Optic Nerve Imaging: Past, Present and Future
Chat Highlights
May 31, 2006

On Wednesday, May 31, 2006, Dr. Jeff Henderer, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Optic Nerve Imaging: Past, Present and Future."

Moderator: The topic tonight is "Optic Nerve Imaging: Past, Present, and Future".

P:    Can you give a brief summary of optic-nerve imaging systems?

Dr. Jeff Henderer:    The three main systems are the HRT (Heidelberg Retinal Tomography), the GDx (scanning laser polarimeter) and the OCT (ocular coherence tomography).  The HRT looks at the optic nerve the way I would look at the nerve; that is, it measures the amount of nerve rim versus the cup.  The GDx and OCT both measure the amount of nerve tissue on the surface of the retina surrounding the nerve.  All three can compare your optic nerve to a normative database to see if yours differs.

[Editor's note:  Optical Coherence Tomography is a non-contact, non-invasive imaging technique used to obtain high resolution cross-sectional images of the retina.]

P:   As a glaucoma suspect for about three months, I had an HRT at Wills performed by Dr. Rick Wilson.  Would an OCT-3 provide more information than the HRT?

Dr. Jeff Henderer:   The OCT-3 is the most recent version of the machine and does, indeed, measure something different from the HRT.  The other side of the coin, so to speak.  The OCT measures the nerve tissue on the retina, whereas the HRT measures the same tissue as it funnels into the nerve.  In theory, the two machines should be in agreement, but in reality that is often not the case.  The best tie-breaker is the experience of the doctor.  Dr. Rick is about as experienced as anyone in the world.

P:   What is the resolution of each of those three machines?

Dr. Jeff Henderer:   The resolution?  Hmm, well the OCT is the only one that really needs resolution the way that you mean (as in seeing cell layers), and it is 9 microns.  The HRT and GDx do not measure the nerve in ways that are dependent upon resolution, as I understand it.

P:   Can these new systems be used as a diagnostic tool, or just monitoring?

Dr. Jeff Henderer:   Both, actually - - in theory.  However, using them diagnostically is not so easy.  We really believe their best application is going to be in follow up.  But the data is still being collected to see if the OCT and GDx are effective.  Some data indicates the HRT is useful.  The HRT was used in the Ocular Hypertension Treatment Study (OHTS), and was found to be able to predict who would go on to develop glaucoma.

P:   So HRT and GDx aren't imaging techniques?  I don't understand.

Dr. Jeff Henderer:   The HRT and GDx are imaging techniques, but they don't work like the OCT.  The OCT measures the depth of the eye using light and so is able to distinguish the different layers of the retina.  The HRT is like a CAT (Computed Axial Tomography) scan of the nerve. It takes serial sections through the nerve and reconstructs them in 3D.  The GDx relies on the polarization of light as it passes through the nerve fiber layer and measures the change in light polarization to determine how much tissue is present.

P:   Do you image the optic nerves of all your glaucoma patients?  I'm asking because I've never had my optic nerves imaged.

Dr. Jeff Henderer:   It is not necessary to image all patients. I tend to do it because I'm curious if the imaging will help me.  I don't believe there have been more than a couple of times when it showed me something that I didn't already know.  But we've only been using it for a few years.  Even the Normal Tension Glaucoma (NTG) study needed seven years to show that treatment helped, so we haven't had enough time yet.

P:   Is a careful examination of the optic nerve by a glaucoma specialist still considered to be as effective as anything available?

Dr. Jeff Henderer:   Yes, it is.  Our brains are still the gold standard.  Remember that someone has to program the machine so it knows what is normal and what is not. If we do the programming, how can the machine be any better than we are?

P:   How often do you recommend having an HRT for patients you think would benefit from it?

Dr. Jeff Henderer:   If I have no suspicion that there is change, I like to image the optic nerve about once a year.  I also like to test the visual field once a year.  There is no science behind that; just my personal practice.  What I try to do on every visit is compare the patient's optic nerve to the original photos of the optic nerve.  Therefore, I guess my exam is a form of optic nerve imaging too!  But I'm qualitative. I'm hoping to quantitate the nerve, as I quantitate the field.

P:   Is fluorescein angiography (FA) still used and why would it be used for glaucoma?  [Editor's note: Fluorescein angiography is a study of the blood vessels that provides information useful for evaluating many eye diseases affecting the retina.]

Dr. Jeff Henderer:   FA's are not really done anymore for glaucoma evaluations, that I'm aware of.

P:   I had stereo photographs taken on my first visit to Wills Eye Hospital.  Are the photos just to document my current state for future comparison, or do the photos show things not visible in direct observation by the doctor?

Dr. Jeff Henderer:   Primarily, the former.  On occasion I've seen photos that revealed something missed at first, but that's not common.  However, as Dr. Spaeth teaches, "You only see what you look for, and you only look for what you know."

P:   How often do you take photos of a patient's optic nerve?

Dr. Jeff Henderer:   I rarely repeat the photos unless there is some item of interest or change. I've come to use yearly nerve imaging instead, but I still like to get photos at the outset.

P:   What other tests may have been done in the past to evaluate glaucoma but now are outdated?

Dr. Jeff Henderer:   Well, there is the water drinking test and tonography.

Moderator: Water drinking test?

Dr. Jeff Henderer:   I guess the point was to see if your IOP would increase if you drank a lot of water.  There is also the dark test to see if your pupils dilated in a room dark enough to cause an attack of glaucoma.  That one is not used much.

P:   What is tonography?

Dr. Jeff Henderer:   The idea was to press on the eye and measure the rate of the decrease in IOP (intraocular pressure) to see whether the resistance to outflow from the eye was abnormal.  I confess I don't know much about these tests though, as I've never used them.

[Editor's note:  Tonography is a continuous measurement of intraocular pressure to determine the facility of aqueous outflow, used to determine the presence of glaucoma.]

P:   Doesn't IOP increase when you press on the eye?

Dr. Jeff Henderer:   Well, that's a good point.  Initially, yes, but do it long enough and eventually you force fluid out of the eye and the IOP goes down.

P:   I have heard about the dark room test and also about having patients hang their heads over a table.  Glaucoma testing sure has changed, and changed in what seems like a short period.

Dr. Jeff Henderer:   Glaucoma is finally catching up to other areas of ophthalmology. We hope we can one day be able to figure out (1) who has the disease, (2) who's going to lose vision, (3) continuous measurement of intraocular pressure to determine the facility of aqueous outflow, used to determine the presence of glaucoma and what treatment will work best, and (4) how not to make patients worse with treatment, which, as you know, can sometimes happen.

P:   Do you always show patients their photos or images, or only if they ask?

Dr. Jeff Henderer:   I like to show patients their photos.  That's not so easy with slides, but in one office I have them displayed on the computer screen, and I can show patients exactly what I'm watching. I find that to be very helpful for patients.

P:   Are there any non-optical imaging techniques in use or development?

Dr. Jeff Henderer:   I personally believe that being able to measure cellular metabolism is the Holy Grail in glaucoma research.  Short of that, if we could detect dying cells, that would be great.  How that will be done is unclear.  The other option would be to simply count the number of ganglion cells.  If you could do that, what more would you need?

P:   I'm intrigued by your comment about measuring cellular metabolism being the Holy Grail in glaucoma research.  I have the vague memory that MRI (magnetic resonance imaging) on P31 (instead of H1 as normally done) can image relative metabolic rate.  P31 is a weaker signal, though, and I'm not sure how detailed an image it would provide.

Dr. Jeff Henderer:   You know more than I do about this, but if we could do a PET (Positron Emission Tomography) scan of the eye -- and the resolution of the PET scans is not good enough to look at the retina in detail, as we'd like --, you might be able to pull it off. But the expense!  And the chemicals you have to take!  We need a surrogate marker, such as oxygen saturation.  Jon Myers is involved with an oxygen saturation device that might help in this way.

P:   I recently had a repeat OCT.  The tech seemed competent, but the comparison analysis showed an increased thickness in many areas, as well as loss in others.  Isn't the increased thickness most likely to be an artifact?

Dr. Jeff Henderer:   Yes, you can't "grow" new optic nerve tissue.  You have illustrated the bane of my existence!  There are no data that tell us how much change is significant, or really even any data to show how much variability you can expect in real patients from test to test. It's clearly not as good as the company would have you believe.  But I'm looking for trends.  Remember that the image quality has to be good, too.

P:   Is there evidence that new optic nerve tissue can be grown?

Dr. Jeff Henderer:   There is data in animals (and in Christopher Reeve) that brain and spinal cord can, in fact, regenerate.  That's very exciting.  But I don't know if, when the mouse regrows the tissue, it's nerve the nerve cells connect to the brain in a meaningful way.

P:   If you had to say what the present state of optic nerve imaging is, what would you say?

Dr. Jeff Henderer:   The present state?  My answer is, on the verge of great things.  The tools have been developed that make a lot of sense.  Now we just need the experience to know how patients behave over time.  That will help us understand how to determine progression.

P:   What is B-scan ultrasound and why is it used?

Dr. Jeff Henderer:   A B-scan is an ultrasound that generates a picture, like a sonogram during pregnancy.  We can image the internal contents of the eye.  An A-scan just measures the distance from the probe to ocular structures.  It's useful for determining the power of lens implant for cataract surgery and for doing OCT.

P:   Is B-scan useful to see the shape of a vitreous detachment?

Dr. Jeff Henderer:   Well, the B-scan can probably see the floater (if you get lucky and capture it and if it's large enough), and it can show a retinal detachment, but we don't usually use one to look for a PVD (posterior vitreous detachment.)

P:   A friend of mine had an AccuMap done at Wills Eye Hospital.  What is that?

Dr. Jeff Henderer:   The AccuMap is a new visual field machine that doesn't require you to push a button.  You wear a brain-wave monitoring device and you stare at the computer screen.  The pattern of brain waves is recorded and your "vision" is then printed out.

Moderator: Is that more accurate than a regular visual field test?

Dr. Jeff Henderer:   The AccuMap test may sound easy, but from what I hear, it's not.  It's a really neat concept though.  It might, indeed, be more sensitive than a regular visual field test, but we don't have enough experience with it to be sure. It's still quite new.

P:   Are there any tests that measure blood flow to the optic nerve?

Dr. Jeff Henderer:   Yes, blood flow can be measured, but it's not as precise as you'd like to think.  Further, you need numerous gizmos to measure the flow at the different locations in the eye and optic nerve.  Then there's the chicken-and-the egg problem. Is ischemia the cause or the effect of glaucoma?  There is no answer to that, but I can tell you that sometimes there is reasonable evidence that blood flow is a problem.  However, in others (such as ischemic optic neuropathy or a stroke of the nerve), the blood flow problem doesn't result in glaucomatous nerve damage.

P:   When, why, and how are endothelial cell counts performed?

Dr. Jeff Henderer:   Endothelial cells are the cells that line the inside of the cornea and serve to dehydrate the cornea so it stays clear.  You'd count them if you had a disease where they died, or if you think a surgery might kill them and cause the cornea to swell.  There is an optical camera that images them.

P:   Since I have ICE (iridocorneal endothelial syndrome), I've always wondered why I've never had an endothelial cell count.

Dr. Jeff Henderer:   Well, there's probably not much point, because there's no way to regrow them and in ICE they are abnormal.  You go by how the cornea is doing and intervene based on clinical reasons, rather than cell counts.

P:   Is it possible to directly image nerve cells in situ, or is the current image more "macro" (just gross structure)?

Dr. Jeff Henderer:   It is possible to view dying ganglion cells in vivo (in animals) using special techniques pioneered in England.  It's not ready for human beings, but is definitely exciting.  The current state of the art is still not at the cellular level.  That may come one day with high-resolution OCT.

P:   Is optic nerve photography still the standard of analysis?

Dr. Jeff Henderer:   I believe so. I should say that some form of recording the nerve is the standard of care.  Photos are my preference.

P:   Historically, wouldn't the first neuro imaging have been photographs?

Dr. Jeff Henderer:   Yes, photos are still considered the standard of care.  Stereo photos that is.

P:   Would you say that optic nerve imaging is in its infancy, and we will see improvement over time?

Dr. Jeff Henderer:   Yes!  I personally believe that in the future, we will measure your genetic profile to see what risk you have and count your ganglion cells to see if you are losing them at a rate that exceeds normal. I don't know if we'll need visual field tests.  But my wishes are a pipe dream at the moment.

Moderator: Thank you, Dr. Henderer.  It's always a pleasure to have you here.

Dr. Jeff Henderer:   You guys are great!  Great questions.  And I look forward to being here again in July.

On June 7, Dr. Werner discussed "Floaters, Flashes and Halos" in the Chat room. Click here for highlights of that meeting.

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