Vision
Chat Highlights
October 5, 2005

On Wednesday, October 5, 2005, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Vision."

Moderator:  Dr. Wilson, it would take much longer than the time we have to discuss a topic as broad as vision.  Among other things, we could discuss visual defects, astigmatism, diplopia (double vision), myopia (nearsightedness), the various types of color blindness, color vision defects, night and day blindness, scotomas, tunnel vision, the Snellen test (eye chart).  Let's start with impairment of vision.  How is that defined?

Dr. Rick Wilson:  Legal blindness varies, but is often considered to be 20/200 or worse in the better eye, or a visual field of less than 10 degrees.

Moderator:  At what point do people with impaired vision need a restricted license for driving a motor vehicle?

Dr. Rick Wilson:  Requirements vary from state to state.  The requirements may require 20/50 in the better eye, and 140 degrees of visual field.

Moderator:  At what stage in glaucoma do problems with depth perception, contrast sensitivity, color vision, and dark adaptation occur?

Dr. Rick Wilson:  People are quite varied about detecting their problems.  Loss of color vision in the yellow-blue spectrum may occur early in many patients and late in others.  All glaucoma patients lose contrast sensitivity, but not all complain about it.  For example, those who do complain usually can read black letters on white paper, but miss seeing a gray curb on an overcast day.

Moderator:  What causes changes in visual acuity and astigmatism in glaucoma patients?

Dr. Rick Wilson:  A lowering of IOP (intraocular pressure), such as after a trabeculectomy, will increase astigmatism.  Visual acuity usually is not lost until the patient's vision is reduced to a small central island.  For instance, I have patients whose central island is reduced to the point that the patient can see only two letters on the 20/20 line on a Snellen chart.  That's a very small island of vision, but the vision is 20/20.

P:  Why would lowering IOP increase astigmatism? I thought that had to do with the shape of the corneal surface.

Dr. Rick Wilson:  In astigmatism, the surface of the cornea is shaped more like a football than a beach ball.  (Lowering the pressure in a football increases the radius of curvature for the curve that joins the two ends, while decreasing the radius of curvature for the curve that extends around the middle.)

P:  Does glaucoma cause loss of contrast sensitivity gradually or suddenly?

Dr. Rick Wilson:  The loss progresses in concert with the severity of the glaucoma.

P:  How is it determined whether the loss of contrast sensitivity is due to glaucoma rather than to development of cataract?

Dr. Rick Wilson:  That is usually a judgment decision by the doctor examining the density of the cataract and the extent of the loss of contrast sensitivity.

Moderator:  What changes occur in the pupil as we age?

Dr. Rick Wilson:  The pupil becomes less reactive and smaller as we age.  The pupils of patients in their 90's may not dilate much at all.

P:  Last February I developed a blind spot in my left eye.  Have you ever heard of a piece of cholesterol settling in the eyes?

Dr. Rick Wilson:  Yes.  Tiny chunks of cholesterol that have broken off the lining of the carotid or other artery, or off a heart valve, can block a small artery in the retina, causing a small "stroke."

The area of the retina supplied by that blocked artery dies and no longer sees.

P:  How does an ophthalmologist manage to observe a cholesterol blockage in the retina?

Dr. Rick Wilson:  Acutely, the ophthalmologist should be able to see the cholesterol plaque in the retina as a bright (reflecting the light) body in one of the vessels.  After a short time, or maybe longer, the cholesterol may break up and be pushed deeper down the drain, until it can't go any further.

P:  Can a cholesterol blockage located in another part of the body affect the optic nerve?

Dr. Rick Wilson:  A blockage in another part of the body should not affect the optic nerve.

P:  Thanks so much for explaining the blind spot in my eye.  No one else was able to give me that kind of information. How can I prevent that from recurring?

Dr. Rick Wilson:  Your internist should already be helping you.  Lowering cholesterol should help, and taking baby aspirin, if approved by your internist.

P:  What might cause an overall loss of sensitivity on a visual field test in a patient with normal-tension glaucoma?

Dr. Rick Wilson:  Generalized loss of sensitivity is most often caused by cataracts or a small pupil.

Moderator:  What visual problems do patients report most often in the early, middle, and late stages of glaucoma?

Dr. Rick Wilson:  In the early and middle stages, patients usually report no visual problems.  Later, difficulty seeing at night (decreased contrast sensitivity) or blurry vision just beside their central vision.

P:  Do glaucoma patients who have problems with bright lights also have problems with contrast sensitivity and night vision?

Dr. Rick Wilson:  The problem with bright lights should not be linked to loss of contrast sensitivity and night vision.  The latter two come from a generalized loss of the retinal ganglion cells in the retina.

P:  Could my problem with bright lights be because I'm aphakic?

Dr. Rick Wilson:  Yes, it could.

Moderator:  When best-corrected vision is 20/20, but it's not sharp, why is that?

Dr. Rick Wilson:  The refraction may not be exact, so that the light is not exactly focused, or an early cataract may be diminishing the acuity.

Moderator:  The new nerve fiber analyzers (GDx, OCT, HRT) are still considered supplemental tests, and their role in glaucoma management is debatable.  We glaucoma patients would prefer tests that don't depend on our responses.  Do you think measuring nerve fiber layer thickness over time will replace visual field testing as the gold standard within the next ten years?

Dr. Rick Wilson:  Yes.  I have seen the next generation OCT (Ocular Coherence Tomography), and the detail in the retina it picks up is amazing.  I do think we will have tests that test the patients' physiology, directly, or their anatomy, looking for loss of function.

P:  Should an ophthalmologist who has been seeing a patient for years be able to recognize an optic nerve that has sustained, say, 60% or more damage?

Dr. Rick Wilson:  It seems that the ophthalmologist should be able to, but that would depend upon the optic nerve being fairly normal in appearance, and not one that has a large physiologic cup.

P:  What's the final stage in a preretinal vitreous detachment, with or without complete synchesis?

Dr. Rick Wilson:  In a preretinal vitreous detachment, the vitreous jelly that fills up the back of the eye shrinks with age and pulls off the retina, from the back to the front.  It is safest to have the entire vitreous pulled off, since then there will be no more tension on the retina.  Vision should be unchanged, except for the increase in floaters.

Moderator:  Why do many glaucoma eyedrops cause blurry vision?

Dr. Rick Wilson:  Often the eyedrop may change the surface of the cornea, causing cloudy vision or change the tension in the muscle of the eye, which affects focusing.

P:  What distinguishes NTG (normal-tension glaucoma) from ION (ischemic optic neuropathy)?  Migraines, reduced oxygen supply to the optic nerve, and blood vessel spasms seem to be associated with both.

Dr. Rick Wilson:  ION is more of an acute event, compared to NTG, which takes years to develop.

Moderator:  Thank you, Dr. Wilson.

On October 12, Dr. Wilson discussed "Communicating With Your Eye Surgeon" in the Chat room. Click here for highlights of that meeting.

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