Vision Defects
Chat Highlights
September 29, 2004

Norma Devine, Editor

On Wednesday, September 29, 2004, Dr. Elliot Werner, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Vision Defects."

Moderator:  Welcome back, Dr. Werner.  Tonight's topic concerns glaucomatous defects in vision.  Are the defects revealed by visual field tests? *

Dr. Elliot Werner:  Vision defects include much more than just defects in the measured visual field.  We know that glaucoma has profound effects on a variety of visual functions.  

P:  Does the visual field test indicate the cause of the defects?

Dr. Elliot Werner:  The visual field test rarely identifies the cause.  Often, a visual field test can localize the lesion; that is, it can tell the doctor where in the eye or brain the problem is located, but determining the cause usually requires other testing or a more thorough eye examination.

P:  How is the visual field measured?

Dr. Elliot Werner:  Visual fields are measured by projecting a small light, called the target, onto an illuminated background in various locations in the field of vision.  The patient is asked whether or not he or she can see the target.  On the basis of which target locations are seen or not seen, a map of the visual field is created and areas of loss are identified.

P:  Does the pattern (shape) and location of visual field defects reveal anything about the cause of a glaucoma patient's loss, such as from trauma? 

Dr. Elliot Werner:  No.  All glaucomas produce pretty much the same type of visual field defects.  You cannot reliably distinguish one form of glaucoma from another by the appearance of the visual field.

P:  Where does visual field loss usually occur first?

Dr. Elliot Werner:  That varies a lot.  Most patients develop loss in the mid-periphery of the visual field on the nasal side (that is, towards the nose), but almost any pattern can be seen in glaucoma.

P:  How many visual field tests are needed for a baseline?  

Dr. Elliot Werner:  Usually, if the patient is a good tester and gives reliable, consistent results, two visual field tests are adequate for a baseline.  Some patients, however, have a harder time learning how to take field tests and need more tests to learn before a stable baseline can be established.

P:  If a visual field test shows a new area of loss, do you accept that or wait until that loss shows up again in one or two subsequent tests?

Dr. Elliot Werner:  Visual field tests are notoriously variable from test to test.  No change should be accepted unless it has been confirmed on at least one more test.

P:  How much of the visual field must be lost before glaucoma is considered to be advanced?  

Dr. Elliot Werner:  The average sensitivity of the visual field is about 30 decibels ( the unit used to measure this sort of thing).  A loss of more than 10 decibels, or about one third, is considered advanced.

P:  How often should a glaucoma patient take a visual field test? 

Dr. Elliot Werner:  That depends upon how bad the glaucoma is and how long the patient has been stable.  Generally, between one and four times per year, based on those factors.

Moderator:  Doctor Werner, have you ever taken a visual field test?  

Dr. Elliot Werner:  Yes, I have taken many visual field tests.  My mother had glaucoma, so I get tested about every other year.  I also take the tests when I am training new technicians to give them practice administering the test.  It's a terrible test.  I hate it.  I recently had a colonoscopy.  That was much easier than the visual field test. 

P:  In reviewing my visual field tests, my doctor bases his judgments on Pattern Deviation, rather than on Total Deviation.  Can you explain this, please?

Dr. Elliot Werner:  The Pattern Deviation is an index of localized loss, the type that occurs in glaucoma.  The Total Deviation measures a combination of generalized and localized loss, so it is less specific.

P:  A patient who takes her visual field tests on an Octopus is concerned because she hears many of us discussing taking tests on a Humphrey.

Dr. Elliot Werner:  The difference between an Octopus perimeter and a Humphrey perimeter is like the difference between a Ford and a Chevy --  the same thing by different manufacturers.  It makes no difference.

P:  What can or should be done if the visual field gets progressively worse?

Dr. Elliot Werner:  That means the glaucoma is not adequately controlled and the pressure needs to be lowered, if possible.

P:  I understand you have to have a visual field of 20 degrees or less to qualify for SSDI (Social Security Disability Insurance).  Is that the same as legal blindness?

Dr. Elliot Werner:  If the peripheral visual field is lost to within 20 degrees of the center, that is considered legal blindness. The normal visual field extends about 90 degrees in each direction from the center.

P:  How is loss of contrast sensitivity measured?  

Dr. Elliot Werner:  Contrast sensitivity, a person's ability to distinguish an object from its background, is measured by what are called "gratings."  These are alternating dark and light stripes.  In low contrast targets, the dark and light stripes are almost the same shade of gray.  In high contrast targets, the stripes are almost black and white.  Depending on which target the patient can see, the contrast loss can be measured.  A new test called FDT depends on contrast testing.

P:  What does "FDT" stand for?  

Dr. Elliot Werner:  FDT strands for Frequency Doubling Technology.  It uses contrast gratings to test the visual field.  FDT is gaining popularity as a screening test, or for evaluating glaucoma suspects to see if they have early damage.

P:  Is there a way to measure depth perception?  

Dr. Elliot Werner:  Depth perception is usually not affected early in glaucoma.  Loss of depth perception is usually a later manifestation, especially if the loss of vision in one eye is more advanced than in the other.   Functions that seem to be affected early in glaucoma include contrast sensitivity, color vision, and dark adaptation.  

P:  Why is color perception lost early in glaucoma?

Dr. Elliot Werner:  Color perception depends on an intact optic nerve.  If the nerve is damaged, the ability of the eye and brain to perceive color decreases, due to decreased input from the eye.  It's like if the cable to your TV is cut in half, the picture quality will suffer.

P:  Can contrast sensitivity, color vision, and dark adaptation be measured?

Dr. Elliot Werner:  Yes.  There are good tests for these and they are sometimes used for evaluating glaucoma patients.  Their best use is in early glaucoma.

P:  How can you tell if loss of contrast sensitivity is from glaucoma or cataracts if both are present?

Dr. Elliot Werner:  The best way to tell is to take out the cataract and see what happens.  Patients with early glaucoma also usually do not have loss of central visual acuity, whereas cataract patients often do.

P:  What is dark adaptation?

Dr. Elliot Werner:  Technically, dark adaptation is the ability of the retina to increase its sensitivity in low light situations. Strictly speaking, dark adaptation is not directly affected in glaucoma, but as visual field and contrast sensitivity and color vision are lost as the disease progresses, the ability of patients to see things in low light situations becomes very poor.

Moderator:  Do you know why those functions are affected?

Dr. Elliot Werner:  Probably because the patient's optic nerve fibers are destroyed.  All visual functions are carried back to the brain by the optic nerve.  So, if you lose some of your optic nerve, your vision will be adversely affected.  

P:  How is blurred or hazy vision measured, and what are some of the causes? 

Dr. Elliot Werner:  Blurred vision is usually measured with a simple visual acuity chart, like the letters on the wall.  Causes of blurred vision can be almost any chronic eye disease, such as cataract, macular degeneration, glaucoma, etc.

Moderator:  Why would a glaucoma patient have hazy vision intermittently?

Dr. Elliot Werner:  It has been observed for a long time that glaucoma patients complain of intermittent fluctuation of vision.  They have good days and bad days. There are a lot of theories why that might be, but no good scientific explanation.

P:  Should the clinician select one of the thresholding tests in automated perimetry and use it as the default test in most cases?

Dr. Elliot Werner:  Yes.  In evaluating and following patients known to have glaucoma, a screening test should not be used. Thresholding strategies should be used.

P:  Have you ever observed an improvement in vision for a glaucoma patient?  If so, to what have you attributed that?

Dr. Elliot Werner:  Yes, if the patient has very high pressures that are lowered with treatment, some improvement can be observed in many patients.

P:  Why do bright lights seem to blind a patient with advanced glaucoma?

Dr. Elliot Werner:  I'm not sure, but probably because of the marginal function of the optic nerve and retina.  The ability of the system to adapt to different levels of light is poor, causing glare and discomfort.

P:  What is the basis for SWAP's putative ability to detect axonal loss earlier than white-on-white testing?  Is it because the axons that mediate perception along the blue/yellow axis are lost earliest, as I think I've read?  Or is there a newer theory?

Dr. Elliot Werner:  SWAP (Short-Wavelength Automated Perimetry) uses colored targets and backgrounds.  It seems to be capable of detecting early damage before the standard white-on-white perimetry.  The reason is that it tests only a small proportion of the ganglion cells of the retina, so that if some are lost it is easier to detect.  It's like looking at a herd of forty cattle.  If two are missing, it's hard to detect.  If you look at a group of five cattle and two are missing, it's a lot easier to see that two are missing.

P:  Is there any machine that can cope with nystagmus and get good results from visual field tests?

Dr. Elliot Werner:  Let me first explain that nystagmus is uncontrolled movements of the eyes, usually from side to side.  It has many possible causes, but is often congenital (present at birth) as a kind of birth defect in the control system of the eye movements.  Now, to answer your question, not really.  If the nystagmus is not too bad, we can usually get usable fields.  Severe nystagmus, however, makes taking the test difficult.

P:  Can the corneal thickness of a nystagmus patient be measured? 

Dr. Elliot Werner:  Yes.  But corneal topography or measuring corneal curvature, as is done in determining the appropriate power for an intraocular lens before cataract surgery, can be a problem.  Interestingly, nystagmus goes away when you sleep or are under general anesthesia.

P:  Can defects of the cornea be measured?

Dr. Elliot Werner:  Yes, quite easily with a routine eye examination using the slit lamp.

[*See: "Understanding Visual Field Testing" by Jeffrey Henderer, M.D. http://www.willsglaucoma.org/testing/vf.html]

End of highlights for September 29, 2004.

On October 6, Dr. Wilson discussed "Stem Cell Research" in the Chat room. Click here for highlights of that meeting.

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