Hypotony
Chat Highlights
October 18, 2006

On Wednesday, October 18, 2006, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Hypotony."

Moderator:    Welcome back, Dr. Rick. Tonight the topic is hypotony, which is too low an intraocular pressure (IOP).  We’re ready to begin if you are.

Dr. Rick Wilson:    Okay. Hypotony can be caused by too much fluid escaping through a man-made drain (trabeculectomy or shunt) or by too little input from an abnormally functioning ciliary body.  The ciliary body is the part of the eye that makes the watery fluid (aqueous) that fills the eye.

The sclera is the thick white coat that forms the wall of the eye.  The sclera is somewhat elastic in young individuals and loses that property with age.  The inside two layers of the eye are not elastic.  Therefore, if the IOP is too low, the sclera will contract, but the two layers inside of it cannot.  They will therefore be thrown into tiny folds.  The retina is the innermost layer of the eye. Obviously, if it is in tiny folds, vision will be distorted and blurred.

P:    Besides surgery, what can cause hypotony?

Dr. Rick Wilson:  Conditions besides surgery that often cause hypotony are (1) advanced diabetes, a situation in which the ciliary body circulation is harmed by the diabetes and does not produce a normal amount of fluid; (2) a retinal detachment, (3) a long history of uveitis (intraocular inflammation), which also harms the ciliary body.

P:  Does hypotony occur often after a trabeculectomy?

Dr. Rick Wilson:  Hypotony is an unusual complication if the procedure is done properly.  It can be seen right away or develop over a few weeks to a month.  It can be temporary, but is much more common with the use of mitomycin.

P:  Why is that?

Dr. Rick Wilson:  Mitomycin reduces the body's healing ability, which usually causes the IOP to rise with normal scarring.  That makes it less likely the IOP will correct itself.

P:  Is there any special care that should be taken after a trabeculectomy to avoid hypotony?

Dr. Rick Wilson:  The surgeon needs to be conservative with the amount of fluid leaking through the trabeculectomy when it is tested on the operating table.  Care must also be taken postoperatively with the timing of cutting sutures with the laser or cutting releasable sutures.

Moderator:   If the use of mitomycin presents risks, why is it used so often?

Dr. Rick Wilson:  There is a risk-reward ratio in everything in medicine.  Mitomycin allows us to get IOPs that are necessary in patients with advanced disease or low-tension glaucoma.  If, however, the patient's reaction to the mitomycin is unusually strong or the doctor is letting out more fluid than he realized, the IOP may end up too low.

Moderator: Can the patient do anything to help avoid hypotony?

Dr. Rick Wilson:  Hypotony is pretty much in the doctors’ hands and, to some extent, to chance.  If I do the same operation exactly the same way in 100 patients, some will end up with too low an IOP, some will end up with too high an IOP, but most will end up close to their target IOP.

P:  Will dilating the eye help to raise eye pressure in someone with no eye pressure?

Dr. Rick Wilson:  Dilating the eye does little to the IOP.  It does relax the muscle to the lens in the eye, so that the front of the eye (the space between the cornea and the iris and anterior-lens surface) deepens.

P:  Could a vitrectomy be used to treat low eye pressure?

Dr. Rick Wilson:  Rarely, fluid that often forms between the layers of the eye in eyes with very low IOPs can be drained and a vitrectomy (removal of the clear jelly in the back of the eye) performed.  Then a bubble of a long-acting gas can be placed to fill up the back of the eye that may help the hypotony situation.

P:  Have you found that early hypotony (within one month) after trabeculectomy is associated with reduced survival time of blebs?

Dr. Rick Wilson:  If the hypotony results in an accumulation of fluid between the layers of the eye and a decreased amount of fluid being made in the eye, then the drain will not have the pressure of fluid to keep it open.  It will heal more than it normally would with a normal amount of fluid keeping it open.

P:  If hypotony occurs after a trabeculectomy with mitomycin, how long do you wait before doing something to correct the hypotony?

Dr. Rick Wilson:  Usually, the doctor waits about six to eight weeks.  If the IOP is not coming up by then, the doctor can add more sutures to the flap to increase resistance to fluid leaving the eye.  That will increase the IOP.

If the eye develops hypotony later, say one or two years later, then the doctor can inject the patient's own blood into the bleb (space under the conjunctiva where the fluid is coming out of the eye).  The objective is to increase the IOP by partially clogging the bleb.

P:  Is there any way to tell who is more likely to have long-term hypotony, other than the young and those with less-than-ideal surgeries?

Dr. Rick Wilson:  The young and myopic (near sighted) are especially prone to problems with hypotony.  Elderly patients usually have petrified sclera that does not shrink in the face of hypotony.  I have 80+ year olds with IOPs of 2 mm Hg who continue to see 20/30.

P:  After surgery eight weeks ago, I have had one synthetic, not blood, injection.  My IOP went up to 9 mm Hg and my vision improved.  Then my IOP dropped to 5 mm Hg. How many synthetic injections could I have to keep the IOP up?

Dr. Rick Wilson:  Injections of hyaluronic acid (Healon) into the front of the eye are not going to be successful unless you have fluid accumulation between the layers of the eye that may respond to a short-term increase of the IOP.  That is an entirely different approach than an autologous blood injection.  I have only done one more (total of 2) blood injection if the first was not entirely successful.

P:  Is injecting blood in a patient with low IOP nine weeks after a trabeculectomy preferable to injecting the synthetic material?

Dr. Rick Wilson:  If there is no fluid between the layers of the eye, I would say yes.  However, the more appropriate course might be to revise the trabeculectomy surgically so that it also does not leak.

P:  Would an IOP of 8 mm Hg be considered hypotony?

Dr. Rick Wilson:  An IOP of 8 mm Hg is usually an excellent result.  However, young patients with myopia can still get retinal problems with an IOP of 8 mm Hg.

P:  What is the difference in the success rate of blood injections and hyaluronic acid injections?

Dr. Rick Wilson:  Since the two techniques are used at very different times after a trabeculectomy, comparing them is difficult.

P:  Why does a retinal detachment cause hypotony?

Dr. Rick Wilson:  Fluid from the inside of the eye can pass through the hole in the retina and gain access to the next layer of the eye, the choroid.  The choroid is almost all vessels and absorbs the intraocular fluid quickly, resulting in low IOPs.

P:  How long after a trabeculectomy and hypotony is it safe to wait before doing something to avoid the risk of losing some central vision?

Dr. Rick Wilson:  The answer is not certain.  Six months does not seem to pose a problem, but most retina surgeons say irreversible changes take place by one year.

P:  I had hypotony for months after my trabeculectomy.  Then I developed a cataract. Did the hypotony hasten its development?

Dr. Rick Wilson:  Probably. It is well known that the progression of cataractous changes speeds up after a trabeculectomy.

P:  If you have a "failed bleb" after a trabeculectomy and treatments for hypotony, what are the treatment options?

Dr. Rick Wilson:  The first is to try medications again.  The doctor can "needle" the bleb if it appears amenable to being opened up.  Or the trabeculectomy can be repeated if a low IOP is needed.  Or an aqueous shunt can be considered if the target IOP is in the 14 to 22 mm Hg range.

P:  If a retina scan after a trabeculectomy shows a “bumpy” retina, what does that mean?

Dr. Rick Wilson:  That depends on the bump.  It could be anything from a sub-retinal hemorrhage, to fluid between the choroid and the sclera, to scarring of the retina, to serous fluid under the retina, to a small mole or a cancer.

P:  Could hypotony cause angles to close even if the patient is not susceptible to angle closure?

Dr. Rick Wilson:  Hypotony would narrow the angle if there is fluid build-up between the middle layer of the eye, the choroid, and the outer layer, the sclera.  The fluid pressure could push the iris forward, narrowing the angle.

P:  Thanks, Dr. Rick Wilson.

P:  I am new to all of this.  Thank you so much.  This is a wonderful site.

Moderator:  We have had hundreds of these chats in the past, which are archived on this Website.

Dr. Rick Wilson:  Have a great two weeks everyone.  I'll see you on the first Wednesday in November.

Moderator: Good night, Dr. Rick.  Get lots of rest and have a speedy recovery.

On November 1, Dr. Wilson discussed "Trabeculectomy" in the Chat room. Click here for highlights of that meeting.

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