P:  In most cases, where are the first defects in glaucoma found?

Dr. Rick Wilson:  It's a popular misconception that side (peripheral) vision is affected first in glaucoma.  The defect, however, usually occurs within 10 to 20 degrees of central vision.  The earliest field loss is often in the nasal area. 

P:  What is a scotoma?

Dr. Rick Wilson:  A scotoma is a medical term for a visual field defect, usually one with more normal vision on all sides of it.

P:  What is the best way to detect early changes in glaucoma?

Dr. Rick Wilson:  The best way is for an expert to look at the optic nerve.  The best way to detect early changes is to test the visual field using Short Wavelength Automated Perimetry (SWAP).  SWAP tests blue-yellow vision, which usually allows loss of vision to be detected sooner than with white-on-white perimetry.  

P:  What causes defects in the visual fields?  

Dr. Rick Wilson:  Glaucoma injures the retina and optic nerve in a characteristic pattern, leaving holes in the vision.

Moderator:  Can anyone interpret a visual field test?

Dr. Rick Wilson:  It is hard for glaucoma patients to notice the visual field changes, because the changes usually occur slowly.  Having a doctor trained in interpreting visual field tests doing the interpreting is obviously important, especially if the visual field is presented as just numbers and not as a gray scale.

P:  Are the first defects detected usually the same in all forms of glaucoma, or are there special patterns that might distinguish one type of glaucoma from another?  

Dr. Rick Wilson:  Normal-tension glaucoma often has dense defects close to the center of the vision, which is characteristic.  Acute angle-closure glaucoma, with its extremely high pressures, may also look different.  The appearance of the visual field in the various slow-onset glaucomas is usually quite similar.

P:  I know that all visual field tests show a "defect" where the normal blind spot is, usually below the horizontal line on the printout.  On my visual field tests, the normal blind spot is doubled, with a mirror image above the horizontal line.  Would that defect above the line be a defect due to glaucoma?  It's the only significant scotoma there.

Dr. Rick Wilson:  The defect that is caused by the optic nerve entering the back of the eye (the normal blind spot) should cross the horizontal axis.  If the normal blind spot is enlarged, there may be a crescent around the nerve caused by nearsightedness, infection, or degeneration, causing the defect to also be enlarged.

P:  Isn't deformation of the blind spot often found in myopic or aged patients?

Dr. Rick Wilson:  Yes.

P:  Does a patient's level of stress have any effect on a visual field test?

Dr. Rick Wilson:  According to one study, a person's IOP (intraocular pressure) after taking a VF test is elevated.  All my patients think that stress affects their IOPs and visual field tests, but that is difficult to prove.

P:  Is testing the central 30 degrees in visual field tests another good way to detect early changes in glaucoma?

Dr. Rick Wilson:  All glaucoma visual field tests concentrate on the central 30 degrees if vision is close to normal.  [Editor's Note:  The normal visual field extends more than 90 degrees temporally, 60 degrees nasally and superiorly, and about 70 degrees inferiorly.] Some tests trim off the outer part of the visual field temporally, since visual field changes are rarely found there in glaucoma.  Patients with advanced disease are often tested with a central 10-degree field.

P:  What is the shortest time interval between visual field tests that will reliably indicate progression of glaucoma?

Dr. Rick Wilson:  That really depends upon the rate of progression.  If the pressure is quite high, changes could occur weekly.  One study found that checking the visual field too frequently resulted in more (apparent) progression of the visual field before it was picked up.  The testing doctor always had the inclination to compare the current VF test with the previous one.  Since there wasn't much time in between the tests, there often wasn't enough change to see.  If the tests were spread out more, the changes were more readily apparent.  I rarely order them closer than every six months.

P:  What is the common formula to calculate the percent of visual loss in an eye, based on visual field tests? 

Dr. Rick Wilson:  Each of the major recent studies had a method for grading visual field loss that was, for the most part, different from the others.  We use those grading scales only for research.

P:  So doctors usually just guess what the percentage of loss is? 

Dr. Rick Wilson:  They estimate from the visual field and the appearance of the optic nerve.

P:  I have 40% nerve damage in one eye, but test normal in the visual field tests.  Why is that?

Dr. Rick Wilson:  Your nerve loss may be diffuse, so that you can see the small lights, but are not functioning normally on tests of other physiologic function besides the VF test.  That is, compared to most glaucoma suspects who have localized loss that does show up on VF tests.

P:  What besides glaucoma can cause peripheral loss seen on a VF test? 

Dr. Rick Wilson:  Retinal detachments, hemorrhages, strokes in the eye, and retinal diseases, such as retinitis pigmentosa.

P:  I had concerns about the validity of my SWAP tests because of the amount of correction needed for my myopia.  I was told that a focal visual field defect (presumably indicated in the pattern standard deviation) would not result from undercorrection or any other problem in taking the test, such as fatigue, inattention, etc.  I was told that these global problems would be measured in mean deviation, but that a repeatable focal defect pretty much cannot be an artifact. What are your thoughts?

Dr. Rick Wilson:  Because SWAPs are more sensitive, they also have more "noise,"  and may be hard to interpret.  If there is a localized loss that is constant on several tests and is in an area that is characteristic for early glaucoma damage, it would seem to be a valid defect.

P:  I have a comparison chart of three visual field tests that I took during a period of 1 1/2 years.   There are four columns on the chart with the following headings:  (1) "Threshold Grayscale," illustrating my vision with a scotoma; (2) "GHT: Outside Normal Limits;" (3) "Total Deviation";  (4) "Pattern Deviation."  My ophthalmologist was most interested in the last column.  He said Pattern Deviation (PD) provided the most important information.  Can you explain that, please?

Dr. Rick Wilson:  If you have a cataract, or small pupil that diminishes the entire VF, the pattern deviation plot will subtract that out, leaving what is more likely to be glaucoma damage.

P:  My VF test taken two years ago, just before my trabeculectomy, showed a general reduction of sensitivity.  My IOP was high, I had angle closure, and corneal edema caused by ICE (irido-corneal syndrome).  Yesterday my VF test results were within normal limits.  I was pleasantly surprised.  Are the good results of the test a reflection of the success of the surgery?

Dr. Rick Wilson:  Yes.  By lowering the pressure, the surgery was able to get rid of the corneal edema and improve your acuity for taking the VF test.  If your IOP was lowered substantially, the retina and nerve can improve and the VF can also improve.  

P:  What does the term "Fovea: Off" on my VF printouts mean?  Also, what do the letters MD, PSD, FN, and FP on the chart refer to?

Dr. Rick Wilson:  "Fovea off" means the VF did not test the fovea.  MD = mean deviation, the average depression of each point in the VF; FN = false negatives, i.e., when the patient didn't hit the button and should have; FP = false positives, when the patient hit the button when a light was not shown.  PSD, pattern standard deviation, is what I was talking about above.  

P:  In comparing several VF tests that I've taken over time, my doctor was most interested in the sections called "Pattern Deviation".  Can you tell us what that means, and explain the difference between that and "Total Deviation"?

Dr. Rick Wilson:  I guess I didn't make myself clear above. Total deviation is the average amount each locus of tested spot on the VF is decreased.  Pattern deviation subtracts out factors like cataracts or small pupils that affect the whole VF, leaving the localized defects of glaucoma more easily seen.

P:  What is considered to be quite high IOP?  

Dr. Rick Wilson:  An IOP of 50+ mm Hg.

Moderator:  High pressure is 50 mm Hg and up?  

Dr. Rick Wilson:  Over 50 mm Hg is quite high.

Moderator:  Is an IOP of 30 mm Hg high enough to cause damage?

Dr. Rick Wilson:  Absolutely; it just takes longer.

P:  Can a pressure of 25 mm Hg cause damage, just slower?

P:  Can IOP over 20 mm Hg cause damage?

P:  In my eyes, 16 mm Hg was high enough to cause damage. 

Dr. Rick Wilson:  Normal-tension glaucoma?  

P:  Yes.  

Dr. Rick Wilson:  As she says, an IOP of 16 mm Hg may be too high for some patients.  I have people who have gotten worse at IOPs of 12 mm Hg.

P:  What about an IOP of 20 mm Hg?  

Dr. Rick Wilson:  People with eyes predisposed to glaucoma will slowly develop glaucoma at that IOP.  Others will do well with an IOP of 20 mm Hg.

P:  Could a relatively consistent pressure of 26 to 32 mm Hg cause significant loss of visual field in one year?  Even if the pressures have been like that off and on in past years with no signs of damage?

Dr. Rick Wilson:  If your health or circulation changed, it could.  Or you could have been getting damage that just now reached the threshold of being visible.

P:  Aspirin is known as a reducer of blood viscosity.  Do you think it helps the eye to take aspirin regularly?  Could this strengthen the retina cells because of increased oxygenation, thus improving vision?

Dr. Rick Wilson:  I would think of aspirin more as a preventive measure, rather than something that would improve my vision. Almost all male doctors over age 50 take aspirin.

End of highlights for February 4, 2004.

On February 11, Dr. Wilson and Dr. Scott Edmonds discussed "Low Vision" in the Chat room. Click here for highlights of that meeting.

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