Examining the Optic Nerve, Man vs. Machine
Chat Highlights
June 20, 2007

On Wednesday, June 20, 2007, Dr. Michael James Pro, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Examining the Optic Nerve, Man vs. Machine."

Moderator:  Welcome, Dr. Pro.  The topic tonight is, “Examining the Optic Nerve: Man VS Machine. ” Do you want to start with questions, or do you have a few comments first?

Dr. Pro:  Let's start with the questions.

Moderator:  What machines are used to visualize the optic nerve?

Dr. Pro:  The oldest is the stereo optic disc photo.  For the last 30 plus years, that has been the gold standard.  Newer imaging modes include the Heidelburg Retinal Tomograph (HRT), Ocular Coherence Tomograph (OCT), and scanning laser polarimetry (GDx).

P:    Which is more reliable for early detection of glaucoma, the experience of a good glaucoma specialist examining the nerve or one of the diagnostic machines?

Dr. Pro:  Great question.  Of course, every test has limitations due to several factors, such as quality of the image. That is often a problem in patients with cataracts, small pupils, or hazy corneas.  Further, the images themselves can’t diagnose glaucoma.  They are only useful pieces of the puzzle, along with visual field testing, family and medical history, and measuring intraocular pressure.

The newer imagining techniques can be useful for detecting early glaucoma.  For instance, the OCT and GDx can detect defects in the retinal nerve fiber layer that are present before damage to the optic nerve (cupping) is apparent.  The HRT also has been shown to correlate well with glaucomatous disc damage in glaucoma suspects.

P:  A few years ago, HRT, for example, was considered useful for tracking progression, but less so for diagnosis. Is that still the case?

Dr. Pro:  HRT is the one machine that has progression software, so changes in the optic nerve over time can be traced.  As to detection, no one machine has proved to be better than the others.

P:  Do you mean that no one machine has proved to be better than the others AND doctors for detecting glaucoma?  Or just better than the other machines?

Dr. Pro:  For detecting glaucoma, no machine is better than the doctor.  Again, that is because the machine is only programmed to record.  The machine interprets the test, based on software of groups of normal patients.  That has some potential flaws.  One flaw is the test quality, as I mentioned.  Another is, who are the "normals" to whom you are being compared?  In the early HRT databases, these "normals" were generally a small group of people of European descent.  So if your nerve was larger or smaller than average, or if your refractive error was smaller or larger, then what was the quality of the comparison?

Finally, the test is useful if there is a certain suspicion for glaucoma.  The machine can never know that a person has a thin cornea, early visual field abnormality, or a strong family history of glaucoma.  That person’s optic nerve can look the same as the optic nerve of a person with normal corneal thickness, normal visual field, and negative family history.  Only the doctor, ultimately, can determine which patient has glaucoma.

P:  Is a baseline image important to these new machines?  Or can they detect defects immediately?

Dr. Pro:  A baseline image is always useful, but without the progression software, it can be difficult to truly know if a follow-up test is worse.

P:  How costly are these machines to have in a primary eye-care office and do most have them?

Dr. Pro:  These machines cost $35,000 and up, so it is expensive for a primary care office to have more than one type of machine.  It generally pays to buy one machine, though, because of test reimbursement.

P:  When you say a primary care office might find it cost effective to have one of these machines, do you mean a general medical practitioner’s office or an ophthalmologist’s office?

Dr. Pro:  An ophthalmologist’s or optometrist’s office.

P:  How operator-dependent are these new machines?

Dr. Pro:  They’re still pretty operator-dependent.  "Garbage in, garbage out" still applies.  With practice, however, most technicians learn to get a good image.

P:  When checking for progression, my doctor makes use of his original notes, HRT results, visual field tests, and a visual examination of the optic nerve.  In looking at the results of my visual field tests, he tells me that he only cares about the absolute numbers.  That is, the change that records the actual decibels at each point.  He says that the other things on the printout -- the gray scale diagram, charts of the relative values for MD (mean deviation), PSD (pattern standard deviation), etc. -- are just pointers for where to look. Is that generally true for interpreting visual field tests?

Dr. Pro:  That's a good point. It’s the absolute numbers that the machine then interprets.  The MD and PSD are useful tools, though, because they can help point out a trend.  I think that looking only at the raw data risks "losing the forest for the trees".

P:  I had a retina detachment that was re-attached. Does that affect the optic nerve or viewing it? Now I have glaucoma in that eye, caused by damage to the drain from another operation.

Dr. Pro:  Retinal disease can affect the imaging in several ways.  First, if the patient's vision is poor, he or she may have difficulty maintaining fixation on the viewing target.  Then the image is off-center or blurred.  Also, if there has been recent surgery or continuing retinal problems, the posterior chamber may be cloudy with blood or debris.  Finally, retinal disease can complicate the data.  A recent article demonstrated that laser treatment for diabetic retinopathy causes decreases in the retinal nerve fiber layer, independent of glaucoma.

P:  Would an OCT III examination be a good way to detect subclinical hypotony maculopathy after trabeculectomy?

Dr. Pro:  Of the tests I mentioned -- HRT, GDx, disc photos, and OCT -- only OCT can look at the macula.  It basically takes a cross-section of the retina, like a CT scan, and can show swelling or folds.

P:  Are stereo photos of the optic nerve still taken?

Dr. Pro:  Yes, and they remain the single best way to image the nerve.  They have a few advantages:  They never become outmoded by a software upgrade, and eye doctors are familiar with looking at them.

P:  I had a photograph of the optic nerve during which I looked at a central target against a red background and swirling lights.  What is the name of that machine, and does it photograph the retina?

Dr. Pro:  That sounds like the OCT.  If so, then, yes, and it also takes an image of the retina.

P:  Can Doppler imaging (or anything else) show blockage or constriction of ciliary arteries?  If so, can anything be done about the blockage?

Dr. Pro:  That is still an area of debate.  The ciliary arteries are hard to scan, because they are so small.  The test seems to be very operator-dependent.  But studies have shown that glaucoma patients (especially normal-tension glaucoma) have abnormal blood flow.  Then what?

Well, there is really no proven treatment.  Some clinicians recommend ginkgo biloba extract to improve blood flow, but it can cause bleeding and other problems.  Some have recommended calcium channel blockers, but this systemic medication has serious potential side effects.  If I can make one recommendation it is this:  Good diet, regular exercise, no smoking, moderation in drinking, and less coffee and caffeine.

P:  As a glaucoma patient, I follow all of the above recommendations.  However, I am still having slow, but continuing, loss of vision.  At what point should I consider surgery?

Dr. Pro:  Some of that depends on your level of pressure control and whether your ophthalmologist feels that you would benefit from a lower pressure.  It is important to know if your visual fields are stable.  Sometimes the loss of vision is from a non-glaucoma cause, such as macular degeneration or cataract.

P:  Is there a standard for how often these imaging tests should be done?

Dr. Pro:  There’s no standard.  Many glaucoma specialists use the OCT to help detect early glaucoma, and follow with yearly, or every other year, HRT on most other glaucoma patients.

P:  Is Bright Light Therapy for SAD (seasonal affective disorder) not recommended for someone with controlled IOP (glaucoma suspect)?

Dr. Pro:  I can't think of why that would pose a problem for a glaucoma patient.

Moderator:  Thank you, Dr. Pro.  You are very kind and very patient to join us patients and share your knowledge.  There were some tough questions tonight.

Dr. Pro:  Yes, there were.  But it was fun, wasn’t it?

Moderator:  Definitely. And valuable.

Dr. Pro:  Goodnight, everyone.

On July 11, Mark Mosterdiscussed "Neurologic Disorders that Can be Mistaken for Glaucoma" in the Chat room. Click here for highlights of that meeting.

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