Advances in Glaucoma Testing
Chat Highlights
September 8, 2004

Norma Devine, Editor

On Wednesday, September 8, 2004, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Advances in Glaucoma Testing."

Moderator:  Tonight's topic is "Advances in Glaucoma Testing."

P:  Dr. Wilson, will you please start with the HRT (Heidelberg Retinal Tomograph)? 

Dr. Rick Wilson:  The HRT is an instrument that uses computerized imaging to map the surface of the optic nerve.  It is hoped that it will give an objective means of following the optic nerve, rather than the doctor observing it and comparing it to previous photographs.

P:  But is it really of much help to a glaucoma specialist? 

Dr. Rick Wilson:  For glaucoma specialists who are expert in looking at optic nerves, the HRT is a slight help.  It can give a more objective way of measuring the size of the optic nerve, IF the doctor himself or herself maps out the outline.  In most offices, a technician does the mapping, and the original numbers may be highly inaccurate.  Since the original outline is maintained from then on, the numbers may be meaningless, but the comparison may be accurate between exams.

P:  Is the optic disc area delineated by the technician at the time the images are made, or by the doctor evaluating the images afterward?  Should that be done by the same observer each time? I don't see how the test can be reliable if the disk isn't drawn by the same person each time.

Dr. Rick Wilson:  The optic disc area is outlined after the images are made by looking at the image on the computer monitor.  Because the images are not in color, it is hard to see the edge of the nerve. When the outline of the disc is made, the computer remembers it from then on and puts it in the same place, right or wrong.

P:  I'm baffled.  My HRT computer images were in color, and the technician altered the outline the second time around.  The new outline showed up on the printout.

Dr. Rick Wilson:  The image you see is not in natural color.  A new outline can be drawn at any time, but then none of the numbers mean anything for comparison.

P:  When clinicians speak of the need for the HRT software to evolve for it to have any real utility, does that necessarily refer to the correct placement of the disc outline, or are there other variables that need to be better evaluated? 

Dr. Rick Wilson:  We need the computer to be able to find the edge of the disc reliably.  We also need the accuracy of the scan to improve, as well as the repeatability (that is, getting the same measurements on two exams just minutes apart). 

P:  What advances have there been in the HRT?

Dr. Rick Wilson:  The advances are slow but sure.  If the test is repeated several times for a baseline, comparisons with repeated tests later may be as accurate as good examinations of the optic nerve.  

P:  What is the GDx nerve fiber analyzer?

Dr. Rick Wilson:  It is a machine that uses the polarization of light as it passes through the retinal nerve fiber layer (NFL) to measure the thickness of the layer.  Since glaucoma kills nerve cells and their fibers, the NFL thins as the disease progress.

P:  Have there been any improvements in the GDx?  

Dr. Rick Wilson:  There was a large improvement about two years ago.  The manufacturer was able to compensate for the polarization induced by the cornea and subtract it, leaving mainly the polarization induced by the NFL.

P:  Have there been improvements in OCT (Ocular Coherence Tomography)?

Dr. Rick Wilson:  We are using the OCT III,  which was only a slight improvement over the OCT II.  However, I have seen images from the prototype of OCT IV, and they were wonderfully detailed  -- a large improvement.  When OCT IV comes out, that will be THE machine.

Moderator:  What kind of imaging is OCT?  

Dr. Rick Wilson:  OCT uses the coherence of light to give 3D images of tissue.  The increased frequency of the light provides finer details.  

P:  What single diagnostic tool for glaucoma available today would you most have liked to have had when you first became a glaucoma specialist?

Dr. Rick Wilson:  Corneal pachymetry.  Corneal pachymeters measure the thickness of the cornea.  I would also liked to have known that the central corneal thickness (CCT) can have such a significant effect on the IOP (intraocular pressure) measurement.  

P:  Dr. Wilson, for two-and-a-half years, I have been treated for normal-tension glaucoma.  My intraocular pressures range between 17 and 22 mm Hg (with and without drops); I have a notch in the rim of the optic nerve, and a scotoma in one eye only.  My visual fields have shown no progression.  Because of several dermatological problems and sensitivities, I can't seem to tolerate the eyedrops.  

Today, my ophthalmologist recommended laser surgery.  My central corneal thickness has not been measured, and I think that should come first.  If I have normal-tension glaucoma, I would like to be monitored without treatment.  Can you comment, please?  

Dr. Rick Wilson:  It's hard to comment without seeing you.  I agree that corneal thickness measurements should come first.  I would also try non-preserved timolol from Merck, if you can take beta-blockers.  Many people who are allergic to many of the eyedrops are allergic to the preservatives in the drops and not the actual medicine. The risk-benefit of the laser would depend upon your age and what the pigment is like in your trabecular meshwork.

P:  Two years ago, during a chat session here, one of your colleagues mentioned a developing form of visual field testing that uses electrodes to directly register brain activity.  He called it "multifocal visual evoked response."  Is that test available anywhere?  If so, how good is it?  

Dr. Rick Wilson:  I heard a lecture about that by Ivan Goldberg from Australia.  It holds promise because it would be wonderful to have a machine that measured vision objectively without the vagaries of patient input.  The problem with the objective visual field is that the distance between the visual cortex that perceives the light stimulus and the electrode on the outside of the skull varies, so there are two variables.  One is the amount of light the patient perceives; the other is the distance from the cortex to the electrode.  

Moderator:  Are there any other advances in visual field testing?

Dr. Rick Wilson:  SWAP (short wavelength automated perimetry), the blue-on-yellow visual field test, has been around for some years now.  It is the most sensitive of the visual field tests for detecting new glaucoma.  Because it is so sensitive, there is more "noise," so it is harder to interpret the results.   

P:  The importance of the central corneal thickness (CCT) measurement can be considered an outcome of the OHTS (Ocular Hypertension Treatment Study).  Are there any other diagnostic tests that have become more valuable because of clinical studies?

Dr. Rick Wilson:  We have learned more about the reliability of white-on-white visual field tests.  In the CIGTS (Collaborative Initial Glaucoma Treatment Study) and AGIS (Advanced Glaucoma Treatment Study), we found that when a visual field looked significantly worse on one test, if it was rechecked two more times, only one time out of three would there be real progression.  Therefore, if I see a visual field that appears to have become worse, I repeat it before acting on the belief that the patient's vision is worse.

P:  In what way is automated perimetry more useful in diagnosing glaucoma than it was 10 or 12 years ago?

Dr. Rick Wilson:  The statistical packages that help us determine if the visual field is abnormal or has progressed are more sophisticated and can be trusted more.  They give a statistical probability of whether each point on the visual field is worse.

Moderator:  Thank you for being here tonight, Dr. Wilson.  

Dr. Rick Wilson:  You're welcome.  Tomorrow I'm going to Columbia to speak at a symposium, and from there to Shenyang, China, for a week, as a visiting professor.  

Moderator:  Have a safe trip. 

End of highlights for September 8, 2004.

On September 22, 2004, Dr. Wilson discussed "Glaucoma Medications" in the Chat room. Click here for highlights of that meeting.

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