Testing Equipment
Chat Highlights
July 30, 2003

Norma Devine, Editor

On Wednesday, July 30, 2003, Dr. Elliot Werner, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Testing Equipment."

Moderator:  Dr. Werner, tonight we would like to discuss the instruments and tests used in the diagnosis and prognosis of glaucoma.  Are the newer instruments -- such as the GDx (scanning laser technology), HRT (Heidelberg retinal tomograph), OCT (optical coherence tomography), and SWAP (blue-on-yellow perimetry) -- better than the older ones?     

Dr. Elliot Werner:  The clinical diagnosis is still largely based on the clinical examination of the optic nerve by the standard techniques and the usual standard visual field testing.  The newer techniques have not been proven to be especially useful in establishing a diagnosis and prognosis.

P:  What is the slit lamp used for?  

Dr. Elliot Werner:  A slit lamp is basically a microscope that gives a magnified image of different structures of the eye.  It can be used to examine all parts of the eye, including the retina and optic nerve. Because of the magnification, the doctor can see many details of the structure of the eye and detect abnormalities.

P:  What diseases besides AIDS, breast cancer, and diabetes are manifested in the eyes? 

Dr. Elliot Werner:  Many diseases can have manifestations in the eyes.  A few others that come to mind are hypertension, thyroid disease, rheumatoid arthritis.  There's a whole list of others. 

P:  What is an ophthalmoscope and what is it used for?

Dr. Elliot Werner:  An ophthalmoscope is a device for examining the retina and optic nerve.  There are several different types of ophthalmoscopes that give images of different parts of the internal structures of the eye (called the fundus).  When certain special lenses are used, the slit lamp can function as an ophthalmoscope.

Moderator:  Which is the best method to examine the optic nerve?

Dr. Elliot Werner:  Most glaucoma docs would consider the slit lamp the best technique for examining the optic nerve to detect glaucoma.  The magnification, clarity, and quality of the image allow a stereoscopic evaluation of the optic nerve that cannot be obtained with any other instrument.

P:  How does dilating the pupils (mydriasis) improve the doctor's view of the eye with the imaging equipment?

Dr. Elliot Werner:  If you can imagine trying to see into a room through a keyhole versus looking though a wide open window, you get the idea of how dilation improves the view.  The bigger the opening into the eye, that is, the pupil, the easier it is to see into the eye. 

Moderator:  Of the several techniques available to measure intraocular pressure (IOP), such as puff tonometers, tonopens, and the Goldmann, which is the most reliable?  

Dr. Elliot Werner:  The standard technique, the most accurate and reliable, is the Goldmann applanation tonometer.  That is the little round thing with the blue light the doctor puts on the eye when he or she measures the pressure.  The Goldmann is the "gold standard."  All the other devices have certain uses in some situations, but none are as accurate or as reliable as the applanation tonometer on the slit lamp.

Moderator:  If a patient's IOP is higher when an air puff or Tonopen is used than when the applanation tonometer is used, should the patient be concerned?  

Dr. Elliot Werner:  The Tonopen is a portable device that takes less skill to use than the standard Goldmann applanation tonometer. The Tonopen's  portability and ease of use are its main advantages.  It is somewhat less accurate and reproducible than the standard tonometer.  If the Tonopen and the Goldmann measurements disagree, I would go with the Goldmann. 

P:  What is SWAP?  How often is it used and why?

Dr. Elliot Werner:  SWAP stands for Short Wavelength Automated Perimetry.  The visual field is tested on a standard perimeter, but instead of using a white spot on a white background, a blue spot is used on a yellow background.  There is some evidence that this type of visual field testing can detect abnormalities at an earlier stage.  The test, however, is much longer and more difficult than the standard test. 

P:  Why is SWAP more difficult? 

Dr. Elliot Werner:  Patients find the test more uncomfortable.  It's harder  to do, more tiring, and takes much longer.   

P:  Does the type of glaucoma influence the perimetry the doctor chooses?

Dr. Elliot Werner:  Not really.  More important is how advanced the glaucoma is.  SWAP is most useful in very early cases with normal or questionable standard visual fields.  Other techniques, such as frequency doubling perimetry, are most useful as screening tests.

P:  Are you aware of any home tonometers, either clinically available or in development, that have or will have any utility? The Proview, a Bausch & Lomb instrument, sounds good on paper, but I've heard that it's not very useful at higher pressures.

Dr. Elliot Werner:  The Proview works fairly well in patients with reasonably good visual function and a fairly high intelligence.  It doesn't work well in patients with advanced vision loss, who are not very adept, or who have problems with manual dexterity.  It is not particularly accurate at high pressure levels.

Moderator:  Are there differences in standard visual field testing machines?

Dr. Elliot Werner:  There are several different manufacturers, so depending on what brand of perimeter the doctor is using, the machine will look different and the test may seem different.  In the final analysis, all the standard perimeters do the same thing and provide the same information.  It's kind of like comparing different brands of autos.  They may look and feel different, but they all drive from here to there on four wheels.

P:  How often should a glaucoma patient have a visual field test?  My ophthalmologist has me take only one visual field test a year, but my cousin, who sees a different ophthalmologist, takes one every six months. 

Dr. Elliot Werner:  That depends on how advanced the glaucoma is and how high the IOP is.  In more advanced or poorly controlled patients, field tests two, three, or even four times a year may be needed.  In early, stable, well-controlled patients, once a year or even every eight months may be adequate.

P:  What is a GDx and HRT, and how do they differ?

Dr. Elliot Werner:  GDx is a device that measures the thickness of the nerve fiber layer of the retina. In glaucoma, the nerve fibers of the retina die and the layer gets thinner.  HRT stands for Heidelberg Retinal Tomograph.  It makes a 3-D map of the surface of the optic nerve to detect cupping.  Although the two machines measure different things, their diagnostic accuracy seems to be about the same, and it probably doesn't matter which is used.

P:  What do you think prevents GDX, HRT, or OCT from achieving the diagnostic significance their proponents claim they have?  Is it simply a matter of the software evolving, as my clinician keeps saying?  Or is there some fundamental flaw in the rationale for their use?

Dr. Elliot Werner:  The main problem is the high variability of the appearance of the optic nerve in the human population.  Every optic nerve is unique, like the fingerprints.  In fact, optic nerve photographs, like fingerprints, can be used to identify people. Because of the tremendous variability of optic nerve appearance in the population, and the wide range of normal values from the machines, it is difficult to define exactly the features that will consistently separate normal from glaucoma when using the tests.

P:  Is there any value in measuring the cornea of an ICE patient whose cornea is damaged?

Dr. Elliot Werner:  There probably is, because you can monitor the development of corneal decompensation by measuring the thickness at regular intervals.

P:  How is corneal thickness measured?

Dr. Elliot Werner:  A pachymeter is used to measure the corneal thickness.  Pachymetry is becoming a standard test in glaucoma, because patients with thick corneas are less likely to progress, while patients with thin corneas are more likely to progress.

P:  Do you always measure the cornea in a glaucoma patient?

Dr. Elliot Werner:  Generally, yes.  I have started doing that within the last year or so.  The problem is that the equipment is expensive and the test is not reimbursed by insurance, so the doctor has to absorb the cost.

P:  When insurance does eventually pay for pachymetry, my understanding is that it will be for only once in a lifetime.

Dr. Elliot Werner:  I would never even try to predict what an insurance company might do.

P:  My glaucoma specialist thinks the SLT is not living up to the hype.  Do you agree?

Dr. Elliot Werner:  My experience with SLT so far is that the results are about the same as they are for the argon laser (ALT).  Most other glaucoma docs seem to agree.  It probably makes little difference which is used.  If the hype is that SLT is much better than ALT, then it has not lived up to that.

P:  Do you find that patients put more stock in testing equipment than in the doctor?  Do patients sometimes pressure the doctor for tests because they've read about them?

Dr. Elliot Werner:  It is certainly possible, but I personally have not experienced that. Usually if I explain to patients what I am doing and why, and how I interpret the results, they rarely ask for more testing just for the heck of it.

P:  Can the HRT do better than a glaucoma specialist? 

Dr. Elliot Werner:  No.  Studies have shown that a well-trained glaucoma specialist looking at good-quality photographs of the optic nerve performs about as well as, if not slightly better than, the HRT.  The doctor also has the advantage of having ALL the clinical information to make a diagnosis, not just one optic nerve image. No good doctor would rely on one test alone.

P:  What is a goniolens and why is it used?

Dr. Elliot Werner:  Gonioscopy is test that allow the doctor to look at the angle of the eye.  The angle is located at the very edge of the cornea, where the white of the eye joins the iris (colored part) and the cornea.  The angle cannot be seen by looking directly at the eye.  A special lens with a mirror, the goniolens, is required to actually see the angle.  The goniolens has to be placed directly on the surface of the eye.  Although an anesthetic drop is used, the test is a little unpleasant.

P:  Can the cornea be damaged by gonioscopy?  Some doctors are so skillful using the goniolens, and some slam it like a manhole cover!  Dr. Tan at Wills performed that test on me today, and he's good. 

Dr. Elliot Werner:  As with any technical skill, some doctors are better than others at gonioscopy, just as some people play the piano better than others.  It is generally a pretty safe procedure.  Occasionally, a patient will get a small corneal abrasion.  I have never seen any serious or permanent damage form gonioscopy.

P:  What is OCT? 

Dr. Elliot Werner:  OCT (optical coherence tomography) is a device that measures the thickness and contour of the different layers of the retina.  Like the GDX, it measures the thickness of the nerve fiber layer.  Studies have not shown conclusively that any one of these techniques is significantly better than the others for diagnosing glaucoma.  

P:  What is fluorescein angiography (FA) and what is its role in diagnosis?

Dr. Elliot Werner:  That is a retina question and I am not an expert. FA is generally used to diagnose a variety of blood vessel diseases affecting the retina.

Dr. Elliot Werner:  The clock on the wall says 9:30, so goodnight Mrs. Calabash.

Moderator:  Good night, Dr. Werner, and thank you.

End of highlights for July 30, 2003.

On August 6, Dr. Wilson discussed "Current Concepts" in the Chat room. Click here for highlights of that meeting.

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