Optical Pachymetry Research Articles

Intraoperative Corneal Thickness Measurement Using Optical Coherence Pachymetry and Corneo-Gage Plus Ultrasound Pachymetry

Purpose To compare the central corneal thickness measured by online optical coherence pachymetry (OCP) and ultrasound pachymetry in normal cornea. Methods Forty-eight right eyes of 48 consecutive patients were enrolled in this prospective study. Central corneal thickness measurements were taken intraoperatively with the online OCP and ultrasound pachymeter before flap creation. The precision repeatability, intraclass correlation coefficient, and correlation of variation for each instrument was calculated. Intraclass correlation coefficient was based on Bland-Altman plot of differences between instruments. Results Mean central corneal thickness for ultrasound pachymetry and online OCP was 559.45±33.05 µm (range: 475.50 to 650.50 µ m) and 521.19±28.97 µ m (range: 447.50 to 606.00 µ m), respectively. The precision (repeatability) was 7.86 µ m and 9.47 µ m, respectively. The intraclass correlation coefficient for the ultrasound pachymeter was 0.997 (95% CI, 0.993–0.998) and 0.993 (95% CI, 0.988–0.996) for the online OCP. The coefficient of variance was 0.50% and 0.66%, respectively. The mean difference between ultrasound pachymetry and online OCP was 38.26±9.96 µ m. The limits of agreement were: upper=58.19 µ m (95% CI, 53.30–63.07 µ m) and lower=18.34 µ m (95% CI, 13.45–23.22 µ m). Conclusions Online OCP can be used as a reliable alternative to ultrasound pachymetry as both instruments gave highly repeatable measurements of central corneal thickness. However, measurements using the OCP were consistently lower than those with the ultrasound pachymeter; therefore, the two techniques should not be used interchangeably. Further studies are needed to determine the implications of thinner central corneal thickness measured with online OCP as well as to resolve the systematic differences in measurements by these pachymetric technologies. [ J Refract Surg. 2008;24:610–614.]

Anterior Chamber Depth in Elderly Chinese: The Liwan Eye Study

To assess the anterior chamber depth (ACD) and its variation with age, gender, and angle width in elderly Chinese in an urban area of southern China. Cross-sectional study. Adults 50 and older were identified using cluster random sampling in Liwan District, Guangzhou. Gonioscopy was performed before ACD measurements to estimate the geometric angle width according to the Shaffer system. ACD was measured using optical pachymetry. True ACD was calculated by subtracting central corneal thickness from the distance between the anterior corneal epithelium and the anterior lens capsule. Data were presented for the right phakic eyes. Anterior chamber depth and gonioscopy. Among 1405 participants in the study, data from 1248 right eyes were available for analysis. The mean ACD values for men and women were 2.59 mm (95% confidence interval [CI], 2.56-2.62; 25th-75th percentile, 2.37-2.82) and 2.42 mm (95% CI, 2.39-2.44; 25th-75th percentile, 2.21-2.63). Mean ACD declined by 0.09 mm (95% CI, -0.011 to -0.008) per decade (adjusted for gender) and was 0.18 mm (95% CI, -0.213 to -0.141) shallower in women than men (adjusted for age). The ACD was found to be monotonically associated with gonioscopic angle width, decreasing from 2.73 mm (standard deviation [SD], 0.26) in Shaffer grade 4 to 1.94 mm (SD, 0.27) in Shaffer grade 0. There was also a relationship between ACD and refractive error; mean spherical equivalent decreased by 0.030 mm ACD per diopter. This study confirms an inverse association between ACD and age, female gender, and spherical refractive error. Eyes with shallower ACDs had narrower angles.

Comparison of Optical Low Coherence Reflectometry and Ultrasound Pachymetry in the Measurement of Central Corneal Thickness Before and After Photorefractive Keratectomy

To compare an ultrasound pachymeter with an optical low coherence reflectometry (OLCR) pachymeter for measuring pre- and postoperative central corneal thickness of patients undergoing photorefractive keratectomy (PRK). In a prospective, noncomparative, interventional study, 48 myopic eyes (mean manifest refractive spherical equivalent: -4.93 +/- 2.93 diopters [D]) of 30 healthy patients underwent PRK. Pre- and postoperative central corneal thickness was measured by two examiners (E1, E2) with an ultrasound pachymeter and an OLCR pachymeter. Agreement and inter-rater repeatability were determined using the comparison method described by Bland and Altman. The limits of agreement between the two devices ranged from 17.8 microm (E1) to 20.5 microm (E2) preoperatively and from 22.4 microm (E1) to 16.9 microm (E2) postoperatively. The coefficient of inter-rater repeatability ranged from 9.1 microm (ultrasound pachymeter) to 5.4 microm (OLCR pachymeter) preoperatively and from 7.1 microm (ultrasound pachymeter) to 4.7 microm (OLCR pachymeter) postoperatively. The OLCR pachymeter seems to show better pre- and postoperative repeatability compared to the ultrasound pachymeter. The agreement between the two devices should be considered acceptable for clinical practice. Photorefractive keratectomy did not affect the postoperative agreement and repeatability of the pachymeters.

Corneal Changes and Wavefront Analysis after Orthokeratology Fitting Test

To evaluate corneal changes and ocular aberrations during an orthokeratology test. A prospective, nonrandomized cohort study. Fourteen myopic patients (26 eyes) underwent an orthokeratology fitting test with the BE contact lens (Ultravision Pty, Ltd, Brisbane, Australia). Best spectacle-corrected visual acuity (BSCVA), uncorrected (Ultravision Pty, Ltd, Brisbane, Australia) visual acuity (UCVA), subjective cycloplegic refraction, biomicroscopy, corneal topography, optical pachymetry, and aberrometry were performed at baseline and one and eight nights orthokeratology. The short-term effect of orthokeratology using corneal topography, tomography, and ocular aberrations was evaluated. The mean spherical equivalent changed from -2.24 +/- 0.98 diopters (D) at baseline to 0.15 +/- 0.76 D after the eight nights of lens wear (P = .001). All patients had an UCVA of 20/30, 69.2% with 20/20. Changes in central corneal pachymetry were not observed. There was a statistically significant increase in the temporal corneal thickness from night one, without any difference between nights one and eight (P > .001). A significant increase of higher-order root mean square values was observed from baseline (0.42 +/- 0.16 mum), night one (0.81 +/- 0.24 mum), and night eight (1.04 +/- 0.24 mum). Increases in coma (Z7+Z8) and spherical aberration (Z12) were observed. Positive horizontal (Z8) coma increased in right eyes, and negative horizontal (Z8) coma increased in left eyes (P < .001). Myopia reduction resulting from rapid central corneal flattening and improvement of UCVA occurred after orthokeratology. Higher-order aberrations (HOAs), particularly spherical aberration and coma, increased significantly during the orthokeratology test. An increase of temporal pachymetry and differences in coma direction induced between the eyes may be related to the subclinical lens decentration temporally.

Preoperative Assessment of Corneal and Refractive Stability in Soft Contact Lens Wearing Photorefractive Candidates

This pilot study compared traditional methodologies; manifest refraction and keratometry, with that of newer technologies; corneal topography, and optical pachymetry, in assessing corneal and refractive stabilization after soft contact lens wear in photorefractive candidates. The timeline differences among these various technologies in determining refractive and corneal stability were investigated. This was a masked prospective observational clinical study of full-time soft contact lens subjects on various wear schedules, all eligible candidates for photorefractive surgery. Subjects discontinued contact lens wear within 30 min of the initial study visit. During each study visit, the same sequence of tests were performed (manifest refraction, keratometry, corneal topography, and optical pachymetry). The timing of the last visit was determined when the four procedures resulted in stable findings when compared with the previous visit. Fifteen soft contact lens wearers and five noncontact lens wearing controls completed the study. The mean number of days until stability of the 15 test subjects were: 10.7 +/- 10.4 days with manifest refraction, 16.2 +/- 17.5 days with keratometry, 28.1 +/- 17.7 days with topography, and 35.1 +/- 20.8 days with pachymetry. Within the control group, intraclass correlation coefficients for all four methods were > or =0.89, indicating very little variability. Analysis with the randomized block design found statistical differences between traditional and newer technologies in their assessment of stability (p < 0.001). One-way Analysis of variance of the various soft contact lenses modalities suggested extended hydrogel wearers taking the longest time to reach stability after discontinuing full-time contact lens use. Corneal curvature and thickness measurements took the longest to achieve consistency. Thus, topography and pachymetry may be better methods to determine ocular stability before photorefractive surgery. In light of this finding, the current protocol in practice of determining the readiness of contact lens candidates for photorefractive surgery may be inadequate.

Comparison of Optical Low-Coherence Reflectometry and Ultrasound Pachymetry in Measuring Corneal Graft Thickness

To compare optical low-coherence reflectometry (OLCR) and ultrasound pachymetry in measuring corneal graft thickness in patients after keratoplasty. We retrospectively measured the central graft thickness in 41 eyes of 41 patients with the OLCR pachymeter (Haag Streit, Koeniz, Switzerland) and the SP-2000 contact ultrasound pachymeter (Tomey, Nagoya, Japan). Five separate measurements were performed on each eye with both methods. Mean, SD, repeatability, and coefficient of variation of measurements were calculated, and the correlation between the 2 methods was studied with Spearman regression. Mean central graft thickness was 546 +/- 51 (SD) microm with the contact ultrasound pachymeter and 546 +/- 47 microm with the OLCR pachymeter. The correlation between both methods was strong (rs = 0.96). No significant differences in mean SD of measurements were observed between OLCR pachymetry (mean SD = 4.66 microm) and contact ultrasound pachymetry (mean SD = 4.88 microm). The repeatability of both methods was comparable (P = 0.06) and high (the average coefficient of variation of the central corneal graft thickness was 0.9% with both pachymeters). The postoperative time did not affect the correlation between both pachymeters (P > 0.05). Central corneal graft thickness values obtained with the OLCR pachymeter were similar to those obtained with a contact ultrasound pachymeter. In some cases of lamellar keratoplasty, the corneal refractive index could change at the interface level that could affect OLCR measurements.

Corneal Hysteresis, Resistance Factor, Topography, and Pachymetry After Corneal Lamellar Flap

To measure prospectively the early changes in corneal hysteresis, topography, and pachymetry after the creation of a stromal flap cut without laser photoablation. A 37-year-old man was referred for a bioptic procedure to correct for compound myopic astigmatism in the left eye. A 159-microm-thick 8x8.5-mm superior hinged flap was created with a mechanical microkeratome in the left cornea. Changes in the corneal hysteresis, corneal resistance factor, Goldmann correlated intraocular pressure (lOP), corneal compensated IOP, anterior and posterior topography, and optical and ultrasound pachymetry were monitored prospectively before and at 1 hour, 1 day, 5 days, and 25 days after flap creation. The right eye served as a control. In the left eye, corneal hysteresis and corneal resistance factor decreased immediately after the flap cut and remained lower than preoperatively at 1 hour, 1 day, 5 days, and 25 days. Corneal compensated IOP varied significantly less than Goldmann correlated IOP in both eyes. Central flattening of the horizontal meridians was observed on the difference topography maps. The values of the left eye posterior best fit sphere increased after the flap cut. Increased central corneal thickness occurred immediately after the flap cut and decreased over time without returning to its preoperative value. The creation of a stromal flap can modify the biomechanical properties of the cornea, including a reduction in corneal hysteresis. The topographic changes were consistent with previously reported cases of flap cut in normal corneas.

Reproducibility of IntraLASIK Flap Thickness Measured with Optical Coherence Tomography

Purpose: To investigate the relationship among optical coherence tomography (OCT), ultrasound pachymetry, and Orbscan in central corneal thickness measurement and to evaluate the reproducibility of flap thickness using an IntraLase femtosecond laser. Methods: Central corneal thickness was measured by OCT, ultrasound pachymetry, and Orbscan in 59 eyes of 30 patients before LASIK. After IntraLASIK, the corneal flap thickness measured using OCT was compared with the intended corneal flap thickness. Results: Central corneal thickness measured by OCT was thinner than that measured by other instruments preoperatively, but there was no significant difference among these methods (p>0.01), and corneal thickness values obtained by ultrasound pachymetry and Orbscan correlated well with those obtained by OCT (r ranged from 0.804 to 0.889, p0.01). Conclusions: OCT is a relatively accurate instrument for measuring corneal thickness and can easily measure the corneal flap thickness after LASIK. Compared with the results of a previous study, the mean measured flap thickness in this study was more reproducible with the IntraLase femtosecond laser.

Online optical coherence pachymetry to evaluate intraoperative ablation parameters in LASIK

The ablation of corneal tissue with the excimer laser can be variable and can lead to miscorrections. The purpose of this study was to evaluate intraoperative ablation parameters during laser-assisted in-situ keratomileusis (LASIK) with online optical coherence pachymetry (OCP). In a prospective, nonrandomized, comparative clinical study, the ablation parameters were continuously assessed intraoperatively with online OCP (Heidelberg Engineering, Lübeck, Germany) in 45 myopic and 10 hyperopic LASIK treatments. The central intraoperative ablation values were compared with the calculated values of the excimer laser (ESIRIS, Schwind, Germany) and the postoperative refraction. The ablation process and the ablation rate in mum per layer, time, and dioptric correction were evaluated in myopic corrections. In myopic LASIK treatments, a linear ablation process was measured with a mean correlation coefficient of -0.968 +/- 0.04. The intraoperative ablation rate was, on average, 0.59 +/- 0.17 microm per layer, 1.45 +/- 0.48 microm per second, and 24.63 +/- 7.81 microm per corrected diopter. These values were 28.7% to 29.6% higher (P < 0.001) than the calculated values. There was a significant correlation (P < 0.001) for the ablation rate per layer (r = 0.823), per second (r = 0.869), and corrected diopter (r = 0.892), but no correlation (r = 0.21, P = 0.239) between the measured linear ablation process and the postoperative refraction. During hyperopic LASIK treatments, without ablation of the corneal center, there was a significant decrease (P = 0.005) of the stromal thickness by 18.34 +/- 14.13 microm, which corresponded to a mean corneal dehydration rate of 0.27 microm per second. Online OCP allowed a clinical evaluation of intraoperative ablation parameters in LASIK. Further studies are needed to assess a possible active control of the excimer laser ablation from these continuous values, which could possibly improve current ablation nomograms.

Laser Peripheral Iridotomy in Primary Angle-Closure Suspects: Biometric and Gonioscopic Outcomes: The Liwan Eye Study

Purpose To assess the immediate effect of laser peripheral iridotomy (LPI) and mechanisms of angle closure in a population-based study of primary angle closure (PAC) suspects. Design Prospective interventional study. Participants People identified as PAC suspects aged 50 to 79 years from a population-based survey in Guangzhou, China. Intervention Laser peripheral iridotomy was performed in 1 randomly selected eye. Examinations were carried out before and 2 weeks after the intervention. Main Outcome Measures Intraocular pressure (IOP), ultrasound biometry, optical pachymetry, and gonioscopy. Results A total of 72 people with bilateral suspected PAC participated in the study. Mean IOP decreased by 3 mmHg (P<0.001), but axial anterior chamber depth did not change significantly (P = 0.784) after LPI. Median limbal anterior chamber depth increased from 15% to 25% of peripheral corneal thickness (P<0.001, Wilcoxon signed-rank test). Median iridotrabecular angle width increased from 0° to 10° in the superior quadrant and from 10° to 30° in the inferior quadrant (P<0.001). Nevertheless, 14 eyes (19.4%) still had 3 or more quadrants in which the posterior (usually pigmented) trabecular meshwork could not be seen after laser iridotomy. Conclusions This study confirms that LPI results in a significant increase in the angle width in Chinese people with narrow angles. However, one fifth of eyes had residual angle closure after LPI. Although this report confirms that iridotomy widens the anterior chamber angle in most PAC suspects, long-term prospective studies with a larger sample size are required to determine if the risks of PAC glaucoma and other related pathologic sequelae are reduced after prophylactic LPI and to investigate the risk-to-benefit ratio before recommending widespread use of prophylactic LPI in this population. To assess the immediate effect of laser peripheral iridotomy (LPI) and mechanisms of angle closure in a population-based study of primary angle closure (PAC) suspects. Prospective interventional study. People identified as PAC suspects aged 50 to 79 years from a population-based survey in Guangzhou, China. Laser peripheral iridotomy was performed in 1 randomly selected eye. Examinations were carried out before and 2 weeks after the intervention. Intraocular pressure (IOP), ultrasound biometry, optical pachymetry, and gonioscopy. A total of 72 people with bilateral suspected PAC participated in the study. Mean IOP decreased by 3 mmHg (P<0.001), but axial anterior chamber depth did not change significantly (P = 0.784) after LPI. Median limbal anterior chamber depth increased from 15% to 25% of peripheral corneal thickness (P<0.001, Wilcoxon signed-rank test). Median iridotrabecular angle width increased from 0° to 10° in the superior quadrant and from 10° to 30° in the inferior quadrant (P<0.001). Nevertheless, 14 eyes (19.4%) still had 3 or more quadrants in which the posterior (usually pigmented) trabecular meshwork could not be seen after laser iridotomy. This study confirms that LPI results in a significant increase in the angle width in Chinese people with narrow angles. However, one fifth of eyes had residual angle closure after LPI. Although this report confirms that iridotomy widens the anterior chamber angle in most PAC suspects, long-term prospective studies with a larger sample size are required to determine if the risks of PAC glaucoma and other related pathologic sequelae are reduced after prophylactic LPI and to investigate the risk-to-benefit ratio before recommending widespread use of prophylactic LPI in this population.

Will Keratectasia be a Major Complication for Lasik in the Long Term?

Whereas the biomechanical factors of the cornea with respect to elasticity and posterior corneal asphericity have been partly elucidated clinically by Roberts et al and experimentally by Muller et al, the effect of the lasik flap on the eventual outcome has not been entirely evaluated. While Barraquer described the importance of total corneal thickness (TCT) and residual stromal bed thickness (RSB) in the prevention of post MKM keratectasia, and Ruiz and Casebeer, the significance of leaving at least 30% TCT beneath an ALK incision for hyperopia, it has been shown by Reinstein using Artemis 3D pacymetry in cases of progressive post lasik keratectasia (PPLK), that an inadvertently deep lasik flap can compromise the RSB resulting in frank keratectasia post operatively. The significance of PPLK in eyes with forme frust keratoconus (KC) has recently been questioned by Binder et al in a recent editorial. Bearing in mind the incidence of ordinary KC in the population which varies from 50–230 per 100000 (Yang et al) and the reported incidences of 0.12% and 0.66% PPLK by Reinstein and Pallikaris, one would expect to find a greater number of PPLK reported in the world literature which consists of a total 87 eyes (Binder). In a recent review of 137 eyes of highly myopic patients (Ave −14.75D) ooperated by the author over a five year period between 1994–1999, where intra operative US pachymetry measuring TCT and RSB were carried out, in only one retreatment eye in which this was not done and the RSB compromised, was PPLK produced. All the other eyes showed a completely stable topgraphy and refraction over the 9 year follow up period. In the future, evaluation of these cases, it will be important that other characteristics of the cornea such as viscoelasticity, shear modulus, and plasticity be analysed with techniques such as dynamic corneal imaging, ocular hysteresis and the use of electronic speckle pattern intrerferometry (ESP). From a practical point of view, the use of non touch online optical coherence pachymetry should help to avoid the possibility of excessively deep flaps especially with the use of some mechanical microkeratomes.

Corneal Pachymetry Mapping with High-speed Optical Coherence Tomography

To map corneal thickness before and after LASIK with optical coherence tomography (OCT). Cross-sectional observational study. Forty-two eyes of 21 normal subjects undergoing LASIK. A high-speed (2000 axial scans/second) 1.3-microm-wavelength corneal and anterior segment OCT prototype was used for corneal scanning. The scan pattern consisted of 10-mm radial lines on 8 meridians centered on the vertex reflection. The entire scan pattern of 1024 a-scans was acquired in 0.5 seconds. We developed automated computer processing for 3-dimensional corneal reconstruction and measurement. Corneal thickness was measured normal to the anterior surface and presented as color pachymetry maps and zonal statistics. The maps were divided into a central zone (<2 mm) and 3 annular areas (pericentral, 2-5 mm; transitional, 5-7 mm; peripheral, 7-10 mm), which were further divided into quadrantal zones. The average, minimum, and maximum corneal thicknesses were computed for zones within the 7-mm diameter. Optical coherence tomography and ultrasound pachymetry were measured 3 times at the preoperative and 3-month postoperative visits. Reproducibility was assessed by the pooled standard deviations (SDs) of the repeated measurements. Optical coherence tomography pachymetric map and zonal statistic, and ultrasound pachymetry. Before LASIK, central corneal thicknesses (CCTs) were 546.9+/-29.4 microm (mean +/- SD) for OCT and 553.3+/-33.0 microm for ultrasound. After LASIK, CCTs were 513.7+/-44.5 microm for OCT and 498+/-46.6 microm for ultrasound. Optical coherence tomography and ultrasound CCT were highly correlated (Pearson correlation r = 0.97 before LASIK and 0.98 afterwards). Optical coherence tomography CCT was slightly less than ultrasound CCT before surgery (mean difference, -6.4 microm; 95% limits of agreement, -23.2 to 10.4 microm) but slightly greater after LASIK (15.7 microm; -1.6 to 33 microm). These differences were statistically significant, but no more than the CCT measurement differences between ultrasound pachymeters. The reproducibility of the OCT zonal pachymetry averages was roughly 2 microm. High-speed OCT provided noncontact, rapid, reproducible pachymetric mapping over a wide area of the cornea. It is equivalent to ultrasound for CCT measurement before and after LASIK. This technology could be valuable for planning keratorefractive procedures and diagnosis of corneal diseases.

Detection of Narrow Angles and Established Angle Closure In Chinese Residents of Singapore: Potential Screening Tests

To assess the performance of peripheral and central anterior chamber depth (ACD) measurements, and autorefraction, in identifying Chinese people with occludable drainage angles or established primary angle closure. Population-based cross-sectional analysis. Subjects aged 40 years and older were drawn from a representative sample of Chinese Singaporean residents. Limbal chamber depth (LCD) was estimated and central ACD was measured using optical pachymetry and ultrasound. An autorefraction was taken for all subjects. Angle width and the presence of peripheral anterior synechiae were determined by gonioscopy. Receiver operating characteristic curves were generated to show the performance of these tests in detecting occludable angles. A total of 1090 subjects were examined. The prevalence of occludable angles was 6.5%. LCD measurement performed best in detecting occludable angles (area under the curve [AUC] 0.904, 95% confidence interval [CI] 0.870, 0.937). Optical pachymetry and ultrasound measurement of ACD performed less well (AUC 0.859 and 0.810, respectively), and autorefraction performed poorly in detecting occludable angles (AUC 0.632). LCD also gave the best performance in detecting primary angle closure (AUC 0.782, 95% CI 0.727, 0.837). Limbal LCD estimation outperforms other methods of measuring ACD as a screening tool for the detection of occludable drainage angles in the Chinese population of Singapore.

Continuous Measurement of Corneal Dehydration With Online Optical Coherence Pachymetry

Online optical coherence pachymetry (online OCP) allows continuous central corneal thickness measurements over time. In this study, the effect of dehydration on corneal tissue was investigated with online OCP. Twelve eyes of 11 patients were examined with online OCP, and the central corneal thickness was registered over 5 minutes after insertion of an eyelid speculum. Online OCP measurements revealed no decrease in reproducibility after 5 minutes of dehydration. The initial mean central corneal thickness was 538 +/- 48 microm. After 5 minutes the central corneal thickness decreased to 483 +/- 43 microm (P = 0.001). This corresponded to a mean corneal thinning of 55 +/- 4 microm (10.2%) at a rate of 0.19 microm/s. Online OCP was suitable for continuous measurements of corneal changes caused by dehydration. These dehydration effects should be particularly considered in refractive corneal surgery.

The Agreement and Repeatability of Tear Meniscus Height Measurement Methods

This study investigated the agreement and repeatability of five techniques of measuring tear meniscus height (TMH). Pairs of TMH measurements were obtained from one eye of 25 normal subjects with five techniques: optical coherence tomography in cross-section (OCT) and optical pachymetry in cross-section (PACH), without fluorescein; and 5 minutes after the instillation of fluorescein, optical pachymetry en-face (SL-F) and in cross-section (SL-X), and video capture en-face (VC). These 10 measurements were taken on different days within a period of 21 days. Additionally, immediately successive replicates were taken at the first visit when VC was used, allowing for the separation of technique repeatability from true day-to-day variation in TMH. Ninety-five percent confidence intervals for the mean TMH and 95% repeatability coefficients for repeated testing on separate occasions were: 0.25 to 0.29 mm, 0.14 mm--OCT, 0.30 to 0.36 mm, 0.13 mm--PACH, 0.27 to 0.34 mm, 0.092 mm--SL-F, 0.35 to 0.41 mm, 0.19 mm--SL-X, and 0.30 to 0.38 mm, 0.088 mm--VC. Reported repeatability coefficients are averages because, despite a relatively narrow experimental range, the measurements of low TMH were generally more repeatable than those of large TMH. Ninety-five percent limits of agreement were wide for all pairwise comparisons and included zero, indicating that the methods used in this study are interchangeable when taking single recordings, at least on subjects without dry eye. The average TMH of the five techniques was similar, although OCT and SL-X tended to record lower and higher values, respectively. However, their agreement for single recordings was poor, an inevitable consequence of the low intertest repeatability found for all methods. A much improved repeatability for immediate replicates indicated that this, in turn, was principally caused by a large day-to-day variation in TMH rather than measurement error.

Central and paracentral corneal pachymetry in patients with normal tension glaucoma and ocular hypertension

The difference in central corneal thickness among subgroups of glaucoma patients, as well as its influence on Goldmann applanation tonometry, has been well documented in several clinical trials. In the present study, possible similarities and differences between central corneal thickness and corneal thickness of paracentral quadrants in patients with normal tension glaucoma (NTG) and ocular hypertension (OHT) were investigated. Central and paracentral corneal thickness was measured by optical slit scan pachymetry (Orbscan II). Fourteen patients (28 eyes) with NTG and 11 patients (22 eyes) with OHT were included in this study. t-Test was performed for statistical analysis. To evaluate overall corneal topography, the mean and SD values of the differences between the central corneal thickness and each peripheral quadrant were analysed. The following data was obtained (microm): (central, upper, temporal, nasal, inferior paracentral quadrant): OHT group 617-695-663-687-660. NTG group 568-629-593-612-616. Corneal thickness of all four paracentral quadrants differed significantly between the OHT and NTG groups. There was a more heterogeneous intraindividual pattern of overall corneal topography in the OHT group, and a more heterogeneous pattern of corneal topography among the individuals of the NTG group (interindividual heterogeneity). A comparison of central corneal thickness and paracentral corneal thickness revealed clinically relevant differences between the OHT and NTG groups. The presented data underlines the importance of correlating the site of applanation with the corresponding corneal thickness, especially in OHT patients. It further substantiates the necessity to obtain individual pachymetric data for each NTG patient.

Central corneal thickness in high myopia

To examine if central corneal thickness (CCT) is different in emmetropia and high myopia. 57 emmetropic subjects (0 to + 1.5 D) and 48 high myopes (all more than - 6 D in spherical equivalent refraction) were studied. CCT was measured by a Haag-Streit Optical Low-Coherence Reflectometry (OLCR) pachymeter, a recently developed high precision pachymeter with a standard deviation (SD) for repeated measurements of 1 microm. Mean CCT for the emmetropic group was 538.6 microm (SD = 32.1), and for the myopic group 527.7 microm (SD = 35.0). Neither the mean CCT nor the variance from the two groups showed a statistically significant different (p > 0.05). CCT is not systematically altered in myopia. The process by which the myopia progresses does not to a measurable degree influence the central cornea.

Corneal Thickness in an Ophthalmologically Normal Japanese Population

To investigate the distribution of central corneal thickness (CCT) and factors correlating with CCT in a large-scale population of ophthalmologically normal Japanese persons. Cross-sectional study. Participants (n = 7313; 2848 men and 4465 women) were selected as an ophthalmologically normal Japanese sample using the following exclusion criteria: abnormal results suggestive of any eye diseases revealed on slit-lamp examination, fundus photography, or visual field test using frequency doubling technology; corrected visual acuity worse than 0.7; and contact lens wear within 1 week. All study participants were identified from The Eye Disease Screening Service in Tajimi city during the period from September, 2000, through October, 2001. The CCT was measured by SP-2000P specular microscopy (Topcon, Tokyo, Japan). Age, gender, body mass index, systolic blood pressure, laterality, refraction, corneal radius, and intraocular pressure (IOP) data were collected from the records. The CCT value and its correlating factors. Average CCT in this ophthalmologically normal Japanese population was 517.5+/-29.8 microm (mean+/-standard deviation; n = 7313) and was greater in men (521.5+/-30.3 microm) than in women (514.4+/-29.0 microm). The CCT positively correlated with IOP and corneal radius and negatively correlated with refraction, although the correlation was weak. Intraocular pressure measured with a Goldmann applanation tonometer positively correlated with CCT, and the correction was not affected by age (corrected IOP = IOP reading -0.012 [CCT (microm) - 520]). Using optical pachymetry, CCT in normal Japanese persons was defined as 517.5+/-29.8 microm. Men had greater CCT measurements than women. The CCT measurements positively correlated with IOP and corneal radius and negatively correlated with age and refraction in men. Intraocular pressure positively correlated with CCT, regardless of age.

Anterior Chamber Depth and the Risk of Primary Angle Closure in 2 East Asian Populations

To evaluate the risk of peripheral anterior synechiae (PAS) and glaucomatous optic neuropathy attributable to primary angle closure at varying anterior chamber depths (ACDs) in two East Asian populations. Participants 40 years and older were from 2 population-based glaucoma surveys in Mongolia and Singapore. Central ACD was measured by optical pachymetry. Presence of PAS was determined by dynamic gonioscopy. Cases with secondary PAS were excluded. Glaucomatous optic neuropathy was diagnosed in subjects with structural and functional evidence of glaucoma. A total of 2032 subjects, consisting of 942 Mongolians and 1090 Chinese Singaporeans, were included in this study. A logistic model of the relationship between ACD and PAS among Singaporeans showed a consistent, incremental increase in PAS across the entire range of ACD. In deeper anterior chambers the rate of PAS exceeded that seen in Mongolians. In Mongolians, there was a clear threshold for ACD (2.4 mm) at or above which PAS were very uncommon. With ACD less than 2.4 mm, the rate of PAS rose rapidly to overtake that seen in Singaporeans. Shallow anterior chambers are a significant risk factor for angle closure in East Asians, although the nature of the association is specific to the individual population. There was a trend toward higher rates of glaucomatous optic neuropathy in people with the shallowest anterior chambers.

Long-term follow-up of ultrathin corneas after surface retreatment with phototherapeutic keratectomy

To evaluate visual acuity and long-term stability after phototherapeutic keratectomy (PTK) in patients with corneal thickness less than 400 microm and cornea-related vision problems. Department of Ophthalmology, Istituto Clinico Humanitas, Rozzano-Milan, Italy. This retrospective study comprised 48 eyes that had PTK for complications after photorefractive keratectomy that removed more than 200 microm of tissue. Phototherapeutic keratectomy was performed with the Nidek EC-5000 excimer laser with 10 Hz frequency, intraoperative topography, and masking fluid over an ablation diameter of 10.0 mm. The follow-up was 5 years. The mean preoperative best spectacle-corrected visual acuity (BSCVA) was 0.2 +/- 0.09 (SD) with -2.53 +/- 2.34 diopters (D), and the mean pachymetry was 390 +/- 38 microm. The efficacy parameters were pachymetry and visual acuity at 1 year and the stability parameters, optical refraction, corneal curvature (calculated as the mean curvature over the entire 3.0 mm and 5.0 mm central zones), and pachymetry from 1 to 5 years. At 1 year, the mean BSCVA was 0.6 +/- 0.72 with -2.15 +/- 1.67 D and the mean pachymetry, 341 +/- 40 microm. At 5 years, the mean BSCVA was 0.7 +/- 0.15 with -2.33 +/- 1.12 D and the mean pachymetry, 339 +/- 48 microm. In all patients, there was an improvement of 4 or more Snellen lines. Statistical evaluation of refraction and corneal curvature values at 1 and 5 years indicated no statistically significant differences. After PTK in eyes with ultrathin corneas, there was significant improvement in BSCVA and long-term stability of the optical refraction, corneal curvature, and pachymetry. In selected cases, 10.0 mm ablation zone PTK may be an alternative to penetrating keratoplasty, offering long-term corneal stability.