Glaucoma and High Myopia
July 21, 2004
Norma Devine, Editor
On Wednesday, July 21, 2004, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed “Glaucoma and High Myopia.”
Moderator: Good evening, Dr. Wilson. Tonight we will be discussing high myopia. How is that defined?
Dr. Rick Wilson: Myopia is the medical term for nearsightedness, a condition in which the eye is larger than normal. Light entering the eye is focused before it reaches the retina. The degree of myopia can vary from low (-1 to -3 diopters) to high (greater than 6 diopters.) People with high myopia are usually in the higher range.
P: What are “diopters?”
Dr. Rick Wilson: “Diopters” refers to the strength of the glasses needed for the person to see clearly. The higher the diopter power needed to correct the vision, the greater the amount of myopia, and the thicker the glasses need to be to correct the myopia.
Moderator: Why are glasses that correct nearsightedness thin in the middle and thick on the edges?
Dr. Rick Wilson: To spread out the light, so that it is focused on the back of the eye. A person with myopia needs to wear glasses to see things far away, but can see things close by without the aid of glasses. That’s why many people with high myopia see better with contact lenses than with glasses. They see a larger, focused image.
P: Is there an association between intraocular pressure (IOP) and increasing degrees of myopia?
Dr. Rick Wilson: In a young person, when the wall of the eye is elastic, a higher than normal intraocular pressure (IOP) will cause the eye to expand; that is, become larger, and therefore more myopic (nearsighted).
P: Is myopia a result of the eye trying to defend itself from glaucoma, because a higher than normal IOP will cause the eye to expand in young people?
Dr. Rick Wilson: No. I think it is more cause-and-effect, like increasing pressure in a balloon.
P: Do people with high myopia have an increased risk for glaucoma? If so, why is that?
Dr. Rick Wilson: The studies differ on whether or not myopia is a risk factor for glaucoma. Most doctors feel that myopia is a mild risk factor for glaucoma. That may have to do with the expanded eye thinning the support structures for the optic nerve, where it goes through the back of the eye.
P: If the eyes are larger in a person with high myopia, is the cornea thinner?
Dr. Rick Wilson: You would think so, but that’s not necessarily so. I am a -8 myope, but my corneas are of average thickness, about 545 microns.
P: Can myopia complicate glaucoma surgery?
Dr. Rick Wilson: Yes. With the thinner sclera, there is more risk if the IOP ends up too low and the sclera shrinks and throws the retina into folds.
P: Are people with high myopia more prone to hypotony after a trabeculectomy for glaucoma?
They are somewhat more prone to hypotony just because their sclera is thin, and trabeculectomies may leak more fluid if the flap is thin.
P: Is a shunt less risky than a trab? Can anything be done to prevent post-operative hypotony, when the IOP is too low?
Dr. Rick Wilson: Shunts have less chance of hypotony in a normal eye, but more of a chance of hypotony in an eye that is not making a normal amount of aqueous humor due to chronic inflammation or poor circulation; for example, diabetes.
P: How do the characteristic retina and vitreous disorders associated with high myopia (for example, lattice degeneration), syneresis, posterior vitreous detachment, and so on, affect glaucoma, either from any effects on the dynamics of what’s going on in the anterior segment, or from an effect on the optic nerve?
Dr. Rick Wilson: The disorders you mentioned have no effect on glaucoma. A retinal attachment will cause low pressure until it is fixed.
P: What is lattice degeneration?
Dr. Rick Wilson: Lattice degeneration is a grouping of small thin areas in the retina that make the retina more prone to retinal detachment.
P: Is lattice degeneration easily visible to the ophthalmologist?
Dr. Rick Wilson: No, lattice degeneration is not that visible to the ophthalmologist. He or she has to dilation your pupils to see the lattice degeneration. Symptoms only occur when there is a pull on the lattice degeneration from the vitreous constricting with age, or if there is a retinal attachment.
P: Are there any symptoms of lattice degeneration?
Dr. Rick Wilson: Symptoms only occur when there is a pull on the lattice degeneration from the vitreous constricting with age, or if there is a retinal attachment.
P: Can the expanded eye of myopia cause problems in the front of the eye, too? Can it cause problems in the angle?
Dr. Rick Wilson: No. Myopes, because they have large eyes, have all the room in the world in their angle, so there’s no problem.
P: Does high myopia occur more frequently with a particular type of glaucoma?
Dr. Rick Wilson: People with pigmentary glaucoma are usually myopic.
P: Is glaucoma treatment different for a highly myopic patient than one with 20/20 vision?
Dr. Rick Wilson: The only treatment that is different in a highly myopic patient is that we rarely use high-dose pilocarpine. That is usually not a problem because pilocarpine is rarely used today.
P: Why isn’t high-dose pilocarpine used for a person with high myopia?
Dr. Rick Wilson: Because pilocarpine puts a stress on the retina. Patients with high myopia already have thin-stretched retinas, so any additional stress is dangerous.
P: Is closed-angle glaucoma uncommon in eyes with myopia?
Dr. Rick Wilson: Closed-angle glaucoma would only be caused by a secondary mechanism
P: Is there anything available to reverse the effects of myopia besides Lasik?
Dr. Rick Wilson: Yes. Intraocular lenses can be used in people without cataracts to correct their myopia. The clear lens in the eye could also be removed and replaced with a much weaker lens, so the person does not need to use glasses for distance vision.
P: Are there non-surgical, non-spectacle means of reducing myopia?
Dr. Rick Wilson: None that work.
P: Does laser surgery to correct vision in people with high myopia increase the risk for glaucoma later in life?
Dr. Rick Wilson: No, I do not think it increases their risk of getting glaucoma. It markedly increases the risk that if they get glaucoma, it will not be picked up, because to render the cornea less powerful in focusing the light, it must be made thinner. The thinner cornea will given an artificially low reading when tested by a Goldmann tonometer — the blue light that tests eye pressure.
P: Would closed angles be less damaging in a patient with myopia? I have closed-angle secondary glaucoma. Is it possible that my myopia lessened the severity of my glaucoma?
Dr. Rick Wilson: No, closed angles would not be less damaging in a person with myopia. It’s possible that myopia lessened the severity. People with pigmentary glaucoma are usually myopic. To what is your glaucoma secondary?
P: ICE (irido-corneal-endothelial syndrome).
Dr. Rick Wilson: With ICE syndrome, myopia would make very little difference.
P: I have high myopia, a -8.00 glasses’ lens, open-angle glaucoma and have had trabeculectomies. I need cataract surgery soon. Can a lens be inserted to help with my vision?
Dr. Rick Wilson: Yes, a lens can be put in that gets you close to seeing without glasses. The other eye would also have to have surgery, since it is nearly impossible to have a – 8.00 glasses’ lens in front of one eye and a 0 lens in front of the other.
P: I don’t have a cataract in the other eye.
Dr. Rick Wilson: If you look through your minus eight lens, you will see that objects are much smaller than without glasses, although much clearer. If the image you see without glasses was clear in one eye because of the change in lens with the cataract extraction, and the image in the other eye was much smaller, your brain would have a tough time putting those two images together.
Moderator: Thank you, Doctor Wilson. Good night.