Refractive Errors & Glaucoma
Chat Highlights
October 29, 2003

Norma Devine, Editor

On Wednesday, October 29, 2003, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Refractive Errors & Glaucoma."

Moderator:  Dr. Wilson, tonight we would like to discuss refractive errors and glaucoma.  

P:  Would you please first define "refraction?"  

Dr. Rick Wilson:  Refraction is the way the light is focused on the retina.  Myopic (nearsighted) people have eyes that are too large for their corneas and lenses.  That means the light is focused before it reaches the retina, and is out of focus on the retina.  Glasses for myopic people are thin in the middle and thick on the edges.  Such lenses spread out the light so that it can focus on the retina correctly.

P:  I wondered why my glasses were thick on the sides and thin in the middle.

Dr. Rick Wilson:  Now you know.  

P:  How often should glaucoma patients be refracted?

Dr. Rick Wilson:  Unless the patient is a child, glaucoma patients should get refracted no more or no less frequently than other people.

P:  Is there an association between myopia (nearsightedness) and glaucoma?  

Dr. Rick Wilson:  Studies about the association between glaucoma and myopia are not consistent.  Most studies show a weak  association.  

P:  Are nearsighted people more prone to normal-tension glaucoma and open-angle glaucoma? 

Dr. Rick Wilson:  There does seem to be a weak association with those two entities.

P:  Is the risk of glaucoma higher if the degree of myopia is higher?  

Dr. Rick Wilson:  Since the association is weak, I am not sure I could say that with any confidence.

P:  Are nearsighted people more likely to have low intraocular pressure (IOP) after surgical intervention?

Dr. Rick Wilson:  Not to my knowledge.

P:  If you have had your lenses removed due to cataracts, do you become myopic or hyperopic as a result?

Dr. Rick Wilson:  When you have your lenses removed, you become very hyperopic -- usually around a +12 ic.  

P:  If the cataract is removed during infancy, can that make the eye elongate?  You said that cataract removal causes hypyeropia,  which is true as far as my refraction is concerned.  However, my doctor keeps saying I'm very myopic and have large, long eyes.

Dr. Rick Wilson:  Removing a cataract during infancy does not make the eye elongate.  If, however, the infant has glaucoma, the added pressure enlarges the eye while the tissues are young and soft.  It is likely that you are much less farsighted than you normally would be, since the glaucoma has shifted you more toward the nearsighted end of the spectrum. 

P:  Does elevated IOP cause expansion of the eye only in infancy when the tissues are, as you say, "young and soft?"  I'm 52, and wonder about the progressive refractive error over the past fifteen or so years only in my worse eye (that is, more pigment, consistently higher pressures, etc.).

Dr. Rick Wilson:  Yes, the effect is only seen in childhood.  We can become progressively more nearsighted, as the natural lens becomes more dense with age.

P:  I have nystagmus, as well as glaucoma.   What can I do to ensure that I get the best possible refraction for glasses? 

Dr. Rick Wilson:  How good is your vision when you get good glasses?

P:  It's 20/40.

Dr. Rick Wilson:  I had thought you might need a low vision expert, but it seems that a good ophthalmologist or optometrist should be able to fit you well with glasses.

[Note:  Nystagmus is an uncontrolled, repetitive eye movement; for example, the eyes moving rapidly back and forth. That is usually seen in patients with reduced vision, especially at the time of birth.]

P:  Are hyperopic eyes the opposite of myopic eyes?

Dr. Rick Wilson:  Yes.  Hyperopic eyes are small, and the light is focused behind the retina.  Magnifying glasses will focus the light closer to the front of the eye and on the retina.  Because the eyes are small, the front of the eye is crowded. The iris ends up too close to the trabecular meshwork, and angle-closure glaucoma can ensue.

P:  Are people who are farsighted more likely to have ocular hypertension than those with no refractive error?

Dr. Rick Wilson:  Not that I know of.  However, farsighted people -- women more than men -- are much more likely to have angle-closure glaucoma.  Angle-closure glaucoma can be acute, with intense symptoms, or slowly progressive and chronic.  The latter has few or no symptoms.

P:  Does glaucoma make refraction difficult? 

Dr. Rick Wilson:  No.

P:  My doctor said I may need new glasses after my trabeculectomy.  How would that affect my myopia?

Dr. Rick Wilson:  Most of the time, a trabeculectomy does not affect the refraction much.  If a person is using pilocarpine, that  makes the pupils tiny, and gives the patient great depth of field.  Stopping pilocarpine after surgery will cause more blurring, and the need for stronger glasses for reading.

P:  Why do high myopes often have large cups?  What's the relationship?  And is there anything else associated with the cup-to-disk ratio of myopes that has relevance to glaucoma?

Dr. Rick Wilson:  As mentioned earlier, myopes have eyes that are larger than normal.  That means the wall of the eye is thin, and the hole that the optic nerve goes through is larger than normal. Since myopes have the same number of nerve fibers (about 1.2 million) as people with a normal refraction, the fibers are spread out in the larger hole and have a big depression in the middle.  That depression is called the cup.

P:  Are aphakic patients (that is, those who have had lenses removed during cataract surgery) also at greater risk of angle-closure problems?  

Dr. Rick Wilson:  Once a patient has the cataract removed, there is plenty of room in the front of the eye.  Angle closure is not a problem, unless the eye is malformed.

P:  For about two years before my diagnosis of glaucoma and since then, my vision has changed.  That is, my nearsightedness has continued to improve, and my astigmatism has shifted somewhat.  Does that mean the shape of the eye is changing, and is there any connection with glaucoma?

Dr. Rick Wilson:  Are you nearsighted or farsighted?  

P:  I'm nearsighted.  

Dr. Rick Wilson:  That blows my theory.  As people grow older, the lens and the eye also continue to grow.  Since there is no room for the lens to expand, it becomes more dense.  As the lens becomes more dense, it increases in power and, after enough change in the lens, people who could not see at near before can often  read without glasses ("second sight").   If you were farsighted,  I thought this might be happening to you.  I can't explain the changes you are seeing now.

P:  How can glaucoma affect astigmatism?

Dr. Rick Wilson:  The higher the pressure in the eye, the less astigmatism would be expected, as the pressure would tend to smooth out the cornea.  That effect, however, should be quite small.

[Note:  In astigmatism, the eye has two different curves, like the top of a football lying on its side.  Each curve needs a different power to correct it. Therefore, the glasses must have two curves to correct the astigmatism.]

P:  Can glaucoma change visual acuity, or does it just limit peripheral vision?  

Dr. Rick Wilson:  I don't think glaucoma changes the vision much centrally, until the glaucoma is fairly far advanced.   It does certainly cause changes in the vision slightly off to the side of center.

P:  Besides cystoid macular edema, what else might cause a slow but substantial decline in vision (a refraction error) after a trabeculectomy?  Could a pressure of 4 mm Hg cause the cornea to wrinkle and astigmatism to be high?

Dr. Rick Wilson:  A pressure of 4 mm Hg could cause wrinkles in the retina, which could make your vision poor.  Cataracts also get worse at a faster rate after trabeculectomy, and could be part of your problem.

P:  The vision in my amblyopic eye shoots to the right.  When I am being refracted or just examined, the doctor is frustrated because he thinks I'm not looking where he tells me to look.  But I am.  It's hard to explain to someone how I see.

Dr. Rick Wilson:  I can understand the problem, and how it would be frustrating to someone who does not grasp it.

[Note:  Amblyopia is the medical term for a "lazy" eye.]

Moderator:  Thank you, again, Dr. Wilson, for all your help.

Dr. Rick Wilson:  You're welcome.  Next Wednesday night, Dr. Mark Moster, a neurologist who specializes in neurologic diseases that affect the eyes,  will be here to answer questions.

End of highlights for October 29, 2003.

On November 5, Dr. Wilson discussed "Neurology and Glaucoma" in the Chat room. Click here for highlights of that meeting.

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