Relationship between corneal thickness and eye pressure

FitEyes member billym wrote on Wed, Jul 9 2008 12:40 AM:

"My doctor told me that thickness of the eye affects pressure and that because of the thickness of mine, he subtracts 5 from total.  Has anyone else heard this?"

Reply: Yes, there is a relationship between the thickness of the cornea and the accuracy of the eye pressure readings obtained from applanation tonometers. However, it is not a straightforward relationship. It is usually not correct to add or subtract from the intraocular pressure readings based on corneal thickness alone. I would encourage your doctor to read the latest research on this subject.

Here is how it works. Most standard machines that measure eye pressure are applanation-style tonometers. The Goldmann is the most commonly used applanation tonometer. The Reichert AT555 is another applanation tonometer, although it is also a non-contact tonometer. All of these tonometers work by pressing in on the corneal. They flatten a certain size (area) on the normally curved cornea by applying pressure to the cornea.

Obviously, the thickness of the cornea is one factor that could affect how much pressure is required to produce this flattening (applantion) effect. If the cornea adds more resistance than the tonometer is designed for, the reported pressure will be higher than the true pressure. This is the justification for billym's doctor to subtract 5 from his intraocular pressure reading.

The reason this is not correct is because the thickness of the cornea alone does not determine how stiff or rigid the cornea is. The topic of corneal stiffness falls under the area of research called corneal biomechanics and this is an active area of reseach in ophthalmology now. It is now known that some people can have a thick cornea that is soft. Other people can have a thin cornea that is actually stiff. So there is no way to correct intraocular pressure based on corneal thickness alone.

You can think about it in terms of other materials. If we assumed any material that is 10 millimeters thick would have a certain strength or rigidity (or elasticity or any other physical property) we would obviously be very wrong when we compared steel to aluminum to balsa wood to glass. Even the two metals (steel and aluminum) have different strengths for the same thickness material. People's corneas are like this.

In fact, the hydration (water retention) of one's cornea can change throughout the day and this can cause a change in corneal thickness. Obviously, a thick cornea that is expanded due to water retention is not going to be a stronger cornea. This is an example of when it would be completely wrong to subtract a value from the intraocular pressure reading for a thick cornea.

With current technology, the best way to find the error of a Goldmann (or other) applanation tonometer is to have one's intraocular pressure measured by either a Pascal Dynamic Contour tonometer or a Reichert Ocular Response Analyzer (or both) on the same doctor visit where the Goldmann IOP is measured and then compare the differences. In fact, repeating this a few times would be good because the correction will vary with time of day and other factors.

I am reminding all the ophthalmologists that I know that intraocular pressure remains the #1 modifiable risk factor for glaucoma and that the profession needs to utilize the best tonometers for monitoring intraocular pressure. Every ophthalmologist or doctor of optometry that treats glaucoma should have both the Pascal Dynamic Contour tonometer and the Reichert Ocular Response Analyzer in their office. Otherwise they will not know the true eye pressure of their glaucoma patients.

Keep in mind that the correction algorithms based on corneal thickness can be completely wrong. They can indicate that a subtraction of X is required when in reality an addition of Y is required. In other words, they can be wrong not only in magnitude, but also in the direction (plus or minus) of the correction. It is not safe to rely on CCT correction algorithms to correct intraocular pressure in glaucoma patients.

A separate issue is the risk of glaucoma progression and its relationship to CCT. A thick CCT reduces risk of glaucoma progression. This has little or nothing to do with IOP as far as I know.

 i really appreciated the info in that reply, dave.

to billym  at least the opthlamologist considered the corneal thickness.  i recently went to two opthamologists that didn't measure it and didn't mentiion it.  and  that is why i am going to a third.

 Nancy,
The doctors I use just found out about it last year or at least that was the first time they told me about it. I think a lot of people who visit doctors are too trusting. They do not ask questions, etc and do not get the info and care they should. 

 you're right billym.  sometimes, i just sit there like a dunce and go home and think of so many things that i should have asked.  but i learn.  after i went to 3 different opth., i started comparing their approaches to my problems and what they considered significant.  that is when i started learning.  the worst is when they take a condescending attitude.  as a woman, i bet i get that more than a man does.  even from a female doc. 

i'll tell you though.  trying to figure this stuff out is a lot of work and sometimes it really is depressing.  so i understand why people give up and have to rely on the doctors. 

anyway, re coreal thickness.  i just got a copy of a 4 year old office visit where the cct was taken 4 times in a matter of minutes.  all different, varied 25 points.  go figure.

Nancy, I am starting to see that IOP varies a lot and is constantly changing according to circumstance. I have heard drinking a lot of water increases pressure but we are told water is good, drink 8 glasses a day  Thanks for your reply to my post.