Intraocular pressure following intrastromal corneal ring segments
This work was supported by grant no 502-03/5-108-05/502-54-189, 502-03/5-108-05/502-54-175 and grant UMO-2016/21/B/NZ5/01411 from Polish National Science Centre.
The effects of ocular structural factors on the measurements provided by tonometers other than Goldmann applanation tonometry (GAT) have been explored in patients with ectatic corneas or those undergoing penetrating keratoplasty (Ceruti et al. 2008; Unterlauft et al. 2011). However, available data on the effects of corneal factors on intraocular pressure (IOP) readings in patients implanted with intrastromal corneal ring segments (ICRS) are scarce (Piñero et al. 2012; Arribas-Pardo et al. 2015).
In this prospective observational cohort randomized study, IOP measurements and corneal parameters determined using Pentacam corneal topography were made in 70 eyes of 70 patients with keratoconus scheduled for ICRS implantation (KeraRings, Mediphacos, Belo Horizonte, Brazil). Intraocular pressure (IOP) measurements were taken using the five tonometers GAT (Haag-Streit AG, Bern, Switzerland), rebound tonometer iCare Pro (RBT, iCare, Tiolat Oy, Helsinki, Finland), dynamic contour tonometer (DCT, SMT Swiss Microtechnology AG, Port, Switzerland), Tonopen XL (Reichert Ophthalmic Instruments, Depew, New York, USA) and ocular response analyzer (ORA, Reichert Ophthalmic Instruments, Depew, NY, USA), before surgery and 1, 3 and 6 months after surgery. Mean of three measurements was used for analysis. Intraocular pressure (IOP) using GAT was not corrected regarding astigmatism. The order of use of the tonometers was randomized using an automatic randomization method (1:1:1:1:1; www.randomization.com).
No changes in IOP measurements using all the tonometers were found after ICRS placement surgery (Table 1). Neither corneal hysteresis (8.1 mmHg± 1.9; p = 0.063) nor corneal resistance factor (6.2 mmHg ± 2.3; p = 0.167) changed after surgery. Central corneal thickness change was statistically significant but not clinically significant (491.8 μ ±47.0; p = 0.027) Compared with GAT, ORA Goldmann-correlated IOP (IOPg) significantly underestimated IOP in more than 5 mmHg. This could be attributed to biomechanical properties of the cornea given the abnormal corneal variables of keratoconic eyes. Ocular response analyzer takes into account corneal hysteresis, which is low in patients with keratoconus and may be useful for diagnosing in such patients. Rebound tonometer iCare Pro (RBT) underestimated, while Tonopen XL overestimated IOP measurements 6 months after surgery. Such differences could in part be explained by the corneal changes that take place in these patients in response to ICRS placement including those produced in corneal curvature and in astigmatism. These changes could affect one tonometer more than another. The lower contact surface of these instruments could play an important role. The smaller corneal contact surface area means, it is easier to obtain IOP readings in patients with irregular corneas due to ectasia. This makes these tonometers useful in keratoconic eyes. Rebound tonometer iCare Pro (RBT) was the tonometer that showed best agreement with GAT (ICC = 0.681) followed by Tonopen XL. This confirms the findings of a prior study in which we compared GAT and RBT IOP measurements in patients with keratoconus implanted with ICRS, (Arribas-Pardo et al. 2015) while ORA IOP measurements corneal-compensated IOP (IOPcc) and IOPg showed the lowest agreement with GAT.
| Preoperative | 1 month postoperative | 3 month postoperative | 6 month postoperative | GAT-tonometer 6 month postoperative | ICC (95% CI) | pa | |
|---|---|---|---|---|---|---|---|
| Mean; SD (p)b | Paired mean differences; SD | ||||||
| GAT | 14.4; 2.9 | 13.6; 2.7 (0.066) | 12.8; 2.2 (0.001) | 13.6; 3.0 (0.173) | – | – | 0.162 |
| DCT | 15.7; 3.0 | 15.7; 3.8 (0.756) | 13.7; 2.5 (<0.001) | 14.7; 2.7 (0.013) | −1.1; 2.6 (0.004) | 0.398 (0.255 to 0.711) | 0.029 |
| RBT | 13.9; 3.8 | 13.5; 5.4 (0.092) | 12.6; 3.6 (0.010) | 12.2; 3.5 (0.001) | 1.3; 2.3 (<0.001) | 0.681 (0.562 to 0772) | 0.143 |
| Tonopen XL | 14.9; 3.2 | 14.7; 3.7 (0.508) | 14.4; 3.0 (0.410) | 14.8; 3.2 (0.881) | −1.2; 2.2 (<0.001) | 0.584 (0.439 to 0.699) | 0.740 |
| IOPg | 8.0; 3.2 | 8.4; 3.1 | 7.3; 3.3 | 8.0; 3.5 | 5.1; 3.3 (<0.0001) | −0.055 (−0.247 to 0.142) | 0.808 |
| IOPcc | 12.5; 3.1 | 12.8; 3.4 | 11.4; 2.9 | 11.9; 2.9 | 1.3; 3.2 (0.004) | 0.358 (0.175 to 0.517) | 0.153 |
| CC (D) | 50.4; 6.0 | 48.5; 5.3 (<0.0001) | 48.1; 5.3 (0.004) | 49.1; 5.4 (0.003) | – | – | – |
| CCT (μ) | 484.4; 42.3 | 496.7; 48.4 (0.004) | 497.5; 51.0 (0.908) | 491.8; 47.0 (0.027) | – | – | – |
| CA (D) | 6.2; 5.5 | 3.1; 2.5 (<0.0001) | 2.9; 2.5 (<0.0001) | 3.1; 2.8 (<0.0001) | – | – | – |
| CH | 7.6; 1.5 | 7.9; 1.7 (0.291) | 8.1; 1.5 (0.037) | 8.1; 1.9 (0.063) | – | – | – |
| CRF | 5.9; 1.7 | 6.0; 1.4 (0.712) | 6.0; 1.8 (0.951) | 6.2; 2.3 (0.167) | – | – | – |
- SD = standard deviation, ICC = intraclass correlation coefficient, CI = confidence interval, GAT = Goldmann applanation tonometer, DCT =dynamic contour tonometer, RBT = rebound tonometer iCare Pro, IOPc = corneal-compensated intraocular pressure (ocular response analyzer), IOPg = Goldmann-correlated intraocular pressure (ocular response analyzer), CC = corneal curvature, CCT = central corneal thickness, CA = corneal astigmatism, CH = corneal hysteresis, CRF = corneal resistance factor.
- a Tested p values for paired mean differences between IOP values recorded preoperatively and 1, 3, and 6 months postoperatively.
- b ANOVA-test.
The corneal variables measured here were similar to reported studies (Piñero et al. 2012). After surgery, corneal curvature (49.1 D ± 5.4; p = 0.003) and corneal astigmatism (3.1 D ± 2.8; p < 0.0001) were significantly lower, but central corneal thickness and corneal hysteresis did not change.
Only two prospective studies have addressed IOP measurement after ICRS implantation. Ruckhofer et al. (2001) observed significantly lower IOP values using GAT in all follow-up visits after ICRS implantation (−0.89 to −1.75 mm Hg; p < 0.02), which despite being statistically significant, were not clinically significant (below 2 mmHg). Our results are consistent with these observations; we did not detect a significant difference in GAT-determined IOP before and after ICRS implant. Also, IOP differences never exceeded 2 mmHg regardless of the tonometer used.
Gorgun et al. (2011) examined several corneal biomechanical factors after ICRS implant using the ORA. Intraocular pressure (IOP) diminished after surgery although differences were not significant at 6 months. However, this study did not compare corneal biomechanics before and after surgery despite the observation of significant postoperative changes produced only in corneal astigmatism and corneal curvature.
In conclusion, our findings indicate that IOP does not change following ICRS implant in keratoconic eyes. iCare Pro provided readings that were fairly consistent with GAT, while ORA-IOP measurements show poor agreement with GAT readings. This could be explained because ORA take into account the biomechanical properties of the cornea, which are affected in keratoconic eyes.
