Optic Disc Imaging
Chat Highlights
May 19, 2010

On Wednesday, May 19, 2010, Dr. Michael Pro, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Optic Disc Imaging".

Moderator: Tonight our topic is Optic Disc Imaging, continuation from March 17, when there was a power failure.
Dr. Pro, does any additional testing need to be done besides a visual field for a glaucoma diagnosis?

Dr. Pro: Yes, as the topic title suggests, both examination and imaging of the optic nerve are important.

Moderator: And for the newer chatters, can you define what the optic nerve is and why it is important to image it and follow what it looks like?

Dr. Pro: The optic nerve is sort of like a cable that connects the eye to the brain. We look at it during an exam. It is very small, measuring about 1.5mm across, and we are able to see only the very end of the nerve. It resembles a donut.

P: What are the tests used to image the optic nerve?

Dr. Pro: Let's start by saying that the "gold standard" of optic nerve imaging remains stereo disc photos. These are color pictures of the optic nerve that create a 3D image when looked at by a stereo viewer. Newer photos of a nerve are compared to earlier pictures to look for any changes that could suggest progression of glaucoma. There have been many attempts to supplement disc photos by other technologies to image the optic nerve. Disc photos have many advantages: it is relatively inexpensive for a practitioner to buy a fundus camera; disc photos are never made obsolete by new technology; and, they archive well.

But there are some disadvantages. The best way to determine if there is disc progression is to have a stereo camera (more expensive) and to view a stereo photo properly you need a stereo viewer which is cumbersome and time-consuming.

P: Are they digital images? I recall slides of my nerve.

Dr. Pro: Today they are digital images. It is more difficult to see a stereo image on a computer screen. Slides were used in the past.

P: I had some photos taken today for an HRT test? What is HRT?

Dr. Pro: Heidelberg Retinal Tomograph (HRT) is one of the disc imaging technologies that is used today. Basically it is a scanning laser that takes a topographic map of the optic nerve. This baseline is then compared with follow-up scans.

P: I have had photos of the optic nerve done and will have more in July to compare. I'm not sure what they use but the photos are in color

Dr. Pro: Photos are always in color.

Moderator: Is an optic nerve head photograph or drawing necessary anymore on a glaucoma patient with all the imaging devises available?

Dr. Pro: Yes, I still like to draw the nerve. Every imaging modality has shortcomings and sometimes a good, detailed exam and drawing are still superior. Additionally, when I draw the nerve I really get a better sense on how severe an individual's glaucoma may be and I can target my therapy better.

P: I have seen my doctor or technician do what looks like connect the dots, and then stuff many images that seem to be processed. Is this doing a computerized comparison?

Dr. Pro: The "connect the dots" occurs with the HRT and is used for the baseline scan to outline the border of the optic nerve. This outline forms the basis of comparison with follow-up scans.

P: Do optic refractive errors distort the imaging results?

Dr. Pro: Oh yes, the quality of imaging is influenced by refractive errors or media opacities. For instance, if there is a cataract present, then the image may be blurry. The technician also has to neutralize the refractive error to get a good quality image.

P: Can Optical Coherence Tomography (OCT) detect evidence of glaucomatous damage earlier than other imaging techniques? Is it better than optic disc photographs?

Dr. Pro: That is still up for debate. So far no single imaging device is proven better for diagnosing glaucoma or detecting glaucoma progression. OCT may have the potential to be the superior imaging technology in the near future.

P: Can progression of glaucoma be followed if different types of imaging are used each year due to the physician changing equipment?

Dr. Pro: That can be like comparing apples to oranges; it is best to try to mainly use one technology.

P: Can the results of two patients’ confocal laser scanning tomographies that appear to be very similar produce different vision for each patient?

Dr. Pro: Can two person's nerves look the same, but one person's glaucoma is much worse than the other's? Absolutely.

P: Can consecutive years of optic nerve imaging predict when a defect will occur on a visual field? Can any of the imaging techniques predict visual field loss?

Dr. Pro: Yes. This was first described in the 1970's when researchers noticed that changes in the optic nerve of stereo discs proceeded visual field changes. This has come to be referred to as structure vs. function.

P
If the optic nerve photos are compared to the visual field test, will they relate?

Dr. Pro: Yes damage on a specific part of the optic nerve will correlate to a visual field defect in a certain area.

P: Do any of the imaging techniques provide information that a physician can conclude the likelihood of blindness?

Dr. Pro: No better than a good exam.

P: Why are these imaging techniques not performed as often as a visual field test?

Dr. Pro: For several reasons. One reason (here in the U.S.) is economic. Some health insurers will not pay for a scan more than once per year. Secondly, visual fields are known to fluctuate from test to test, so you need more fields to know if there is glaucoma progression.

P: How much does human subjectivity play a role in imaging results?

Dr. Pro: Well that is the benefit of imaging. It is more objective. Even so, it is not always clear that the nerve is getting worse.

P: Dr. Pro, I had an HRT today. I have (traumatic) glaucoma in just my right eye, but the HRT said borderline for the other eye, even though my visual fields have consistently showed no damage over the years. Here's my question: the technician said she had to draw "contour lines" on the image first. It seems to me that anytime you get someone drawing a line on something then that introduces an element of subjectivity in what should be an objective test. Is that an accurate sentiment?

Dr. Pro: Quite right! But one of the strengths of that particular device remains and that is the ability to compare future tests to that baseline test. Regardless of how the baseline contour was drawn, future tests will use that same contour line and could detect change in the optic nerve referenced to that baseline exam.

P: And why does a tech have to draw contour lines on the image?

Dr. Pro: The tech does a better job than the software might and the unit needs a contour line to evaluate the disc parameters.

P: Dr. do you have any closing remarks about optic nerve imaging?

Dr. Pro: Just to say that imaging technology keeps getting better, but glaucoma management remains a case by case process.

Moderator: Good night Doctor Pro. Thank you.

Dr. Pro: You're welcome. Good night.

On June 2, Dr. Pro discussed "Am I a Candidate for a "New" Glaucoma Surgery?" in the Chat room. Click here for highlights of that meeting.

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