Editor,
In the discussion Bechmann and colleagues mention the results of Ehlers et al[1] and compare them with the results of Whitacre et al[2] without regard for generally accepted principles of interpretation.[3] Some biometrical considerations will be found in the following. In figure 4 in the paper by Ehlers et al the correlation coefficient between the correction value and corneal thickness is 0.768 at n=29. In the comparable figure 2 of Whitacre et al no correlation coefficient is given at n=15. This coefficient was calculated by us after digitalizing the data points. It equals 0.51. According to Klemm[3] (page 97) the estimate of regression is extremley unreliable and thus useless at r <_0.6. xmlns:study="urn:x-prefix:study" the="the" data="data" of="of" ehlers="ehlers" et="et" al="al" therefore="therefore" are="are" much="much" more="more" convincing="convincing" than="than" whitacre="whitacre" al.="al." this="this" fact="fact" does="does" not="not" reduce="reduce" merit="merit" whitacres="whitacres" study="study" which="which" brought="brought" problem="problem" corneal="corneal" influence="influence" on="on" tonometry="tonometry" to="to" our="our" attention.="attention." it="it" escaped="escaped" attention="attention" bechmann="bechmann" that="that" figure="figure" _4="_4" al1="al1" and="and" _2="_2" al2="al2" one="one" hand="hand" differ="differ" fundamentally="fundamentally" from="from" in="in" paper="paper" by="by" wolfs="wolfs" al4="al4" other="other" reports="reports" results="results" rotterdam="rotterdam" study:_="study:_" ordinate="ordinate" shows="shows" correction="correction" value="value" according="according" thickness="thickness" measurement="measurement" error="error" thickness.="thickness." these="these" two="two" ordinates="ordinates" sign="sign" show="show" result="result" subtraction="subtraction" intracamerally="intracamerally" measured="measured" iop="iop" applanation="applanation" values.="values." however="however" tonometry.="tonometry." is="is" a="a" fundamental="fundamental" difference="difference" absolutely="absolutely" forbids="forbids" comparison="comparison" same="same" breath.="breath." provide="provide" correlation="correlation" coefficient="coefficient" shown="shown" may="may" be="be" interpreted="interpreted" as="as" cloud="cloud" points.="points." we="we" have="have" similar="similar" calculated="calculated" r="0.17." case="case" concluded="concluded" estimate="estimate" regression="regression" playing="playing" with="with" figures="figures" only3page="only3page" _97.p="_97.p"> In summary, the data of Ehlers et al presently show the association of measurement error and corneal thickness in the most convincing way. Although Bechmann et al have (erroneously) seen a small influence of CCT in IOP measurement in the literature they attribute an important part to corneal thickness in the diagnosis and understanding of various types of glaucoma. It can be concluded from the context that the authors treat corneal thickness as a new quantity in the diagnosis of glaucoma-like optic disk parameters. They have nicely shown different values of corneal thickness in the various types of glaucoma. However, they do not believe that corneal thickness influences applanation tonometry. Therefore, they have to explain their findings in a more complicated way. A description of this behaviour is given by the psychologist and philosopher Watzlawick[5] (page 67) who states that we prefer declaring undeniable facts (which are inconsistent with our explanation) to be untrue or unreal instead of fitting our explanation to these facts. The application of biometric knowledge in judging the data Whitacre et al[2] and a reinterpretation of the figure 2 of the Rotterdam study[4] may fit the opinion of the authors to the most likely explanation[6-8] that corneal thickness influences the results in applanation tonometry to a clinically relevant degree. And that recommends the application of OCT in the diagnosis of glaucoma if available.

1. Ehlers N, Bramsen T, Sperling S. Applanation tonometry and central corneal thickness. Acta Ophthalmol (Copenh) 1975;53:34-43.
2. Whitacre MM, Stein RA, Hassanein K. The effect of corneal thickness on applanation tonometry. Am J Ophthalmol 1993;115:592-596.
3. Klemm PG. Keine Angst vor Biomathematik. Berlin: Ullstein Mosby, 1993.
4. Wolfs RC, Klaver CC, Vingerling JR, et al. Distribution of central corneal thickness and its association with intraocular pressure: The Rotterdam Study. Am J Ophthalmol 1997;123:767-772.
5. Watzlawick P. Wie wirklich ist die Wirklichkeit? München: Piper, 1976.
6. Bron AM, Creuzot-Garcher C, Goudeau-Boutillon S, et al. Falsely elevated intraocular pressure due to increased central corneal thickness [In Process Citation]. Graefes Arch Clin Exp Ophthalmol 1999;237:220-224.
7. Stodtmeister R. Applanation tonometry and correction according to corneal thickness. Acta Ophthalmol Scand 1998;76:319-324.
8. Shah S, Chatterjee A, Mathai M, et al. Relationship between corneal thickness and measured intraocular pressure in a general ophthalmology clinic. Ophthalmology 1999;106:2154-2160.

Editor,
I read the article by Bechmann et al with interest, and I congratulate the authors on their work. In the discussion, they cover the entire subject of tonometry on the basis of the central corneal thickness (CCT). With the increasing number of corneal refractive procedures performed every year, this point will be associated with much uncertainty for diagnosing glaucoma in the near future. Apparently, ophthalmologists are slowly becoming aware of this problem.[1] In their conclusion, the authors emphasize the "need for a combined measurement of IOP and CCT in order to be able to classify the different types of glaucoma." There is, as we perceive it, a slight misunderstanding of the problem. The Goldmann tonometer[2] measures the force required for flattening the cornea. From this, it concludes the pressure in the eye. From the original article of Goldmann, it is well known that this relationship is only valid for a corneal thickness of roughly 520 micrometers. However, the problem is not the thickness itself of the cornea, but rather the cornea's biomechanical properties (e.g., elasticity), which are somehow related to its thickness. Thus, a better solution for this problem would be a measurement of the eye pressure that was independent of the thickness. In this case, each eye could be measured correctly irrespective of the biomechanical properties of the cornea. With the contact lens tonometer, now called SmartLens®, we have been able to demonstrate that after LASIK the true IOP can be measured,[3] thereby excluding any possible false negative glaucoma cases. The SmartLens® tonometer directly measures the pressure in the middle of the applanated area and, therefore, is independent of any corneal property, including its thickness.[4] To the best of our knowledge, this is the only way to prevent the ophthalmologist from possible incorrect diagnosis and follow-up of glaucoma patients in the future.

YVES C.A. ROBERT, MD
CLAUDE KAUFMANN, MD
UniversitätsSpital Zürich
Augenklinik
Frauenklinikstr. 24
CH-8091 ZÜRICH
Email: yrobert@opht.unizh.ch

References

1. Lewis, RA. Refractive surgery and the glaucoma patient (customized corneas under pressure). Ophthalmology 2000;107(9):1621-1622.
2. Goldmann, H, Schmidt TH. Über Applanationstonometrie Ophthalmologica. 1957;134:221-241.
3. Kaufmann C, Schipper I, Robert YCA. SmartLens® Tonometry compared to Goldmann Tonometry before and after Refractive Surgery. Invest Ophthalmol Vis Sci 2000;41(4):S598. Abstract #2475.
4. Dekker PW, Robert YCA, Kanngiesser H, Pirani P, Entenmann B. Principles of contact lens tonometry. International Ophthalmol 1999;22:105-111.