Determination of the corneal thickness profile by optical pachometry

Impact of age, systemic glucocorticoids, and progressive knee arthritis on specific mRNA levels in different areas of the rabbit cornea

PURPOSE

To determine the influence of factors such as age, osteoarthritis (OA), and glucocorticoid treatment on total RNA and mRNA regulation in the cornea and how these factors differ between prepupillary and peripheral areas of the cornea.

METHODS

Molecular analyses of corneal tissue were performed using rabbits of different age groups and skeletally mature animals that had undergone anterior cruciate ligament (ACL) transection, an established model of knee OA. Systemic glucocorticoids were administered to cohorts of the osteoarthritic and control animals to determine the influence of distal joint disease on the corneal response. Corneal tissue was analyzed for changes in mRNA levels for several relevant genes: collagen I, collagen III, collagen V, decorin core protein, cyclooxygenase-2 (COX-2), glucocorticoid receptor, and the housekeeping gene beta-actin.

RESULTS

The corneal tissue was found to have diminishing total RNA with age, which is consistent with previous studies in the literature. Interestingly, in skeletally mature animals, distal joint OA was found to affect corneal mRNA levels for several important structural and inflammatory genes (collagen I, decorin core protein, and COX-2) in a manner that progressed with OA progression. Although systemic glucocorticoid treatment did not alter mRNA levels in the normal cornea, it did counteract the changes observed early after OA induction (3 weeks) while having less of an effect in later, more established arthritis (6 weeks).

CONCLUSIONS

This study reveals that distal joint OA can affect mRNA levels for several structural and inflammatory genes of the cornea, changes that seem to be suppressed by systemic glucocorticoid treatment. These findings indicate that OA has associated systemic factors that influence corneal cell metabolism.

Corneal thickness measurement by confocal microscopy, ultrasound, and scanning slit methods

PURPOSE

To measure corneal thickness by using a calibrated confocal microscope and to compare this measurement to thickness determined by ultrasonic and noncontact scanning slit pachymetry.

DESIGN

Comparison of corneal thickness measured by using four instruments in normal subjects.

METHODS

Thickness measured by a clinical confocal microscope (Tandem Scanning) was calibrated from measurements of polymethylmethacrylate contact lenses with known thickness. Corneal thickness was measured in one eye of 24 normal subjects by using this instrument, two ultrasonic pachymeters (DHG-1000 and Sonogage), and a noncontact optical scanning slit pachymeter (Orbscan II).

RESULTS

Mean corneal thickness measured by confocal microscopy was 516 +/- 30 microm (+/-SD). This was less than the mean thickness measured by both ultrasonic pachymeters, 554 +/- 28 microm by the DGH, and 555 +/- 28 microm by the Sonogage (P <.001). Thickness measured by the Orbscan II pachymeter was 540 +/- 35 microm (P <.001, compared with either confocal or ultrasound) after applying an "acoustic factor" of 0.92, a default correction of the software.

CONCLUSION

Corneal thickness measured by calibrated confocal microscopy is approximately 39 microm (7.0%) less than thickness measured by two commonly used ultrasonic pachymeters and approximately 24 microm (4.4%) less than thickness measured by the corrected Orbscan II pachymeter. These differences are important for planning and measuring the effects of refractive and other surgical procedures. The precision of confocal microscopy is limited by corneal motion in an anterior-posterior direction. The difference between instruments suggests that verification of clinical ultrasonic pachymeters should be revisited.

Reliability of Corneal Thickness and Endothelial Cell Density Measures

PURPOSE

We evaluated the reliability and agreement between the Orbscan, an ultrasonic pachymeter (Humphrey 855), and the Konan SP 9000-LC in terms of central corneal thickness. The Konan was also used to study the reliability and agreement between endothelial cell density measures.

METHODS

Twenty-five normal subjects were examined on two occasions (mean separation = 9 +/- 5 days) by a single examiner using all three instruments for central corneal pachymetry. The Konan Center Method and a manual counting method were performed by two examiners to determine endothelial cell density. Reliability and agreement were assessed by calculating the 95% limits of agreement (LoA) and intraclass correlation coefficients (ICC).

RESULTS

For corneal pachymetry test-retest reliability, the 95% limits of agreement were -20 to +17 microm for the ultrasound, -27 to +22 microm for the Konan, and -13 to +13 microm for the Orbscan. There was fair-to-good agreement between the pachymeters (intraclass correlation coefficients range = 0.85 to 0.92). For endothelial cell density test-retest reliability, the 95% limits of agreement for the Konan Center Method was -498 to +530, and -482 to +333 cells/mm2 for examiners 1 and 2, respectively. The test-retest 95% limits of agreement for the manual overlaid grid method was -355 to +355, and -535 to +670 cells/mm2 for examiners 1 and 2, respectively.

CONCLUSIONS

The reliability and agreement of the Orbscan and Konan corneal pachymeters was good, although the reliability of the Konan for estimating endothelial cell density was fair, at best.

Central and peripheral corneal thickness measured with the TOPCON specular microscope SP-2000P

PURPOSE

Modern refractive surgery and follow up relies on a knowledge of corneal thickness (CT) and shape, and the reliability of modern instrumentation providing such data is important. This study sought to determine the performance of the TOPCON SP-2000P specular microscope in measuring CT. The aims of this study were: (a) to determine if there is any difference between the CT results obtained from the first image, the clearest of three images and the mean of measurements from three images; (b) to determine the correlation between central and peripheral CT and (c) to investigate the reliability (repeatability and reproducibility) of the SP-2000P in the determination of central and peripheral CT.

METHODS

The central and peripheral CT measurements shown on the first (f), the clearest (c) of three images and the mean (m) CT shown in three images captured with the SP-2000P of 43 subjects were compared. All images were captured by the same examiner. Nineteen of the subjects (male) returned on another day and measurements were taken by two examiners.

RESULTS

fCT, cCT and mCT were not significantly different from each other. The intraclass correlation coefficients (ICC) between the three values were> 0.9 for all corneal locations measured. However, the variability in the differences between fCT and cCT was relatively greater for peripheral CT (except temporal). The peripheral CT values obtained were all significantly greater than the central CT, and statistically significant correlations were found between the central and each of the peripheral thickness. There was no statistically significant between-visit or between-examiner differences in the central or peripheral CT. The ICC values for between-visit differences for central and temporal measurements were > 0.9 but for the other corneal locations, the ICC values were 0.81 to 0.88. For between-examiner differences, the ICC value was 0.82 for inferior CT and > 0.9 for the other four CT.

CONCLUSIONS

Differences in CT measurements obtained from the first and clearest images captured by the SP-2000P were not statistically significant but can be clinically significant for peripheral CT. We therefore suggest the use of the CT value obtained from the clearest of three images. For every unit change in central CT, there is an approximately equivalent change in peripheral CT. Based on the ICC values, the SP-2000P showed very good repeatability and reproducibility in the determination of central CT. For the determination of peripheral CT, the repeatability and reproducibility of the SP-2000P were good and reasonably good respectively. These findings have relevance to the measurement of the cornea before and after refractive surgery.

Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach

We determined the "normal" central corneal thickness (CCT) value in human corneas based on reported literature values for within-study average CCT values, and used this as a reference to assess the reported impact of physiological variables (especially age and diurnal effects), contact lens wear, pharmaceuticals, ocular disease, and ophthalmic surgery on CCT. With the expected CCT and its variance defined, it should be possible to determine the potential impact of differences in CCT in intraocular pressure (IOP) assessments, especially by applanation tonometry, using a meta-analysis approach. Some 600 sets of CCT data were identified from the worldwide literature over the period of 1968 through mid-1999, of which 134 included IOP measures as well. The within-study average CCT values and reported variance (SD) was noted along with the number of eyes and any special characteristics, including probable ethnic origin of the study subjects. Various sets of data were subjected to statistical analyses. From 300 data sets from eyes designated as normal, the group-averaged CCT was 0.534 mm. From 230 data sets where interindividual variance was reported, the group-averaged CCT was 0.536 mm (median 0.536 mm; average SD of 0. 031 mm, average coefficient of variation = 5.8%). Overall, studies using slit-lamp-based pachometry have reported marginally lower CCT values (average 0.530 mm, average SD 0.029 mm) compared to ultrasound-based studies (average 0.544, average SD 0.034 mm), which perhaps reflects the type of individual studied (non-surgical vs. pre-surgical patients) rather than the technique itself. A slight chronological increase in reported average CCT values (approximately 0.006 mm/decade) was evident, but a substantial chronological increase was evident for ultrasound pachometry studies (approximately 0.015 mm/decade). Within the meta-analysis-generated average and variance, age had no obvious impact on CCT measures for *whites, although an age-related decline in CCT is evident for non-whites. Any diurnal effects are likely concealed within the expected variance in CCT. Contact lens wear and pharmaceuticals generally produced changes in CCT that were well within the expected variance in CCT. Of the ocular diseases, only those associated with collagen disorders (including keratoconus) or endothelial-based corneal dystrophies (e.g., Fuchs) were likely to result in decreases or increases, respectively, of CCT beyond the normal variance. Routine contact lens wear and diseases such as diabetes seem unlikely to produce changes in CCT of a magnitude that would justify pachometry as a monitoring method beyond routine slit-lamp evaluation. Increases in CCT beyond the expected variance were reported after a range of intraocular surgeries (cataract operations, penetrating keratoplasty), whereas photorefractive surgery produces a measurable decrease in CCT. A meta-analysis of possible association between CCT and IOP measures of 133 data sets, regardless of the type of eyes assessed, revealed a statistically significant correlation; a 10% difference in CCT would result in a 3. 4 +/- 0.9 mm Hg difference in IOP (P </= 0.001, r = 0.419). The observed phenomenon was much smaller for eyes designated as healthy (1.1 +/- 0.6 mm Hg for a 10% difference in CCT, P = 0.023, r = 0. 331). For eyes with chronic diseases, the change was 2.5 +/- 1.1 mm Hg for a 10% difference in CCT (P = 0.005, r = 0.450), whereas a substantial but highly variable association was seen for eyes with acute onset disease (approximately 10.0 +/- 3.1 mm Hg for a 10% difference in CCT, P = 0.004, r = 0.623). Based on the meta-analysis, normal CCT in white adults would be expected to be within +/-11.6% (+/-2 SD) of 0.535 mm, i.e., 0.473-0.597 mm (95% CI, 0.474-0.596). The impact of CCT on applanation tonometry of healthy eyes is unlikely to achieve clinical significance, but for corneas of eyes with chronic disease, pachometry should be performed if the tonometry reveals IOP readi

A simple technique for measurement of corneal thickness

Measurement of corneal thickness has potential benefit both in the fitting of contact lenses and in monitoring any pathology that could affect corneal thickness. Corneal thickness measurement is undertaken using an accessory to the biomicroscope, the optical pachometer, or by means of expensive apparatus such as the ultrasonic pachometer. There are other complex methods, such as laser Doppler interferometry, or ultrasonic rasters of the cornea. In this study, an easier and low cost method, based on the measurement of the optical section formed of the cornea by the biomicroscope illumination system, is proposed. The advantages of this new method are the simplicity of its experimental set-up which consists of a calibrated graticule in an eyepiece of the biomicroscope, the speed of the measurement which increases with practice and the low cost, due to the fact that the biomicroscope is standard equipment in the ophthalmic consulting room.

Posterior corneal curvature and its influence on corneal dioptric power

An algorithm for estimation of the posterior corneal curvature is presented and applied on data from normal and keratoconic eyes. Radius of central posterior corneal curvature are demonstrated to be (mean +/- SD) 6.71 +/- 0.23 mm and 5.58 +/- 0.78 mm in normals and keratoconic eyes, respectively. This corresponds to a ratio between posterior and anterior corneal curvature at 0.85 and 0.83 in the groups mentioned. Both these ratios are significantly smaller than the corresponding ratio at 0.88 in Gullstrand's schematic eye which on corneal dioptric power results in offset errors at 0.20 D and 0.46 D in normal and keratoconic eyes. It is further demonstrated that the ratio is not constant over the corneal surface, resulting in central peripheral dioptric offset errors between 0.2 D and -0.31 D in normals and between 0.46 D and -0.38 D in keratoconic eyes. On corneal dioptric power it is finally shown that a variation in the refractive index of aqueous humor has a 30 times larger influence than a similar variation in corneal refractive index.

Interferometric measurement of corneal thickness with micrometer precision

The recently developed partial coherence laser Doppler interferometry technique was improved to measure central and peripheral corneal thickness with high precision. Corneal thickness profiles were measured on 18 eyes of health, volunteer subjects. All of these eyes were measurable at angles (between visual axis and measuring direction) ranging from 20 degrees nasal to 25 degrees temporal. At larger angles (up to 35 degrees) only part of the eyes was measurable. The thickness profiles of the 18 corneas have a nearly perfectly parabolic shape within the measured region. The precision (standard deviation) was 1.6 microns for central measurements and decreased somewhat to about 3.5 microns at measuring angles in the range of 25 to 30 degrees. No significant interobserver variability was found on 14 eyes measured by three different observers. This study indicates that the new technique is likely to be superior to currently used ultrasound and conventional optical pachymetry techniques, especially for refractive procedures.

Corneal pachymetric topography

PURPOSE

The authors developed a system for producing topographic pachymetric maps of the corneal epithelium and anterior scar tissue.

METHOD

The system uses high-frequency ultrasound scanning enhanced by digital signal processing. Ultrasonic echo data from consecutive parallel B-scans of the cornea spaced at 250-microns intervals are digitized and stored. Using the I-scan (obtained by computing the analytic signal magnitude of the deconvolved ultrasound signal), layer thickness measurements are made with a precision of 2 microns (standard deviation) at 120-microns intervals along each scan plane. The data are stored as an array, z(x,y), mapping thickness, z, onto horizontal and vertical (x,y) spatial coordinates. Pachymetric maps are then constructed by plotting local thickness, represented by a color scale, against measurement point position.

RESULTS

Examples of a normal cornea, a contact lens-wearing cornea, Reis-Bückler dystrophy, and postphotorefractive keratectomy are presented. Areas with significant subepithelial scarring and general epithelial thickening in a subject with Reis-Bückler dystrophy are mapped. Unevenness in the epithelial thickness profile of the cornea in a subject after photorefractive keratectomy is shown, relative to the fellow (untreated) cornea.

CONCLUSION

This technique provides the corneal surgeon with a new tool for the topographic evaluation of the thickness of anterior corneal layers in normal and pathologic corneas with high precision. In addition, the technique is not limited to optically transparent tissue.

Epithelial and corneal thickness measurements by high-frequency ultrasound digital signal processing

PURPOSE

The authors determine the mean central corneal and epithelial thickness in a group of normal human subjects using a new high-frequency ultrasound technique, incorporating digital signal processing.

METHOD

Both eyes of ten volunteers (age range, 23-44 years) were scanned through a normal saline standoff. Digitized ultrasonic echo data were mathematically transformed to produce a plot, the I-scan, which optimally localizes acoustic interfaces to provide improved measurement precision. System precision was determined by analysis of variance of repeated measures. Central epithelial thickness was obtained by averaging multiple measurements. Central corneal thickness was determined by fitting measurements of apparent corneal thickness in consecutive parallel B-scans to a mathematically modeled cornea. A speed of sound of 1640 m/second was used.

RESULTS

Epithelial pachymetric precision using A-scan and I-scan was 4.8 and 2.0 microns (standard deviation), respectively. The mean epithelial thicknesses for the right and left eyes were 50.7 +/- 3.7 microns and 50.3 +/- 3.4 microns, respectively. The mean corneal thicknesses in the right and left eyes were 514.6 +/- 38.4 microns and 516.2 +/- 37.8 microns, respectively. The root mean-square differences in epithelial and corneal thickness between the left and right eyes of each subject were 1.3 and 7.7 microns, respectively (neither was statistically significant).

CONCLUSION

This system provides a pachymetric precision superior to current optical and ultrasound methods. Epithelial and corneal pachymetry is obtained noninvasively by a method that is not limited to optically clear media.

Variation of corneal thickness with age in young New Zealanders

The central thickness of the left eyes of 1082 New Zealand students aged 5 to 20 years were measured using optical pachometry. No significant variation in corneal thickness was found with increasing age. The mean corneal thickness of the left eye was 540 +/- 25 microns. No significant differences in corneal thickness were found when the effects of sex or cultural groupings were examined. Corneal thickness appears to remain constant between the age of 5 and 20 years, irrespective of sex or cultural grouping.

The effect of corneal thickness on applanation tonometry

Simultaneous manometry and Perkins tonometry were performed at 10.0, 20.0, and 30.0 mm Hg on 15 eyes on which intraocular procedures were performed. There was a statistically significant relationship between corneal thickness and the error of Perkins tonometry. Thin corneas produced underestimations of the intraocular pressure by as much as 4.9 mm Hg, whereas thick corneas produced overestimations by as much as 6.8 mm Hg. Measuring the corneal thickness is necessary to interpret properly the results of Goldmann applanation tonometry, particularly in eyes with thin corneas.

Mêasurement of corneal thickness by low-coherence interferometry

A special interferometric technique, which uses light of low-coherence length and the Doppler principle, is applied to measurement of the thickness of the human cornea in vivo. The special construction of the instrument eliminates any influence from eye motions on the thickness results. With a superluminescent diode as a light source, a precision of ~ 1.5 microm is obtained. This is ~ 3-8 times better than the precision of existing instruments. Since interobserver and interinstrument variability are avoided by the measurement principle, the improvement in total accuracy, compared with that when existing instruments are used, should be even better.

The glaucoma suspect: differentiation of the future glaucomatous eye from the non-glaucomatous suspect eye. 1. Ultrasonic measurements and eye-wall stress

We examined the eyes of a group of patients entered into and still continuing in a glaucoma suspect study. Over time, some remained unchanged while others developed chronic open-angle glaucoma (COAG). By comparing the data obtained by ultrasonic measurement of the axial length (A) and central corneal thickness (CCT), along with measurement of intra-ocular pressure (IOP) from these patients and normal control subjects, we found no differences. However, by combining these factors in a calculation of eye-wall stress we found a significant difference between the glaucoma suspect and the future glaucomatous eye.

Corneal tissue mass in normal and keratoconic eyes. In vivo estimation based on area of horizontal optical sections

Based on measurements of the corneal diameter, radius of curvature variation, and thickness profile a method for calculation of the horizontal meridian's sectional tissue mass area (M) is described. Pooled values of 52 normal eyes demonstrate M to be 8.57 mm2 +/- SD. Comparing one eye of 27 keratoconic patients with one eye of 28 normals established no significant difference in M. This result is in accordance with a hypothesis in which the keratoconic corneal thinning is a result of an increased sliding of collagen fibers rather than a result of increased collagen degradation.

Corneal elasticity and ocular rigidity in normal and keratoconic eyes

The elastic properties of a given tissue may be characterized by Youngs Modulus (Y) which relates the stress (force per unit cross sectional area) and the resultant strain (the relative linear deformation). Cornea and sclera react to stress by a biphasic viscoelastic response consisting of a quick immediate deformation followed by a further slow deformation. Thus, it is necessary to distinguish between Y determined from the immediate elastic response (Yi) and Y determined in steady state (Ys). In a normal (n = 29) and a keratoconic group (n = 27) the ocular rigidity (E) was determined as estimate of the Yi of the ocular tunics. Further based on measurements of intraocular pressure, corneal diameter, corneal shape and thickness profile that value of Ys which represents elastic minimum energy content in the actual corneal membrane was estimated in the two groups. E as well as Ys were found to be significantly lower in keratoconic eyes compared with normals and uncorrelated to each other in the two groups.

Assessment of an elastic model in the pathogenesis of keratoconus

One eye of 27 patients with keratoconus and 37 normals were examined by photokeratoscopy and topographic pachometry to obtain comparative informations about the corneal shape and thickness profile, characterized by radius of the central curvature K, the coefficient of radius variation RV, the central corneal thickness T and the coefficient of thickness variation TV. RV and TV express the corneal central-peripheral variation in radius of curvature and thickness, respectively. Compared with normals the keratoconic corneas demonstrate a decrease in K and T and an increase in RV and TV. In the keratoconic eyes the decrease in K is positively correlated to the decrease in T. Further the decrease in K and T are both negatively correlated to the increase in RV and TV. Finally the increase in RV is positively correlated to the increase in TV. The results are in good accordance with an elastic model which implies increased distensibility of the keratoconic corneal tissue as a pathogenetic factor in the development of keratoconus.