The Optic Disk
Chat Highlights
December 19, 2007

On Wednesday, December 19, 2007, Dr. Michael James Pro, a glaucoma specialist at Wills, and the glaucoma chat group discussed "The Optic Disk".

Moderator:  Tonight's topic is “The Optic Disc.” Dr. Pro, do you have some opening remarks, or do you want to start with questions?

Dr. Pro:  Let's start with questions.

P:  Is the optic disc the same as the optic nerve?

Dr. Pro:  Well, the disc really refers to the last millimeter of the nerve that is visible on examination. The nerve could include the entire length to the brain.

P:  How do you examine the optic disc?

Dr. Pro:  First, a little history. The optic nerve was not seen until the middle of the 19th century, when the first ophthalmoscopes allowed the development of modern ophthalmology. For a long time, the disc was examined with a hand-held device (the direct ophthalmoscope). Even today, many specialists, such as pediatricians, neurologists, and others, view the nerve that way. Most ophthalmologists and optometrists, however, use the slit lamp, which allows a stereo view.

P:  What do you look for when you examine the optic disc?

Dr. Pro:  As a glaucoma specialist, I am looking for various details. First, what is the general size of the nerve? Is it larger than average or smaller? Are disc hemorrhages present? They can indicate glaucomatous nerve damage. Is the rim of the nerve intact? That can be tough to explain to patients.

I like to compare the nerve to a tire on a rim. The neural tissue is like the tire; it is on the outside. It should be symmetrical and even, like a tire. It should be thick and intact. But if the neural tissue is thin in one spot, that can indicate glaucoma. Then the empty space in the middle looks larger. (That is what we call "cupping".)

I also look at the color of the nerve tissue and the retina around the nerve. The nerve tissue should be pink; if it is pale, there could be a neurological problem. If the retina around the nerve is thin or damaged, that can also indicate glaucoma.

P:  If a patient has a large disc, how is it determined whether it is glaucomatous or congenital?

Dr. Pro:  Some nerves are easy to categorize. You look at them and really have a good idea that they are glaucomatous. But large nerves can be tough. Remember the wheel and tire analogy? Well, if the wheel is really big, then the tire may look small. So a big nerve seems to have less pink nerve tissue, but it may still be normal. That is where comparing photos of the optic disc from year to year can help, and other tests, such as visual fields, are useful.

P:  If six highly qualified, well-trained ophthalmologists, using the same instrument, examined the same patient's optic nerve for glaucomatous damage, would you expect agreement among them?

Dr. Pro:  I would expect pretty good agreement. It gets a lot harder for "funny-shaped" nerves. We see those in people who are high myopes (near-sighted) or have large nerves, such as I described above. That's why confirming the diagnosis often requires demonstrating some change in the nerve over time, or a visual field defect, or both.

P:  When I have a GDX, is that a picture of my optic disc?

Dr. Pro:  The GDx is a trade name. The device is actually a scanning laser polarimeter. It measures polarization of the retinal nerve fiber layer, the thin layer of nerve tissue that spreads out over the retina after exiting the nerve head (disc). It measures whether the nerve layer is thin. Patterns of nerve- layer thinning are seen in glaucoma.

[Editor’s note: For a discussion with Dr. Pro of Heidelberg Retinal Tomograph (HRT), Ocular Coherence Tomography (OCT), and scanning laser polarimetry (GDx), see the Chat Highlights, “Examining the Optic Nerve; Man vs. Machines. June 27, 2007. http://www.willsglaucoma.org/supportgroup/20070620.htm ]

P:  Which of the new machines -- HRT, OCT, GDx -- do you find most useful in examining the optic rim? My glaucoma specialist says the state of the software doesn't yet allow HRT to be of much clinical relevance.

Dr. Pro:  Of the three, the HRT has the most data about looking at the state of the rim (neural tissue). The software is continually evolving. But the real problem is that sometimes the scans are not good quality, which makes the interpretation difficult.

P:  Are disc photos still being taken? I have not had any lately.

Dr. Pro:  Less often than before, because the new machines are being used to image the nerve. I think the photos are still excellent for documentation. Because they are not at the mercy of constant software changes, it is possible to compare a patient's disc photos from 1991 to the present time.

P:  What is “cup-to-disc ratio” and how important is it?

Dr. Pro:  Cup-to-disc (C/D) refers to the ratio of the "empty space" in the middle of the nerve to the neural tissue, or rim, on the outside. Maybe the wheel-tire analogy is useful for some. How useful? Well, giving an arbitrary value doesn't really describe all those parameters that I mentioned above. You could have a cup/disc ratio of 0.7 that is completely normal in one patient, or have the same ratio in another patient and have advanced glaucoma. So it is not really that useful.

P:  Does the optic disc have any function other than simply being the end of the optic nerve?

Dr. Pro:  It is the end of the nerve, and it is the part of the nerve specifically damaged in glaucoma. So, for me, it is the most important part!

P:  Are discs larger at birth or smaller? Do they grow?

Dr. Pro:  The entire eye grows from birth. If you are referring to the optic disc, I think it also grows a bit.

P:  What do you mean that the disc is the part that is specifically damaged in glaucoma? I always thought it was the fibers of the optic nerve that are destroyed, not just the end.

Dr. Pro:  True, but some kind of structural damage also occurs right at the disc. You see, there are other diseases that affect the nerve fibers and even cause thinning, such as optic neuropathy. But for some reason, these other processes do not lead to the "cupping" seen in glaucoma. For that reason, some researchers have proposed a localized (in the area of the disc) vascular disorder and a structural abnormality, or both.

P:  Can illegal drugs like cocaine, heroin, and methadone damage the optic nerve and vision of glaucoma patients?

Dr. Pro:  Cocaine could lead to vaso-constriction. That could reduce the blood flow to the nerve. In a glaucoma patient, reduced blood flow could theoretically cause worsening of the damage to the nerve. I would think methadone could have a similar effect.

P:  I'm acute myopic and apparently have a notch. Does a notch always appear on the outside of the optic rim or can it appear on the inside where cupping has taken place?

Dr. Pro:  The notch goes from the inside toward the outside. The notch can correlate to defects on the visual field.

Moderator:  Dr. Pro, thank you for being with us for this last chat of 2007. Happy holidays from all of us.

Dr. Pro:  Happy holidays to all and to all a good night.

On January 2, Dr. Wilson discussed "Glaucomatocyclitic Crisis and Other Inflammatory Glaucomas" in the Chat room. Click here for highlights of that meeting.

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