How do pachymetry results influence IOP readings?

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Carmen - 23 Dec 2004 11:33 GMT

Hello
I finally did the pachymetry (with specular microscope): the result was 517
for the left eye and 521 for the right one. The doctor said that the mean is
about 550, so my cornea is slightly thinner than the average. In this case,
how do you reconsider the IOP values? If, as happens to be my case, the
cornea is approximately 5% thinner than the average, do you raise the
pressure value by 5% as well? I have a pressure of 20 in both eyes: do I
have to consider it as a 21? (I have myopia: - 8 in the left eyes / -8,50 in
the right one: does this influence cornea thickness?)
Thank you very much
I really wish you all Merry Christmas and a Happy New Year!

Carmen

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Halterb - 24 Dec 2004 12:30 GMT

Carmen inquired about pachymetry results:

>I finally did the pachymetry (with specular microscope): the result was 517
>for the left eye and 521 for the right one. The doctor said that the mean is
[quoted text clipped - 4 lines]
>have to consider it as a 21? (I have myopia: - 8 in the left eyes / -8,50 in
>the right one: does this influence cornea thickness?)<

Since no one has picked up on this, I'll make a stab at it, particularly since
corneal thickness is another of my favorite topics!

My understanding, if my memory is correct, is that the Goldmann applanation
tonometer (the blue light deal) uses 520 as a corneal thickness reference base.
So, in that case, Carmen's eyes would appear to be about as normal as eyes
could be, so no adjustment in readings would be needed. My own eyes come in a
tad thick at 539, so I have concluded that adds somewhat over 1 mm to my
readings. There's disagreement, like in everything else, over how much pressure
is affected per each micron difference of thickness, and even on what the
normal is.

Bottom line is it would not appear that readings in the 520 range suggest
corneal thickness is a significant in this case. As far as the myopia is
concerned, those considerations are beyond my familiarity--but a pressure of 20
(if based on a Goldmann reading, not a pen type hand held instrument or air
puff unit) wouldn't seem to be a problem IF THERE'S NO EVIDENCE OF DAMAGE such
as suspicious field test results, abnormal appearance of the inner parts of the
eye (especially the optic disc) or vision problems, but I wouldn't go several
years without seeing a good ophthmalogist.

(Again, I caution readers that I am only a patient and have no medical
training.)

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eyeguyrc@aol.com - 25 Dec 2004 05:26 GMT

Different reports of "average" corneal thickness, depending on which
article you read. Personally (and I can't tell you from which article
I picked up this number...I really don't remember), I use an average
corneal thickness of 540 microns (0.540 millimeters). The nomogram I
use to figure out the variation in IOP adds or takes away 1 mm of IOP
for every 20 micron variation off 540. For instance, if your corneal
thickness were 500 microns (40 microns thinner than average), your
actual IOP would probably be 2 millimeters of mercury higher than what
we measure by Goldmann tonometry. If your cornea were 600 microns, I
would assume that your actual IOP was about 3 mm Hg lower than what was
measured. Again, these are rough estimates, and few of us make major
treatment decisions based on corneal thickness. The Ocular
Hypertension Treatment Study (OHTS study) showed that patients with
thin corneas are significantly more likely to progress from ocular
hypertension to glaucoma than those with average or thick corneas.
That means if a patient has a borderline pressure and a very thin
cornea, I would be more likely to treat that patient. Hope that helps.
Rick Cohn, MD
Glaucoma Specialist
Winter Park, FL

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Carmen - 27 Dec 2004 13:29 GMT

Thank you very much Halterb and Dr. Cohn

Carmen

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