Tonometry after laser in situ keratomileusis treatment

Intraocular pressure after myopic laser refractive surgery measured with a new Goldmann convex prism: correlations with GAT and ORA

Background

The purpose of this study is to describe measurements using a newly developed modified Goldmann convex tonometer (CT) 1 year after myopic laser refractive surgery. Intraocular pressure (IOP) measurements were compared with IOP values obtained by Goldmann applanation tonometer (GAT), and Ocular Response Analyzer (ORA).

Methods

Prospective double-masked study performed on thirty eyes of thirty patients that underwent laser in situ keratomileusis (LASIK; n = 19) or photorefractive keratectomy (PRK; n = 11). IOP was measured before and 3 and 12 months after surgery. Intraclass correlation coefficient (ICC) and Bland-Altman plot were calculated to assess the agreement between GAT, CT, IOPg (Goldmann-correlated IOP) and IOPcc (corneal-compensated IOP) from ORA.

Results

Twelve months after LASIK, IOP measured with CT showed the best correlation with IOP measured with GAT before surgery (GATpre) (ICC = 0.886, 95% CI: 0.703–0.956) (15.60 ± 3.27 vs 15.80 ± 3.22; p < 0.000). However, a moderate correlation was found for IOP measured with IOPcc and CT 12 months after LASIK (ICC = 0.568, 95% CI: − 0.185 – 0.843) (15.80 ± 3.22 vs 12.87 ± 2.77; p < 0.004). Twelve months after PRK, CT showed a weak correlation (ICC = − 0.266, 95% CI: − 3.896 – 0.663), compared to GATpre (17.30 ± 3.47 vs 16.01 ± 1.45; p < 0.642), as well as poor correlation (ICC = 0.256, 95% CI: − 0.332 – 0.719) with IOPcc (17.30 ± 3.47 vs 13.38 ± 1.65; p < 0.182).

Conclusions

Twelve months after LASIK, IOP measured with CT strongly correlated with GAT before surgery and could therefore provide an alternative method for measuring IOP after this surgery. More studies regarding this new convex prism are needed to assess its accuracy.

Reliability of Intraocular Pressure Measurement by Goldmann Applanation Tonometry After Refractive Surgery: A Review of Different Correction Formulas

Myopia is one of the main risk factors for the onset of open-angle glaucoma. One of the first steps to assess glaucoma occurrence is the measurement of the intraocular pressure (IOP) by the Goldmann applanation tonometry (GAT). Even if this device is considered to be the gold standard for such measurements, it is affected by several sources of errors. Among these, there are the corneal thickness and curvature, both modified by corneal refractive surgery (CRS), that nowadays has become a very popular method to treat refractive errors. Indeed, CRS, by modifying the corneal shape and structure, causes an underestimation of the IOP measurements. In the literature, several IOP correction formulas to utilize with different devices have been proposed to overcome this problem. This paper aims to review the various correction formulas applied to the GAT in the attempt to improve the reliability of this measurement.

New applanation tonometer for myopic patients after laser refractive surgery

This study assesses the agreement between intraocular pressure (IOP) measurements taken with the Goldmann applanation tonometer (GAT) and a new experimental applanation tonometer with a convexly shaped apex (CT) after laser myopic refractive surgery. Two different CT radii (CT1 and CT2) were designed with a finite element analyser, and a prospective double masked study on 102 eyes from 102 patients was carried out. A Bland-Altman plot and intra-class correlation coefficient (ICC) were calculated to assess the agreement between GAT measurements and the measurements of both CT1 and CT2 before and after myopic laser assisted in situ keratomileusis (LASIK; n = 73) and photorefractive keratectomy (PRK; n = 29). We evaluated a subset of two subgroups (n = 36 each) for intra and inter-observer (IA/IE) error. From the whole cohort, the best IOP agreement was observed between GATpre and CT1post surgery: 16.09 ± 2.92 vs 16.42 ± 2.87 (p < 0.001); ICC = 0.675 (95% CI: 0.554-0.768). In the analysis of LASIK vs PRK, GATpre and CT1post showed the highest agreement, although LASIK measurements were more accurate than PRK, as the ICC = 0.718 (95% CI: 0.594-0.812) and ICC = 0.578 (95% CI: 0.182-0.795) respectively. Excellent agreement was observed for IA/IE, and there was an ICC > 0.8 (95% CI) in all cases. CT1 proved more accurate in the LASIK subgroup. In conclusion, our new version of GAT could be used with post-surgery LASIK patients as a more accurate measurement device compared to the current reference tonometer.

Pneumotonometer Accuracy Using Manometric Measurements after Radial Keratotomy, Clear Corneal Incisions and Lamellar Dissection in Porcine Eyes

Purpose/Aim of the study: Measured intraocular pressure (IOP) after corneal incisions may not be reflective of the true IOP because of changes in corneal biomechanical properties. The purpose of this study is to investigate the effect of various corneal incisions on pneumotonometer accuracy in enucleated porcine eyes.Materials and Methods: A pneumotonometer was used to measure IOP (IOPp) at manometrically controlled pressure levels of 10, 20, 30 and 40 mmHg in enucleated porcine eyes. IOP measurements at each level were repeated after one of the following corneal incisions: radial keratotomy (8 eyes), lamellar dissection (10 eyes), clear cornea standard phacoemulsification incisions (10 eyes). The pneumotonometer error, defined as the difference between IOPp and manometric pressure (IOPm), was calculated for each pressure level. The error before the corneal incisions was compared to the error after the corneal incisions to assess the accuracy of the pneumotonometer.Results: The pneumotonometer underestimates true IOP at all pressure levels, both before and after the corneal procedures. There was a statistically significant greater underestimation of IOP after radial keratotomy incisions at pressure levels of 20, 30 and 40 mmHg (p = .013, 0.004, and 0.002, respectively). There was no statistically significant difference in the amount of pneumotonometer underestimation error after lamellar dissection or standard cataract incisions.Conclusion: The pneumotonometer underestimates true IOP in enucleated porcine eyes at all pressure levels between 10-40 mmHg. Radial keratotomy incisions caused a statistically significant greater underestimation error in pneumotonometry measurements at pressures of 20-40 mmHg. Lamellar dissection and clear corneal cataract incisions did not cause an additional error in pneumotonometry measurements in enucleated porcine eyes.

Delayed-onset interface fluid syndrome after LASIK following phacotrabeculectomy

BACKGROUND

Interface fluid syndrome (IFS) is an unusual complication after laser-assisted in-situ keratomileusis (LASIK). We report the first case of IFS after uncomplicated phacotrabeculectomy in a patient who had undergone LASIK 10 years previously. This case emphasizes the importance of intraocular pressure (IOP) interpretation in eyes that have undergone LASIK.

CASE PRESENTATION

A 30-year-old woman with a history of LASIK surgery presented to glaucoma clinic due to uncontrolled IOP despite of maximally tolerable medical treatment. After receiving phacotrabeculectomy, IOP decreased to 3 mmHg on the first postoperative day, but again increased up to 21 mmHg and a diffuse corneal edema with cloudy flap interface was demonstrated by slit-lamp microscopy. Corneal edema was sustained even after the IOP was lowered to 14 mmHg. Spectral-domain optical coherence tomography scanning of the cornea revealed a diffuse, thin fluid pocket in the corneal interface. After laser lysis of the scleral flap sutures, IOP was further decreased to 9 mmHg and interface fluid was resolved.

CONCLUSION

IFS should be considered as a possible cause of postoperative corneal edema despite of low IOP in the eyes that underwent LASIK surgery. Additional IOP lowering may be helpful for resolving the corneal edema.

Is the PASCAL®-Tonometer Suitable for Measuring Intraocular Pressure in Clinical Routine? Long- and Short-Term Reproducibility of Dynamic Contour Tonometry

Purpose

Dynamic contour tonometry (DCT) is a new technique to measure intraocular pressure (IOP). In several studies no correlation between IOP values and corneal thickness was shown with DCT. This is in contrast to the gold standard, Goldmann applanation tonometry (GAT). The authors evaluated the reproducibility (RP) of DCT compared to GAT, a prerequisite for its introduction into clinical routine.

Methods

IOP was measured with both DCT and GAT in 50 subjects with normal cornea. To evaluate the short-term RP, four DCT and four GAT measurements were performed at day 1 in a randomized order. Long-term RP was determined by one additional measurement per method at day 2, 5, and 8.

Results

The short-term RP was defined as the mean value of all standard deviations (SD) of the individual measurements at day 1. Short-term RP was 1.1 mmHg for GAT and 1.2 mmHg for DCT. For long-term reproducibility, mean SD was 1.2 mmHg for GAT and 1.5 mmHg for DCT. Bland-Altman revealed a good agreement of the two methods. However, mean DCT values were on average 0.8±1.1 mmHg higher than GAT values. A significant correlation was observed between GAT and CCT (r2=0.15, p=0.006), but not between DCT and CCT (r2=0.032, p=0.21).

Conclusions

Short- and long-term reproducibility of DCT is comparable to that of GAT. GAT is more affected by CCT than DCT, measuring higher IOPs in eyes with higher central corneal thickness.

A sample predictive model for intraocular pressure following laser in situ keratomileusis for myopia and an "intraocular pressure constant"

PURPOSE

To describe a sample predictive model for intraocular pressure (IOP) following laser in situ keratomileusis (LASIK) for myopia and an IOP constant.

METHODS

The records of patients that underwent LASIK for myopia and myopic astigmatism via WaveLight Allegretto Wave Eye-Q 400 Hz excimer laser and Hansatome XP microkeratome were retrospectively reviewed. Patients with no systemic or ocular disease other than myopia or myopic astigmatism were included in the study. Preoperative and postoperative month 1 data and intraoperative data were used to build the predictive model for IOP after LASIK. The IOP constant was calculated by subtracting the predicted IOP from preoperative IOP. The paired samples t test, Pearson's correlation analysis, curve estimation analysis, and linear regression analysis were used to evaluate the study data.

RESULTS

The study included 425 eyes in 214 patients with a mean age of 32 ± 7.8 years. Mean spherical equivalent of the attempted correction (SE-ac) was -3.7 ± 1.7 diopters. Mean post-LASIK decrease in IOP was 4.6 ± 2.3 mmHg. The difference between preoperative and postoperative IOP was statistically significant (P < 0.001). SE-ac, preoperative IOP, and central corneal thickness had highly significant effects on postoperative IOP, based on linear regression analysis (P < 0.001 and R ² = 0.043, P < 0.001 and R ² = 0.370, and P < 0.001 and R ² = 0.132, respectively). Regression model was created (F = 127.733, P < 0.001), and the adjusted R ² value was 0.548.

CONCLUSIONS

Evaluation of IOP after LASIK is important in myopic patients. The present study described a practical formula for predicting the true IOP with the aid of an IOP constant value in myopic eyes following LASIK.

Ageing and ocular surface immunity

Foreword

It gives me pleasure to introduce the 4th edition of the EGS Guidelines. The Third edition proved to be extremely successful, being translated into 7 languages with over 70000 copies being distributed across Europe; it has been downloadable, free, as a pdf file for the past 4 years. As one of the main objectives of the European Glaucoma Society has been to both educate and standardize glaucoma practice within the EU, these guidelines were structured so as to play their part.

Glaucoma is a living specialty, with new ideas on causation, mechanisms and treatments constantly appearing. As a number of years have passed since the publication of the last edition, changes in some if not all of these ideas would be expected.

For this new edition of the guidelines a number of editorial teams were created, each with responsibility for an area within the specialty; updating where necessary, introducing new diagrams and Flowcharts and ensuring that references were up to date. Each team had writers previously involved with the last edition as well as newer and younger members being co-opted.

As soon as specific sections were completed they had further editorial comment to ensure cross referencing and style continuity with other sections.

Overall guidance was the responsibility of Anders Heijl and Carlo Traverso. Tribute must be made to the Task Force whose efforts made the timely publication of the new edition possible.

Roger Hitchings

Chairman of the EGS Foundation

www.eugs.org

The Guidelines Writers and Contributors 

Augusto Azuara Blanco

Luca Bagnasco

Alessandro Bagnis

Keith Barton

Christoph Baudouin

Boel Bengtsson

Alain Bron

Francesca Cordeiro

Barbara Cvenkel

Philippe Denis

Christoph Faschinger

Panayiota Founti

Stefano Gandolfi

David Garway Heath

Francisco Goni

Franz Grehn

Anders Heijl

Roger Hitchings

Gabor Hollo

Tony Hommer

Michele Iester

Jost Jonas

Yves Lachkar

Giorgio Marchini

Frances Meier Gibbons

Stefano Miglior

Marta Misiuk-Hojo

Maria Musolino

Jean Philippe Nordmann

Norbert Pfeiffer

Luis Abegao Pinto

Luca Rossetti

John Salmon

Leo Schmetterer

Riccardo Scotto

Tarek Shaarawy

Ingeborg Stalmans

Gordana Sunaric Megevand

Ernst Tamm

John Thygesen

Fotis Topouzis

Carlo Enrico Traverso

Anja Tuulonen

Ananth Viswanathan

Thierry Zeyen

The Guidelines Task Force 

Luca Bagnasco

Anders Heijl

Carlo Enrico Traverso

Augusto Azuara Blanco

Alessandro Bagnis

David Garway Heath

Michele Iester

Yves Lachkar

Ingeborg Stalmans

Gordana Sunaric Mégevand

Fotis Topouzis

Anja Tuulonen

Ananth Viswanathan

The EGS Executive Committee 

Carlo Enrico Traverso (President)

Anja Tuulonen (Vice President)

Roger Hitchings (Past President)

Anton Hommer (Treasurer)

Barbara Cvenkel

Julian Garcia Feijoo

David Garway Heath

Norbert Pfeiffer

Ingeborg Stalmans

The Board of the European Glaucoma Society Foundation 

Roger Hitchings (Chair)

Carlo E. Traverso (Vice Chair)

Franz Grehn

Anders Heijl

John Thygesen

Fotis Topouzis

Thierry Zeyen

The EGS Committees 

CME and Certification 

Gordana Sunaric Mégevand (Chair)

Carlo Enrico Traverso (Co-chair)

Delivery of Care 

Anton Hommer (Chair)

EU Action 

Thierry Zeyen (Chair)

Carlo E. Traverso (Co-chair)

Education 

John Thygesen (Chair)

Fotis Topouzis (Co-chair)

Glaucogene 

Ananth Viswanathan (Chair)

Fotis Topouzis (Co-chair)

Industry Liaison 

Roger Hitchings (Chair)

Information Technology 

Ingeborg Stalmans (Chair)

Carlo E. Traverso (Co-chair)

National Society Liaison 

Anders Heijl (Chair)

Program Planning 

Fotis Topouzis (Chair)

Ingeborg Stalmans (Co-chair)

Quality and Outcomes 

Anja Tuulonen (Chair)

Augusto Azuara Blanco (Co-chair)

Scientific 

Franz Grehn (Chair)

David Garway Heath (Co-chair)

Changes in biomechanical properties of the cornea and intraocular pressure after myopic laser in situ keratomileusis using a femtosecond laser for flap creation determined using ocular response analyzer and Goldmann applanation tonometry

PURPOSE

To compare intraocular pressure (IOP) measurements before and after laser in situ keratomileusis (LASIK) using a femtosecond laser for flap creation, and to identify factors that may influence the preoperative and postoperative IOP, and the change in IOP after LASIK.

METHODS

Forty eyes from 20 patients who underwent treatment for myopia using a femtosecond laser for flap creation were enrolled in this study. The IOP and corneal biomechanical markers were prospectively measured preoperatively and 1 month after LASIK with Goldmann applanation tonometry (GAT) and the ocular response analyzer (ORA). Manifest refraction spherical equivalent, central corneal thickness, ablation depth, and mean corneal curvature (K reading) were also recorded.

RESULTS

After LASIK, there was a significant reduction in the IOP measurement with respect to the corneal-compensated intraocular pressure (IOPcc) (Δ=-0.67±2.07 mm Hg), Goldmann-correlated pressure (IOPg) (Δ=-3.92±2.19 mm Hg), and GAT (Δ=-2.6±2.51). Multiple regression analysis revealed that the corneal hysteresis (CH) and corneal resistance factor (CRF) are statistically significant predictors of IOPcc, IOPg, and GAT (P<0.000). In this analysis, the preoperative manifest refraction spherical equivalent, CH, and CRF were significant predictors of ΔIOPcc (adjusted R²=0.401) and ΔIOPg (adjusted R²=0.386). The preoperative SE and central corneal thickness significantly predicted ΔGAT (adjusted R²=0.464).

CONCLUSIONS

ORA provides a more complete measurement of IOP after LASIK with a femtosecond laser than GAT because ORA provides greater knowledge of the corneal biomechanics in terms of CH and CRF.

Factors that influence intraocular pressure changes after myopic and hyperopic LASIK and photorefractive keratectomy: a large population study

PURPOSE

To describe the factors that influence the measured intraocular pressure (IOP) change and to develop a predictive model after myopic and hyperopic LASIK and photorefractive keratectomy (PRK) in a large population.

DESIGN

Retrospective, observational case series.

PARTICIPANTS

Patients undergoing primary PRK or LASIK with a refractive target of emmetropia between January 1, 2008, and October 5, 2011.

METHODS

The Optical Express database was queried for all subjects. Data were extracted on procedure specifics, preoperative central corneal thickness (CCT), IOP (using noncontact tonometry), manifest refraction, average keratometry, age, gender, and postoperative IOP at 1 week, 1 month, and 3 months. A linear mixed methods model was used for data analysis.

MAIN OUTCOME MEASURES

Change in IOP from preoperatively to 1 month postoperatively.

RESULTS

A total of 174 666 eyes of 91 204 patients were analyzed. Hyperopic corrections experienced a smaller IOP decrease than myopic corrections for both PRK and LASIK (P<0.0001). Patients who underwent LASIK had a 0.94 mmHg (95% confidence interval [CI], 0.89-0.98) greater IOP decrease than patients who underwent PRK (P<0.0001), reflecting the effect of the lamellar flap. The decrease in IOP was linearly related to preoperative manifest spherical equivalent (MSE) for myopic PRK and LASIK (P<0.0001), weakly correlated with preoperative MSE after hyperopic LASIK, and not related to preoperative MSE after hyperopic PRK. The single greatest predictor of IOP change was preoperative IOP across all corrections. By using the available data, a model was constructed to predict postoperative IOP change at 1 month; this was able to explain 42% of the IOP change after myopic LASIK, 34% of the change after myopic PRK, 25% of the change after hyperopic LASIK, and 16% of the change after hyperopic PRK.

CONCLUSIONS

Myopic procedures lower measured IOP more than hyperopic procedures; this decrease was proportional to the amount of refractive error corrected. Independent of the refractive correction, the creation of the lamellar LASIK flap decreased measured IOP by 0.94 mmHg. A best-fit model for IOP change was developed that may allow better interpretation of post-laser vision correction IOP values.

Comparison of different intraocular pressure measurement techniques in normal eyes, post surface and post lamellar refractive surgery

Background

The purpose of this study was to determine the accuracy of intraocular pressure (IOP) measurement after laser in situ keratomileusis (LASIK) or epithelial laser in situ keratomileusis (Epi)-LASIK using Goldmann applanation tonometry, air puff tonometry, ocular response analyzer corneal compensated IOP (ORA IOPcc) and Pentacam corrected IOP.

Methods

A prospective comparative clinical study was conducted between February and September 2011 on 30 eyes divided into four groups, i.e. 20 corneas of 10 patients before LASIK (group A), 20 corneas of the same patients 2 months postoperatively (group B), 10 corneas of five patients before Epi-LASIK (group C), and 10 corneas of the same patients 2 months postoperatively (group D). Patient age ranged from 20 to 50 years. IOP was measured using Goldmann applanation and air puff tonometry, ORA corneal compensation, and Pentacam correction (which also measured central corneal thickness).

Results

Significant positive linear correlations were found between IOP values measured by Goldmann applanation tonometry and other techniques, and with preoperative pachymetry in group A. The correlation between preoperative Pentacam-corrected and preoperative ORA corneal-compensated IOP was strongest for Goldmann applanation tonometry (r = 0.97 and r = 0.858 respectively, P < 0.001). Compared with preoperative values, postoperative IOP measured by the four methods were significantly lower. The difference was statistically significant when IOP was measured using Goldmann applanation and air puff tonometry compared with the ORA and Pentacam methods (P < 0.001 for LASIK patients and P = 0.017 for Epi-LASIK patients). Nonsignificant correlations were found between the degree of lowering of postoperative IOP and postoperative pachymetry in groups B and D.

Conclusion

Refractive surgery causes significant lowering of IOP as measured using Goldmann applanation tonometry, air puff tonometry, ORA compensation, and Pentacam correction. LASIK has a greater effect than Epi-LASIK on IOP measurement error following refractive surgery.

Evaluation of intraocular pressure according to corneal thickness before and after excimer laser corneal ablation for myopia

Intraocular pressure is affected by corneal thickness and biomechanics. Following ablative corneal refractive surgery, corneal structural changes occur. The purpose of the study is to determine the relationship between the mean central corneal thickness (CCT) and the change in intraocular pressure measurements following various corneal ablation techniques, using different measurement methods. Two hundred myopic eyes undergoing laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) were enrolled into a prospective, non-randomized study. Corneal parameters examined included full ocular examination, measurement of CCT, corneal topography, corneal curvature and ocular refractivity. Intraocular pressure measurements were obtained using three different instruments-non-contact tonometer, Goldmann applanation tonometer and TonoPen XL (TonoPen-Central and TonoPen-Peripheral). All measurements were performed pre-operatively and 4 months post-operatively. Post-operative intraocular pressure was significantly lower than pre-operative values, with all instruments (p value < 0.001, Student's t-test). The post-operative intraocular pressure decrease was smallest using the Tonopen-XL compared to the Goldmann applanation tonometer and non-contact tonometer (p value < 0.001, ANOVA). Intraocular pressure readings are significantly reduced following corneal ablation surgery. We determined in our myopic patient cohort that the TonoPen XL intraocular pressure measurement method is the least affected following PRK and LASIK as compared to other techniques.

Comparison of dynamic contour tonometry and Goldmann applanation tonometry in keratoconus

PURPOSE

We performed a comparative study using Goldmann applanation tonometry (GAT) and dynamic contour tonometry (DCT) to measure intraocular pressure (IOP) in eyes with keratoconus.

METHODS

IOP was measured in 114 eyes of 75 patients with keratoconus (51 men and 24 women; mean age, 36.1 ± 11.2 years) using GAT and DCT in randomized order. Central corneal thickness (CCT), minimal corneal thickness (MCT), and corneal topography were recorded using the Pentacam. Four groups according to Amsler's keratoconus classification were composed and analyzed for significant differences of CCT, MCT, GAT, and DCT results.

RESULTS

Mean CCT in the 114 keratoconus eyes was 481.1 ± 46.2 μm (range, 334-601 μm). Mean MCT was 453.3 ± 56.3 μm (range, 239-573 μm). Mean IOP measured using GAT was 13.1 ± 2.9 mm Hg, whereas mean IOP measured using DCT was 14.8 ± 2.6 mm Hg. Neither the results for GAT nor those for DCT showed a significant correlation with CCT (Pearson correlation: P < 0.05). Multifactorial analysis revealed that CCT and MCT, but not GAT and DCT, results were significantly different in corneas of varying curvatures.

CONCLUSIONS

This study shows that DCT measures IOP higher than GAT in eyes with keratoconus. In keratoconus, both methods seem to be independent of CCT and therefore are equally, but not interchangeably, applicable when monitoring IOP. Further analysis revealed that CCT and MCT are significantly different in corneas of varying Amsler grade.

Intraocular pressure assessment after laser in situ keratomileusis: a review

This paper aims to review the current methods available for the measurement of intraocular pressure after myopic laser in situ keratomileusis for the correction of myopia. Searches were performed for studies that assessed or compared various methods of intraocular pressure assessment. There were 20 eligible studies that explored the use of pneumotonometry, pressure phosphene tonometry, rebound tonometry, dynamic contour tonometry, statistical modeling, mathematical formulae, ocular response analyzer and even measuring intraocular pressure on the nasal cornea. Our review shows that an ideal method would be one that is independent of corneal factors. Dynamic contour tonometry and pressure phosphene tonometry held promise in research settings. More studies need to be done to validate the new methods of intraocular pressure assessment, especially in glaucoma patients. It is important to empower laser in situ keratomileusis patients with knowledge of these difficulties and potential implications for the future.

Effect of corneal thickness on intraocular pressure measurements with the Pascal dynamic contour, Canon TX-10 non-contact and Goldmann applanation tonometers in healthy subjects

PURPOSE

To investigate the effects of central corneal thickness (CCT) on intraocular pressure (IOP) measurements of the Pascal dynamic contour tonometry (DCT), Canon TX-10 non-contact tonometry (NCT) and Goldmann applanation tonometry measurements (GAT) in healthy subjects.

METHODS

IOP values of 135 eyes with normal corneas of 135 healthy volunteers were determined by DCT, NCT and by GAT. The CCT was measured using an ultrasonic pachymeter after all IOP determinations had been made.

RESULTS

When DCT measurements were compared (IOP = 17.52 +/- 2.0 mmHg) with NCT measurements (IOP = 16.54 +/- 2.77 mmHg) and GAT measurements (IOP = 15.07 +/- 2.35 mmHg), DCT measurements were significantly higher than NCT and GAT (p < 0.001). There was a significant correlation between CCT with both NCT (r = 0.260, p = 0.003) and GAT measurements (r = 0.257, p = 0.005). There was a weak correlation that was not statistically significant between CCT and DCT (r = 0.160, p = 0.079).

CONCLUSION

The IOP measurements with DCT seem to be less dependent on CCT. NCT appears to be more affected by variation in CCT than GAT.

Poor agreement between Goldmann and Pascal tonometry in eyes with extreme pachymetry

AIM

To compare the intraocular pressure (IOP) measurements obtained using the Pascal dynamic contour tonometer (PDCT) with the standard Goldmann applanation tonometry (GAT) and to correlate them with the central corneal thickness (CCT) in a non-glaucomatous population.

METHODS

We prospectively measured IOP using PDCT and GAT in random order in 100 normal eyes. CCT was analysed with an ultrasonic pachymeter in each case. Statistical analysis of baseline and stratified data included intraclass correlation coefficient (ICC), Lin correlation, and Bland-Altman analysis to evaluate the agreement between both techniques.ResultsGAT was used first in 51 eyes and PDCT in 48 cases. Mean IOP was 14.8 mm Hg with GAT and 20.3 mm Hg with PDCT. Mean pachymetry was 553.23 micrometres (microm) (SD: 4.7 microm). Global agreement of IOP between GAT and PDCT was 0.09 by ICC and 0.170 by Lin correlation. When CCT values ranged between 540 and 545 microm, the agreement between both tonometers was optimal (ICC 0.54 and Lin 0.61). Outside these pachymetry values, agreement between both tonometers diminished dramatically.

CONCLUSION

Statistically significant agreement between both devices was reached only at intermediate pachymetry readings in contrast with other studies that show excellent global agreement between GAT and PDCT.

Comparison of dynamic contour tonometry and goldmann applanation tonometry in deep lamellar and penetrating keratoplasties

PURPOSE

To compare the measurements of intraocular pressure (IOP) with dynamic contour tonometry (DCT) and Goldmann applanation tonometry (GAT) in eyes with corneal graft and to evaluate the influence of corneal thickness (CCT), corneal curvature (CC), and astigmatism on these methods.

DESIGN

Prospective, observational cross-sectional study.

METHODS

Eighteen eyes of 18 patients after penetrating keratoplasty (PKP) and 14 eyes of 14 patients after deep lamellar keratoplasty (DLKP) underwent IOP evaluation with DCT and GAT, and measurements of CCT, CC, and astigmatism. Bland-Altman plots were used to evaluate the agreement between tonometers. Multivariate regression analysis was used to evaluate the influence of ocular structural factors and running suture on IOP measurements obtained with both tonometers.

RESULTS

IOP values obtained by DCT and GAT were strongly correlated in all eyes (r = .91; P < .001). DCT values measured 2.5 +/- 1.7 mm Hg higher than GAT readings (P < .001). A reduction of the mean IOP difference between DCT and GAT with an increase in IOP values (P < .001) was found. Regression analysis showed no effect of CCT, CC, astigmatism, and running suture on both DCT and GAT readings, either in DLKP or in PKP eyes.

CONCLUSIONS

We found a good overall correlation between both tonometers but the agreement between instruments differs in high or low IOP ranges. The wide and varying 95% limits of agreement between DCT and GAT indicates that DCT provides IOP measurements on deep lamellar and penetrating keratoplasties which can be used in the clinical practice.

Effects of antiglaucoma drugs on refractive outcomes in eyes with myopic regression after laser in situ keratomileusis

PURPOSE

To assess effects of antiglaucoma drugs on refractive outcomes in eyes with myopic regression after laser in situ keratomileusis (LASIK).

DESIGN

Prospective, nonrandomized clinical trial.

METHODS

We examined 27 eyes with mean myopic regression +/- standard deviation of -1.26 +/- 0.48 diopters (D; range, -0.50 to -2.25 D) after LASIK. Nipradilol 2.5% was administered topically twice daily to these regressive eyes. We obtained the refraction (spherical equivalent, astigmatism), intraocular pressure (IOP) measurements, pachymetry, geometry, and refractive power of the cornea before and three months after treatment.

RESULTS

Mean manifest refraction was improved significantly from -1.02 +/- 0.52 D to -0.44 +/- 0.39 D (P < .001). However, mean manifest astigmatism was changed from -0.55 +/- 0.30 D to -0.49 +/- 0.22 D, but the difference was not significant (P = .23). The IOP was decreased significantly from 11.4 +/- 2.4 mm Hg to 9.4 +/- 1.3 mm Hg (P < .001). Central corneal thickness was not changed significantly from 505.2 +/- 39.3 microm to 503.6 +/- 38.7 microm (P = .61). The posterior corneal surface was shifted posteriorly by 9.1 +/- 8.2 microm, and the total refractive power of the cornea was decreased significantly, by 0.63 +/- 0.62 D (P < .001), at three months after application.

CONCLUSIONS

The preliminary data show that antiglaucoma drugs are effective for the reduction of the refractive regression, especially of the spherical errors, after LASIK. It is suggested that backward movement of the cornea may occur, possibly flattening the corneal curvature by lowering the IOP. Reduction of the IOP may contribute to improving regression after keratorefractive surgery.

[Assessment of the nerve fiber layer thickness after LASIK using scanning laser polarimetry with variable corneal compensation]

AIM

To evaluate the reliability of scanning laser polarimetry with variable corneal compensation after laser-assisted in-situ keratomileusis (LASIK).

METHODS

Thirty-six eyes of 18 consecutive patients who had LASIK where included in the study. For each eye, one scanning laser polarimetry with variable corneal compensation (GDxVCC) was performed before LASIK (GDx no 1). One month after surgery, two other scanning laser polarimetry procedures were performed: one used the corneal compensation measured preoperatively (GDx no 2) and one used a new corneal compensation, measured postoperatively (GDx no 3). The values measured preoperatively were compared first to the values obtained postoperatively with GDx no 2, then to the values obtained postoperatively with GDx no 3 (Wilcoxon test).

RESULTS

When corneal compensation was re-measured postoperatively (GDx no 3), the measurements were reliable for 35 eyes (97%). We found no significant differences between the preoperative and postoperative values except for the NFI (p=0.032). When the preoperative corneal compensation value was used in the postoperative measurements (GDx no 2), the exam was not reliable for 13 eyes, and it could not be performed at all for two eyes. For the remaining eyes, the values of four parameters were significantly modified: superior thickness (p=0.03), superior ratio (p=0.0005), inferior ratio (p=0.009), and ellipse modulation (p=0.39). The values of the other parameters (average thickness and inferior average) remained unchanged.

CONCLUSION

The use of a preoperative customized corneal compensation for the realization of post-LASIK measurements does not provide reliable results. The GDxTMVCC can be considered a reliable tool after LASIK only when the corneal compensation is re-evaluated postoperatively.

Pressure Phosphene Tonometry Versus Goldmann Applanation Tonometry for Measuring Intraocular Pressure Before and After LASIK

PURPOSE

To compare pressure phosphene tonometry with Goldmann applanation tonometry for measuring intraocular pressure (IOP) before and after LASIK.

METHODS

Forty-three (18 men and 25 women) consecutive healthy patients underwent complete pre- and postoperative LASIK ophthalmologic assessments including manifest and cycloplegic refraction, keratometry, and central corneal thickness. Three repetitive sets of pressure phosphene tonometry and Goldmann applanation tonometry measurements were performed the day before and 3 months following uneventful LASIK.

RESULTS

Mean preoperative spherical equivalent refraction was -4.70 +/- 2.50 diopters (D) (range: -1.50 to -12.90 D) and mean preoperative keratometry was 43.95 +/- 1.08 D. After LASIK, spherical equivalent refraction was +0.23 +/- 0.11 D and mean keratometry was 39.46 +/- 2.28 D. Preoperative pressure phosphene tonometry (12.16 +/- 1.58 mmHg) and Goldmann applanation tonometry (12.01 +/- 1.55 mmHg) IOP measurements were similar. Postoperative IOP was 10.30 +/- 1.16 mmHg with Goldmann applanation tonometry and 12.20 +/- 1.62 mmHg with pressure phosphene tonometry. The postoperative IOP difference between Goldmann applanation tonometry and pressure phosphene tonometry was 0.15 +/- 1.12 mmHg (P = .41). The mean change in pachymetry after LASIK was 68.73 +/- 73 microm. The change in Goldmann applanation between preoperative and postoperative LASIK values was 1.71 +/- 1.43 mmHg (P < .0001), a change that was strongly correlated with changes in corneal thickness (R = 0.75, P < .0001) and keratometry (R = 0.72, P < .0001). No such correlations were found with pressure phosphene tonometry.

CONCLUSIONS

Goldmann applanation tonometry-measured IOP decrease after LASIK is strongly correlated with a decrease in central corneal thickness and changes in keratometry, whereas pressure phosphene tonometry-measured IOP is independent of corneal thickness. Pressure phosphene tonometry appears to be a more reliable method for recording tonometry in these patients.

Changes in corneal biomechanics and intraocular pressure following LASIK using static, dynamic, and noncontact tonometry

PURPOSE

To compare the preoperative and postoperative measurement of corneal biomechanical properties and intraocular pressure (IOP) using Goldmann applanation tonometry (GAT), the ocular response analyzer (ORA), and the Pascal dynamic contour tonometer (PDCT) in eyes undergoing myopic laser in situ keratomileusis (LASIK).

DESIGN

Prospective, nonrandomized clinical trial.

METHODS

IOP was measured in 66 myopic eyes before and after LASIK by GAT, ORA, and PDCT in a randomized sequence. Metrics of corneal biomechanical properties (corneal hysteresis [CH], corneal resistance factor [CRF], and ocular pulse amplitude [OPA]) were recorded.

RESULTS

After LASIK, there was a reduction in mean corneal pachymetry of 90.2 mum and in IOP measurements with GAT (Delta = -1.8 +/- 2.8 mm Hg; P < .01), ORA-Goldmann (Delta = -4.6 +/- 2.8 mm Hg, P < .01), and ORA-corneal compensated (Delta - 2.1 +/- 2.6 mm Hg; P < .05). However, there was no statistically significant difference between preoperative and postoperative IOP measurements taken by PDCT (Delta = -0.5 +/- 2.6 mm Hg). Postoperatively, CRF decreased by 28.6% (P < .01), CH by 16.2% (P < .01), and OPA by 1.8% (P = .32).

CONCLUSIONS

Measurement of IOP with PDCT appears to be relatively immune to changes in corneal biomechanics and pachymetry after LASIK, in comparison to GAT and ORA measures of IOP. PDCT and ORA both showed statistically lower variation in measurement than GAT. LASIK produced a marked decline in CH and CRF, which may reflect respective changes in the viscous and elastic qualities of the post-LASIK cornea. In contrast, there was no statistical change in OPA.

Measurement of intraocular pressure after LASIK by dynamic contour tonometry

Changes of corneal properties induced by laser in situ keratomileusis (LASIK) results in low inaccurate intraocular pressure (IOP) readings by Goldmann applanation tonometry (GAT). Before and after LASIK, the applied value of IOP, measured by dynamic contour tonometry (DCT) in comparison to GAT, was evaluated. Before and 1, 4 weeks after LASIK, the IOP in 30 cases (60 eyes) was measured by GAT and DCT respectively. The obtained results were statistically processed by SPSS11.5 statistical software. The results showed that central corneal thickness (CCT) could affect GAT measurements but not DCT measurements. The comparison of IOP one and 4 weeks after LASIK revealed that the readings from GAT was separately decreased by 5.00 +/- 1.12 and 5.45 +/- 1.13 mmHg as compared with those before LASIK, while those from DCT had no significant difference. It was concluded that LASIK-induced changes of CCT could influence the accuracy of GAT measurements, but had no influence on those from DCT. DCT was more beneficial to the measurements of IOP in normal eyes and those subject to LASIK surgery.

The relationship among race, iris color, central corneal thickness, and intraocular pressure

PURPOSE

Central corneal thickness (CCT) influences applanation intraocular pressure (IOP) measurement. The present study sought to determine whether iris color might represent a qualitative surrogate for CCT or race, and therefore differential risk for elevated IOP and, consequently, developing glaucoma.

METHODS

Eligible patients included those with best-corrected visual acuity (BCVA) better than 20/40 and who had not worn contact lenses within 24 hours. Exclusion criteria were prior ophthalmic surgery, topical ocular or systemic medication that would influence IOP, previous ocular inflammatory conditions, or current treatment for ophthalmic treatment. Data collection included demographic (name, date of birth, race), BCVA, and iris color. Iris color was judged according to a purpose-developed chart (white: blue, green, brown or black: brown) and patients were assigned to one of four groups. Goldmann applanation tonometry and pachymetry measurements were performed consecutively. To attain a power of 90% to find a difference of 40 microm with alpha < 0.05, we examined at least 14 subjects (28 eyes) for each group.

RESULTS

Comparing pachymetry measurements among iris colors revealed no statistically significant difference among the three groups of whites: blue (552 microm), green (552 microm), and brown (562 microm). The same held true when comparing IOP and CCT-adjusted IOP with iris color: blue-15.2, 15.1, green-15.4, 15.2, and brown-14.7, 14.0. When comparing CCT between whites and blacks, CCT was significantly thinner in blacks (533 microm), whether evaluating all whites (555 microm, p = 0.03) or comparing only the brown-iris white group with the black group (562 microm vs. 533 microm, p = 0.03). Mean CCT-adjusted IOP was barely significantly different between whites (14.8) and blacks (16.7) (p = 0.04).

CONCLUSION

These results suggest that iris color is not associated with CCT and apparently iris color does not influence measured IOP. We were able to establish a relationship between race and IOP when adjusting IOP for CCT. Our data show a significantly higher CCT-adjusted IOP for blacks than whites demonstrating a racial difference in CCT-corrected IOP.

Interface Corneal Edema Secondary to Steroid-induced Elevation of Intraocular Pressure Simulating Diffuse Lamellar Keratitis

PURPOSE

To describe interface corneal edema secondary to steroid-induced elevation of intraocular pressure (IOP) following LASIK.

METHODS

Retrospective observational case series. Diffuse interface edema secondary to steroid-induced elevation of IOP was observed after LASIK simulating diffuse lamellar keratitis (DLK) in 13 eyes. Mean patient age was 31.4 +/- 5.3 years. Patients were divided into two groups according to provisional misdiagnosis: DLK group (group 1) comprised 11 eyes and infection group (group 2) comprised 2 eyes (microbial keratitis). Mean follow-up was 8.1 +/- 0.5 weeks.

RESULTS

In the DLK group, typical diffuse haze was confined to the interface and extended to the visual axis, impairing vision in all eyes. Provisional diagnosis was late-onset DLK and topical steroids were started. Repeat examination showed elevated IOP as measured at the corneal center and periphery using applanation tonometry (mean 19.1 mmHg and 39.5 mmHg, respectively), causing interface edema with evident interface fluid pockets. Steroids were stopped and topical anti-glaucoma therapy was started. The interface edema decreased and at the end of follow-up the corneal transparency was restored and IOP dropped to normal values. The infection group demonstrated a microbial keratitis-like reaction and underwent flap lifting and interface wound debridement and biopsy with administration of fortified antibiotics and steroids. After elevated IOP was detected, steroids and antibiotics were stopped and topical anti-glaucoma therapy was started, resulting in the resolution of the interface edema.

CONCLUSIONS

Interface fluid syndrome secondary to steroid-induced elevation of IOP might develop in steroid responders after LASIK with a misleading clinical picture simulating DLK or infectious keratitis. Management includes stopping topical steroids and starting topical antiglaucoma therapy.

Avoidance, recognition, and management of LASIK complications

PURPOSE

To provide important concepts of the latest developments in laser in situ keratomileusis (LASIK) complication avoidance, recognition, and management.

DESIGN

A perspective.

METHODS

A comprehensive literature search and review of a total of 816 publications that discussed LASIK complications from 1992 to 2005 was conducted.

RESULTS

The risk of visually threatening complications is inherent in any ophthalmologic surgical procedure. Not only does LASIK require the use of several complex medical devices, but there can be significant human variation in response to this surgical intervention. As a result, many potential complications can occur after LASIK. The risk of many complications can be mitigated by appropriate patient selection and preoperative, surgical, and postoperative care. Unforeseen complications will occur, despite meticulous planning, and must be managed. Important current developments in the avoidance, recognition, and management of LASIK complications are reviewed.

CONCLUSIONS

Complications as a result of LASIK can threaten vision and may cause debilitating symptoms in an otherwise healthy eye. Advancing our understanding of the prevention and management of the complications of LASIK is an endeavor that must be continued as long as refractive surgery is performed.

Measuring intraocular pressure-adjustments for corneal thickness and new technologies

PURPOSE OF REVIEW

Central corneal thickness has become an important biometric factor and is an essential part of the evaluation of glaucoma. Goldmann applanation tonometry is the most widely used method of measuring intraocular pressure, but it is well known that corneal parameters affect the accuracy of this instrument. Intraocular pressure reduction is currently the only treatment available for decreasing the risk of glaucoma progression, so it is important to have an accurate reading of the true intraocular pressure.

RECENT FINDINGS

New tonometers such as the dynamic contour tonometer and the Proview tonometer do not appear to be influenced by central corneal thickness. The ocular response analyzer provides a new measure of corneal biomechanics called corneal hysteresis. Rebound tonometry has been used in animal models of glaucoma, but has recently been compared to Goldmann applanation tonometry in humans. All of these technologies are presented in this review and the influence of corneal thickness on their accuracy is examined.

SUMMARY

Confronted with the expanding evidence that central corneal thickness is an important ocular parameter that should be measured in clinical practice, eye-care professionals understandably wonder how to best obtain the measurements and what to do with the information. There is wide disagreement among investigators as to if there is an adequately validated 'correction algorithm'; without a validated algorithm, the argument goes, clinicians cannot use the data. These newer technologies may allow clinicians to be able to accurately determine the true intraocular pressure without having to rely on a correction algorithm.

A predictive model for postoperative intraocular pressure among patients undergoing laser in situ keratomileusis (LASIK)

PURPOSE

The aim of this study was to develop a predictive model based on preoperative variables for estimating postoperative intraocular pressure (IOP) of those eyes undergoing LASIK surgery, to predict the amount of underestimated IOP after LASIK for myopia and myopic astigmatism.

DESIGN

Pretest-post-test longitudinal study.

METHODS

Both eyes of 193 eligible subjects who underwent LASIK procedures at the Department of Ophthalmology, National Taiwan University Hospital, from July 2000 to December 2002 for myopia and myopic astigmatism were identified to build up the predictive models. IOPs were measured with noncontact air-puff tonometry. Information on age, gender, preoperative central corneal thickness (CCT), preoperative central corneal curvature (CCK), preoperative spherical equivalent refractive error, and ablation depth was collected and applied for predicting postoperative IOP after LASIK based on linear mixed model.

RESULTS

Significant predictors for postoperative IOP after myopic LASIK procedures included age, gender, preoperative IOP, ablation depth, preoperative CCT, and preoperative spherical equivalent refractive errors. The linear mixed model, taking into account these significant preoperative correlates and the correlation of IOPs between both eyes of the same patient, explained 91% of the variation of postoperative IOP.

CONCLUSIONS

A statistical model was developed for predicting the amount of underestimated IOP after LASIK for myopia and myopic astigmatism, which is of clinical importance to uncover ocular hypertension among patients whose information on postoperative IOP immediately after LASIK is not available.

Validity of Pachymetric Measurements by Manipulating the Acoustic Factor of Orbscan II

PURPOSE

To assess the validity of pachymetric measurements by examining the constancy of the acoustic factor (AF) of the Orbscan II (Orbtek, Bausch & Lomb, Rochester, NY) after overnight rigid gas-permeable (RGP) contact lens wear.

METHODS

Twenty participants wore CRT (Paragon Vision Sciences, Mesa, AZ) HDS 100 contact lenses on one eye and control lenses on the contralateral eye for one night while sleeping. Another 24 participants wore CRT lenses on both eyes for one night. Central corneal thickness was measured using optical coherence tomography and Orbscan II on the night before lens use, immediately after lens removal on the following morning, and 1, 3, 6, and 12 hours later. By using optical coherence tomography as a reference, the adjusted AF was calculated by using a least squares method over time.

RESULTS

The adjusted AF depended on the corneal thickness in normally hydrated corneas. The adjusted AF and the percentage change of the adjusted AF varied before and after overnight lens wear. There was a strong and significant correlation between the corneal swelling and the percentage change of the adjusted AF (all r at least 0.91, P<0.05).

CONCLUSIONS

The adjusted AF is a variable, not a constant. The AF is a function of the corneal thickness and its alteration with, for example, corneal swelling. The validity of the adjusted Orbscan II pachymetric measures using a single AF is untenable.

Effect of Corneal Curvature and Corneal Thickness on the Assessment of Intraocular Pressure Using Noncontact Tonometry in Patients After Myopic LASIK Surgery

PURPOSE

To evaluate the effect of corneal curvature and corneal thickness on the assessment of intraocular pressure (IOP) using noncontact tonometry (NCT) in patients after myopic LASIK surgery.

METHODS

All patients who had myopic LASIK in a university-based eye clinic between February 2002 and May 2002 were retrospectively analyzed. Preoperative NCT was compared with postoperative NCT, postoperative corneal thickness, and postoperative corneal curvature.

RESULTS

The difference between the mean preoperative NCT (15.46 +/- 2.50 mm Hg) and postoperative NCT (6.30 +/- 1.57 mm Hg) was significant (9.16 +/- 1.96 mm Hg, P < 0.010). Preoperative NCT significantly correlated with postoperative NCT (P < 0.001), postoperative corneal thickness (P = 0.006), and postoperative anterior corneal curvature (P < 0.010).

CONCLUSIONS

Both corneal thickness and anterior corneal curvature affect IOP assessment in patients with myopic LASIK. Although correction formulas can be used to estimate the actual IOP, alternative methods should be investigated to assess IOP independent of corneal thickness and curvature.

Evaluation of Tonometric Correction Factors

PURPOSE

To investigate the efficacy of currently available correction factors in correcting intraocular pressure (IOP) measurements for the errors induced by the normal variations in corneal structural characteristics.

MATERIALS AND METHODS

Central corneal thickness (CCT) and corneal radius of curvature were measured on 324 individuals (175 normal: group 1 and 149 had either open angle glaucoma or ocular hypertension: group 2). IOP was measured in all normal subjects with the Goldmann applanation tonometer and the highest recorded IOP was obtained from patient charts for subjects with either open angle glaucoma or ocular hypertension. Regression analysis was performed on IOP, CCT, and corneal radius of curvature. The corrected IOP was also calculated using the models proposed by Ehlers and Orssengo and Pye. Linear regression analysis was used to calculate the residual association between corneal parameters and corrected IOP.

RESULTS

There was a significant positive correlation between IOP measured using Goldmann applanation tonometer and the CCT in both groups. There was no significant correlation between corneal radius of curvature and IOP in either group. There was a significant negative correlation in both the groups between CCT and corrected IOP calculated using the models of Ehlers and Orssengo and Pye. This indicates that the Ehlers and Orssengo and Pye models may significantly overestimate the effect of CCT on IOP measurement.

CONCLUSION

The effect of CCT and IOP as observed in the present study and by other studies in literature is less than predicted by both the Ehlers formula and the Orssengo and Pye model. Correcting IOP for the effect of CCT using these models could be erroneous and lead to overcorrection of IOP, thus resulting in erroneously low corrected IOP eyes with thicker cornea and erroneously high corrected IOP in eyes with thinner cornea.

Ex vivo evaluation of Tono-Pen and pneumotonometry in cat eyes

PURPOSE

To evaluate the validity and intraobserver reliability of intraocular pressure (IOP) measurements with both pneumotonometry and the Tono-Pen in a closed ex vivo system in cat eyes.

METHODS

IOP was increased step by step in 5 enucleated cat eyes, while taking IOP measurements with the Tono-Pen and pneumotonometry. The outcomes were compared to readings of a digital manometer simultaneously measuring the actual pressure in the anterior chamber.

RESULTS

Pneumotonometry overestimated IOP below 15 mm Hg and underestimated pressures above 20 mm Hg. Tono-Pen tonometry considerably underestimated IOP over the whole spectrum in all of the eyes tested. The pneumotonometer was identified as the more valid and reliable instrument for cat eyes.

CONCLUSION

Both tonometers are clinically useful tools to assess IOP for glaucoma studies using a cat animal model. However, one has to consider underestimation of IOP in the upper ranges. A correction formula can be used to calculate the actual IOP.

Comparison of dynamic contour tonometry with goldman applanation tonometry over a wide range of central corneal thickness

PURPOSE

To compare dynamic contour tonometry with Goldmann applanation tonometry in structurally normal corneas over a wide range of central corneal thickness (CCT).

PATIENTS AND METHODS

Twenty-five patients each with normal CCT (group A), thin corneas (group B), and thick corneas (group C) had IOP measured with the Goldmann (GAT) and dynamic contour tonometer (DCT).

RESULTS

In group A (mean CCT = 552 +/- 16 microm) the mean GAT was 15.9 +/- 3.1 mm Hg and mean DCT was 16 +/- 3.3 mm Hg (P = 0.91). In group B (mean CCT = 491 +/- 19 microm) the mean GAT was 13.2 +/- 3.5 mm Hg and the mean DCT was 15.9 +/- 3.5 mm Hg (P = 0.009). For group C (mean CCT = 615 +/- 22 microm), the mean GAT was 17.4 +/- 3.8 mm Hg and the mean DCT was 17.4 +/- 3.5 mm Hg (P = 0.95). The 95% agreement limits for DCT were -3.1 mm Hg to 2.9 mm Hg. The mean GAT-DCT difference was -2.6 mm Hg in thin corneas and -0.06 mm Hg in thick corneas. Below 520 microm reduction of 10 microm in CCT appears to result in a significant underestimation of the GAT IOP by 0.7 mm Hg (P < .001) and above 580 microm a non-significant overestimation of 0.2 mm Hg per 10 microm increase in CCT (P = 0.27).

CONCLUSION

Dynamic contour tonometer agrees well on average with GAT but the agreement limits are wide. In structurally normal thin corneas DCT may give a more accurate assessment of the true IOP but it does not appear to have any benefit over GAT in thick corneas.