New devices and clinical implications for measuring corneal thickness

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  • Clinical and Experimental Ophthalmology 2006; 34: 729731doi:10.1111/j.1442-9071.2006.01395.x

    2006 The AuthorsJournal compilation 2006 Royal Australian and New Zealand College of Ophthalmologists

    Editorial

    New devices and clinical implications for measuring corneal thickness

    devices to become incorporated into an ophthalmic practice.When comparing different devices one needs to look at howeach will function in the real world. Ultrasound measure-ments in this study were performed by using a hand-heldprobe applied to the centre of the cornea. While describedas the centre what generally occurs is that the physician ortechnician positions the probe by eyeballing (no pun here)the cornea and touches the apex or high point while tryingto hold the probe perpendicular to the corneal surface. Inthis study five readings were averaged. It should also berealized that each individual ultrasound reading is actuallyan average of up to 50 separate readings. What is beingreported is an average of averages. Actual individual mea-surement accuracy is typically not known.

    Each OCT measurement was taken only after calibration,which were performed by certified engineers from the man-ufacturer using a calibration test-eye tool which was a glassplate whose thickness and index of refraction were accu-rately known. How the system responds without the teamof engineers is not reported. In actuality, the Visante OCTis not overly complicated and a team of engineers does nothave to be kept on retainer, but it does raise the question ofwhether the results here reflect which one can expect toobtain in routine clinical use.

    Finally, does all this matter? Are a few microns here anda few microns there really important? When evaluating aglaucoma suspect will the treatment differ if the thickness isreally 545 instead of 560? Probably not, it is, however, adifferent story with refractive surgery.

    The fear of all corneal refractive surgeons is iatrogenicectasia. While certain guidelines exist, the literature isreplete with case reports of postoperative ectasia in spite ofwhat appeared to be a normal preoperative evaluation andoperative plan.2,3 To date, most of the discussion has focusedon the inaccuracy of the LASIK flap (inadvertent thick flap)leading to an overestimation of the residual bed thickness.4

    Many cases, however, may also be due to the limitation ofrelying solely on anterior curvature analysis and ignoringpreoperative changes that may be present on the posteriorcorneal surface (Fig. 1).5 Another, often ignored potentialsource of error may be the preoperative pachymetry itself.6,7

    Risk assessment typically involves utilizing a worst caseconstruct or what happens when more than one system fails.What happens if the flap is too thick and the patient had anundiagnosed abnormal posterior corneal surface? Almost allreported cases of post-refractive ectasia have utilized ultra-

    While reading the paper by Leung et al. in this issue of Clinicaland Experimental Ophthalmology (Comparison between centralcorneal thickness measurements by ultrasound pachymetryand optical coherence tomography)1 a number of issues cameto mind. The paper serves a very practical function (evalu-ating new technology) and compares central corneal thick-ness readings by a newly available device using opticalcoherence tomography (OCT) (Visante OCT, HumphreyInstruments, Dublin, CA, USA) to ultrasound pachymetry,the reported Gold Standard for which all new systems shouldbe compared.

    First, what exactly is a Gold Standard and how did ultra-sound pachymetry obtain that lofty status? The term GoldStandard has its roots in world finances. Its original purposewas to prevent inflationary expansion of the money supply(i.e. the printing of useless or overvalued paper money) andto serve as a common standard by which all currencies couldbe judged. The gold standard has actually not been in usesince 1971. Currently, there is no circulating paper currencyredeemable in gold. Most contemporary economists actuallyview the gold standard as archaic and counter-productiveand believe a system gauging trade supply and demand abetter means of judging value. One only needs to look at theeconomics of oil to understand this. We now use the termto designate some standard that is believed to be the best,most accurate, and most reliable.

    Ultrasound is clearly the most commonly used device formeasuring corneal thickness. It is relatively inexpensive, por-table and easy to operate, but does that make it a standardby which to judge all newer technology? If one had to designa device to measure thickness of a biologic tissue that mayvary in hydration, consistency and shape one would notchoose the speed of sound through the tissue as the bestyardstick (or metre stick for those in Europe and down-under). Typically, what normally occurs is that we have somevery expensive, overly complex, difficult to use device thatis highly accurate that is used to judge a simpler and lesscostly device, not the other way around. We need to throwout the concept of ultrasound as the Gold Standard. We haveto recognize the obvious limitations of ultrasound pachym-etry. That is not to say we should abandon its use, just thatit cannot serve as the standard by which to judge the accu-racy of newer technology.

    As noted above, the advantages of ultrasound is that it isaffordable, reasonably reproducible, and can be performedwith a minimal of training; features necessary for most

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    2006 The AuthorsJournal compilation 2006 Royal Australian and New Zealand College of Ophthalmologists

    sound pachymetry.2,3,5,8 As noted above, ultrasound readingsare typically done at the corneal apex (or as close to the apexas the technician can estimate). Laser refractive procedures,however, are normally centred on the pupil. Is this differencebetween the corneal thickness at the apex and the pupilsignificant? We recently looked at 1436 preoperative eyesusing the Oculus Pentacam9 and recorded corneal thicknessat the apex, pupillary centre and the thinnest reading.

    The Pentacam uses a rotating Scheimpflug camera imag-ing system to measure anterior and posterior corneal eleva-tion, corneal thickness and numerous other anterior chamberparameters. The average thickness readings at the apex(539.3 m), pupil centre (538.8 m) and the thinnest read-ing (536.3 m) were similar. The differences between theapex and both the pupillary centre and the thinnest regionwere also small with a relatively tight standard deviation(1.06 1/73 m, 2.99 4.34 m, respectively). The range,however, did show a few significant outliers. At least onepatient had a 31 m difference between their apex and pupilcentre reading and up to 93 m comparing the thinnestregion to the apex.

    Does this matter? Could the difference between the cor-neal apex and the pupil centre or the apex and the thinnestzone account for some of the cases of ectasia without appar-ent cause? The incidence of a significant difference is rare.

    The standard deviation of the difference between apicalthickness and pupillary centre readings is only 1.73 m and4.34 m comparing the apex to the thinnest reading. Thismeans that only about 1% of patients should be expected tohave more than a 5 m difference between the apex andpupil centre, but up to a 13 m difference when comparingthe apex to the thinnest area. Estimates on the frequency ofpostoperative ectasia run from a low of 1/6208 to a high of1/25003 with the later being a more recent estimate. In ourstudy of 1436 eyes at least one eye had a difference betweenapex and pupil centre and apex and thinnest area of 31 mand 93 m, respectively, more than enough to be a signifi-cant confounding variable that could account for some casesof iatrogenic ectasia of unknown cause.

    Newer methods for measuring corneal thickness (e.g.Scheimpflug, OCT) offer advantages by providing a betterunderstanding of the cornea than were previously availableby ultrasound. It is hoped that this new information whencombined with other available data will lead to a betterunderstanding of the corneal response to refractive surgeryand to a reduction in the incidence of iatrogenic ectasia.

    Michael W Belin MD FRANZCO andStephen S Khachikian MD

    Albany Medical Center, Albany, New York, USA

    Figure 1. Pre-operative four pic-ture display (Oculus Pentacam).The anterior elevation (upper left),anterior curvature (bottom left) andpachymetry distribution (bottomright) are all normal for a moderateastigmatic cornea. However, theposterior elevation (upper right)is distinctly abnormal showinga prominent island of elevation(>+30 m) in the central region.

  • Editorial 731

    2006 The AuthorsJournal compilation 2006 Royal Australian and New Zealand College of Ophthalmologists

    REFERENCES1. Dexter YL, Leung DY, Lam DK, Yeung BY, Lam DS. Compar-

    ison between central corneal thickness measurements by ultra-sound pachymetry and optical coherence tomography. ClinExperiment Ophthalmol 2006; 34: 7514.

    2. Moshirfar M, Marx DP. Incidence of Post-LASIK Ectasia in Patientswith Unidentified Preoperative Risk Factors. Presented World CorneaCongress, Washington, DC, 2005.

    3. Klein SR, Epstein RJ, Randleman JB, Stulting RD. Cornealectasia after laser in situ keratomileusis in patients withoutapparent preoperative risk factors. Cornea 2006; 25: 388403.

    4. Giledi O, Mulhern MG, Espinosa M, Kerr A, Daya SM. Repro-ducibility of LASIK flap thickness using the Hansatome micro-keratome. J Cataract Refract Surg 2004; 30: 9378.

    5. Binder PS, Lindstrom RL, Stulting RD et al. Keratoconus andcorneal extasia after LASIK. J Refract Surg 2005; 21: 74952.

    6. Reader AL, Salz JJ. Differences among ultrasonic pachyme-ters in measuring corneal thickness. J Refract Surg 1987; 3: 711.

    7. Gordon A, Boggess EA, Molinari JF. Variability of ultrasonicpachymetry. Optom Vis Sci 1990; 67: 1625.

    8. Murta JN, Quadrado M, Carvalheira F, Van Velze R. CornealEctasia after LASIK. Presented World Cornea Congress,Washington, DC, 2005.

    9. Belin MW, Khachikian SS. Reducing Risk and Improving Outcomesin Refractive Surgery. Presented ESCRS Annual Meeting, London,2006.

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