IOP Measurement and Central Corneal Thickness

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  • Letters to the Editor

    paper, we agree that the effect of applying the 50% adjustment to the second eye would need to be prospectively validated to quantify theactual benefit, not only in terms of the postoperative refractive result but also to assess patient reported outcomes and satisfaction. Jivrajkaet al4 in 2012 demonstrated in a prospective study that the 50% adjustment for the second eye improves refractive outcomes in patientswho had FEPE higher than 0.50 diopters (97 of 250 patients [38.8%] had an FEPE higher than 0.50 diopters). The study usedIOLMaster biometry, the hydrophobic acrylic single-piece intraocular lens (IOL) Alcon SN60WF and the Haigis Formula.4 This level ofclinical evidence supporting the 50% adjustment to the second eye far exceeds the quality of evidence base used so far in shaping practicefor cataract surgery, such as the assessment of the performance of IOL power formulae, which is largely based on small, retrospective,theoretical studies.

    We can now reply to Arshinoffs comments in sequence. (1) There is now ample evidence1-4 to support the benefit of adjusting thesecond eye IOL power and this should be taken into account when choosing between simultaneous and delayed surgery. (2) There was atheoretical clinical benefit in 477 (25.5%) out of 1867 patients with interocular differences in keratometry within 0.60 diopters. (3) Thestudy by Jivrajka et al4 has prospectively demonstrated the same finding on a single piece hydrophobic acrylic IOL model. (4) A differencein 0.12-diopters in mean absolute error (MAE) may seem small because it is not expected to translate to a change in IOL power choicefor 100% of patients. Nevertheless, if this reduction in MAE results in a change in IOL power choice for a significant proportion of patients,then it is a clinically important difference. Olsen2 has explained the clinical significance of even smaller changes in MAE of 0.08 diopters.In another example, in 71 eyes with axial length between 27.00 and 27.99 mm, the MAE for Hoffer Q was 0.51 and 0.37 diopters for theSRK/T. This difference in 0.14 diopter of MAE can also be translated to 35%, 56%, and 87% for the Hoffer Q and 51%, 75%, and 93%eyes for the SRK/T within 0.25, 0.50, and 1.00 diopters, respectively.5 Even with the current selection of most IOL models, whichcome in 0.5-diopter steps, most surgeons believe that these small improvements in MAE afforded by IOL power formula choice do havean impact on refractive outcome. No study has prospectively validated refractive outcomes in a randomized, controlled manner to comparethe performance of different IOL power formulae. However, the study by Jivrajka et al4 has prospectively validated the benefit of the 50%adjustment for the second eye, using an IOL model that comes in 0.5-diopter steps. (5) Electronic databases eliminate transcription errorsin the data extraction process, and the large numbers of patients improve the statistical power of the results. Good quality, retrospectivestudies can be reliable; if their findings are clinically relevant, it is easier to fund and ethically justify prospective trials to validate thesefindings.

    We agree with Arshinoff that refractive results in fellow eyes correlate highly. Indeed, patients with good refractive outcome in 1 eye are morelikely to have a good outcome in their fellow eye and the same applies for poor refractive outcomes.3 Around 30% of eyes have a prediction errorgreater than 0.5 diopters and 5% higher than 1.00 diopters.2 A rough calculation would give a figure of 17.5% of eyes with hyperopic errorsgreater than 0.5 diopters and myopic errors of below 1.00 diopter. This group of patients with suboptimal results is more prone to poorrefractive outcome in their fellow eye and would benefit the most from the 50% adjustment for the second eye.

    Bilateral simultaneous cataract surgery still has a role in clinical practice provided that patients are consented and made aware that theymay be missing out on an opportunity to improve refractive outcomes for 1 eye.

    PETROS ARISTODEMOU, FRCOPHTH,1 NATHANIEL E. KNOX CARTWRIGHT, MRCOPHTH,2

    JOHN M. SPARROW, DPHIL, FRCOPHTH,1,2 ROBERT L. JOHNSTON, FRCOPHTH31Academic Department of Ophthalmology, University of Bristol, Bristol, United Kingdom; 2Bristol Eye Hospital, Bristol, United Kingdom;3Cheltenham General Hospital, Cheltenham, United Kingdom

    References

    1. Covert DJ, Henry CR, Koenig SB. Intraocular lens power selection in the second eye of patients undergoing bilateral, sequentialcataract extraction. Ophthalmology 2010;117:4954.

    2. Olsen T. Use of fellow eye data in the calculation of intraocular lens power for the second eye. Ophthalmology 2011;118:17105.3. Aristodemou P, Knox Cartwright NE, Sparrow JM, Johnston RL. First eye prediction error improves second eye refractive outcome

    results in 2129 patients after bilateral sequential cataract surgery. Ophthalmology 2011;118:17019.4. Jivrajka RV, Shammas MC, Shammas HJ. Improving the second-eye refractive error in patients undergoing bilateral sequential cataract

    surgery. Ophthalmology 2012;119:1097101.5. Aristodemou P, Knox Cartwright NE, Sparrow JM, Johnston RL. Formula choice: Hoffer Q, Holladay 1, or SRK/T and refractive

    outcomes in 8108 eyes after cataract surgery with biometry by partial coherence interferometry. J Cataract Refract Surg2011;37:6371.

    IOP Measurement and Central Corneal Thickness

    Dear Editor:Park et al1 declare it reasonable that Goldmann applanation tonometry intraocular pressure remains the clinical gold standard, at least untilanother method . . . is accessible for most clinicians. That means: For the time being, this method should not be up for discussion.

    Why are we worshiping applanation tonometry like the golden calf? The reason is a historical one: At the time when Goldmann andSchmidt2 introduced their new method, ophthalmologists were very aware of the fact that the Schitz tonometry was influenced by therigidity of the ocular walls. Goldmann presented a method that was not influenced by this property of the eye. Hans Goldmann was veryfamous. His method, therefore, was eagerly adopted as the salvation of all pitfalls in tonometry. Weekers and Prijot3also famous in their

    timestated that applanation gives the true value of intraocular pressure from the outset. I suspect that almost no one critically read the

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  • Ophthalmology Volume 119, Number 12, December 2012

    papers of Goldmann and Schmidt, which contained the limitations of the method outlined by the authors. Today, the basic biophysical andstatistical details are forgotten and applanation tonometry is used with the same assurance as a ruler to measure length.

    Would a reviewer of a manuscript submitted to Ophthalmology accept a paper on a new method of tonometry based on the resultsobtained in 10 freshly excised human cadaver eyes and in 25 eyes that were excised a longer, but not clearly defined time postmortem?In eyes where the age of the patients is not given and the refractive state is not known as well as the central corneal thickness? A wisereviewer ought to ask for more details or reject the paper whose description applies to the paper of Goldmann and Schmidt.2

    The judgment of the influence of corneal thickness by Park et al1 may have been different if they would have included the results ofKohlhaas et al4 obtained in 125 eyes and if the influence of age would have been taken in account as well as the influence of the 3 centralcorneal thickness values 640 m in Figure 6 of the Kohlhaas et al4 paper. Park et al lump together the regressions of applanation readingson central corneal thickness (Doughty and Zaman; Shimmyo et al, as referenced by Parks et al) and the regressions of the differences(manometric pressure minus applanation readings) and central corneal thickness. Applanation readings are basically readings of force andnot of pressure. Park et al lump together regressions with low coefficients of correlation (Shimmyo et al; r 0.238) and relatively highcoefficients (Ehlers et al5; r 0.768) omitting the results of Kohlhaas et al4 (r 0.91). They lump together regressions with 29 pairs ofdata (Ehlers et al5) and with 7 pairs (Whitacre et al subgroup). These imprecisions in biophysics and statistics make it difficult to followthe conclusions of the authors who put into question the widely used practice of the correction of readings by influences of knowndisturbances, such as the adjustment for age in analysis of variance, which is done also by applying regression. The Bland-Altman diagramsare not shown. They are assessed by the authors only whose generosity in interpreting diagrams is obvious. Perhaps we can follow theconclusions if we have seen these diagrams.

    It is time now that applanation tonometry is dealt with according to the presently acknowledged standards of scientific thinking andaccording to the principles of biophysics and statistics. In short: Lets go to the sources of our science remembering the appeal of Erasmusof Rotterdam: Ad fontes!

    RICHARD STODTMEISTER, MDDepartment of Ophthalmology, Technical University of Dresden, Dresden, Germany

    References

    1. Park SJ, Ang GS, Nicholas S, Wells AP. The effect of thin, thick, and normal corneas on Goldmann intraocular pressure measurementsand correction formulae in individual eyes. Ophthalmology 2012;119:4439.

    2. Goldmann H, Schmidt T. Weiterer Beitrag zur Applanationstonometrie. Ophthalmologica 1961;141:44156.3. Weekers R, Prijot E. Tonometrie und Tonographie. In: Straub W, ed. Die ophthalmologischen Untersuchungsmethoden. Stuttgart:

    Ferdinand Enke; 1970:30982.4. Kohlhaas M, Boehm AG, Spoerl E, et al. Effect of central corneal thickness, corneal curvature, and axial length on applanation

    tonometry. Arch Ophthalmol 2006;124:4716.5. Ehlers N, Bramsen T, Sperling S. Applanation tonometry and central corneal thickness. Acta Ophthalmol (Copenh) 1975;53:3443.

    Author reply

    Dear Editor:We appreciate the interest in our article1 shown by Dr Stodmeister. In contrast with the opinion that the basic biophysical and statisticaldetails of applanation tonometry have been forgotten, there has been an enormous interest regarding the influence of corneal biomechanicson intraocular pressure (IOP) measurement in the ophthalmic literature in the last 15 years.

    As we explored in our paper, several IOP correction formulae have been published, largely from regression measurements of groupsof eyes. Because it is inherent in creating a regression formula that individual variations are mashed into a line of best fit, the effect of otherparameterssuch as the non-central corneal thickness (CCT) biomechanical properties of individual eyesmust be ignored. Applying aregression formula to an individual cannot properly describe that individual, because of the inherent variability in any complex biologicalsystem.

    The point of our paper was to explore how often such an erroneous assumption might impact decisions for individual patients. In ourfirst paper on this topic, we demonstrated that adjusting IOP from Goldmann applanation tonometry (GAT) for CCT with Kohlhaassformula2 did not result in an improvement in the agreement with dynamic contour tonometry in a Caucasian glaucoma and glaucomasuspect population.3 We found that this CCT correction approach suffered from the same limitation as the others: That, for an individualeye, a correction formula for IOP based on one parameterCCTcould at best be nearly right most of the time. In our more recent andlarger study,1 the adjustments for CCT based on the different formulae were conducted separately and were not lumped together, althoughthe results were presented on the same Table.

    We have repeated the analyses (Table 1, available at http://aaojournal.org) using Kohlhaass formula as Stodmeister suggested (theadjusted IOP herein referred to as IOPKOHL). This, like the other formulae, resulted in poorer agreement (95% limits of agreement [LOA] 5.5 mmHg) with Pascal Dynamic Contour Tonometer (PDCT) IOP, as well as increasing the proportion of IOP with difference of 20% fromPDCT IOP (28%), than using unadjusted GAT IOP.

    In the thinnest tertile, IOPKOHL produced a mean IOP value that was closer to the mean PDCT IOP (1.1 mmHg) but with similar 95%LOA (5.5 mmHg) when compared with taking GAT IOP alone. In this group, 11.3% of IOP had a difference of 20% from PDCT

    KOHLIOP, compared with 36.1% for unadjusted GAT IOP.

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    http://aaojournal.org

    IOP Measurement and Central Corneal ThicknessReferences

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