Effect of Laser in Situ Keratomileusis on Schiøtz, Goldmann, and Dynamic Contour Tonometric Measurements

Predictive models for IOPs measured with NCT, GAT, and ORA among patients undergoing SMILE

Purpose: To develop predictive models for the intraocular pressure (IOP) of patients undergoing small incision lenticule extraction (SMILE) procedures, measured with a noncontact tonometer (NCT), Goldmann applanation tonometry (GAT), and an ocular response analyzer (ORA). Methods: In this prospective study, a total of 104 eyes (-6.23 ± 2.06 diopters) of 52 patients (24.38 ± 4.76 years) undergoing SMILE procedures were included. The intraocular pressure was measured (IOPNCT with NCT, IOPGAT with GAT, and IOPcc and IOPg with ORA) before surgery and at postoperative 6 months. Information on age, preoperative and attempted spherical equivalent (SE), ablation depth, preoperative values and postoperative changes in central corneal thickness (CCT), K1, K2, Km, corneal hysteresis (CH) and corneal resistance factor (CRF) values was collected in order to predict IOPs. Results: All surgeries were uneventful. At postoperative 6 months, the efficacy and safety index were 1.04 ± 0.15 and 1.08 ± 0.18, respectively. Significant decreases were detected in postoperative IOPNCT, IOPGAT, IOPcc, and IOPg compared to preoperative values (all p < 0.001). No relationship was found between any IOP and ablation depth, attempted SE, and preoperative SE, as well as CCTdifference (all p > 0.05). Predictive models for IOPs were constructed to predict preoperative values, and R 2 values were 67.5% (IOPNCT), 64.5% (IOPGAT), 78.7% (IOPcc), and 82.0% (IOPg). The prediction band of IOPNCT and IOPGAT was 7.4-15.1 mmHg and 8-16 mmHg, respectively. Conclusion: Predictive models for IOP measurements after SMILE procedures can be helpful in clinical practice.

A Critical Assessment of Friedenwald’s Technique for Estimating the Coefficient of Rigidity of the Cornea

Purpose

To determine if Friedenwald's technique for estimating the coefficient of corneal rigidity (Ko, units mmHg/μL), could differentiate between the cornea in keratoconus, normal eyes, and after crosslinking (CXL).

Methods

Two operators (1 and 2) independently measured Ko in three groups (keratoconus, normal, and post-CXL corneas), and repeated the procedure in some where their care remained unchanged and others after routine CXL (>28 days postop, epi-off treatment, 3.0 mW/cm², 30 min). The data were subsequently used to quantify interoperator error, test-retest/intersessional reliability for estimation of Ko, the significance of intergroup differences, and the effect of CXL on Ko.

Results

The major findings were: (i) Ko values were not normally distributed; (ii) mean (±sd, 95% CI) interoperator error was -0.002 (±0.019, −0.006 to 0.003, n = 95) and the limit of agreement between the operators was ±0.039; (iii) RMS differences in the intersessional estimation of Ko values were 0.011 (operator 1) and 0.012 (operator 2); (iv) intergroup differences in Ko were not significant (p > 0.05); (v) intersessional change in Ko (y) was linearly related to Ko estimated (x) at 1^st session (for operator 2 y = 1.187x−0.021, r = 0.755, n = 16, p < 0.01); and (vi) change in Ko (y₁) after CXL was linearly related to Ko (x₁) at preop (for operator 2 y₁ = 0.880x₁−0.016, r = 0.935, n = 20, p < 0.01).

Conclusion

Friedenwald's technique for estimating the Ko is prone to substantial interoperator error and intersessional differences. According to the technique, the change in Ko following CXL is on par with the expected intersessional change observed in controls.

The effect of corneal crosslinking on the rigidity of the cornea estimated using a modified algorithm for the Schiøtz tonometer

Purpose:

The aim of this study was to test a method for estimating corneal rigidity before and after cross-linking (CXL) using a Schiøtz tonometer.

Methods:

The study was performed in the Kyiv City Clinical Ophthalmological Hospital “Eye Microsurgical Center”, Ukraine. This was a prospective, consecutive, randomized, masked, case-by-case, clinical study. Corneal rigidity, indicated by the gradient (G) between lg applied weight and corresponding lg scale reading during Schiøtz tonometry, were obtained by increasing (A-mode) then reducing (D-mode) weights by two operators [A] in keratoconus, post-CXL and control subjects for estimation of (i) interoperator and (ii) intersessional errors, (iii) intergroup differences; [B] before and after CXL. Central corneal thickness CCT was measured by scanning slit pachymetry. ANOVA, t tests, linear regression were the statistical tools used.

Results:

Average interoperator difference (ΔG) was –0.120 (SD = ±0.294, 95%CI = –0.175 to –0.066). A significant correlation between ΔG and the mean of each pair of G values was found (r = –0.196, n = 112, P = 0.038). Intersessional differences in mean G values were insignificant (P > 0.05). There was a significant correlation between G at first session (X₁) and difference between sessions (ΔG) [Operator 1, ΔG = 0.598x₁–0.461, r = 0.601, n = 27, P = 0.009]. Significant intergroup differences in G were found (Operator 1, one-way ANOVA, F = 4.489, P = 0.014). The difference (Δ) between the pre-(X₂) and post-CXL treatment G values was significantly associated with the pre-CXL treatment value (Operator 1, Δ = 1.970x₂-1.622, r = 0.642, n = 18, P = <.001). G values were correlated with CCT in keratoconus and post-CXL.

Conclusion:

Corneal rigidity (G) estimated using the Schiøtz tonometer can be useful for detecting changes after CXL. However, G values are linked to CCT, can vary from time-to-time and the procedure is operator dependent.

Association between corneal refractive surgery and the prevalence of glaucoma: Korea National Health and Nutrition Examination Survey 2010-2012

AIM

To determine the relationship between corneal refractive surgery and the prevalence of glaucoma in the Korean population.

METHODS

Data were obtained from the Korea National Health and Nutrition Examination Survey (KNHANES), a population-based cross-sectional study using a complex, stratified, multistage, probability-cluster survey. This study included 604 eyes that had undergone myopic corneal refractive surgery, and 3389 control eyes without a history of any ocular surgery and having a spherical equivalent (SE) <-3.00 D, obtained from the KNHANES database for the years 2010-2012. Glaucoma diagnosis was based on the International Society of Geographical and Epidemiological Ophthalmology criteria. The association between a history of corneal refractive surgery and the prevalence of glaucoma was analysed using logistic regression analysis, after adjusting for potential confounding factors.

RESULTS

Glaucoma prevalence did not differ between eyes that had and had not undergone corneal refractive surgery (p=0.675). After adjusting for age, sex, SE, and intraocular pressure, multivariate logistic regression analysis found that corneal refractive surgery was significantly associated with an increased risk of glaucoma (OR 9.14, p=0.002; 95% CI 2.22 to 37.69). Subgroup analysis that only included control eyes with a refraction cut-off <-3.70 D found that corneal refractive surgery was not significantly associated with glaucoma.

CONCLUSIONS

History of corneal refractive surgery was associated with a higher prevalence of glaucoma in the Korean population. However, this association was not observed in eyes with a higher degree of myopia.

The Effect of Corneal Thickness, Densitometry and Curvature on Intraocular Pressure Measurements Obtained by Applanation, Rebound and Dynamic Contour Tonometry

Evaluate the effect of corneal thickness, densitometry and curvature on intraocular pressure (IOP) measurements obtained by Goldmann applanation tonometry (GAT), non-contact tonometry (NCT), rebound tonometry (RT), and dynamic contour tonometry (DCT). A cross-sectional prospective study involving 40 participants was performed. Corneal measurements were obtained using Pentacam (Oculus GMbH, Wetzlar, Germany), densitometry was measured at annuli of 0–2, 2–6, 6–10 and 10–12 mm. The relationship between corneal thickness (central, 4 and 6 mm), corneal astigmatism and corneal densitometry and IOP was examined. There was a significant relationship between corneal thickness (central, 4 and 6 mm) and GAT180, GAT90, RT, and NCT (P < 0.001 for all comparisons) but not for DCT. Higher corneal densitometry (6–10 mm and 10–12 mm zones) was associated with higher IOP from GAT180 and GAT90, and higher densitometry in the 6–10 mm zone correlated with higher IOP from NCT, however corneal densitometry increased with age. Accounting for age, the relationship between corneal densitometry and IOP measurements was not significant. In eyes with greater corneal astigmatism there was a greater difference between GAT90 and GAT180 measurements. IOP measurements may be affected by corneal thickness, densitometry and curvature. DCT was less affected by properties of the cornea compared to other devices.

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.

Effectiveness of 4 tonometers in measuring IOP after femtosecond laser-assisted LASIK, SMILE, and transepithelial photorefractive keratectomy

PURPOSE

To test the performance of 4 tonometers in estimating intraocular pressure (IOP) after 3 forms of refractive surgery.

SETTING

Eye Hospital, Wenzhou Medical University, China.

DESIGN

Prospective case series.

METHODS

Patients matched for preoperative age, corneal thickness, and myopic correction enrolled for femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK), small-incision lenticule extraction (SMILE), or transepithelial photorefractive keratectomy (tPRK) were included in the study. For each patient, 4 measurements of IOP were obtained preoperatively and 3 months postoperatively, using the Goldmann applanation tonometer (GAT-IOP), the Dynamic Contour Tonometer (DCT-IOP), corneal-compensated IOP (IOPcc) from the Ocular Response Analyzer, and biomechanically corrected IOP (bIOP) from the Corvis ST. Overall corneal stiffness was also estimated based on the stiffness parameter (SP-A1) provided by the Corvis ST.

RESULTS

The study included 144 eyes of 144 patients. Among the 3 procedures, the smallest variances between preoperative and postoperative IOP estimates and SP-A1 values were observed with the tPRK, followed by SMILE and FS-LASIK. In the tPRK group, no significant differences were observed in both bIOP (-0.18 ± 1.63 mm Hg) and DCT-IOP (-.64 ± 2.34 mm Hg), whereas they were larger and significant in GAT-IOP (-1.78 ± 2.29 mm Hg) and IOPcc (-2.77 ± 1.84 mm Hg). In FS-LASIK and SMILE groups, although there were similar significant reductions in IOP postoperatively, these reductions were still lower in bIOP and DCT-IOP than those in GAT-IOP and IOPcc.

CONCLUSIONS

The bIOP and DCT-IOP were the least affected IOP estimates between the 3 refractive surgery procedures considered. It was evident that tPRK produced significantly smaller reductions in IOP readings than did FS-LASIK and SMILE.

Effectiveness of the Goldmann Applanation Tonometer, the Dynamic Contour Tonometer, the Ocular Response Analyzer and the Corvis ST in Measuring Intraocular Pressure following FS-LASIK

Purpose: To test the performance of the four tonometers in providing IOP measurements that were free of the effects of corneal biomechanics changes caused by refractive surgery.Methods: Four tonometers were employed to provide IOP measurements for 65 participants who accepted Femtosecond laser-assisted LASIK (FS-LASIK). The measurements included GAT-IOP by the Goldmann Applanation Tonometer, DCT-IOP by the Dynamic Contour Tonometer, Goldmann-correlated IOP (ORA-IOPg) and corneal-compensated IOP (ORA-IOPcc) by the Ocular Response Analyzer, and uncorrected IOP (CVS-IOP) and biomechanically corrected IOP (CVS-bIOP) by the Corvis ST. Statistical analyses were performed to assess the association of the differences in IOP caused by FS-LASIK with central corneal thickness (CCT), mean corneal curvature (Km), age, refractive error correction (REC), optical zone diameter (OZD), ablation zone diameter (AZD), residual stromal bed thickness (RSB) and RSB ratio (RSB/CCT). Multiple linear regression models were constructed to explore factors influencing IOP changes.Results: All four tonometers exhibited significant differences between IOP measurements taken pre and post-surgery except for CVS-bIOP in the low to moderate myopia group (t = 1.602, p = .12). CVS-bIOP, followed by DCT-IOP, provided the best agreement between pre and post-FS-LASIK measurements with the lowest differences in IOP and the narrowest limits of agreement. The pre-post IOP differences were also significantly associated with the reduction in CCT in only GAT-IOP, ORA-IOPg, and CVS-IOP. CVS-bIOP and ORA-IOPcc were the only measurements that were not correlated with CCT, Km or age both before and after FS-LASIK.Conclusions: The biomechanically corrected bIOP from the Corvis ST provided post-FS-LASIK measurements that were in closest agreement with those obtained before surgery. In comparison, GAT-IOP, ORA-IOPg, ORA-IOPcc, and CVS-IOP appeared to be more influenced by the changes in corneal biomechanics caused by FS-LASIK.

Influence of LASEK on Schiøtz, Goldmann and dynamic contour Tonometry

PURPOSE

Our purpose was to assess the effect of LASEK surgery on ocular rigidity and to compare its effect on intraocular pressure (IOP) readings with Goldmann applanation tonometry (GAT), Schiøtz indentation tonometry (ST) and dynamic contour tonometry (DCT).

METHODS

Prospective, observational, single-masked cohort study. One eye per patient from 31 consecutive subjects who underwent LASEK refractive surgery to correct myopia, as well as from 108 non-operated myopic patients, were included. IOP was measured using GAT, DCT, and ST. The coefficient of ocular rigidity (K_o) was obtained from the regression line of the three readings obtained with each weight of the ST. Linear multiple regression analysis was performed with dummy variables to assess the effect of age, central corneal thickness (CCT), and previous refractive surgery on IOP values.

RESULTS

Age, CCT, and LASEK surgery explained 23% of the IOP readings with GAT, 11% with DCT, and had no significant influence on ST readings. The IOP readings obtained with GAT and DCT, but not with ST, were significantly different between LASEK and control groups. Significant differences in K_o were observed between LASEK (0.014 mmHg/μL ±0.007) and control groups (0.021 mmHg/μL ±0.013) (P = 0.001). K_o values were not related with age or CCT, in either LASEK or control groups.

CONCLUSIONS

ST readings seems to be less affected by LASEK surgery than the other tonometers evaluated. There is a difference in the ocular rigidity between the control and the LASEK eyes that it is not correlated with CCT, so it seems that ST can detect changes in the biomechanical behavior of the LASEK operated corneas.

Considerations for refractive surgery in the glaucoma patient

PURPOSE OF REVIEW

Given the popularity of keratorefractive surgery, and an aging populous of patients who have undergone these procedures, there is an increasing need for updated management protocols. This is particularly relevant for patients with chronic progressive diseases such as glaucoma, due to the variety of related diagnostic and management challenges inherent to these diseases. Here, we will review the current literature to provide an update on the management of patients with glaucoma who are undergoing, or have had laser ablative refractive surgery. Preoperative testing and eligibility considerations, intraoperative factors, and postoperative observation and follow-up will be discussed.

RECENT FINDINGS

Intraoperative intraocular pressure (IOP) rise during flap creation is associated with low risk of acute complications, and furthermore do not appear to have significant long term effects. Modern technologies have improved our ability to determine accurate IOP after refractive surgery despite postoperative changes in corneal architecture. Furthermore, advances in structural imaging allow for earlier detection of even subtle glaucomatous nerve damage.

SUMMARY

Although glaucoma remains a relative contraindication to refractive surgery, it is a safe procedure for many patients with appropriate perioperative management and follow-up. Advancements in diagnostic modalities have allowed for earlier detection of glaucomatous disease, and subsequent earlier intervention when appropriate. Standardized diagnostic algorithms and rigorous perioperative assessment are critical to safe management of glaucoma patients undergoing refractive corneal surgery.