Visual Fields & Functional Vision
Chat Highlights
October 26, 2005

On Wednesday, October 26, 2005, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Visual Fields & Functional Vision."

Moderator:  Many patients have a poor understanding of vision testing.  For instance, some wonder why a glaucoma patient who reads 20/20 on the Snellen chart would complain about poor vision.

Dr. Rick Wilson:  There are several reasons for that.  One is the loss of contrast sensitivity.  Patients may be able to see dark black letters on a bright white background in a dark room, but those are not normal conditions.  That is high contrast.  In our daily lives, we more often have gray-on-gray, especially in the evening under twilight conditions or at night.  Patients with glaucoma have trouble under low-contrast conditions.

Another problem is their reduced visual field.  I have patients who can see only two or three letters on any line because their central visual field is that small.  While they may have 20/20 vision, they can be legally blind because of their small field of vision.  Intermediate to that stage are many where the holes in the vision are very annoying.  Some patients lose the blue-yellow discrimination, which causes more complaints about their vision.

P:  Does glaucoma destroy contrast sensitivity as well as peripheral vision?

Dr. Rick Wilson:  Yes, it destroys the ability to discern between two objects of low contrast.

P:  Can the damage that has been done to the visual field ever be reversed or improved?

Dr. Rick Wilson:  At any stage of glaucoma, there seems to be retinal ganglion cells that are destroyed and cannot be returned to function, and a much smaller group of the cells that are not quite dead.  If the IOP (intraocular pressure) is lowered far enough, the cells can be partially restored to function.

P:  What causes the loss of contrast sensitivity?

Dr. Rick Wilson:  The loss of contrast sensitivity is related to the generalized loss of retinal ganglion cells throughout the retina.  Some areas are preferentially damaged, which results in blind spots in the vision.  Patients with cataracts also lose contrast sensitivity, but that is regained when the cataract is removed.

P:  Then how is it determined whether glaucoma or cataracts are causing the decreased vision?

Dr. Rick Wilson:  Sometimes it is difficult to tell.  If the cataract is not too dense, machines like the PAM (Potential Acuity Meter) or the laser interferometer can focus light through the cataract and test the retina.

P:  On the printout of a visual field test, is the gray area more important than the scotoma area in determining the visual field loss?  How is the test used to determine how bad the vision in the eye is?

Dr. Rick Wilson:  The mean deviation can be affected by small pupils, cataracts, etc.  It is not specific for glaucoma.  The area and depth of the scotomas determine the extent of the glaucoma loss.

P:  If you're anticipating a focal nerve defect, as might occur with glaucoma, isn't the pattern standard deviation the most powerful indicator of glaucomatous damage, all other things being equal?  That's usually the first thing I look at when they hand me my print-out.

Dr. Rick Wilson:  If there is any kind of generalized depression, as seen with a small pupil or cataract, then the Corrected Pattern Standard Deviation would be the diagram I look at.  But a focal defect should be easily visible in the gray scale and the actual numbers.

P:  Glaucoma doctors are expert at evaluating the eye's visual functioning, but patients are often more interested in using vision to function.  What's the relationship between damage to the visual field and functional vision?

Dr. Rick Wilson:  Usually, the damage to the visual field has to be quite serious before the patient even notices.  That is because the loss is so slow it is like watching hair grow. The patient doesn't notice the week-to-week changes, and one eye often can see where there are holes in the visual field of the other eye and compensate for it.  Often, it is only when a blind spot is noticed that the patient becomes aware of the loss.

P:  How often should glaucoma patients have a visual field test, and how much change is worrisome?  I return next month for my second appointment with a glaucoma specialist.

Dr. Rick Wilson:  All patients with definite glaucoma should have a visual field once a year.  Patients in whom there is a question of progression should have visual field tests as often as every six months.  If there seems to be progression in one visual field, the test should be repeated, because two times out of three, the next visual field will be back to baseline.

P:  Is it asking too much to have three visual field tests to confirm a loss shown on one of the tests?

Dr. Rick Wilson:  Probably. I would doubt that an HMO (Health Maintenance Organization) would pay for more than two visual field tests in a six-month period.

P:  On the printout of my visual field test, a small area is white, but most of the area is a light gray, except for where it is black for the natural blind spot.  My doctor said my test was normal.  Does almost completely light gray indicate normal vision?

Dr. Rick Wilson:  Yes.

P:  Do you go by the grayscale image or the numbers?

Dr. Rick Wilson:  I use the numbers, unless there is a great deal of damage throughout the visual field.  Then I look at the grayscale to key me in on which numbers are most important to compare.

P:  Which part of the visual field corresponds to the macula?

Dr. Rick Wilson:  The center of the visual field is the part seen by the macula.

P:  Does the visual field test measure contrast sensitivity?

Dr. Rick Wilson:  Spatial contrast sensitivity is measurable to some extent, but is usually checked with printed charts.  Temporal contrast sensitivity tests the ability to detect changes of contrast over time.  A video monitor is usually used for that.

P:  Should glaucoma patients be seeing retina specialists regularly for evaluations?

Dr. Rick Wilson:  Only if they have a retinal disease, like macular degeneration, or traction on the retina from the vitreous gel in the eye, which could pull a hole in the retina and cause a retinal detachment.

P:  I take my visual field tests on an Octopus 101.  What numbers are important to understand the deviation?  The right is 1.9 and left is 0.6.  Does that mean I am legally blind?

Dr. Rick Wilson:  Are those numbers the RMS (root mean square) or the average deviation from normal, or what?  I haven't used the Octopus in years, although it remains an excellent machine and the one I started on.

P:  I have absolutely no idea.  It shows a graph with a line going down below normal.  After the word "deviation" there is "dB."

Dr. Rick Wilson:  Then that seems to be the average deviation from normal at each point on your curve.  If so, the numbers you gave may not be too bad.  What you need to look for is the difference (space) between your line and the normal line.

P:  Since it's said that anatomical damage or loss precedes functional loss, shouldn't you be able to correlate a defect on a visual field test with measurable defect on nerve examination, whether visually or by HRT (Heidelberg Retinal Tomograph), etc?  In other words, by the time nerve damage is sufficient to cause a detectable field deficit, shouldn't it be observable to the clinician, and its quadrant location corroborate that observation?

Dr. Rick Wilson:  Yes, about 85% of the time.

P:  I wear trifocals, and although I have 20/20 vision through the distance portion of my eyeglasses, that's only when looking at distant objects.  Close vision through that part of the lens is difficult-to-impossible, particularly for my right eye.  For my last visual field test, the technician had me wear my eyeglasses. Of course, I was looking at the stimulus (the little flashes of light) through my distance lens.  The technician added a corrective lens for the right eye, but not for the left eye.  I would have seen the stimulus better, I think, without my glasses, because I'm myopic.  Before taking visual field tests, what kinds of lens adjustments should be made?

Dr. Rick Wilson:  Patients who wear glasses have lenses put in front of their eyes in the holder equal to their distance prescription, with added power for near, depending upon an age chart supplied by the company.  You are correct that the technician did not know what she was doing if you wore your trifocals during the examination.

P:  Are you using the AccuMap?  If so, what is your opinion of it?

Dr. Rick Wilson:  We are using it and find it intriguing, but not a replacement for standard visual field testing.

P:  Can floaters in the vitreous affect visual field tests?

Dr. Rick Wilson:  If they are large and almost stationary.  They usually are fleeting and do not affect the tests.

P:  I read about a study that found a daily intake of .15 g of lipoic acid improved the visual sensitivity of glaucoma patients.  Can you comment, please?

Dr. Rick Wilson:  Sorry, if I read the study, I have forgotten it.  In general, antioxidants, vitamins, and herbal supplements have not proved particularly helpful in glaucoma.  Some of the latter need to be used with care, as they may interfere with the action of systemic medications or augment them.

On November 2, Dr. Wilson discussed "Early and Late Complications of Trabeculectomies" in the Chat room. Click here for highlights of that meeting.

Click here for the most recent glaucoma chat highlights and links to the chat archives.

Click here for upcoming glaucoma chat events.