Corneal Thickness and IOP
March 6, 2002
Norma Devine, Editor
On Wednesday, March 6, 2002, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed “Corneal Thickness and IOP.”
Moderator: Welcome back, Dr. Rick. We are discussing corneal thickness and intraocular pressure (IOP). Have there been any recent studies on the relationship of corneal thickness to intraocular pressure?
Dr. Rick Wilson: Yes, recent studies have shown that African Americans not only have four times the prevalence of glaucoma as whites do, but they also have thinner corneas that give falsely low IOP readings. Also, studies on patients who have had LASIK have found those patients have much thinner corneas and artifactually low IOPs.
P: Sculpting the cornea with the laser makes the cornea thinner?
Dr. Rick Wilson: Yes, often much thinner.
Moderator: How is the thickness of the cornea measured?
Dr. Rick Wilson: It’s measured with a pachymeter. Some pachymeters use light; the newer ones use ultrasound. The average cornea is 561 microns thick, a little more than half a millimeter.
P: Is there a set scale for adjusting IOP according to corneal thickness, or is it an estimate?
Dr. Rick Wilson: There are different scales. My rule of thumb is to adjust the IOP 2.5 mm for every 50 microns the cornea is thinner or thicker than 560 microns.
P: Do most ophthalmologists have a pachymeter?
Dr. Rick Wilson: No.
P: In your view, if corneal thickness were taken into account, what percentage of normal-tension glaucoma (NTG) patients would be diagnosed as high-tension glaucoma patents?
Dr. Rick Wilson: There aren’t any studies on that, and I would not hazard a guess. Some studies have shown that ocular hypertensives, on average, have thicker corneas, meaning many of them may have artifactually elevated IOPs.
P: In my doctor’s practice in Connecticut, I was told that I am one of the few normal-tension glaucoma patients. How common is this type of glaucoma and is it more difficult to treat?
Dr. Rick Wilson: NTG patients account for one in six primary open-angle glaucoma (POAG) patients. NTG is harder to treat, because it is more difficult to know how low to get the IOP, since the original IOP is in the normal range.
P: Is measuring corneal thickness part of a standard eye exam by glaucoma specialists?
Dr. Rick Wilson: Not at this time, but it’s becoming more common.
P: Would it be a good idea for ocular hypertensives to have their corneas measured if more aggressive treatment is being considered?
Dr. Rick Wilson: Yes.
P: Is the thickness of the cornea the same across the entire surface?
Dr. Rick Wilson: Normally, it is thinner in the center than in the periphery.
P: What markers suggest that corneal measurements are indicated?
Dr. Rick Wilson: IOPs that don’t agree with the severity or progression of the disease, or lack of it.
P: After pachymetry, my doctor revised my tonometer-measured IOP of 26 down to 22 mm Hg. Which is the more significant figure?
Dr. Rick Wilson: It is hoped that the corrected 22 mm Hg is your actual IOP, and would be the meaningful number.
P: Thanks, that’s great. But my first doctor was unimpressed by the idea of correcting the tonometer reading.
P: Does a scarred cornea affect the IOP measurement?
Dr. Rick Wilson: Yes, it may. Although Goldmann tonometry readings are usually significantly more accurate than the Tonopen’s, on scarred corneas, the Tonopen is better.
P: How does a cornea transplant affect a glaucomatous eye?
Dr. Rick Wilson: Often the act of doing the transplant may cause scarring or inflammation that causes a rise in IOP. It becomes harder to check the IOP after a corneal graft, as the cornea is no longer smooth. Also, glaucoma medicines and surgery, if needed, are hard on the cornea.
P: Does the chronic use of eyedrops thin the cornea? What causes it to thin or thicken?
Dr. Rick Wilson: The chronic use of drops may be harmful to the cells lining the cornea over the long term. The drops themselves do not thin the cornea. If too many cells are lost, then the cornea becomes swollen and thick.
P: How many cyclophotocoagulations can be safely done on an eight-year-old child before the cornea is seriously injured? How does cyclophotocoagulation affect the cornea?
Dr. Rick Wilson: The corneas are not my first worry with cyclophotocoagulation. The real worry is that too much of the part of the eye that makes the fluid (the ciliary body) will be injured , and the eye will partially collapses because of too low a pressure.
P: If corneal thickness were taken into account in the current Ocular Hypertension Treatment Study by the National Institutes of Health, what percent of the subjects would actually NOT have ocular hypertension? Could you comment on how this might affect the outcome of this important study?
Dr. Rick Wilson: Corneal thickness is being investigated in that study. I don’t have the figures on that percentage, and would not hazard a guess.
P: Is an extra-thick but apparently healthy and uninjured cornea just a random physical feature, or does it correlate with any other congenital structures or developed conditions in the eye? (I’ve never taken eyedrops or been scarred, and just wonder if the thickness itself has any health-related implications, beyond the problem with getting an accurate IOP measurement.)
Dr. Rick Wilson: In most people, there are corneal conditions like corneal icthyosis and corneal dystrophies where the cornea may not be of normal thickness. There are no health implications, unless the corneal dystrophy is systemic.
P: I take it you mean extra thickness is usually a random feature. Thanks.
P: Are there any within-patient variations in corneal thickness, such as diurnal variations?
Dr. Rick Wilson: Yes. One of the studies showed that holding the eye open too long while examining before taking the IOP dries out the cornea, thinning it and giving a falsely low reading. Conversely, taking the IOP just after awakening means that the cornea has been moist all night and will be thicker, giving a higher IOP reading.
P: How long would be too long to hold the eye open before measuring IOP?
Dr. Rick Wilson: The longer, the more chance of drying, especially if the eye is dry to begin with.
P: Do you mean minutes or hours?
Dr. Rick Wilson: A minute or two.
P: What conditions, other than meds, surgery, or congenital, would cause a scarred cornea?
Dr. Rick Wilson: Trauma or exposure. Trachoma is one of the leading blinding diseases in the world. It causes the lids to turn inward, so that the lashes are always on the cornea, causing corneal scarring with time.
P: Speaking of diurnal variations: Do studies show that there’s a seasonal IOP cycle? I’ve read on the Internet that summertime IOP readings may be a few millimeters of mercury lower than wintertime IOP readings, presumably because of the miotic effect of increased sunlight.
Dr. Rick Wilson: IOP readings are higher in women in the morning, and in the winter, on average.
P: What IOP do you consider “too low?”
Dr. Rick Wilson: Under 8 mm Hg in the young, who have elastic and flexible sclera and under 5 mm Hg in the elderly.
P: My doctor tells me that the three most useful diagnostic facts relating to glaucoma are (1) the condition of the optic nerve, (2) a visual field test, and (3) IOP, in that order. We lay people tend to focus on IOP because it’s a numerical variable, but it’s the least useful fact to know. Right?
Dr. Rick Wilson: Your doctor is correct.
Moderator: Dr. Rick, before you leave, would you tell us a little about the American Glaucoma Society conference you attended recently in Puerto Rico?
Dr. Rick Wilson: It was a good conference. The Mayo group presented a population study on pigmentary-dispersion syndrome (PDS). The study found a lower change to pigmentary glaucoma than the 35 to 50% figures most books show. I don’t remember the exact percent, but I think it was in the high teens.
Moderator: Thanks, Dr. Rick.
Dr. Rick Wilson: Good night. Everyone have a great week.
End of highlights for March 6, 2002.