Chat Highlights
HRT - Heidelberg Retina Tomograph
August 16, 2000

Norma Devine, Editor

On Wednesday, August 16, 2000, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "HRT - Heidelberg Retina Tomograph." 

Moderator:  Dr. Wilson is with us to discuss the Heidelberg Retinal Tomograph.  A copy of an HRT can be viewed at:  http://www3.telus.net/783836/stevehrt.html.  Dr. Rick, what does "tomograph" mean?

Dr. Wilson:  Tomograph means anatomical cuts through a physical structure.

Moderator:  How thin are the cuts?  Are they similar to an MRI (Magnetic Resonance Imaging) or CT (computed tomography) scan? 

Dr. Wilson:  Yes, the test is similar to those two,  but the HRT only looks at the surface of the nerve and the thickness of the retinal nerve fiber layer.

P:  Are  HRTs done in an ophthalmologist's office or in a more specialized setting?

Dr. Wilson:  Usually in a doctor's office.

Moderator:  Is this a common test?

Dr. Wilson:  It is becoming more common, because it is reimbursable.  The companies are promoting it as a way for doctors to add to their incomes and offset growing office expenses.

Moderator:  Should anyone diagnosed with glaucoma, or a glaucoma suspect, start to have HRTs on a regular basis?

Dr. Wilson:  If they are seeing a glaucoma specialist or a good general ophthalmologist, the doctor should do better looking at the nerve than the machine does, at this point. If  they live where continuing medical education is less available and the doctor doesn't spend much time looking at nerves, then the machine will be better.  At this point, visual field tests are more important in patients with serious nerve damage, and good disc photos are more important in early glaucoma.

P:  How does the HRT differ from fluorescein testing?

Dr. Wilson:  There is little to compare.  Fluorescein angiography looks at the vessels;  HRT looks at the surface configuration of the nerve.

P:  When you look at the results of an HRT, what is the most useful information?

Dr. Wilson:  The most useful information is the dimension of the cup in the nerve and, conversely, the rim of the nerve and the configuration of the cup. 

P:  Those are the numbers used to calculate the cup-to-disk ratio?

Dr. Wilson:  Yes.  

P:  Does the "rim" go all around the nerve, or just around the cup?

Dr. Wilson:  The rim goes around both.  The nerve is approximately a disc about 1 mm by 1.5 mm. The depression (cup) is in the middle, with a doughnut of healthy nerve tissue around it for 360 degrees, in most cases.

P:   Is the HRT used at Wills?

Dr. Wilson:  Yes.  We have been investigating it for many years, and trying to help the company improve its software.  In fact, I had an HRT at Wills last week. The machine gave me the diagnosis of glaucoma, which I do not have and I actually have a little cup. The problem with this type of technology is that one gets a digital printout that looks official. It is very easy to put a lot of stock in the numbers, even if they are wrong.

P:  Why would the numbers be wrong? 

Dr. Wilson:  The technology is young, and cataracts, eye movements, etc. can throw the measuring device off.  

P:   How does a cataract 'throw off' the image?

Dr. Wilson:   It makes the image hazy, so that the computer does not get enough information to calculate the topography of the nerve accurately.

P:   If  I do not hold steady during the HRT, does it not register the movements?

Dr. Wilson:  It ignores that image and requires another one.

P:  So how does that generate a false result?  And can't it differentiate between cataract damage and optic nerve damage?

Dr. Wilson:  If there is any movement, the machine cannot get a consistent image and disregards it.  It can differentiate between cataract and optic nerve damage.

P:  Is a comparative analysis possible with HRT? Could it be used to measure progression? 

Dr. Wilson:  That is its major usefulness.  It is completely worthless at diagnosing glaucoma. It may well be able to detect change at the level of  an average general ophthalmologist. The test must be given the same way each time and the tech marking the nerve disc margin on the screen must do it the same way. If multiple technicians are doing the sequential scans, the meaning drops dramatically.

P:  How can a company get something this important on the market with invalid results?

Dr. Wilson:  Any company trying to get new drugs or technology to market supports doctors sympathetic to its cause to do research that shows the abilities of the machine. Problems are usually downplayed.  The HRT is a nascent technology, improving all the time.

Moderator:  I am looking at a HRT printout.  What are the green, blue, and red areas on the left side?

Dr. Wilson:  The red area is the cup in the middle, the green area is "healthy" nerve rim, and the blue area is sloping down.

P:  What do you mean by "sloping down?"

Dr. Wilson:  The cup in not like a real coffee cup, but usually more like a valley with sloping sides. 

P:  So the sloping doesn't indicate nerve tissue on it's way to being damaged?

Dr. Wilson:  No, it may or may not be.

P:  I had my contact lens in for one HRT, which completely threw off the results.

Dr. Wilson:  The refraction (glasses) has a great deal to do with the size of the image the computer sensor sees. Your contact lens changed the size of what the computer was seeing.

P:  Do the notched-looking bits around the edge of the cup indicate damage?

Dr. Wilson:  They represent unevenness in the rim, which may be damage or just vessels going over the rim of the cup. 

Moderator:  Are pupils dilated  for this test?

P:   I notice dark-eyed people get dilated.  Is  that true? 

Dr. Wilson:  Light-colored irises (irises) have larger pupils and may not need to be dilated as much as dark irises.

P:  Why do light-colored irises have larger pupils?

Dr. Wilson:  I don't know.  Good question.  I'm not sure anyone knows.

P:  Do you commonly use HRT as a tool in following your glaucoma patients?

Dr. Wilson:  I'm using it selectively on people for whom I feel the computer may have the ability to add to the follow-up, such as patients with large canals through the back of the eye for the nerve to go through.  Such people would have large cups, since the amount of nerve tissue is the same whether the canal is small or large.  

Moderator:  Does the test hurt?

P:  No, it does not hurt.  Red lights flash in your eye for a few seconds.  It's fast and much easier than visual field tests.  

Moderator:  Are the flashes of red light bright?

Dr. Wilson:  Yes, but not as bright as a photo.

P:  Pretty bright, but not as bad as a flash from a regular camera.

Dr. Wilson:  Night all. Have a great week.

Moderator:   Good night.  Thank you.

End of highlights for August 16th chat.

On August 23rd, Dr. Jeff Henderer discussed "GDx Nerve Fiber Analyzer" in the Chat room. Click here for highlights of that meeting.

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