Optic Nerve Imaging
Chat Highlights
August 20, 2008

On Wednesday, August 20, 2008, Dr. Michael Pro, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Optic Nerve Imaging".

Moderator: Tonight's topic is "Optic Nerve Imaging". Dr. Pro what are the different methods of imaging?

Dr. Pro: The oldest method is optic nerve photos, either monoscopic or stereoscopic.
Newer methods include:

• Scanning Laser Polarimetry–GDx-VCC, Laser Diagnostic Technologies, CA;
• Confocal Scanning Laser Ophthalmoscopy–Heidelberg Retina Tomograph, HRTII/HRT3, Heidelberg Engineering, Germany;
• Optical Coherence Tomography–Stratus OCT, Carl Zeiss, Meditec, CA;
• And the newest of all is the Fourier (Frequency) Domain OCT.

P: Do you get different information from the different types of imaging?

Dr. Pro: Yes. Let's start with the optic nerve photos. They obviously give the best detail of the nerve, with true color, but photos can not tell us what is going on under the surface. The OCT and GDx can tell us what is happening to the nerve fiber layer, which is where damage and change is really occurring in glaucoma. The HRT is sort of like a topographic map. In that sense it is more like a photograph than the OCT and GDx which are more able to see below the surface.

P: Which is appropriate for the different kinds and stages of glaucoma? Are some better for diagnostics and some for monitoring?

Dr. Pro: Great question! The HRT and nerve photos may be best for monitoring. The HRT in particular has a great software package that allows for comparison of follow-up scans to a baseline scan.

The OCT has been mostly used for retinal diseases, but has also found a home in glaucoma; it may be more useful at early detection of glaucoma.

The GDx has a new software package that may allow better comparison to baseline scans which will help in monitoring change.

P: Does that mean the OCT would show optic nerve damage before the photos would?

Dr. Pro: Nerve photos are still the "Gold Standard," with the most data from years of scholarly glaucoma articles, but the OCT has the potential to show damage earlier. I am very excited about the newest generation of OCT, which has tremendous resolution.

All three scanning devices have drawbacks, such as trouble with tilted or abnormal nerves, but with better resolution we could evaluate the tissue planes in better detail, and we'd have the ability to thoroughly check the same area on follow-up scans.

P: How many times does a patient need to have an imaging done in order to make a comparison of damage?

Dr. Pro: In general three scans are the minimum I use.

P: How frequently should these tests be done?

Dr. Pro: I guess it depends on the suspicion that glaucoma progression may be happening. In general I scan my patients once/year, but I increase the frequency if I am worried that my patient could progress.

P: If I have a copy of all my imaging, what is the most important area of each report for me to look at and what does the data mean?

Dr. Pro: Well, different studies point to different measures and these machines are capable of generating lots of data points. For the HRT the Rim Area is important. If the rim is thinning over successive tests there may be change.

For the OCT the total RNFL (retinal nerve fiber layer) as well as inferior, superior, and temporal RNFL measures are important; a decrease over 10 microns on follow-up scans is suggestive of progression.

For the GDx the Nerve Fiber Index (NFI) is an important measure, with a value over 35 being very suspicious for glaucoma.

P: Are some of these changes just a normal part of aging of the eyes, and may never be classified as glaucoma?

Dr. Pro: It's true; the eye changes over time and to be honest we don't know enough about normal aging. In fact there is increasing evidence that glaucoma is simply accelerated aging in the eye.

P: As the nerve imaging equipment is advancing for better performance, can the old tests and the new tests be compared for the same piece of equipment?

Dr. Pro: Unfortunately they are usually not backwards compatible, which is part of the appeal of good, old-fashioned nerve photos.

P: Does EMR (Electronic Medical Records) effect the quality of images in Ophthalmology and the ability of the physician to interrupt the image? Is the quality of a fundus photo, for example, better if printed out, viewed on slide, or if viewed via an electronic record?

Dr. Pro: The best image quality for a nerve photos is probably with a slide and slide viewer, which gives the best 3D image. EMR is the future and I expect the image quality to continue to improve. Ultimately EMR may allow better comparison from test to test.

P: Do you interpret nerve photos with slides and slide viewer or within your ERM application?

Dr. Pro: I do not routinely use a slide viewer. They are mostly used in glaucoma research.

P: Do you store your images long term for reference? Within your EMR application?

Dr. Pro: I have a paper chart and we store the photos in a digital format for planned conversion to EMR and we also print large glossy images for the chart.

P: Are any of these imaging techniques as good or better then a trained physician's eye?

Dr. Pro: No, not yet, but they can be faster and have the advantage of generating quantitative data which is easier to compare.

P: Do doctors vary in their interpretations of the photos, or is the information pretty straight-forward, with no room for error?

Dr. Pro: There is lots of variation. Studies have shown that viewers do interpret photos differently, but in glaucoma studies, change in the optic nerve is often an endpoint for diagnosing glaucoma. To get around the problem of different viewers the studies use two trained viewers. If they disagree on an nerve description a third person is the "tie-breaker."

P: Which of the three newer pieces of equipment (GDx, HRT, OCT) is the favored one among specialists? Would you rank them most to least popular?

Dr. Pro: Tough to do that. I guess the most popular is the HRT, followed by the OCT and GDx. I do not mean to pass judgment on the instruments, but am simply noting popularity.

P: Near the beginning of the chat, you said the newest machine is the Fourier (Frequency) Domain OCT. How new is it?

Dr. Pro: It is just coming out now. It's faster and with higher resolution than the Stratus OCT, but for now most of its applications and software is for retinal imaging.

P: How do you know which type of imaging to order? Is there a standard protocol you follow for a particular type of glaucoma?

Dr. Pro: Not really. I usually order HRT to follow-up and OCT for glaucoma suspects, but I would be quick to change my habits if any technology were proven to be superior.

Moderator: Dr. Pro, there are no more questions in the queue. You've done a great job of educating us on Optic Nerve Imaging!

Dr. Pro: I hope you enjoyed it!

Moderator: It was very informative. Thank you.

On September 3, Dr. Pro discussed "Corneal Thickness" in the Chat room. Click here for highlights of that meeting.

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