Chat Highlights
What is Glaucoma?
February 21, 2001

Norma Devine, Editor

On Wednesday, February 21, 2001, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "What is Glaucoma?" 

Moderator:  Doctor Rick,  how can we explain to others what glaucoma is?  

Dr. Wilson:  Glaucoma means a collection of diseases that damage the nerve in a characteristic way, causing characteristic visual field defects.  Eye pressure plays a role in most of  the glaucomas. 

Moderator:  What is the most common type?

Dr. Wilson:  In the garden variety of glaucoma, the intraocular pressure (IOP) is too high for the health of the optic nerve, damaging it.  There seem to be a variety of vascular, autoimmune, and other problems that can lead to what we would consider glaucoma damage.   If we could eliminate the causes of glaucoma one at a time, it would be wonderful.

P:  If the optic nerve isn't showing damage, then a diagnosis of glaucoma isn't made? 

Dr. Wilson:  Usually not, unless the visual fields are very characteristic. Almost all the time, the appearance of the optic nerve matches the (appearance of the) visual field.

P:  In normal-tension glaucoma (NTG), is there the characteristic cupping of the optic nerves?

Dr. Wilson:  Yes, glaucomatous cupping of the optic nerve is characteristic.

Moderator:  How does the angle of the eye play a role in the diagnosis?  

Dr. Wilson:  The two main kinds of glaucoma are open angle and closed angle. Looking into the angle of the eye will usually reveal the difference.

P:  I have difficulty understanding "cupping."  Is there a rating?  What is considered high?  I hear the numbers read out, but I don't understand whether they are okay or not.  

Dr. Wilson:  The cupping of the optic nerve means the size of the depression in the middle of the nerve when viewed from the front of the eye.  Normally, the ratio between the size of the cup and the size of the whole nerve or optic disc (cup-to-disc ratio) is 0.4 to 0.6.,  which is the ratio of the cup to the whole nerve. When there is damage to the optic nerve, the cupping increases. Therefore, 0.1 or 0.3 might be normal, whereas 0.7 might mean significant damage. That is why optic nerve photos are so important. If you have a 0.4 cup now, it might seem normal unless it is known that you started with a 0.1 cup-to-disc ratio.

P:  What does open angle mean?  

Dr. Wilson:  Open angle means the iris is not blocking the trabecular meshwork,  or the outflow channel of the eye.  Something else must be making the IOP go up.

P:  If  there is a bleeder in the eye and it causes the dead cells to block something, can that be corrected?  I'm on Neptazane, Cosopt, Alphagan and Xalatan, but my  pressure spiked to 42 from the low teens one evening.  It hurt and my vision was blurred.  The doctor tried adding two drops of Timoptic, and Diamox, and had me drink some bad-tasting stuff.  The pressure only went down to 38 and he's concerned.  

Dr. Wilson:  Yes,  the blood cells or dead cells can be washed out of the eye if they are not absorbed.   I, too, would be concerned.  Your doctor should be able to tell whether it is the iris, inflammation or something else that is blocking the outflow passages.

P:  How long does a pressure spike last?  I have pigmentary glaucoma.  

Dr. Wilson:  My guess is that a spike usually lasts for 3 to 12 hours.  

P:  Studies by Zeimer and others using ambulatory monitoring have shown that people who have large variations in IOP, not seen during office visits when  pressure is normal, have a higher risk of nerve damage.  Could this be one explanation for normal-tension glaucoma or do you believe that these patients' pressure is really always normal?

Dr. Wilson:  I was taught that most "normal-tension glaucoma" patients really have a pressure spike that nobody is catching.  We now know that about one out of six patients with open-angle glaucoma have normal-tension glaucoma. They often have a problem with circulation or the regulation of circulation to the optic nerve.

P:  Is there a way to determine whether a circulation problem, and not necessarily elevated IOP,  is causing optic nerve damage?

Dr. Wilson:  It is difficult. One way is to blow expanding CO2 or NO2 over the fingernail  beds to see if they show spasms of the small vessels under the nails.

P:  When would such a test be indicated?

Dr. Wilson:  Usually in low-tension glaucoma, to see if a calcium channel blocker, which limits vasospasm, is needed. 

P:  How is corneal thickness measured?

Dr. Wilson:  The cornea is measured with an A-scan ultrasound.

P:  There is one theory that the thickness of the cornea affects the measurement of IOP.  If that is true, does a normal exam take that into consideration?

Dr. Wilson:   If the IOP is high but the nerve seems O.K., then the cornea should be measured.  If it is too thick, then falsely high readings may be given.  On the other hand, if the patient is getting worse with what seem to be normal IOPs, then the cornea should be measured to see if it is too thin,  giving falsely low readings.

P:  If glaucoma is a group of diseases, why aren't patients treated for the specific disease?

Dr. Wilson:  Mainly because we are not very good at telling what the etiology or cause of the glaucoma is. 

P:   My father is supposed to decide whether to have the YAG laser procedure or cryotherapy.  He was told that there is a small risk of a sympathetic drop of  IOP in the untreated eye.   Is the risk greater in patients who have aniridia?  

Dr. Wilson:  No, there is no reason to think so. 

P:  My 16-year-old daughter has ectopia lentis, which caused closed-angle glaucoma.  She is  scheduled for lensectomy.  What is the prognosis for such a young person?

Dr. Wilson:  With the improving shunts and medication, I think that the glaucoma should be able to be controlled well. Tonight the residents presented the cases of  two Amish boys with a similar situation who had to have big surgery on their eyes and are doing superbly now.

P:  How long does a pressure spike last and what can I do to keep spikes to a minimum?  I have pigmentary glaucoma.  

Dr. Wilson:  I use pilocarpine Ocuserts to prevent pressure spikes in patients with pigmentary glaucoma.  If tolerated by the patient, they work very well.

P:  Why are some over-the-counter medications, such as sleeping pills, okay for patients with open-angle glaucoma to use, but not for those with closed angle.  

Dr. Wilson:  Those medications can dilate the pupil, which makes the angle more crowded with iris and more likely to close.  Usually, if you have narrow angles, you have had something done about them or have been warned about them.  If you have open-angle glaucoma, there is no risk with cold pills.

P:  What about dryness of the cornea? If it's been marginally deprived of Vitamin A and its metabolites, is there a possibility of a higher IOP reading without clear evidence of xeropthalmia?

Dr. Wilson:  If the cornea is thickened because of swelling, it softens.  The pressures measured are artificially low because the tonometer tip can push in the corneal surface more easily.

P:  How many mmHg change would typically result from a swollen or dry cornea?

Dr. Wilson:  One to three.  

P:  If the cornea is swollen because of Fuchs' dystrophy (corneal disorder) could that be an explanation for a low IOP of six and fuzzy vision five months after a trabeculectomy?  

Dr. Wilson:  Yes, although your IOP could be six after a trab.  I would bet the IOP is low because of the trab and that is causing the poorer vision.

On February 28, Dr. Katz discussed "Current Research" in the Chat room. Click here for highlights of that meeting.

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