Chat Highlights
How Aqueous Humor Flows
May 23, 2001

Norma Devine, Editor

On Wednesday, May 23, 2001, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "How Aqueous Humor Flows." 

Moderator:  Welcome, Dr. Wilson.  Tonight we're discussing "How Aqueous Humor Flows."  

P:  To start with, could you please explain how the blood-aqueous barrier (BAB) works and the effect that increased BAB permeability may have on IOP?

Dr. Rick Wilson:  The blood flows through the small blood vessels in the ciliary body.  Those vessels are quite leaky.  The serum that leaks out is pushed mostly by hydrostatic pressure across two layers of cells into the posterior chamber of the eye. There are other osmotic and active transport mechanisms, but the hydrostatic pressure mechanism is the main one. The blood-aqueous barrier prevents most proteins and other macromolecules from getting into the eye.  If there is inflammation, the blood-aqueous barrier is lowered and serum proteins and white cells can get into the eye.

P:  Would you explain "hydrostatic pressure?" 

Dr. Rick Wilson:  Hydrostatic pressure just means the fluid pressure in one compartment is higher than in another. The pressure in the blood vessels in the ciliary body of the eye is higher than in the eye, so fluid has a tendency to filter through the pores in the wall of the blood vessel and move into the posterior chamber (behind the iris) of the eye.

P:  So, theoretically, would a substance that reduces the permeability of the blood-aqueous barrier (without unwanted side effects) reduce the formation of aqueous humor and lower IOP?  If so, would that be through a different mechanism of action than other drugs aimed at reducing aqueous production, such as beta-blockers and carbonic anhydrase inhibitors?

Dr. Rick Wilson:  If  you are asking whether a substance that made the blood-aqueous barrier less leaky (i.e., raised the barrier) would reduce IOP, the answer is yes.  It would be a different mechanism of action.

P:  What does osmosis mean?

Dr. Rick Wilson:  If two compartments, one filled with water and salt and the other just with water, are separated by a membrane permeable to only water but not by salt molecules, the compartment with the greater osmotic load (the salt) will pull water from the other side to try to equalize the osmotic load between the two compartments.

P:  Is there a way to increase the amount of aqueous?

Dr. Rick Wilson:  We have plenty of medications that reduce the making of aqueous, but no practical ones that increase the flow of fluid.

Moderator:  How does the eye maintain a balance between aqueous production and outflow?

Dr. Rick Wilson:  As the fluid pressure in the eye goes up, there is less difference in the pressure in the vessels in the ciliary body from that of the posterior chamber of the eye and therefore less hydrostatic pressure pushing fluid into the eye.  This lessens aqueous production but does not stop it.  Other than this main mechanism, the amount of inflow and the amount of outflow seem to be independently controlled so that if a blockage occurs in outflow, the amount of inflow is not regulated down.

P:  How do blebs created by a trabeculectomy work in this process?

Dr. Rick Wilson:  Fluid exits through a small hole in the white wall of the eye (the sclera) and comes out under the clear layer (the conjunctiva).  The aqueous (fluid) is then absorbed into blood vessels or lymphatic vessels.  In blebs that have been in place for some time and are quite thin, the fluid may filter through the conjunctiva into the tears.

In a normal eye, the fluid in the front part of the eye, the aqueous humor, is produced and exits the eye in such a way that it exerts enough pressure to keep the eye properly formed without damaging it. That is, the amount of new aqueous that is constantly being created by the eye is balanced by the amount that is constantly draining out of the eye at a place in the eye called the "angle."

The human eye, showing the main features of the front part (anterior chamber) of the eye and the optic nerve.	Normal open anterior chamber angle showing the flow of aqueous humor through the pupil and out the Canal of Schlemm.
Narrow anterior chamber angle, open but capable of closing.	Closed anterior chamber angle with flow blocked.

[Note:  Since participants asked few questions on the topic, some questions unrelated to the topic are being included.]     

P:  Could a lot of tears after a trabeculectomy mean a leak in the bleb?  

Dr. Rick Wilson:  Yes.

P:  Would a patient know a bleb is leaking by the tears?  

Dr. Rick Wilson:  If the leak were very slow, the patient would not notice.  But if there is a tear or hole in the bleb, then the amount of tears would be abnormally high and quite evident.

P:  Does the excess flow of tears cause eye irritation?

Dr. Rick Wilson:  No.  Patients with dry eyes have eyes that feel more comfortable.

P:  Is there pain associated with a tear or a hole in a bleb? 

Dr. Rick Wilson:  Usually not. 

P:  If crying produces no tears, is that a cause for concern? 

Dr. Rick Wilson:  It could be related to dry eyes or Sjogren's syndrome.

P:  What is Sjogren's syndrome? 

Dr. Rick Wilson:  It's a connective tissue disease with autoimmune components.  Patients have a dry mouth, dry eyes, and may have arthritis.

P:  Is Sjogren's a syndrome primarily seen in older people?

Dr. Rick Wilson:  No, its seen commonly seen in middle-aged women as well as the elderly again with a female predisposition.

P:  After I massage my eye, it is filled with tears.  The excess amount of tears does not last too long.

Dr. Rick Wilson:  The tears you mention are probably coming from the tear gland and not the bleb.

P:  Can a glaucoma specialist detect a leaking bleb without painting it?

Dr. Rick Wilson:  Only if he can see a hole in the bleb.

P:  How often should a doctor check the bleb for leaks?

Dr. Rick Wilson:  That depends upon the appearance of the bleb.  If it's thin without any blood vessels, I would check maybe every three months if no symptoms developed.  Otherwise, I would check much less often.

P:  Will massaging the eye really increase aqueous flow? 

Dr. Rick Wilson:  I feel it helps to keep the trabeculectomy patent.  Not everyone is as big a believer in massage as I am.

P:  If there are no tears after massage, does that indicate there was no flow?

Dr. Rick Wilson:  No.

P:  I have never heard of massaging the eye before.  Is that something all glaucoma patients should do?  

Dr. Rick Wilson:  Massaging an eye (I refer to it as "flushing the eye") means putting a firm, steady pressure on the cornea, toward the back of the head, forcing aqueous through the trabeculectomy hole under pressure to keep it open as wide as possible.  A patient might flush the eye for two, ten-second periods, five seconds apart, every hour or two.  

P:  When considering surgery in normal-tension glaucoma, does the finding that progression of damage in untreated eyes can be slow change the risk/benefit analysis?   

Dr. Rick Wilson:  Probably not, because it is impossible to tell ahead of time whether a particular patient's progression will be fast or slow.  Therefore, we err on the side of being more aggressive, rather than accept nerve damage. 

P:  Hypothetically, in an aphakic eye, could bits of vitreous (or vitreous floaters) travel through the pupil into the anterior chamber and get stuck in the angle? Enough to clinically raise IOP?

Dr. Rick Wilson:  Yes, but vitreous usually does not do that. 

Moderator:  How would you know if vitreous had leaked through to the anterior chamber?

Dr. Rick Wilson:   You can see it with a slit lamp and magnification.

P:  After I used the new medication, Lumigan, for three weeks, my IOP had not changed.  Now I am using Travatan and will have another pressure check in three weeks.  Is that enough time to determine if the drops are effective, and are they really different from Xalatan?  

Dr. Rick Wilson:  A maximum effect from those medications will be seen in four to six hours.   Therefore, waiting three weeks will not accomplish much, except to see if you  develop side effects from the medication.   There should be little difference in intraocular pressure between the effect of Xalatan, Lumigan, and Travatan -- perhaps 1 or 1 1/2  mm Hg, at most.

P:  Could astigmatism have anything to do with glaucoma?

Dr. Rick Wilson:  It can confuse the tonometry (measuring the pressure in the eye). 

Moderator:  How can astigmatism confuse the tonometry?

Dr. Rick Wilson:  Astigmatism means the cornea is not the same shape in both directions.  Imagine trying to remove a piece from the side of a football, rather than a baseball, which has the same curve in all directions.  If you measure the pressure in the football along the long axis of the ball, it is easier to indent the ball than if you try to indent the steeper curve. The same holds true for the cornea and eye.

Dr. Rick Wilson:  If there are no more questions, I will go see my wife, whom I haven't seen in three days.  Have a great week.  Thanks to all for their birthday wishes to me.

Illustrations Copyright 2003 Tim Peters and Company, Inc. Peapack NJ 07977 USA. All Rights Reserved. www.timpetersandcompany.com

End of chat highlights for May 23, 2001.

On June 4, Dr. Henderer surprised the Monday night support group with a Glaucoma Q and A session.  Click here for highlights of that meeting.

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