Optic Disc
Chat Highlights
June 4, 2003

Norma Devine, Editor

On Wednesday, June 4, 2003, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Optic Disc."

Moderator:  Dr. Wilson, will you please start by explaining what the cup and disc are?

Dr. Rick Wilson:  The optic nerve comes into the back of the eye through a canal in the sclera.  After entering the eye, the optic nerve  spreads out onto the surface of the inside of the eye as the nerve fiber layer over the retina. The circle or disc that this appears as is the disc.  The cup is the depression in the center of the disc.  It is naturally there, but as nerve fibers are killed by high pressure, the cup expands.

Moderator:  What instrument is used to examine the optic nerve?  

Dr. Rick Wilson:  A direct ophthalmoscope or a slit lamp with a 60 to 90 diopters lens are used to look at the optic nerve. The slit lamp gives a stereoscopic view that helps to detect early glaucoma.

P:  Why would the stereoscopic view help?  Wouldn't glaucoma be indicated more by errors in the visual field test, rather than a particular cup size?

Dr. Rick Wilson:  The visual field test is a very subjective test, influenced markedly by the person taking it.  The retina, with 1.2 million fibers, has so much redundancy that early loss is not picked up by visual field tests.  Usually, around 1/3 of the nerve has to be lost before reproducible visual field loss is seen.  Loss in the disc can be seen earlier.

P:  What is a normal cup-to-disc (C/D) ratio?

Dr. Rick Wilson:  The C/D ratio varies greatly, and is larger in African-Americans than in Caucasians.

P:  I know from reading highlights of previous chats that you consider examination of the optic nerve head (ONH) by a skilled clinician to be among the most important diagnostic tools early in the course of the disease.  Can you elaborate on what characteristics of the ONH, specifically, you're evaluating when you make such an assessment?  And is rim thinning necessarily synonymous with progressive cupping? 

Dr. Rick Wilson:  The C/D ratio is usually 0.5 or less, and enlarges slightly with age as retinal ganglion cells are lost.  When I was trained, we looked at the vessels lining the cup to outline the cup and the edge of the cup to give the extent of the cup. Asymmetry between the eyes and vertical elongation of the cup, or notches in the cup rim, were diagnostic of glaucoma.  Now we know that transparency of the disc (nerve) tissue is a very early sign of glaucoma, but special viewing conditions and experience are needed to see that.   

P:  Has the C/D ratio become less significant in diagnosing glaucoma?

Dr. Rick Wilson:  The C/D ratio has become less helpful, and the quality of the disc tissue more important.  Asymmetry is still an important early finding in patients with asymmetric glaucoma. 

P:  So it isn't the size of the disc that counts, but the flatness of the cup that is a concern? 

Dr. Rick Wilson:  Discs come in a variety of sizes, all the way from slightly over a 1 mm- to a 6 mm-square area.  Large discs have large cups naturally.  Therefore, it's important to know the size of the disc to get the best idea of whether the ratio of cup to disc is abnormal.

P:  Is the C/D ratio different in near-sighted people?

Dr. Rick Wilson:  As a rule, near-sighted people have large eyes and therefore large scleral canals.  That means larger-than-average discs and cups. 

P:  When nerve fibers are being lost because of high IOPs, does the visual field test reflect that loss? 

Dr. Rick Wilson:  Usually the loss shows up first in the optic nerve head (disc), then in the visual field. 

P:  What is the best method for determining the quality of disc tissue?

Dr. Rick Wilson:  A stereoscopic examination of the disc with a small beam of light directed on the surface of the optic nerve.

P:  Does Heidelberg Tomography (HRT) help to determine C/D ratio and the quality of disc tissue?

Dr. Rick Wilson:  It helps with the C/D ratio, but not with the quality of the disc tissue.

P:  What is the significance of a difference between the C/D ratios in a patient's eyes?

Dr. Rick Wilson:  The difference in the size of the C/D ratio between a patient's discs should be within 0.1.  More asymmetry than that often signifies damage.

P:  How does change in the C/D ratio affect vision?  Can the change be detected in visual field tests? 

Dr. Rick Wilson:  The C/D change comes first, then the visual field change.  The early changes in glaucoma may be a decrease in contrast sensitivity or color vision in some patients. In all there is a gradual decrease in sensitivity that often is more prominent nasally and about 15 degrees above the center of the vision.

P:  Since the C/D ratio increases with age, at what age is .5 or less the usual ratio? 

Dr. Rick Wilson:  The C/D ratio enlarges from infancy to adulthood, as the scleral canal enlarges.  As a person moves from middle age, the change is very subtle, so that someone born with a 0.1 cup may have a 0.3 cup by age 5, but only a 0.35 to 0.4 cup at age 90.

P:  Are all changes in the size and shape of the cup synonymous with vision being lost?

Dr. Rick Wilson:  Changes due to glaucoma mean a loss of nerve tissue and consequently vision, even if the loss cannot be detected by our presently crude tests.

P:  Does HRT help to determine C/D ratio and the quality of disc tissue?

Dr. Rick Wilson:  It helps with the C/D ratio, but not with the quality of the disc tissue.

P:  What is the best method to determine the quality of disc tissue?

Dr. Rick Wilson:  A stereoscopic examination of the disc with a small beam of light directed on the surface of the optic nerve.

P:  How important is excavation of the cup?  My glaucomatous eye has a cup that's not only larger than the other, but also very deep.

Dr. Rick Wilson:  Excavation is a sign of the extent of nerve damage, and therefore an important parameter.

P:  Can there be changes to the cup that are not related to glaucoma?

Dr. Rick Wilson:  Yes, brain tumors or a tiny stroke to the optic nerve can cause swelling, which fills in the cup.  Diabetes may cause abnormal vessels to grow on the disc and fill in the cup.

P:  How much does high myopia complicate determining glaucomatous damage when evaluating the disc?

Dr. Rick Wilson:  It may complicate it quite a bit in first-time patients.  Once the doctor has established a baseline, then subsequent visits mean looking for change from that first baseline.

P:  Which, if any, of the newer, objective tests (for example, OCT (optical coherence tomography), GDx, (nerve fiber analyzer), HRT) have significant diagnostic value in evaluating the optic nerve head and retinal nerve fiber layer?  My specialist claims there are problems with the software, etc., that need to be solved.  The problem is, he's been saying that for three years.

Dr. Rick Wilson:  The HRT allows a sizing of the optic nerve that is helpful.  Other than that, the machines are no better than an expert disc observer with good photo documentation.  The machines are getting better. The OCT 3 is presently in use.  I have seen the OCT 4, and it is a large leap forward and will be a big help.

P:  I had OCT last week.  If I ask for a copy of the test what will I see?

Dr. Rick Wilson:  A map of the thickness of the nerve fiber layer surrounding the disc.

P:  Isn't research being conducted about protecting the optic nerve from high pressures? I forget the name.

Dr. Rick Wilson:  We are starting to understand more about how the nerve is injured in glaucoma, as well as how some compounds might offer some protection to the optic nerve endangered by high intraocular pressure.  That is called neuroprotection.

Moderator:  Thank you, Dr. Wilson. 

Dr. Rick Wilson:  Good night all.  Have a great week.

End of highlights for June 4, 2003.

On June 11, Dr. Wilson discussed "The Optic Nerve" in the Chat room. Click here for highlights of that meeting.

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