Chapter 3: Material and methods
3.9 Contribution to biostratigraphy and sedimentation rates
Just as normative elevation data guides the interpretation of topographic images, accurate pachymetry measurements are used to guide the surgeon. Corneal thickness, along with residual bed calculations are used in selecting the most appropriate refractive surgical procedure. Normal values for central corneal pachymetry have been established12 and values outside the accepted normal range may suggest that the patient is a poor candidate for refractive surgery or that further evaluation is warranted before undergoing a refractive procedure.
When using pachymetry to assess a patients’ potential candidacy for refractive surgery, the thickness of each cornea is traditionally evaluated independently. Unlike other ocular parameters such as intraocular pressure or cup-to-disc ratio, where a certain degree of asymmetry is considered “abnormal”,13-15 there are no such guidelines for corneal pachymetry.
Intrasubject OD/OS pachymetry comparison and interpretation, is not routinely performed and values that constitute a normal amount of asymmetry are not well known. Despite a thorough preoperative refractive surgery evaluation, a large difference in pachymetry between
eyes may go unnoticed if the corneal thickness and the calculated residual bed thickness are adequate in each eye.
Recently, we assessed intrasubject pachymetric asymmetry and established normative values for this measurement.16 Given that corneal thickness can reflect overall corneal health, significant differences in pachymetry between fellow eyes may suggest underlying or as of yet unidentified corneal abnormalities.
In a review of over 700 patients, data for corneal pachymetry was obtained using the Pentacam Eye Scanner. We compared the pachymetric measurements of fellow eyes at the apex, thinnest point and pupil center. We also looked at the relative pachymetry difference between these three points in the same eye. Corneal thickness at the apex, thinnest point and pupil center are often assumed to be very similar, and as such, the pachymetry at these locations should have little variation. This is not always the case. Table 3 shows that the average thickness readings at the apex (539.3 µm), pupil center (538.8 µm) and the thinnest points (536.3 µm) were similar. The differences between the apex and both the pupillary center and the thinnest region were also small with a relatively tight standard deviation (1.06
± 1.73 µm, 2.99 ± 4.34 µm respectively). The range, however, did show a few significant outliers. At least one patient had a 31µm difference between their apex and pupil center reading and up to 93µm comparing the thinnest region to the apex.
When evaluating pachymetric symmetry (Table 4), we found the average amount of corneal asymmetry was about 9 µm at the corneal apex, thinnest and pupil center. Individuals with a greater than 23.2µm difference in apical thickness between eyes represent less than 5% of the population. Individuals with a greater than 30.4µm difference in apical thickness between eyes represent less than 0.3% of the population. Values for the pupil center and
TABLE 3 - Distribution of corneal pachymetry at the apex, pupil center, and thinnest point.
Pupil - Thin (µm) Apex - Thin
(µm)
Patients with a large degree of asymmetry can therefore be classified using pachymetry comparisons even when unilateral central corneal thickness falls within the normal range.
This may be an important finding in certain patients when placed in context with a complete preoperative assessment. The clinical significance of this variation, however, is yet unknown and likely warrants further evaluation.
One question this data raises is could the pachymetry difference between the corneal apex and the pupil center or the apex and the thinnest zone account for some of the cases of ectasia without apparent cause? Estimates on the frequency of post-operative ectasia run from a low of 1/2500 to a high of 1/620 with the former being a more recent estimate.17 In the greater than 1,400 eyes studied, at least one eye had a difference between apex and pupil center and apex and thinnest area of 31µm and 93µm respectively, more than enough to be a significant confounding variable that could possibly account for some cases of iatrogenic ectasia of unknown cause.
It has been shown that patients with keratoconus and forme fruste keratoconus are at increased risk for ectasia after LASIK.18,19 These diagnoses are contraindications to refractive surgery. Even patients with unremarkable examinations at presentation, however, may develop corneal ectasia over time. There is extensive clinical and topographic data that help establish the diagnosis of keratoconus.20 The challenge has become detecting patients with primary corneal abnormalities that have thus far gone undetected. Biomechanical evaluation and more reliable posterior corneal surface topography that can detect subtle corneal irregularities are now available. In many cases, however, this level of preoperative evaluation may not be readily accessible. It is therefore important to identify patients who warrant a more extensive preoperative evaluation in light of an otherwise normal screening for refractive surgery. By classifying normal values for elevation, normal variance for pachymetric asymmetry, and identifying differences between apical thinnest and pupil center readings we can now identify patients who have subtle corneal abnormalities and warrant further examination.
AVERAGE OD / OS
TABLE 4 - Pachymetric asymmetry at the corneal apex, pupil center, and thinnest point.
REFERENCES
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2. Litoff D, Belin MW, Winn SS, et al: PAR Technology Corneal Topography System. Inv Ophthalmol Vis Sci 1991;32:922
3. Fam HB, Lim KL. Corneal elevation indices in normal and keratoconic eyes. J Cataract Refract Surg 2006;32:1281-7
4. Wei RH, Lim L, Chan WK, Tan DT. Evaluation of Orbscan II corneal topography in individuals with myopia.
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5. Nawa Y, Masuda K, Ueda T, et al. Evaluation of apparent ectasia of the posterior surface of the cornea after keratorefractive surgery. J Cataract Refract Surg 2005;31:571–573
6. Cairns G, McGhee CNJ. Orbscan computerized topography: attributes, applications, and limitations. J Cataract Refract Surg 2005;31:205–220
7. Cairns G, Ormonde SE, Gray T, et al. Assessing the accuracy of Orbscan II post-LASIK: apparent keratectasia is paradoxically associated with anterior chamber depth reduction in successful procedures. Clin Exp Ophthalmol 2005;33:147–152
8. Ciolino JB, Belin MW. Changes in the posterior cornea after laser in situ keratomileusis and photorefractive keratectomy. J Cataract Refract Surg 2006;32:1426-1431
9. Ciolino JB, Khachikian SS, Cortese MJ, Belin MW. Long-term stability of the posterior cornea after laser in situ keratomileusis. J Cataract Refract Surg 2007;33:1366-70
10. Ciolino JB, Khachikian SS, Belin MW. Comparison of Corneal Thickness Measurements by Ultrasound and Scheimpflug Photography in Eyes That Have Undergone Laser In Situ Keratomileusis. Am J Ophthalmol 2007 (publication ahead of print)
11. Kim JT, Cortese M, Belin MW, Ambrosio R Jr, Khachikian SS. Tomographic Normal Values for Corneal Elevation and Pachymetry in a Hyperopic Population. J Clinic Experiment Ophthalmol. 201; 2:130.
12. Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach. Surv Ophthalmol. 2000;44:367-408.
13. Vernon SA, Jones SJ. Intraocular pressure asymmetry in a population tested with the Pulsair non-contact tonometer. Eye. 1991;5:674-7.
14. Yablonski ME, Zimmerman TJ, Kass MA, Becker B. Prognostic significance of optic disk cupping in ocular hypertensive patients. Am J Ophthalmol. 1980 Apr;89:585-92.
15. Quigley HA, Enger C, Katz J, Sommer A, Scott R, Gilbert D. Risk factors for the development of glaucomatous visual field loss in ocular hypertension. Arch Ophthalmol. 1994;112:644-9.
16. Khachikian SS, Belin MW, Ciolino JB. Intrasubject corneal thickness asymmetry. J Refract Surg. 2008;24:606-9.
17. Klein SR, Epstein RJ, Randleman JB, Stulting RD. Corneal ectasia after laser in situ keratomileusis in patients without apparent preoperative risk factors. CORNEA. 2006; 25(4):388-403.
18. Seiler T, Quurke AW. Iatrogenic keratectasia after LASIK in a case of forme fruste keratoconus. J Cataract Refract Surg. 2000;26:629-30.
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20. Krachmer JH, Feder RS, Belin MW. Keratoconus and related non-inflammatory corneal thinning disorders. Surv Ophthalmol. 1984;28:293–322.
Corneal thickness represents an important variable when planning keratorefractive surgeries,1,2 evaluating ectatic diseases3-6 and assessing corneal endothelial function.7 Additionally, corneal thickness affects intraocular pressure measurements,8 and pachymetry may be an independent risk factor for glaucoma.9,10
The need for accurate pachymetric evaluation has been a major stimulus for the development of new technologies. Optical pachymetry was the first technique used, but its accuracy was very dependent on the skills and experience of the examiner. Ultrasound (US) pachymetry was introduced in the early 1980’s and replaced optical pachymetry because of its higher accuracy and reproducibility.1,3,6 However, US is limited to a single point measurement, and is sensitive to probe positioning and angulation.
Ultrasonic central corneal thickness (US-CCT) measurements refers to measurements at the geometric center or apex of the cornea, which is not always the thinnest point.2,11 In
> 10% of normal patients, the difference between the thinnest point to the geometric center of the cornea is over 10 µm.11 There is also a significant correlation relating the distance between these points (central and thinnest) and their quantitative difference.2,11 The distance between the thinnest point and the geometric central point is also significantly higher in keratoconus compared to normals.9 A reliable pachymetric map is therefore essential for determining the localization and value of the cornea’s thinnest point.
Cross-sectional imaging (Corneal Tomography) provides for a three-dimensional reconstruction of the cornea, enabling the evaluation of the anterior and posterior corneal surfaces. Accurate assessment of both anterior and posterior corneal surfaces allows for the creation of a full pachymetric (corneal thickness) map as the corneal thickness is determined by the spatial difference between anterior and posterior corneal surfaces.11 There are at least four different commercially used imaging modalities which allow for the reconstruction of the anterior segment: horizontal slit-scanning (Orbscan II, Bausch & Lomb), rotating Scheimpflug camera (Pentacam, Oculus; Galilei, Ziemer), very high frequency ultrasound (Artemis, Ultralink) and high-speed anterior segment optical coherence tomography (AS-OCT – Artemis, Zeiss).