Corneal Thickness
Chat Highlights
September 3, 2008

On Wednesday, September 3, 2008, Dr. Michael Pro, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Corneal Thickness".

Moderator: Welcome back to chat Dr Pro. Tonight's topic is Corneal Thickness. What is the normal range for corneal thickness and how is it measured?

Dr. Pro: OK, well let's explain some basics. The best recent description of corneal thickness was from data in the Ocular Hypertension Treatment Study (OHTS), but the concept of measuring corneal thickness has been known for a long time. In fact when Hans Goldmann invented his tonometer over 50 years ago (which is still the standard in use today), he calibrated his device against an average corneal thickness of 550 microns. Thin corneas cause the IOP to be underestimated, and that thicker corneas would lead to an overestimation of IOP, but over time these facts were forgotten. By the time of the OHTS study the concept of routinely measuring the CCT (corneal thickness) was unheard of—and the OHTS study was published only five years ago!

It showed that “Central corneal thickness was found to be a powerful predictor for the development of POAG.” The mean thickness was 573 microns with persons of African descent having on average thinner corneas. Eyes with corneal thickness of 555 microns or less had a threefold greater risk of developing glaucoma than those who had corneal thickness of more than 588 microns.

P: Have those results changed treatment and diagnosis Dr. Pro?

Dr. Pro: Absolutely. It is now standard practice to measure the CCT with an ultrasound pachymeter. There is a bit of disagreement on how to treat the reading, however. Some glaucoma specialists feel that every effort should be made to calculate a "correction factor" to adjust the Goldmann reading to reflect the "true IOP," while others simply take the measurement of the CCT and group patients into 3 categories—thin, average, and thick--and adjust the patients estimated risk for developing glaucoma, or progressing, based on those groups.

P: Is corneal thickness hereditary?

Dr. Pro: Yes, but to be honest I can not remember a source article.

P: Does corneal thickness change with time?

Dr. Pro: Not in healthy eyes, but the corneal thickness is sensitive to change due to swelling (edema). This can be seen in congenital glaucoma or cases where the IOP is extremely high in a short amount of time (such as an angle closure attack). Fluid can swell the cornea and cause it to be very thick in these cases. In such instances the cornea can go back to its normal thickness when the IOP is lowered.

Of course the cornea can also be thinned surgically; this is the principle behind refractive surgery such as LASIK.

P: How does previous LASIK surgery effect IOPs with a diagnosis of glaucoma?

Dr. Pro: Well, the "true IOP" is probably higher than the measured value. There have been various correction factors proposed, but in general one should consider that the corrected IOP may be 3-4 points higher.

P: I was told I have thicker corneas, but don't know the measurement. It was also recommended I have an iridotomy as I have narrow closures. My pressures were 20 and 22 at last reading. Is that considered high for persons with thicker cornea and is the decision to do the iridotomy usually based more on pressure or on the look of the closure through the lens?

Dr. Pro: The decision to do the iridotomy is based on the appearance of the angles (ie, are they narrow enough to put you at risk for angle closure), the thickness of the cornea is not a part of the equation.

P: Is the natural lens the same thing as the cornea?

Dr. Pro: Yes. The cornea is in front (the window of the eye) and the lens sits behind the iris (the colored part of the eye).

P: Can childbirth or pregnancy thin corneas because of hormonal changes?

Dr. Pro: I have not found any references to articles on corneal thickness and pregnancy.

P: Dr. Pro, my cornea went back to its normal thickness after my trab for ICE syndrome. It was a nice surprise for me. My vision also improved, I no longer felt like I was looking through wavy glass.

Dr. Pro: Great!

P: If thinner corneas are predictive of more severe glaucoma, how thin do the corneas have to be in order for this occur?

Dr. Pro: Well I have a few thoughts on this based on some journal articles:
CCT was significantly thinner in glaucomatous eyes than in normal fellow eyes in phakic children with congenital glaucoma. (Authors: Wygnanski-Jaffe T, Barequet IS.)
Predictors of long-term progression in the early manifest glaucoma trial showed that another new factor was thinner CCT, with results possibly indicating a preferential CCT effect with higher IOP. (Leske MC, Heijl A, Hyman L, Bengtsson B, Dong L, Yang Z; EMGT Group.) A CCT measurement 548 or less is considered thin with approximately one and a half times the risk for progression for each 40 microns thinner. The Barbados Eye Study found an almost identical result for developing open angle glaucoma.

P: Dr. Pro, can thicker corneas interfere with the absorption of glaucoma medications or otherwise affect medications?

Dr. Pro: When dosed with IOP lowering drugs, eyes with thinner corneas had lower intraocular pressure than eyes with thicker corneas. This suggests a reduced efficacy of some glaucoma medications in ocular hypertensive patients with thick corneas.

P: May I ask the definition of phakic?

Dr. Pro: Phakic is when the natural lens is present, pseudophakic is when the natural lens was removed (as in cataract surgery) and replaced with an artificial lens.

P: What portion of the population have thick or thin corneas? Are there differences between the sexes in central corneal thickness?

Dr. Pro: The distribution of CCT is a bell-shaped curve with the average at about 550 microns, the average was higher in the OHTS study and CCT has been shown in other studies to be higher in ocular hypertensive patients.

Moderator: And distribution between sexes, doctor?

Dr. Pro: I don't know, but will try to find an article about that.

P: Are you born with a certain central corneal thickness? Do we reach it at a certain age?

Dr. Pro: Like the body in general, the eye grows from birth, although it reaches nearly its adult size much sooner than other organs in the body. As such, it is certain that the CCT changes a bit from birth, but I don't know all the parameters.

P: Should everyone have CCT checked, or just glaucoma suspects?

Dr. Pro: There is no recommendation from the American Academy of Ophthalmology that all persons have their CCT measured, but all glaucoma suspects should.

Moderator: Dr. Pro, thank you again for your time and generosity in coming to answer our questions.

Dr. Pro: You are welcome. Goodnight and thanks.

On September 17, Dr. Spaeth discussed "New Surgical Treatments: Express Shunt, Canaloplasty and Trabectome" in the Chat room. Click here for highlights of that meeting.

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