The Optic Nerve in Glaucoma
Chat Highlights
January 18, 2006

On Wednesday, January 18, 2006, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "The Optic Nerve in Glaucoma."

Moderator:  Dr. Wilson, many patients are confused about the cup-to-disc ratio.  Will you please explain that?   

Dr. Rick Wilson:  The cup is the depression in the middle of the face of the optic nerve inside the eye. The disc is the face of the entire optic nerve. The ratio is the diameter of the cup divided by the diameter of the whole disc.  As glaucoma damages the optic nerve, the dead fibers wither away, increasing the size and depth of the cup, increasing the cup-to-disc ratio.

Moderator:  What's the best way to measure the cup-to-disc ratio?  By observation or by measurement with HRT (Heidelberg Retinal Tomography) or some other instrument?  

Dr. Rick Wilson:    The HRT or OCT (Ocular Coherence Tomography) is probably more accurate, but estimating the ratio is also common. 

P:  What does it mean when a doctor tells a patient, "You have lost optic nerves?"  

Dr. Rick Wilson:    The canal that the optic nerve passed through the sclera at the back of the eye remains, but the nerve fibers shrivel up and disappear, leaving the support structures remaining.

Moderator:  Does that mean the optic nerve dies from the center out, hence the "cupping?"

Dr. Rick Wilson:    No, there is a generalized loss of optic nerve fibers from all over the retina, which causes a generalized enlargement of the cup.  There usually is a preferential loss of nerve fibers at the vertical poles of the optic nerve, resulting in notches at the 6 o'clock and 12 o'clock positions in the remaining nerve tissue.

P:  What does "atrophy" of the optic nerve mean? 

Dr. Rick Wilson:    Atrophy is the loss of tissue due to injury or death.

P:  Would you please describe what a "notch" looks like?

Dr. Rick Wilson:    It is like a doughnut with a small bite out of the inside of the ring.

P:  What is the relevance of the color of the optic nerve? 

Dr. Rick Wilson:    In typical glaucoma, the remaining optic nerve maintains a healthy salmon color.  If there is loss of blood supply or compression on the nerve by a tumor, for instance, then the optic nerve is pale. 

P:  How can you tell the difference between damage to the optic nerve caused by glaucoma and some other cause?    

Dr. Rick Wilson:    By the appearance of the optic nerve, as we discussed earlier.  Central vision is maintained, as is red-green color vision. The visual field is characteristic for glaucoma. With neurologic disease, the central vision may be affected, red-green vision is often affected early, and the visual field will not be characteristically glaucomatous.

P:  Does a patient's visual field test indicate the part of the optic nerve that has died?

Dr. Rick Wilson:    Yes.

P:  Is pallor (paleness) of the optic nerve head permanent in some instances, but changes to a normal pink in others? For example, in an ischemic event, doesn't the pallor fade away?  If so, how long does that take?

Dr. Rick Wilson:    The pallor usually is long-lasting after an ischemic (loss of blood supply) event.

P:  Is information about the cup-to-disc ratio usually given to patients?  My cup-to-disc ratio has not been mentioned during my appointments.

Dr. Rick Wilson:    The doctor usually talks about the extent of the damage.  Information about the visual field is more appropriate for patient education.  Normal cups can be almost non-existent, or may be large: 0.85 to 0.9, which my son was born with.  It is, therefore, difficult to show a drawing of a disc without knowing what it looked like normally, and really know where you stand.

P:  Does .7 OD (right) and .5 OS (left) indicate a person is at risk for developing glaucoma? 

Dr. Rick Wilson:    As I mentioned before, numbers like that don't mean too much unless we know what is normal for the patient. Certainly, the larger the cup,  the greater the chance that it is enlarged pathologically.  It could, however, be normal for that person, as it is for my son. 

P:  What symptoms might a patient with optic nerve damage notice? 

Dr. Rick Wilson:    Usually, the damage in open-angle glaucoma progresses very slowly, and remains unnoticed until the disease is quite advanced.

P:  Is there any way to tell clinically, either during a visual examination or by using HRT or OCT, whether the cupping and pallor are the result of a reversible dysfunctional state or irreversible cell death?

Dr. Rick Wilson:    Reversal of cupping is less and less evident the older the patient is, and may not mean much in terms of how well the nerve is working. The critical factor is how well the ganglion cells are working, which is not something we can see. 

P:  Are there guidelines for eye doctors to take stereo photos of patients' optic nerves?  If so, how often should those photos be taken for comparison?

Dr. Rick Wilson:    Photos should be taken soon after diagnosis as a glaucoma suspect or patient, and at least whenever there is a noticeable change in the optic nerve from the original photo.

P:  My ophthalmologist has stopped photographing optic nerves at his office, because the technicians had trouble getting decent images.  I located a photographer who specialized in that kind of thing and had photos taken.  It seems to me that periodic photographing of the optic nerves would be critical for charting progression.  How difficult is it, really, to get clear images of the optic nerve?  

Dr. Rick Wilson:    Most technicians could not get the excellent images necessary to show subtle changes.  At places such as Wills, we use specially trained medical photographers.

P:  Does asymmetry of cupping make someone a glaucoma suspect? 

Dr. Rick Wilson:    Yes.  Most people's cups are within 0.1 of each other.  A greater disparity makes people glaucoma suspects.  

P:  I have a tumor in my skull and am a glaucoma suspect.  Could it be determined whether the tumor is obstructing blood flow to the optic nerve or whether the obstruction is due to some other cause? 

Dr. Rick Wilson:    All I can say without examining you is that it can usually be determine whether there has been a substantial loss of blood supply to the optic nerve as it enters the eye or, over time, if there is compression on the optic nerve, injuring it.

P:    I have megalopapillae (an abnormally large optic nerve) and high intraocular pressure.  How can that affect my treatment?  

Dr. Rick Wilson:     Because your optic nerve looks abnormal in its normal state, it will be harder to detect early change and evaluate how much damage there is, compared to if you had a normal nerve.

[Editor's Note:  "Megalopapillae has led to more misdiagnoses of glaucoma than probably any other anomaly." (Handbook of Ocular Disease Management.)]

P:  Does the death of part of the optic nerve affect the longevity of the remaining live part?  

Dr. Rick Wilson:    Great question.  The answer is unknown, but most of us believe it does, especially if there has been significant injury.

Moderator:  It's said that 35 to 40% of the optic nerve is lost before the damage shows up on a visual field test.  Is the subsequent progression linear? 

Dr. Rick Wilson:    The damage to the ganglion cells may be linear, but will look more logarithmic.  That is, the same increment of ganglion cell loss will show less visual field change early, rather than later, in the disease. 

P:  What is a peripapillary halo?

Dr. Rick Wilson:    That is a loss of the retinal pigment epithelial (pigmented) layer of the retina surrounding the optic nerve.  That results in the appearance of a lighter colored ring of tissue around the optic nerve.

P:  I am from Argentina,  I'm 21-years old, and my IOP is 15 to 20 mm Hg.  Is that in the normal range? 

Dr. Rick Wilson:    The average IOP is 16 mm Hg, so 15 to 20 mm Hg is in the normal range.

P:  If there are signs of optic nerve damage with my pressures of 15 to 20 mm Hg, should glaucoma medication be considered? 

Dr. Rick Wilson:    If you have progression of optic nerve damage, or a definitely damaged nerve, then drops would be considered.

P:  If, at my age of 21 years, my pressures are 15 to 20 mm Hg, will my pressures increase without medication as I get older? 

Dr. Rick Wilson:  First you need to have the thickness of your corneas measured to be sure the reading of 20 mm Hg is accurate and doesn't need to be adjusted because the cornea is thicker than normal.   

P:  How is progression determined?  

Dr. Rick Wilson:    Any visible change of the optic nerve or visual field is progression and needs to be aggressively treated.  Good night all. Have a good week.

Moderator:  Thank you, Dr. Wilson.  As always, your generosity in sharing time and knowledge is greatly appreciated.

On January 25, Dr. Henderer discussed "Overcoming Treatment Challenges in Glaucoma" in the Chat room. Click here for highlights of that meeting.

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