Changes in Intraocular Pressure After Laser in situ Keratomileusis

Goldmann and modified Goldmann tonometry measuring intraocular pressure changes in eyes which underwent myopic laser in situ Keratomileusis and photorefractive keratectomy

PURPOSE

Compare intraocular pressure (IOP) measured by a standard Goldmann applanation tonometer prism (IOPg) and a modified correcting applanation tonometer surface Goldmann prism (IOPc) before and after laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK).

METHODS

Goldmann tonometry was analyzed in a retrospective, cross-sectional study, using both GAT and modified-GAT prisms pre-operatively and at the 3 month post-operative appointment on 120 eyes (64 patients) who received LASIK (n = 58) or PRK (n = 62). Demographics, central corneal thickness (CCT), manifest refraction and corneal curvature (CC) data was collected at each visit as well as surgical parameters, including maximum ablation depth.

RESULTS

Mean paired IOP following LASIK decreased by - 3.28 ± 3.2 mmHg measured by IOPg and - 1.93 ± 3.3 mmHg by IOPc (p ≤ 0.0001). Mean paired IOP following PRK reduced by - 1.92 ± 3.6 mmHg measured by IOPg and - 1.06 ± 3.6 mmHg by IOPc (p ≤ 0.0001). Increased LASIK ablation depth and post-procedural change in CCT trended toward a statistically significant reduction in IOPg (p = 0.07,p = 0.12), but not IOPc (p = 0.18,p = 0.32). PRK ablation depth was not associated with a reduction in IOPg or IOPc.

DISCUSSION

The modified Goldmann (IOPc) prism measured less of an IOP reduction following LASIK and PRK compared to the standard (IOPg) prism, and the IOP reduction with both prisms was associated with the degree of myopic correction.

WHAT IS ALREADY KNOWN AND THE RESIDUAL QUERY

Corneal refractive surgery generally demonstrates significant postoperative Goldmann IOP reductions. Presumably, this is due to corneal biomechanical changes for which a newer method of Goldmann IOP measurement may be able to compensate.

WHAT THIS STUDY ADDS

A modified, corneal conforming Goldmann prism demonstrates significantly less IOP reduction following myopic LASIK and PRK compared to the standard flat Goldmann prism.

HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

A newer, modified Goldmann prism may help detect glaucoma and OHT at an earlier stage in patients which have undergone LASIK or PRK. The findings corroborate predicted corneal biomechanical changes following the most common corneal refractive procedures.

[Alterations in intraocular pressure and the CorVis parameters after LASIK]

INTRODUCTION

The reduction of corneal thickness following laser-assisted in-situ keratomileusis (LASIK) requires a correction of intraocular pressure (IOP) measurements. The corneal visualization Scheimpflug technology (CorVis ST, Oculus, Wetzlar, Germany) not only measures the IOP and central corneal thickness (CCT) but also determines 10 additional corneal parameters, such as the time to first and second applanation and velocity. Besides CCT we compared various corneal parameters before and after LASIK in order to detect possible correlations and to correct IOP measurements.

METHODS

Measurements with CorVis ST were made before and after LASIK in 45 myopic patients (45 left eyes). We compared the IOP and CorVis ST parameters using a paired t‑test before and after LASIK and corrected for possible correlations in a multifactorial linear model. Finally, we correlated the changes in IOP to changes in biomechanical parameters.

RESULTS

We observed a direct correlation between the IOP measurements and the corneal thickness. The IOP was underestimated by 0.039 mm Hg per micrometer in reduction of corneal thickness. The multifactorial linear model showed a correlation of IOP change to A2 velocity and the radius of applanation.

CONCLUSION

Surgical thinning of the central cornea via LASIK demonstrated a direct correlation between corneal thickness and IOP measurements using the CorVis ST technique. Postoperative changes of the A2 velocity and the applanation radius also had a statistically significant influence on post-LASIK IOP measurements. Our findings could be useful to obtain more precise post-LASIK IOP measurements.

Corvis ST Tonometer for Measuring Postoperative IOP in LASIK Patients

PURPOSE

To compare the postoperative measurements of intraocular pressure (IOP) using the Corvis ST Tonometer (CST), ocular response analyzer (ORA), and Goldmann applanation tonometry (GAT) in eyes undergoing laser in situ keratomileusis (LASIK), as well as to analyze the relationship between the corneal biomechanical parameters of the CST and the ORA.

METHODS

Fifty participants who had undergone LASIK to treat myopia in the previous 3 months were enrolled. Postoperative IOP measurements of these participants were obtained using the CST, ORA (corneal-compensated IOP [IOPcc], Goldmann-correlated IOP [IOPg]), and GAT. Device agreement was calculated by Bland-Altman analysis. The metrics of corneal biomechanical properties were recorded using the ORA and the CST. Corneal biomechanical parameters were compared.

RESULTS

The Bland-Altman analysis revealed a significant bias between CST and GAT, between CST and IOPcc, and between CST and IOPg of 3.4, 1.0, and 3.8, mm Hg, respectively, with 95% limits of agreement of -0.7 to 7.5 mm Hg, -2.1 to 4.2 mm Hg, and -0.4 to 8.0 mm Hg. The ORA-derived IOP measurements, CST-derived IOP, and GAT IOP values showed good correlation with each other. The CST IOP and IOPcc were higher than the GAT IOP (all p < 0.05), whereas IOPg did not differ from the GAT IOP readings. Ocular response analyzer-derived corneal biomechanical parameters (corneal hysteresis and the corneal resistance factor) showed significant correlations with CST-derived parameters, including the maximum deformation amplitude at the corneal apex and the time from start until the first applanation.

CONCLUSIONS

The CST offers an alternative method for measuring postoperative IOP in LASIK patients, and it appears to obtain higher IOP values than other tonometry techniques. The technique may facilitate the investigation of corneal biomechanical property changes in LASIK-treated eyes.

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.

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.

[Mobile intraocular pressure measurement. From palpation to initial clinical experience with the handheld dynamic contour tonometer]

Goldmann applanation tonometry is still the gold standard of intraocular pressure measurement (IOP) and an essential part in the diagnosis of glaucoma. Applanation tonometry is usually performed on a sitting patient at the slit lamp. However, under certain circumstances it is necessary to measure the IOP outside the office setting. With handheld devices the measuring procedure is brought to bedside and surgery theatres, as well as to patients who are not able to sit behind the slit lamp. The dynamic contour tonometer (DCT) represents a new method of direct IOP measurement. Its physical principle is based on piezo-electronic contour matching tonometry and is claimed to be less dependent on biochemical properties of the cornea. Besides the IOP, the ocular pulse amplitude can also be measured. Until now, the DCT had been available as a slit lamp mounted device. In this report, we present a portable prototype of the device. In comparison with the Perkins tonometer and the TONO-PEN XL, the handheld DCT shows smaller intra- and inter-examiner variability. Additionally, the device offers the unique possibility to display the ocular pulse amplitude while the patient is in a horizontal position.

Five-Year Outcome of LASIK for Myopia

PURPOSE

To investigate the efficacy and safety of LASIK over a 5-year postoperative period.

DESIGN

Observational case series.

PARTICIPANTS

We examined 779 eyes in 402 patients with myopia or myopic astigmatism who underwent LASIK to correct their refractive errors and received regular postoperative assessments for 5 years.

METHODS

Postoperative examinations were performed 1 day; 1 week; 1, 3, and 6 months; and 1, 2, 3, 4, and 5 years after LASIK surgery.

MAIN OUTCOME MEASURES

We evaluated changes in uncorrected visual acuity (UCVA) (logarithm of the minimum angle of resolution [logMAR]), manifest refraction, best-corrected visual acuity (BCVA) (logMAR), intraocular pressure, corneal thickness, corneal endothelial cell counts, and complications.

RESULTS

Preoperative UCVA of 1.27 improved to -0.03 at 1 day after surgery and -0.08 at 1 month and revealed minimal but significant decreases thereafter. Postoperative manifest refraction was also improved by surgery, showing minimal but significant regression after 1 year. Final BCVA loss was seen in 10 eyes of 7 patients; in 7 cases, there were obvious reasons such as the progression of cataracts in 3 eyes, epithelial disintegrity due to dry eye in 2 eyes, irregular astigmatism due to flap striae in 1 eye, and age-related macular dystrophy in 1 eye. Intraocular pressure and corneal thickness decreased by 4.0 mmHg and 76.9 microm, respectively, due to surgery, but remained stable throughout the follow-up period. Corneal endothelial cell counts (2689.0+/-232.9 cells/mm(2) before surgery) showed a statistically significant decrease at 5 years after surgery (2658.0+/-183.1 cells/mm(2); 1.2% loss for 5 years), likely within the range due to physiological age-related loss. No serious, vision-threatening, irreversible complication such as keratectasia or progressive endothelial cell loss was observed.

CONCLUSION

LASIK surgery is an effective and safe procedure for correcting myopia/myopic astigmatism as long as inclusion and exclusion criteria are strictly respected. However, minimal regression occurred during the 5-year investigative period.

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.

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.

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 Correction Formula for the Real Intraocular Pressure After LASIK for the Correction of Myopic Astigmatism

PURPOSE

To create a correction formula to determine the real intraocular pressure (IOP) after LASIK considering the altered corneal thickness, corneal curvature, and corneal stability.

METHODS

This prospective clinical trial comprised 101 eyes of 59 patients (34 women and 25 men) that underwent LASIK with a mean preoperative spherical equivalent refraction of -6.3 +/- 2.17 diopters (D) (-3.0 to -11.5 D). Mean patient age was 32 +/- 9 years. Preoperatively and 6 months postoperatively, IOP (by Goldmann applanation tonometry), keratometry (by topography), and central corneal thickness (CCT) (by ultrasound pachymetry) were evaluated. These parameters were measured in all patients between 8 and 11 o'clock in the morning.

RESULTS

Due to the LASIK procedure, IOP was reduced from 16.5 +/- 2.1 mmHg (range: 12 to 22 mmHg) to 12.9 +/- 1.9 mmHg (range: 8 to 16 mmHg). Multiple linear regression analysis of the IOP values before and after LASIK showed a significant correlation between the measured IOP and CCT and keratometry values (R2=0.631; P<.001). After LASIK, the biomechanical bending strength of the cornea is reduced by the cut so that the measured IOP must be additionally corrected by 0.75 mmHg. An equation containing all three changes is given: IOP (real) = IOP (measured) + (540-CCT)/71 + (43-K-value)/2.7 + 0.75 mmHg.

CONCLUSIONS

Intraocular pressure measurements after LASIK for the correction of myopia are inaccurate as a consequence of changes in CCT, corneal curvature, and corneal flap stability. After LASIK, the measured IOP should be corrected to avoid false low IOP applanation readings.

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.

Evaluation of the Orssengo-Pye IOP corrective algorithm in LASIK patients with thick corneas

BACKGROUND

The objective of this study was to evaluate the Orssengo-Pye central corneal thickness (CCT) Goldmann applanation tonometry (GAT) corrective algorithm by observing changes in GAT and CCT before and after laser in situ keratomileusis (LASIK) surgery in patients with CCT that remains greater than 545 microm postoperatively.

METHODS

Tonometric and pachymetric measurements were made on 14 patients (28 eyes) before and after LASIK surgery. The selected patients were required to have average or above average postoperative central corneal thickness values in both eyes (not less than 545 microm). Preoperatively, all patients had CCT and GAT measurements taken. Postoperatively patients had CCT, GAT, and dynamic contour tonometric (DCT) measurements taken.

RESULTS

Preoperatively, median CCT values were 589.536 microm. Median GAT values were 16.750 mmHg. Median corrected preoperative GAT values were 14.450 mmHg. After LASIK treatment, median CCT values were 559.417 microm. The decrease in median CCT was 30.119 microm. Median postoperative GAT values were 11.500 mmHg (decrease, 5.250 mmHg). Median corrected postoperative GAT values were 10.775 mmHg (decrease, 3.675 mmHg). Median postoperative DCT values were 17.858 mmHg.

CONCLUSIONS

LASIK treatment causes a significant reduction in measured GAT intraocular pressure (IOP) values. The Orssengo-Pye formula, which attempts to correct for GAT error associated with individual variation in CCT, appears to yield misleading results in these circumstances. An unexpected 3.675-mmHg decrease in "corrected IOP" by the Orssengo-Pye method seen in this study may be attributed to some limitation or error in the formula. After adjusting for the approximate1.7-mmHg difference, which has been demonstrated between DCT and GAT, postoperative DCT values were similar to preoperative measured GAT values.

Intraocular pressure change after laser in situ keratomileusis (LASIK)

To analyze the relationship between change in intraocular pressure (IOP) after laser in situ keratomileusis (LASIK), and preoperative central-corneal thickness (CCT), and central-corneal ablation depth (CCAD), a prospective study was conducted of 30 patients (60 eyes) with myopia or myopic astigmatism who underwent LASIK and who were followed up for a mean of 6 months. The parameters evaluated included IOP 1 week preoperatively and 3 months postoperatively, pre- and postoperative CCT, and CCAD; in addition, the anterior ocular segment and the fundus were examined, as was the apparent and cycloplegic refraction. There were 11 male patients and 19 female patients (mean age, 28.3 years) involved in the study. Mean IOP readings were 14.9 mmHg 1 week preoperatively, and 8.3 mmHg 3 months postoperatively; thus, the mean decrease in IOP after LASIK was 6.6 mmHg. Mean values for preoperative CCT, postoperative CCT, and CCAD were 537.2+/-34.7 microm, 434.0+/-32.5 microm, and 101.6+/-23.90 microm, respectively. IOP correlated with CCT both pre- and postoperatively. The mean change in IOP after LASIK for myopia and myopic astigmatism was statistically significant, but did not correlate with CCAD or age. We postulate that CCAD is not a significant predictor of IOP after LASIK.

Considerations of Glaucoma in Patients Undergoing Corneal Refractive Surgery

Glaucoma patients present a unique set of challenges to physicians performing corneal refractive surgery. Corneal thickness, which is modified during corneal refractive surgery, plays an important role in monitoring glaucoma patients because of its effect on the measured intraocular pressure. Patients undergo a transient but significant rise in intraocular pressure during the laser-assisted in situ keratomileusis (LASIK) procedure with risk of further optic nerve damage or retinal vein occlusion. Glaucoma patients with filtering blebs are also at risk of damage to the bleb by the suction ring. Steroids, typically used after refractive surgery, can increase intraocular pressure in steroid responders, which is more prevalent among glaucoma patients. Flap interface fluid after LASIK, causing an artificially low pressure reading and masking an elevated pressure has been reported. The refractive surgeon's awareness of these potential complications and challenges will better prepare them for proper management of glaucoma patients who request corneal refractive surgery.

Intraocular pressure measurement in patients with previous LASIK surgery using pressure phosphene tonometer

PURPOSE

To compare intraocular pressure (IOP) assessment in post-LASIK patients using non-contact tonometry, pressure phosphene tonometry and applanation tonometry.

METHODS

Sixty-two consecutive LASIK patients were analysed preoperatively and postoperatively with non-contact, pressure phosphene and applanation tonometry. Comparisons among these values were assessed with paired sample Student t-test, Pearson's correlation test and Bland-Altman plotting.

RESULTS

There was no significant difference for preoperative IOP measurement between non-contact, pressure phosphene and applanation tonometry. The mean +/-SD difference between the preoperative non-contact tonometry and postoperative pressure phosphene tonometry IOP measurements was 0.80 +/- 2.77 mmHg (P < 0.01). Postoperative applanation tonometry significantly underestimated IOP measurement by 5.45 +/- 2.96 mmHg (P < 0.001) and postoperative non-contact tonometry significantly underestimated IOP measurement by 9.96 +/- 2.25 mmHg (P < 0.001).

CONCLUSION

Pressure phosphene tonometry may provide an alternative method for the assessment of IOP in post-LASIK patients.

Dynamic contour tonometry versus Goldmann applanation tonometry: a comparative study

BACKGROUND

Various sources of error, including central corneal thickness (CCT) and structural corneal rigidity, have been proposed for Goldmann applanation tonometry (GAT). The Pascal dynamic contour tonometer (DCT) is a novel device designed for intraocular pressure (IOP) measurements assumed to be largely independent of CCT and corneal curvature. We compared DCT with GAT in eyes with normal corneas of various thickness.

METHODS

We prospectively measured IOP using DCT and GAT in random order in 100 eyes of 100 subjects (M:F=46:54; mean age 42+/-19, range 23-88 years).

RESULTS

Mean DCT values were about 1 mm Hg higher than GAT readings (16+/-3 vs 15+/-3 mmHg, p=0.001). Bland-Altman analysis of individual pairs of DCT and GAT measurements revealed a bias of -1.0 mmHg [95% confidence interval (CI): +/-1.2]. Neither GAT nor DCT showed a significant correlation with CCT (533+/-48, range 399-641 microm).

CONCLUSIONS

In eyes with normal corneas, DCT allows suitable and reliable IOP measurements which are in good concordance with GAT. Comparison of DCT with intracameral manometry is desirable in the future.

Influence of corneal biomechanical properties on intraocular pressure measurement

PURPOSE

To understand and quantify intraocular pressure (IOP) measurement errors introduced by corneal variables during applanation tonometry using a cornea biomechanical model.

SETTING

Department of Ophthalmology, Biomedical Engineering Center, The Ohio State University, Columbus, Ohio, USA.

METHODS

The model assumed an overall resultant pressure that was based on the summation of the applanation pressure, the true IOP, and the surface tension caused by the tear film to determine the final deformation of the corneal apex during IOP measurement. Corneal resistance was varied according to the cornea's biomechanical properties, thickness, and curvature, and the effect of each variable on the accuracy of IOP tonometry readings was examined quantitatively.

RESULTS

The model demonstrated that tonometry readings do not always reflect true IOP values. They deviate when corneal thickness, curvature, or biomechanical properties vary from normal values. Based on the model, predicted IOP readings have a 2.87 mm Hg range resulting from the variation in the corneal thickness in the normal population and a 1.76 mm Hg range from the variation in the corneal radius of curvature. Considering that Young's modulus of the corneal varies from 0.1 to 0.9 MPa in the normal population, the model predicts tonometry IOP readings will have a range of 17.26 mm Hg because of the variation in this corneal biomechanical parameter alone.

CONCLUSIONS

The simulation based on the model demonstrated quantitatively that variations in each corneal variable cause errors in tonometry IOP readings. The simulation results indicate that differences in corneal biomechanics across individuals may have greater impact on IOP measurement errors than corneal thickness or curvature.

Goldmann tonometry after hyperopic laser in situ keratomileusis: comparison between retreated and nonretreated patients

PURPOSE

To identify differences in applanation tonometry between retreated and nonretreated eyes (primary LASIK eyes) 6 months after hyperopic laser in situ keratomileusis.

METHODS

In a prospective study 100 eyes (100 patients) underwent conventional hyperopic laser in situ keratomileusis under a 160-microm flap. Central Goldmann applanation tonometry was determined before and 6 months after surgery in 78 (78%) primary LASIK eyes and before surgery and 6 months after the retreatment date in 22 (22%) retreated eyes.

RESULTS

Preoperative mean tonometry was 14.96 +/- 1.96 mm Hg and 15.30 +/- 1.95 mm Hg in primary LASIK and retreated eyes, respectively. Six months after surgery it was 12.99 +/- 2.03 mm Hg (P < 0.001) and 12.67 +/- 2.20 mm Hg (P < 0.001), respectively. No significant differences in mean tonometry were found between retreated and primary LASIK eyes 6 months after surgery (P = 0.537). Decreased tonometric values were found in 20 retreated eyes (90.91%) and in 62 primary LASIK eyes (79.48%). Increased tonometric values higher than 1 mm Hg were found in 3 primary LASIK eyes (3.84%), but no retreated eyes showed increased values 6 months after surgery. Two retreated eyes (9.09%) and 10 primary LASIK eyes (79.49%) presented the same tonometric values before surgery and at the end of the study.

CONCLUSIONS

After hyperopic laser in situ keratomileusis there was no significant difference in Goldmann applanation tonometry between retreated and primary LASIK eyes.

Assessment of the Pascal dynamic contour tonometer in monitoring intraocular pressure in unoperated eyes and eyes after LASIK

PURPOSE

To assess the performance of the Pascal dynamic contour tonometer (PDCT) (Swiss Microtechnology AG) by comparing it to Goldmann applanation tonometry (GAT) and noncontact air tonometry (NCT) before and after laser in situ keratomileusis (LASIK).

SETTING

Vlemma Eye Institute, Athens, Greece.

METHODS

Intraocular pressure was measured in 118 eyes before and 1 and 4 weeks after LASIK using GAT, NCT, and the PDCT, which allows direct, digital, transcorneal measurement of intraocular pressure.

RESULTS

Preoperatively, central corneal thickness (CCT) correlated with GAT and NCT measurements but not with PDCT measurements. After LASIK, the mean reduction in CCT ranged from -3.0 to -171.0 microm (median 78 microm). The mean GAT measurement dropped by -4.9 mm Hg +/- 2.7 (SD) at 1 week and was -5.4 +/- 3.0 mm Hg at 4 weeks. Similar drops were observed in NCT measurements. The observed post-LASIK changes in GAT and NCT IOP measurements were not directly proportional to the change in CCT, refractive error, or mean keratometric readings. The preoperative and postoperative PDCT measurements did not differ significantly.

CONCLUSIONS

The structural corneal changes induced by LASIK appeared to influence GAT and NCT measurements but not PDCT measurements. Therefore, PDCT may be better suited for monitoring IOP in unoperated eyes and in eyes that have had LASIK.

Refractive surgery in glaucoma

PURPOSE OF REVIEW

The purpose of this review is to discuss the topics relevant to the topic of LASIK in glaucoma patients or suspects. The safety of LASIK and the implications of LASIK on glaucoma diagnostic testing are reviewed.

RECENT FINDINGS

Most studies have found no change in the nerve fiber layer thickness or optic disc after LASIK. Several reports of steroid-induced glaucoma after LASIK have been published. Important new syndromes such as the interlamellar stromal cyst have been described. Most investigators, but not all, have found intraocular pressure to be lower after LASIK, most likely an artifact of measurement as a result of reduced central corneal thickness.

SUMMARY

The safety of LASIK in patients with glaucoma has not been proved. However, based on the published literature, LASIK may be a viable option for some glaucoma patients. Although not an absolute contraindication, glaucoma is a relative contraindication, and careful patient education and life-long follow-up is mandatory. The clinician must be ever mindful of the important effects that LASIK may have on diagnostic testing.

Intraocular pressure, Goldmann applanation tension, corneal thickness, and corneal curvature in Caucasians, Asians, Hispanics, and African Americans

PURPOSE

This is to investigate whether there are differences in Goldmann applanation tonometry (GAT), central corneal thickness, and corneal curvature among four racial groups. If differences are present, they may alter GAT reading, diagnosis, and management of glaucoma in the population.

DESIGN

Observational retrospective cross-sectional study.

METHODS

Charts of patients who have had keratorefractive surgery were examined. Central corneal thickness, corneal curvature, refractive power, and GAT were measured in 1482 Caucasian, 172 Asian, 204 Hispanic, and 118 African-American eyes (total 1976 eyes). Refractive components and GAT were compared. We compared intraocular pressure (IOP) adjusted by GAT, central corneal thickness, and corneal curvature among the four groups.

RESULTS

There was a statistically significant difference between the mean (+/- standard deviation) central corneal thickness of African American (535.46 +/- 33.39) and Caucasian (552.59 +/- 34.48) eyes. Mean central corneal thickness was near 550 microm in Caucasians, Asians, and Hispanics. No significant difference was noted in corneal curvature in the four groups. There was a significant correlation between central corneal thickness and corneal curvature, and GAT was similar among the four groups. When IOP was adjusted for central corneal thickness, it was significantly greater in African Americans (16.12 +/- 3.27) than in Caucasians (14.32 +/- 2.93). Corneas of women were significantly thinner than corneas of men.

CONCLUSIONS

African Americans had significantly thinner central corneal thickness than Caucasians, Asians, or Hispanics, causing the underreading of true IOP. Significant correlation between central corneal thickness and corneal curvature was demonstrated. Uncorrected GAT underreading of African Americans may lead to delay in diagnosis, inadequate treatment target setting, and higher morbidity. Goldmann applanation tonometry needs to be corrected by central corneal thickness and corneal curvature for proper diagnosis and management of glaucoma.

Corneal ectasia detected 32 months after LASIK for correction of myopia and asymmetric astigmatism

We report a case of corneal ectasia detected 32 months after laser in situ keratomileusis (LASIK) for correction of -4.25 diopters (D) of myopia associated with -2.00 D of regular but slight asymmetric astigmatism. The patient retained stable visual acuity for 15 months postoperatively. The preoperative corneal thickness was 540 microm, and the postablation untouched stroma was assumed to be 290 microm. Although a rare complication of LASIK, corneal ectasia can occur, and there is no consensus regarding how much stroma should be left intact to avoid it. Until we have a better understanding of corneal strength, we think surface photorefractive keratectomy or laser-assisted subepithelial keratectomy ablations should be considered instead of LASIK in borderline cases.

Corneal flap complications in refractive surgery: Part 2: postoperative treatments of diffuse lamellar keratitis in an experimental animal model

PURPOSE

To induce diffuse lamellar keratitis (DLK) and investigate a prophylactic treatment with pharmacological agents.

SETTING

Magill Research Center for Vision Correction, Storm Eye Institute, Charleston, South Carolina, USA.

METHODS

In 100 eyes of 50 Dutch Belted rabbits, a nasal corneal flap was cut and the interface inoculated with Pseudomonas aeruginosa lipopolysaccharide endotoxin (n = 50) or Palmolive Ultra soap (n = 50). The eyes were randomly assigned to treatment with a mast-cell stabilizer, a nonsteroidal antiinflammatory drug (NSAID), a fluoroquinolone antibiotic agent, a corticosteroid, or left without treatment as a control. Slitlamp examinations and photographs were performed 1, 3, 5, and 7 days postoperatively, and DLK was graded by a masked observer from 0 (no DLK) to stage 4.

RESULTS

At the end of the study, 80 eyes were available for evaluation. Ninety-four percent of the untreated eyes developed DLK compared to 56% of eyes treated with NSAIDs and 63% of eyes treated with steroids (P<.05, Fisher exact test). The DLK rates with the mast-cell stabilizer and fluoroquinolone antibiotic agent were 86% and 76%, respectively. The DLK incidence in the latter 2 groups was not significantly different from that in the control eyes (P>.05, Fisher exact test).

CONCLUSIONS

Postoperative prophylactic treatment with NSAIDs and corticosteroids led to a statistically significantly lower incidence of postoperative DLK. The study demonstrated that corticosteroids and NSAIDs can be used to treat DLK after LASIK. While steroids are a generally accepted treatment for DLK, NSAIDS may offer an additional potent modality.

Some possible longer-term ocular changes following excimer laser refractive surgery

While the short- and medium-term refractive and acuity results of excimer laser refractive surgery may now be generally satisfactory, the relatively brief history of the procedures involved (around 10 years or less) means that those concerned with eye care must remain vigilant to the possibility of longer-term problems. This paper reviews some relevant studies of potential post-surgical effects, including imperfect corneal healing and recovery of innervation, reduced corneal sensitivity and dry eye problems, changes in corneal rigidity leading to slow refractive change, possibly misleading reductions in measured intraocular pressure, and retinal and vitreous pathology.