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GDx Nerve Fiber Analyzer

Chat Highlights
GDx Nerve Fiber Analyzer
August 23, 2000
Norma Devine, Editor

On Wednesday, August 23, 2000, Dr. Jeff Henderer, a glaucoma specialist at Wills, and the glaucoma chat group discussed “GDx Nerve Fiber Analyzer.”

Dr. Henderer: Hello all. Dr. Wilson is in surgery, so he asked me to be here.

Moderator: Welcome, doctor. Tonight the topic is the GDx Nerve Fiber Analyzer.

P: What sort of test is the GDx?

Dr. Henderer: The GDx is a tool that uses laser to determine the thickness of the nerve fiber layer.

P: Do most glaucoma specialists have access to GDx?

Dr. Henderer: Some doctors have access to the GDx.

P: How does information about the thickness of the nerve fiber layer help?

Dr. Henderer: The nerve fiber layer is important because it is the tissue that the optic nerve is made of. Damage to the nerve layer could be a sign of glaucoma.

Moderator: What does the thickness indicate?

Dr. Henderer: In theory, a thinning of the nerve fiber layer is bad. It is not always from glaucoma, so care must be taken.

P: Do you need to have your eyes dilated for this? If not, would it be especially useful for angle-closure?

Dr. Henderer: You do not need to be dilated for this exam.

P: If my specialist wanted to use the GDx and/or the HRT for me (a very long- term, stabilized patient), would I be justified in refusing them in favor of visual fields and photos?

Dr. Henderer: I don’t know the answer to that. I think that it would depend on your age, the level of pressure, the existing damage, and how important detecting change would be.

P: How can you tell the nerve fiber layer is bad? Isn’t each person’s thickness different to start with?

Dr. Henderer: The answer is that you are compared to other “normal” people your age. Then a statistical calculation is performed to determine if you are in the “normal” range or not.

P: Can it detect damage that cannot be seen by an ophthalmologist using an ophthalmoscope?

Dr. Henderer: The answer is probably yes, the machine can measure changes in thickness that the MD cannot. Whether this is important is not yet determined, as far as I know.

P: Can the test determine if the thinning is from glaucoma or another reason?

Dr. Henderer: The test cannot distinguish glaucoma from other causes of nerve fiber damage, such as a vein occlusion. Care must be taken when interpreting the results.

Moderator: At what point might you order a GDx for a glaucoma patient? Do you think it will become a standard diagnostic tool for testing glaucoma in the future?

Dr. Henderer: I am not sure if it is the test of the future. More work needs to be done, and the system has some problems that need to be ironed out.

P: Is the layer uniformly thin, or is it just affected in spots?

Dr. Henderer: The idea is that in glaucoma you have focal loss of tissue, whereas aging causes diffuse loss.

P: This thinning would show up before loss is seen with a visual field test?

Dr. Henderer: This is a test that has promise. It may offer important information that is not obvious on clinical examination. It may be most useful in following patients over time to see if they are losing tissue at a rate faster than they should. The thinning may show up earlier than with a field test, but remember that until there is a field defect, you are unaware of a problem. Is it important to know this? Maybe not for some people, but maybe yes for others.

P: Which numbers in the GDx test is a glaucoma specialist interested in?

Dr. Henderer: As far as I know, the numbers that are most important are the actual thickness maps. There is some sort of derived number that is supposed to indicate “level of abnormality.” I have my doubts about this, and would rather see just the raw data and draw my own conclusion. Check out the GDx image on the banner above. The thickness maps are the “M” lines down at the bottom.

P: Are both nerves damaged in that example?

Dr. Henderer: The “M’s” indicate the expected range of normal, and the middle line in the shaded area is the patient in question. I don’t see evidence of damage in either of these eyes.

Moderator: How important is the technician who performs the test?

Dr. Henderer: The technician’s capability is critical. Garbage in, garbage out. If the patient is not aligned properly, you get garbage and can’t compare one exam to another performed later. This is a huge problem.

Moderator: What should patients do if they can tell the technician does not have a clue? That is my fear.

Dr. Henderer: Just make sure you are focused on the target. That’s all you can do. That should be enough to make it a good test.

P: How does GDx differ from the HRT?

Dr. Henderer: The GDx and the HRT are both laser instruments. The GDx measures nerve fiber thickness and the HRT is like a CAT scan of the optic nerve. It makes a 3D map of the nerve and measures its parameters.

P: Is the technician’s skill critical in visual field testing, too?

Dr. Henderer: The technician is critical in field testing, but not as important as the patient’s level of experience. I think that the technicians are fine, but you just have to be careful. It should be relatively easy to get good data with either machine. It is not that hard to do.

Moderator: Have they ever tried to do a high resolution CT scan of the optic nerve, like 1 mm, or maybe an MRI, to look at the nerves?

Dr. Henderer: CT and MRI don’t have the resolution (remember that the nerve is only 1.5 mm or so in diameter) to provide good information for glaucoma. They are very helpful for other diseases of the nerve though.

P: Does the HRT measure thickness at all?

Dr. Henderer: The HRT does measure thickness of the nerve fiber layers, but that is not its primary job. Therefore, I am not so confident about it. It may be fine, but I would rather look at things like the area of the rim.

P: Does the GDx give more information than the HRT?

Dr. Henderer: No, they just give different information about the same thing: The optic nerve or the nerve fiber layer — sort of two sides of the same coin.

P: What is the meaning of the colors and the green circle in the GDx example above?

Dr. Henderer: The colors are a computer enhancement of the thickness. The circles represent the nerve. The orange areas on the pictures are areas of thicker nerve fiber layer. Looks pretty but, like the grayscale plot on a visual field, it’s really not as helpful as the other information on the printout.

P: So I guess the blue is the thinner, isn’t it?

Dr. Henderer: Yes. Notice that the thickness is greater at the top and bottom of the nerve. That is normal. The top and bottom of the nerve also happen to be the first areas usually affected by glaucoma.

Moderator: Do insurance companies reimburse for the GDx test?

Dr. Henderer: Yes, you can bill for it and it should be covered, just like optic nerve photos.

P: When would you use GDx, HRT, or just photos?

Dr. Henderer: I don’t know. I think the new machines have great potential but, for now, optic nerve photos are the gold standard. I think the real potential of these machines may be following people for change over time. They may be better than photos or fields for this, but the work to prove this is ongoing.

P: Are these machines relatively commonplace or still experimental?

Dr. Henderer: Both machines are becoming more common. But both are still experimental.

P: Do you happen to know how the database for this program was set up?

Dr. Henderer: I don’t know. This is a big concern because if your racial group was not included in the “normal” database, you could be compared to the wrong standard. I don’t know if this is true for the GDx though.

P: Isn’t there a test that does not require dilation?

Dr. Henderer: The HRT and clinical exams do not require dilation.

Moderator: I am looking at that printout and have no idea what I am looking at.

Dr. Henderer: The pictures at the top are a color “photo” of the nerve and the nerve fiber layers. The rest is analysis of the level of abnormality. The maps at the bottom show how this patient’s nerve fiber layer compares to “normals.”

P: Which areas are affected first by glaucoma?

Dr. Henderer: The superior and inferior poles are usually affected first.

P: Does that mean top and bottom? Sorry to be dense.

Dr. Henderer: Yes, the top and bottom of the nerve. Sorry not to talk so people can understand. That is a constant battle for all MDs, and we should be corrected.

P: Are the top and bottom where the nasal step would be?

Dr. Henderer: The nasal step is caused by the damage usually occurring in this region that serves the nasal field. That’s why it’s often the first field defect to show up.

Moderator: Is the GDx in the the example normal or does the patient have glaucoma in both eyes?

Dr. Henderer: The way I see it, both eyes are normal. I have enjoyed talking to you all. Goodnight! E-mail other questions later. They’re always welcome.

End of highlights for August 23rd chat.

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About the Author:

The Glaucoma Service Foundation’s mission is to preserve or enhance the health of all people with glaucoma and to provide a model of medical care by supporting the educational and research efforts of the physicians on the Wills Eye Institute Glaucoma Service, the largest glaucoma diagnosis and treatment center in the country.
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