The Optic Nerve
Chat Highlights
September 18, 2002

Norma Devine, Editor

On Wednesday, September 18, 2002, Dr. Jeffrey Henderer, a glaucoma specialist at Wills, and the glaucoma chat group discussed "The Optic Nerve."

Dr. Jeff Henderer:  Hello all!

Moderator:  Good to have you back, Dr. Henderer.

Dr. Jeff Henderer:  I'm happy to be here!

Moderator:  Tonight's topic is "The Optic Nerve."  Doctor, can you start by telling us what the optic nerve is? 

Dr. Jeff Henderer:  Sure.  The optic nerve is the structure in the eye that contains a bundle of small nerve fibers wrapped into one large nerve fiber that takes the signal from the retina to the brain.  It's sort of like a fiber optic cable.

P:  Can the optic nerve be considered part of the brain itself?

Dr. Jeff Henderer:  Yes, it is part of the brain for all intents and purposes.  The optic nerve is located at the back of the eye and is most famous for being the cause of everyone's "blind spot."  

P:  Approximately how many nerve fibers are there in the optic nerve?

Dr. Jeff Henderer:  The teaching is that everyone has about 1.2 million nerve fibers but, in fact, large nerves have a bit more and small nerves a bit less.

P:  Approximately what percentage of those 1.2 million fibers needs to be damaged for loss of field to occur?

Dr. Jeff Henderer:  Great question.  It turns out that about 30 to 40 percent need to be lost to pick up damage, depending on the type of field test used.  That is why it is so useful to look at the nerve.  You will miss early glaucoma if you wait for a field defect.  Now, the patient doesn't see early glaucoma, so that is a cushion, but we like to find it.

P:  Can very sensitive field perimeters sometimes pick up defects before a good glaucoma specialist can see them?  I mean the blue-on-yellow and the FDP (frequency doubling perimetry).

Dr. Jeff Henderer:  Perhaps. I still think that a field defect should have some associated nerve finding, but if you look at the results of the Ocular Hypertension Treatment Study (OHTS), about half of the people were judged to be worse by field criteria only and half by disc criteria.  I guess there were no associated nerve findings in the group that got worse by field criteria only.

P:  What is frequency doubling perimetry?  

Dr. Jeff Henderer:  It's a type of field test that uses vertical bars that flicker, instead of a light, to test the field.  It tests a different population of ganglion cells than conventional fields and might be able to find defects earlier.  I have found it very useful for screening in the community. 

P:  It appears from the OHTS that field damage is an indicator, even though no optic nerve damage is visible. And that was half the group! That's pretty substantial.

Dr. Jeff Henderer:  What was interesting was that was the case in half the group in the OHTS.  In the Normal-Tension Glaucoma Study, only 3 of the 35 eyes that got worse showed disc changes. All the rest were field changes.

P:  Oh, my gosh!  That means field change in normal-tension glaucoma is extremely indicative of glaucoma, even though the nerve looks normal.

Dr. Jeff Henderer:  Well, wait.  In that study, that was the case.  Those were special eyes, and I'm not sure those nerves would have been easy to follow, because the more damaged the nerves, the harder it is to follow the nerves.  Field progression is often an artifact and needs to be reproducible to believe it.

P:  How useful is the Heidelberg Retinal Tomograph (HRT)?  I'm scheduled for that test, but my insurance won't reimburse me for it.

Dr. Jeff Henderer:  I think of HRT as a means to follow glaucoma.  It is probably useful for that, but it doesn't replace either my exam or visual fields.  Perhaps one day.

P:  What do glaucoma specialists mainly look for when they visually examine the optic nerve?  It seems like color, shape, and cup-to-disc ratio are often  discussed. 

Dr. Jeff Henderer:  That is the essence of glaucoma.  The features of a glaucomatous nerve are:  thinning or loss of tissue, changes in color (getting more pale), a change in the blood-vessel pattern, holes in the nerve tissue, and changes in the retina surrounding the nerve.

P:  What are the known causes of optic nerve damage, and how well are they understood?

Dr. Jeff Henderer:  The only thing we've been able to demonstrate that causes nerve damage is eye pressure.  People have suggested other causes, too, like poor blood flow, toxic chemicals, and free radicals.  These are being investigated, but have not been really proven yet.

P:  Is it believed that the intraocular pressure directly injures the nerve, or that it is a problem because it affects perfusion in association with blood pressure, or other things that affect blood flow? 

Dr. Jeff Henderer:  It's unclear what the relationship is.  Associations between low blood pressure and glaucoma have been well documented.  But there are also good reasons to think that elevated IOP may not only prevent blood from entering, but may also deform the nerves and prevent nutrients from the brain from passing back into the eye to keep the nerves healthy. 

P:  If IOP makes it difficult for certain nutrients from the brain to get back to the optic nerve, what are the nutrients?  Is it possible they could be "primed" in other ways, to be more available, or is it basically that one route regardless of "supply"?

Dr. Jeff Henderer:  The one that I have heard the most about is brain-derived neurotrophic growth factor (BDNF).  People are working on ways to deliver nutrients or to prevent toxic chemicals from causing damage.  However, it's not an easy thing to deliver a drug to the inside of the eye without causing side effects.

P:  How is the retina mapped by the optic nerve?

Dr. Jeff Henderer:  There is a very specific way that the nerve fibers from the retina feed into the optic nerve.  The fibers from the temporal retina enter from the north and south poles of the nerve.  The temporal side of the nerve serves the macula (where we do fine vision), and the nasal nerve serves the nasal retina.

P:  What would it mean if the rim of the optic nerve were missing temporally and inferiorly?  

Dr. Jeff Henderer:  Generally, loss of rim tissue means that you have suffered damage from glaucoma, and that is an area of the nerve that will likely show a field defect.

P:  Is it known at what pressure damage to the optic nerve occurs?

Dr. Jeff Henderer:  It turns out that everyone seems to have a different threshold for damage.  Some can tolerate very high pressures and some cannot tolerate even "normal" pressures.  We cannot predict who is who yet, but it is almost certainly genetic and we may find genes for that.

P:  Is there a difference between the visual field defects in normal-tension glaucoma and high-tension glaucoma?

Dr. Jeff Henderer:  The field defects of glaucoma seem to be the same whether it's high tension or low tension.  They follow optic nerve defects generally and match the area of the nerve that is damaged. In advanced cases of nerve damage, the field may show change while the nerve stays the same, but that is only because we cannot see the individual nerve fibers that are certainly being damaged.

P:  Are splinter hemorrhages that occur in the optic disc or on the disc margin more common in normal-tension glaucoma than in other types?  And does visual field loss always follow disc hemorrhages?

Dr. Jeff Henderer:  Those hemorrhages are reported to be more common in normal-tension glaucoma, and it is my understanding that they will eventually lead to a defect, although that can take up to 18 months.

P:  Do you find that the results of visual field tests are consistent with the condition of the optic nerve?

Dr. Jeff Henderer:  Well, that's certainly what I look for.  Visual fields are fraught with artifact, so if the picture doesn't make sense, then I get suspicious there might be something else at work.  The other thing is that in early glaucoma the field defect may lag behind the nerve changes.  In advanced disease, the nerve is already so damaged that the examiner can't see change well.  The visual field may be the best way to follow some of those eyes.

P:  We know that optic nerve damage can manifest itself to the patient as a scotoma (blind spot), but can it also manifest itself as a general graying of the visual field?  If so, why does that happen? 

Dr. Jeff Henderer:  It turns out that glaucoma does not cause complete loss of vision until all the nerves are dead in that one spot.  Did you know that there are far more nerve fibers than rods and cones?  There are about 20 nerve fibers to every cone in parts of the retina (or more) and the density decreases further away from the center.  This means that if you lose half of the nerve fibers you will still see, but it will be gray.  There is enough redundancy to cover the loss.

P:  Does damage to the optic nerve as indicated by a visual field test have any relation to a retinal vein occlusion?  I have a hemi-retinal occlusion and have lost most useful vision in the eye that showed optic nerve damage.

Dr. Jeff Henderer:  Any cause of poor retinal function will manifest as a field defect.  Glaucoma is only one cause.  Others include vein occlusions or artery occlusions or even laser treatment.  Of course, brain disease can do it too.

P:  Is blindness from macular degeneration also related to the optic nerve?

Dr. Jeff Henderer:  No, it turns out that macular degeneration is a problem of the retina itself, not the connection between the retina and the brain (glaucoma). 

P:  In a normal rim of the optic disc, can the horizontal be larger than the vertical?  And is larger vertical cupping more typical of glaucomatous damage?

Dr. Jeff Henderer:  There is a useful rule to remember that Elliott Werner developed.  It's called the ISNT rule.  It means the Inferior rim should be the thickest, followed by the Superior rim, then the Nasal, then the Temporal.  Vertical cupping (really, vertical rim thinning), is classic for glaucoma, as the poles seem to be the first places to get damaged for most eyes with glaucoma.

P:  If the size of the cups is different in the two eyes of a patient, what significance does that have?

Dr. Jeff Henderer:  That is an excellent question.  Generally, there is some variation between eyes.  Most believe a difference of more than 0.2 c/d (cup-to-disc) ratio is suspicious for glaucoma.  But remember that some people have different-sized optic nerves and for them this asymmetry may be normal.

P:  Since the physiologic cup in normal eyes varies, how can you determine that cupping is abnormal only on the basis of the cup-to-disc ratio?

Dr. Jeff Henderer:  Now you ask the hard questions!  Determining if the disc has glaucoma is only possible if it is obvious, or if you see change.  Many discs are suspicious-looking, as there is a lot of physiologic variability.  The key is to look for a pattern of findings.  Pupillary defects, field loss, disc damage, IOP levels -- that sort of thing.  Sometimes it just isn't easy to tell.

P:  Without the advantage of previous documentation or photographs, is it possible to recognize that a disc has sustained glaucomatous damage?

Dr. Jeff Henderer:  Yes, I think so, if the pattern of damage is classic (and the other eye seems normal or at least doesn't have that same pattern) or you see progression on the field.

P:  Once there is optic nerve damage, does damage seem to progress more quickly even with lower pressures?  

Dr. Jeff Henderer:  People have said that, but I'm not so sure.  Perhaps I don't have the advantage of having watched patients for years, but I see a lot of very damaged nerves hanging on for years.  If you can get the pressure down, most nerves will be helped.

P:  Is an eye migraine part of having glaucoma?

Dr. Jeff Henderer:  I don't think so, but eye pain can be a part of glaucoma!

P:  In the case of sudden vision loss, what tests (if any) would you recommend for examining the nerve area behind the eye?

Dr. Jeff Henderer:  Well, the area behind the eye is only visible by ultrasound, CT scans, or MRI.  Each one has advantages.  They are all useful, but I suspect CT or MR is more widely available.

P:  Is it always the case that, once an eye shows the beginning of glaucomatous change, it will be more vulnerable to IOP increases than it was before?  In other words, once resilience is compromised, is there a kind of vulnerability progression or can it re-stabilize even with some damage?

Dr. Jeff Henderer:  Well, that is not entirely clear from scientific studies (at least, I'm not aware of any).  My hunch is that, yes, once damaged, you are more vulnerable.

P:  Could a decrease in the oxygen supply to the brain, such as from a mini stroke or transient global amnesia, cause an increase in the optic pressure gradually over a period of a year?

Dr. Jeff Henderer:  Hmm.  I don't know much about global amnesia and oxygen supply, but chronic hypoxia can't be a good thing.  I have never thought about looking for glaucoma in smokers with chronic bronchitis who are always in this state.  Good idea!

P:  Does the appearance of an orbitally compressed optic nerve (from Graves' disease) differ from that of a nerve damaged by primary-open angle glaucoma?  If so, could you please explain how?

Dr. Jeff Henderer:  That is a tough one.  In fact, the changes that you see in glaucoma are pretty specific to glaucoma.  Optic nerve compression generally shows up as pallor of the nerve, not cupping.  The same can be said of a "stroke" of the nerve.  When the blood supply is cut off, the changes are not generally the same as you see in glaucoma.  We can not reconcile that yet.

P:  Why is it that one doctor sees a notch in an optic nerve, yet another doctor does not?

Dr. Jeff Henderer:  You only see what you look for, and you only look for what you know.  For example, I am not going to see patterns of corneal disease that a corneal specialist would see.  

P:  We've often been advised to exercise in order to increase blood flow.  However, blood flow has not been demonstrated, only suspected, to cause nerve damage.  Is exercise, then, a safeguard?

Dr. Jeff Henderer:  I believe exercise helps in two ways.  First, it lowers eye pressure.  Second, it promotes a healthy cardiovascular system.  I suppose the more blood pumped to the eye, the better. 

P:  Is something else -- something curable -- ever mistaken for glaucoma?  This is wishful thinking on my part.

Dr. Jeff Henderer:  Well, remember that glaucoma is treatable.  Maybe not curable, but for most quite treatable.  Yes, other things are confused for glaucoma, but the pattern of loss of rim tissue is really pretty specific to glaucoma and to arteritic ischemic optic neuropathy, which has other findings as well.

P:  You talked earlier about pupillary change.  Can you explain further?

Dr. Jeff Henderer:  It is an idea of Dr. Spaeth's, mainly, that an afferent pupillary defect is quite common in glaucoma.  I agree.  What does this mean? Well, when you shine a light into one eye and then the other, the pupils will stay constricted.  If one eye has damage to the nerve (whatever cause) more than the other, the eye with more damage will perceive the light to be "dimmer" so it will dilate instead of constrict.  I find it a very useful test in early glaucoma, when typically there is damage in one eye and not the other.

P:  Would there be some developmental delays in a child who had severe cupping of the optic nerve for the first year of his life, because the brain didn't get a clear picture from the eyes?  

Dr. Jeff Henderer:  I'm not sure. There might be, but if the other eye is okay, probably not.  And I have seen plenty of blind kids who seem to be maturing fine.

End of highlights for September 18, 2002.

On September 25, Dr. Werner discussed "Secondary Glaucoma" in the Chat room. Click here for highlights of that meeting.

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