Interesting IOP + CCT experience

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Anon E. Muss - 13 Sep 2006 21:55 GMT

I had an interesting experience with an ocular hypertensive (OHTN)
patient the other day I thought I would relate. It is ONE patient and
ONE experience, so I do not wish to draw any conclusions -- just to
relate some anecdotal information.

IOPs by Goldmann tonometry (GAT):
    OD 20mm Hg
    OS 22mm Hg

Central Corneal Thickness (CCT) (Heidelberg IOPac):
    OD 593 microns (mean of 8 readings with a SD of 6.4 microns)
    OS 616 microns (mean of 8 readings with a SD of 13.7 microns)

IOPs by Pascal Dynamic Contour Tonometry* (DCT <see
http://tinyurl.com/kraew>):
    OD 13.3mm Hg (Ocular pulse amplitude: 2.1mm Hg)
    OS 13.6mm Hg (Ocular pulse amplitude: 3.3mm Hg)

Obviously, the interesting part is the HUGE discrepancy between the
Goldmann and DCT readings.

As I have wrote here before, I am *NOT* a fan of adjusting IOPs based
on adjustment algorithms. I believe these algorithms are not precise
and they are based on the false assumption that the only important
factor is CCT. However, it does appear, in this instance, that CCT is
having a major impact of GAT readings.

* DCT is promoted as being free of several factors/characteristics of
the cornea (including CCT) which influence the IOP on almost all
tonometers.

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William Stacy - 13 Sep 2006 22:14 GMT

I think it is more evidence for my side of our previous discussion, and
thank you for posting it. To me, even if the correcting tables are
flawed, or innaccurate, it is far better than making no correction at
all. (btw, I don't throw out the raw Goldmann readings, I record them as
measured, but show pachymetry with them when appropriate.

w.stacy, o.d.

>I had an interesting experience with an ocular hypertensive (OHTN)
>patient the other day I thought I would relate. It is ONE patient and
[quoted text clipped - 27 lines]
>tonometers.
>

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Anon E. Muss - 14 Sep 2006 02:43 GMT

>I think it is more evidence for my side of our previous discussion

I think so too.

It is important to realize that this data is "anecdotal" as I wrote,
which means <http://tinyurl.com/zcw8y>:

    [...]
    "a single case report not yet substantiated by studies
    using large numbers of people."
    [...]

My interests are in providing excellent patient care and knowing the
truth in order to better manage my glaucoma patients, not in promoting
any particular "IOP/CCT agenda".

>and thank you for posting it.

I figured you would like it. :)

I ordered some tests on a different (true OHTN -- I suspect POAG also
-- I'll decide after the tests confirm my optic nerve evaluation
findings) patient and they were supposed to have been performed today
(GATs as high as 33mm Hg OS) -- HRT3, CCT, GDx/VCC, optic nerve,
retinal nerve fiber layer photos. Since the clinic I sent him to is
either (1) testing out a DCT or (2) has a DCT, I'm sure they'll use
that and send me back the results.

If DCT is performed, I'll plan on reporting the intraocular tension
findings here when I get the results sent back in a week or two, how's
that sound?

>To me, even if the correcting tables are flawed, or innaccurate, it is
>far better than making no correction at all.

Heh. We've gone over this before.

>(btw, I don't throw out the raw Goldmann readings, I record them as
>measured, but show pachymetry with them when appropriate.

Fair enough.

BTW, I'll be looking at pachymeters (among a lot of other stuff) at
Vision Expo this weekend.

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William Stacy - 14 Sep 2006 05:41 GMT

> BTW, I'll be looking at pachymeters (among a lot of other stuff) at
> Vision Expo this weekend.

Good for you. I'm betting you'll buy one within a week.

w.stacy, o.d.

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jab - 03 Oct 2006 14:00 GMT

I am not a doctor. I am working for Ziemer, manufacturer of the PASCAL
DCT.
This is a typical case as we see it on an almost daily basis:

A thick cornea, probably rigid. Since the GAT is not measuring direct
IOP but the force needed to applanate the cornea on an area of 3.06mm,
in this case, the GAT clearly overestimates the pressure, whereas the
DCT, not being influenced by CCT or any other corneal bio-mechanical
properties, is directly measuring the true IOP.
Now today it's common to correct the GAT readings. In this case, one
would correct this result downwards.

But what if this thick cornea is soft or mushy?

We saw cases with thick corneas with GAT readings of 19mmHG. Because of
the CCT, results were corrected downwards. The Pascal measured 25mmHG -
again and again.

This patient had a thick but very soft cornea. The GAT did not
overestimate the pressure as one would have expected in a thick cornea.
Quite to the contrary - because there was no resistance on this cornea,
the GAT underestimated the IOP and in addition to this was then
corrected in the wrong direction.... - down to 16mmHG. Quite a
gap......

> I had an interesting experience with an ocular hypertensive (OHTN)
> patient the other day I thought I would relate. It is ONE patient and
[quoted text clipped - 26 lines]
> the cornea (including CCT) which influence the IOP on almost all
> tonometers.

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jab - 03 Oct 2006 14:53 GMT

As regards correction of GAT readings read the following statement

quote from James Brandt, MD, (participant in the OHTS study; University
of California at Davis UCSD)

"Assuming that CCT can be used as a correction factor for GAT is a
misinterpretation of the results of OHTS...that couldn't be further
from the truth. Adjusting IOP based on CCT is attempting to instill a
degree of precision into a flawed measurement. You may actually correct
in the wrong direction. The issues related to the most accurate
tonometry need to include the material properties of the cornea." This
quote was a slide in one of the presentations at the American Glaucoma
Society Meeting.

Anon E. Muss schrieb:

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