GLAUCOMA QUESTION

#1: Is an "intraoccular pressure measurement" (IOP) with a tonometer correlated to (a) the pressure within an eye -- or (b) the thickness (and toughness) of the cornea? ( I, Martin Carbone, have come to the conclusion that IOP readings are correlated with the thickness and toughness of the cornea). Read the discussion below.

[This information was originally sent to my ophthalmologist, Dr. Zelko of Santa Barbara, on 10/30/99. I was trying to convince him that I did not have glaucoma and was in no particular danger of getting glaucoma in the future. He had, on the basis of an IOP reading of about 20 mm Hg, advised me that I was in danger of developing glaucoma and should therefor be treated to lower my IOP. I considered that advice hasty and ill-considered. I subsequently studied the subject of glaucoma in some detail. This letter and the other material on this website (A), (B), (C), is a result of that study.]

Consider:
1.0: Average thickness of cornea = ~ 0.5 mm in center; ~ 1 mm in periphery (Foster, C.S., Azar, D.T. and Dohlman, C.H. Smolin and Thoft’s "The Cornea. Scientific Foundations and Clinical Practice", 4th edition, Philadelphia: Lippincott Williams & Wilkins, 2005) Found at << http://faculty.washington.edu/chudler/facts.html>>
1.1: One micron = .001 mm
1.2: In other words -- the average cornea is about 500 microns thick

2.0: IOP was found to increase by 2.9 mm Hg/100 microns Central Corneal Thickness in males ... see study below: "Positive Correlation between Tono-Pen Intraocular Pressure and Central Corneal Thickness".

3.0: From the above --- if you multiply 500 microns x 2.9 mm Hg/100 microns -- you get 14.5 mm Hg -- which is almost exactly the "pressure" in the average eye (IOP).

4.0: Doesn't this lead to the logical conclusion that a tonometer does not really measure pressure within the eye -- but instead reads the thickness of the cornea?

5.0: I do not think this is surprising because,
5.1: The cornea thickness is probably directly related to the tensile strength of the cornea in the circumferential direction.
5.2: When the tonometer presses on the eye, it can't compress the vitreous fluid in the eye -- because that fluid is incompressible -- being 98% water.
5.3: Therefore, the pressure from the tonometer is transferred to the outer wall of the eye where it stresses the wall in a circumferential direction.
5.4: It is the resistance to this stress that is read on the tonometer. That resistance to stress is what is normally known as tensile strength.
5.5: If the tonometer were not being applied, there would no circumferential stress in the wall of the eye.
5.6: The tensile stress of the wall in a closed sphere is, of course, directly related to the force that is applied by the pressure within the sphere against the walls of the sphere. The forces in the wall and the forces against the wall are always balanced and are always equal to zero according to the law of physics that says (paraphrased) forces on a body always add to zero where there is no motion.
5.7: Consider a balloon which is filled with water at sea level (760 mm-Hg atmospheric pressure) which is then weighted and allowed to sink to the bottom of a pool or ocean. Since the water is incompressible, the pressure within the balloon would exactly equal the water pressure outside the balloon and there would be no stress in the wall of the balloon. If one applied a tonometer to the wall of the submerged balloon one would read the pressure applied by the tonometer and not the true enormous balanced pressure within and outside the balloon. We contend that essentailly the same forces come into play in the human eye when tonometer readings are taken.

6.0: One might say that it makes no difference whether the reading actually defines the pressure within the eye or the stress in the wall of the eye -- because these forces exist together and in any event they should be reduced.
6.1: But it makes an enormous difference if you are looking for the cause of what is called the pressure within the eye (IOP). In that case you will naturally look for a malfunctioning source of the pressure and/or a malfunctioning mechanism that should be releasing or controlling the pressure.
6.2: If, on the other hand, you look for a cause of thickening or hardening of the wall of the eye, perhaps as in aging --- you would look for entirely different mechanisms.

Positive Correlation between Tono-Pen Intraocular Pressure and Central Corneal Thickness: By Ali A. Dohadwala, Rejean Munger, PhD, Karim F. Damji, MD
Objective: To examine the relationship between intraocular pressure (IOP) readings taken by the Tono-Pen tonometer (Mentor O&O, Norwell, MA) and central corneal thickness (CCT).
Design: Prospective cross-sectional population study.
Participants: There were 651 eyes of 332 healthy subjects.
Main Outcome Measures: A questionnaire was given to each subject requesting information on gender, age, race, and other factors that can influence IOP. The IOP then was measured using the Tono-Pen followed by measurements of CCT using an ultrasonic pachymeter.
Results: The IOP was found to increase by 2.9 mmHg/100 µm CCT in males and 1.2 mmHg/100 µm in females. For males, CCT was found to be statistically significant in predicting IOP (P < 0.001 in the right and left eyes) and diabetes was of borderline significance (P = 0.012 in the right eye, P= 0.089 in the left eye). For females, CCT was of borderline significance (P = 0.064 in the right eye, P = 0.019 in the left eye). In females, a family history of glaucoma (P = 0.021 in the right eye, P = 0.022 in the left eye) and hypertension (P = 0.010 in the right eye, P = <0.001 in the left eye) were also significant in the prediction of IOP. Race was found to be a significant predictor of CCT (P < 0.001 in both right and left eyes) for both males and females.
Conclusion: Clinicians should be aware that, as with the Goldmann applanation tonometer, the Tono-Pen has a systematic error in IOP readings caused by its dependence on CCT. Tono-Pen IOP readings are positively correlated to CCT in males and, to a lesser extent, in females as well. The CCT measurements should be considered to ensure proper interpretation of IOP measurements in the diagnosis and management of disorders in which the CCT or IOP readings are outside normal limits.

Originally received: September 19, 1997.
Revision accepted: May 15, 1998. Manuscript no. 97677.
From the University of Ottawa Eye Institute, Ottawa General Hospital, Ottawa, Ontario, Canada.

Address correspondence and reprint requests to Karim F. Damji, MD, University of Ottawa Eye Institute, 501 Smyth Rd, Ottawa, ON, K1H 8L6, Canada.-------------------------

Am I overlooking something????

Marty Carbone

Find out more about Glaucoma

A variety of global patient advocacy organizations claim, in ads through Pfizer << http://www.pfizeropthalmics.com >>, that they can provide a wealth of up-to-date information, support, and resources that will help you to protect your vision.

Write to them and ask them what evidence they have that high Intra Ocular Pressure (IOP) causes glaucoma.

Click on the links below to visit their individual Web sites and learn more about them.

World Glaucoma Association
http://www.globalaigs.org/

World Glaucoma Patient Association
http://aigpo.org/

International Glaucoma Association
http://www.glaucoma-association.com

The Glaucoma Foundation
http://www.glaucomafoundation.org/

Glaucoma Research Foundation
http://glaucoma.org/

Glaucoma Australia
http://www.glaucoma.org.au/

Glaucoma Friends Network
http://www.gfnet.gr.jp