Comparison between Pascal dynamic contour tonometer and Goldmann applanation tonometer after different types of refractive surgery

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1 DOI /s REFRACTIVE SURGERY Comparison between Pascal dynamic contour tonometer and Goldmann applanation tonometer after different types of refractive surgery Antonios P. Aristeidou & Georgios Labiris & Andreas Katsanos & Michalis Fanariotis & Nikitas C. Foudoulakis & Vassilios P. Kozobolis Received: 28 February 2010 /Revised: 26 May 2010 /Accepted: 3 June 2010 # Springer-Verlag 2010 Abstract Background To evaluate and compare the recorded IOP values of the Pascal dynamic contour tonometer (PDCT) and the Goldmann applanation tonometer (GAT) after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). Methods Three groups of 84, 182 and 43 patients each were treated by PRK for myopia, LASIK for myopia and LASIK for hyperopia respectively. Intraocular pressure (IOP) measurements were performed in all eyes 1 day before and 1, 3, 6 and 12 months after treatment with PDCT and GAT. Ultrasound pachymetry was performed in all eyes preoperatively and at 1st month postoperatively. Results Preoperatively and postoperatively, GAT readings were lower than PDCT in all groups (all p < 0.05). Postoperatively in the PRK group, compared to the preoperative value, the mean differences of IOP recorded with GAT at 1st, 3rd, 6th and 12th month were 1.4 mmhg, 1.7 mmhg, 1.7 mmhg and 1.9 mmhg respectively (all p<0.05). In the myopic LASIK group; the corresponding values with GAT were 3.6 mmhg, 3.6 mmhg, 3.6 mmhg and 3.5 mmhg (all p<0.05), while in the hyperopic LASIK group the corresponding values were 1.1 mmhg, 0.7 mmhg, 1.1 mmhg and 0.9 mmhg (all p <0.05). The mean IOP difference (GAT-PDCT) for A. P. Aristeidou : G. Labiris : N. C. Foudoulakis : V. P. Kozobolis Ophthalmology Department, Democritus University of Thrace, Alexandroupolis, Greece A. P. Aristeidou (*) : G. Labiris : A. Katsanos : M. Fanariotis : N. C. Foudoulakis : V. P. Kozobolis Eye Institute of Thrace (E.I.T.), Democritus University of Thrace, Dragana, Alexandroupolis, Greece aaristeidou@yahoo.gr myopic PRK, myopic LASIK and hyperopic LASIK were respectively 3.8 mmhg, 4.1 mmhg and 1.5 mmhg at the 12-month follow-up. No statistically significant changes were found for any group with the PDCT. Conclusions GAT-determined IOP values were significant lower at all time-points after hyperopic LASIK, as well as myopic PRK or LASIK. The Pascal tonometry values remained unaffected for all groups. Keywords Pascal dynamic contour tonometer. Goldmann tonometer. PRK. LASIK. Myopia. Hyperopia Introduction Laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) have gained considerable acceptance as means of correcting myopia, hyperopia and astigmatism. The outcomes regarding refractive error correction are usually satisfactory, but some issues have been raised concerning the accuracy of postoperative tonometry readings [1 8]. The accuracy of IOP measurements after refractive surgery procedures is of a great clinical importance because some of these patients will eventually develop glaucoma in the future. Underestimation of IOP values may result in serious visual loss. Recent studies show that IOP readings derived by Goldmann applanation tonometry (GAT) underestimate intraocular pressure (IOP) after LASIK or PRK surgery [1 10]. Nonetheless, published research concerning the effects of hyperopic refractive treatments in IOP measurements is limited [10 15]. In addition, other types of tonometers like common non-contact tonometers (NCT) and the ocular response analyzer (ORA) have also been

2 shown to underestimate the IOP after myopic LASIK [10, 16]. The aim of our study was to compare the IOP readings derived by PDCT and GAT in 309 healthy eyes that underwent either PRK for the correction of myopia or LASIK for the correction of myopia or hyperopia. Patients and methods Setting The study adhered to the tenets of the Declaration of Helsinki, and written informed consent was given by all participants. The institutional review board of the Democritus University of Thrace approved the protocol, and the study was conducted at the Eye Institute of Thrace in the period between April 2007 and April Participants Three hundred and nine eyes of 309 consecutive healthy white adults with no other ophthalmic problem except ametropia were enrolled in this study. Eighty four of the participants underwent PRK for the correction of myopia with or without astigmatism, 182 underwent LASIK for myopia with or without astigmatism and 43 underwent LASIK for hyperopia with or without astigmatism. The decision to perform PRK or LASIK in myopic eyes was reached after a detailed preoperative examination and discussion with the patient. Conditions such as refractive error, corneal thickness, tear film condition, corneal abnormalities, intraocular pressure, and the patient s needs and preferences were all taken into account. A series of exclusion criteria were applied: previous incisional eye surgery, evidence of any type of corneal pathology, evidence or history of any type of optic neuropathy, and diabetes mellitus. Procedures Preoperative control All participants underwent the same preoperative examination protocol which included: assessment of uncorrected and best-corrected visual acuity (BCVA) before and after instillation of cycloplegic drops (cyclopentolate), slit-lamp biomicroscopy, corneal topography, pupillometry, stereoscopic Lang test, tear film break-up time, Schirmer test, Goldmann applanation tonometry, and Pascal contour dynamic tonometry (PDCT, Swiss Microtechnology AG) preoperatively and during all visits postoperatively. The mean of two IOP measurements was recorded as the final IOP value. If the difference between the two IOP measurements exceeded 3 mmhg, a third measurement was made, and the mean of the three measurements was recorded as the final IOP value. Ultrasound pachymetry (Optikon pachymeter, Optikon, Roma, Italy) was also performed preoperatively and postoperatively at the 1st, 3rd, 6th and 12th month. In all cases, the same experienced examiner (AA) performed Pascal tonometry after Goldmann tonometry after an interval of 15 minutes. Surgical procedure With regard to the operation itself, ofloxacin drops were instilled in both eyes and povidone iodine solution was used for the disinfection of the eyelids and surrounding tissues. Proxymetacaine hydrochloride drops provided the required local anaesthesia. The Pal Brush (University of Crete, Greece) was used for the removal of the epithelium in the PRK group. For the LASIK group, the Carriazo- Pendular microkeratome (SCHWIND Eye-tech-solutions GmbH & Co. KG, Kleinostheim, Germany) with the 130 μm cutting head was used for the creation of the flap. The normal value of the negative pressure of the suction ring was between mmhg, and the velocity of the microkeratome head was constant at 3 mm/sec. The hinge was created at the 12 o clock position. The Allegretto 200 Hz (WaveLight AG, Erlangen, Germany) excimer laser was used for the ablation in all groups. After LASIK, the flap was repositioned with an irrigation cannula and the interface was irrigated. Proper alignment was ensured by gentle handling with a wet microsponge. All operations were performed by the same surgeon (VPK). Postoperative management All patients were administered ofloxacin and dexamethazone q.i.d. for a period of 7 days and preservative-free artificial tears for 2 months. Patients who underwent PRK were discharged with a therapeutic contact lens and additional fluorometholone drops for 3 weeks. Statistics Analysis of the data was performed with the Statistical Package for the Social Sciences (SPSS), version 17.0 (SPSS, Inc., Chicago, IL, USA). Repeated measures ANOVA after Greenhouse Geisser correction for unequal variances was used to test for differences in readings between the Goldmann and Pascal tonometers after each type of refractive surgery. The paired samples t-test was used for the within-group comparisons. Bland Altman plots were used to evaluate the agreement of measurements with GAT and PDCT. Only one randomly-selected eye per

3 Table 1 Patient demographics participant was used in the analysis. Statistical significance was considered for p values of less than Results PRK LASIK myopia Participants enrolled Eyes included LASIK hyperopiaa Mean ± SD age (years) 31.1± ± ±7.0 Range Sex PRK: photorefractive keratectomy 43 men, 41 women LASIK: laser in situ keratomileusis 83 men, 99 women 22 men, 21 women Patient demographics are presented in Table 1. Data were completed for all 309 subjects. Forty-three men and 41 women (mean ± SD age: 31.1±10.2 years) were included in the PRK group, 83 men and 99 women (mean ± SD age: 32.4±9.1 years) were included in the myopic LASIK group, and 22 men and 21 women (mean ± SD age: 34.5±7.0 years) were included in the hyperopic LASIK group. The mean pre- and postoperative values for central corneal thickness, mean spherical equivalent and mean keratometry readings are presented in Table 2. In the PRK group, the mean IOP values with PDCT and GAT are shown in Fig. 1. The mean ± SD PDCT values were: preoperatively: 14.9±2.0 mmhg, 1st month: 15.1± 1.8 mmhg, 3rd month: 14.8±2.2 mmhg, 6th month: 14.8± 2.1 mmhg, and 12th month: 14.9 ± 2.0 mmhg. The respective GAT-determined values were: 12.9±2.8 mmhg, 11.4±2.9 mmhg, 11.1±2.3 mmhg, 11.1±2.5 mmhg, and 11.0±2.8 mmhg. In the myopic LASIK group, the mean PDCT-determined values were: preoperatively: 14.9±2.2 mmhg, 1st month: 15.0±2.3 mmhg, 3rd month: 14.8±2.4 mmhg, 6th month: 15.0±2.1 mmhg, and 12th month: 14.9±2.1 mmhg. The respective GAT-determined values were: 14.5±2.8 mmhg, 10.8±1.8 mmhg, 10.8±2.1 mmhg, 10.9±2.1 mmhg, and 10.9±2.1 mmhg (Fig. 1). In the hyperopic LASIK group, the PDCT-determined values were: preoperatively: 14.6±2.0 mmhg, 1st month: 14.7±1.8 mmhg, 3rd month: 14.7±2.2 mmhg, 6th month: 14.4±2.1 mmhg, and 12th month: 14.9±2.4 mmhg. The respective GAT-determined values were: 14.2±2.0 mmhg, 13.0±2.2 mmhg, 13.4±1.7 mmhg, 13.0±2.0 mmhg, and 13.2±2.2 mmhg (Fig. 1). Figure 2 shows Bland Altman plots of the measurements with PDCT and GAT for each treatment group preoperatively and at 6 months postoperatively. Similar plots were observed for the other postoperative time-points (detailed data not shown). A significant difference between the preoperative and all postoperative readings was found when the Goldmann tonometer was used (paired samples t-test). In detail, p values of repeated measures ANOVA after Greenhouse Geisser correction for the PRK, myopic LASIK and hyperopic LASIK groups were respectively <0.001, <0.001 and However, there was no significant difference between any postoperative GAT-determined values for any treatment group. On the other hand, there was no difference between any pre- and postoperative measurements when the Pascal tonometer was used (all p values >0.05, detailed results not shown). A comparison of the mean IOP values between PDCT and GAT for each treatment group revealed a statistically significant difference (p<0.05) in all postoperative time points. The mean IOP difference (d IOP, GAT-PDCT) in the three groups were preoperatively for myopic PRK 2.0 mmhg, for myopic LASIK 0.4 mmhg and for hyperopic LASIK 0.4 mmhg, while the mean IOP difference postoperatively for myopic PRK was 3.8 mmhg, for myopic LASIK 4.0 mmhg and for hyperopic LASIK 1.5 mmhg (Table 4). Finally, a moderate positive correlation between preoperative GAT and CCT values (r=0.237, p<0.001) and a weak correlation between GAT measurements and K readings Table 2 Pre- and postoperative data (mean ± SD) for each treatment group CCT preop μm Mean K preop D Mean SE preop D Treatment zone mm CCT reduction postop μm Mean K postop D Mean SE postop D PRK myopia 508.7± ± ± ± ± ± ±0.4 LASIK myopia 554.8± ± ± ± ± ± ±2.2 LASIK hyperopia 542.7± ± ± ± ± ± ±0.7 CCT: Central corneal thickness SE: spherical equivalent preop: preoperative, postop: postoperative SD: Standard deviation

4 Fig. 1 Mean IOP values with PDCT and GAT in the three groups Fig. 2 Bland Altman plots of the measurements with PDCT and GAT for each treatment group preoperatively and at 6 months postoperatively

5 Table 3 Pre- and postoperative intraocular pressure (IOP) readings obtained with Goldmann applanation tonometer (GAT) and Pascal dynamic contour tonometer (PDCT) for each treatment group PRK LASIK, myopic LASIK, hyperopic IOP (mean±sd), mmhg IOP (mean±sd), mmhg IOP (mean±sd), mmhg GAT PDCT GAT PDCT GAT PDCT Preoperative 12.9± ± ± ± ± ±2.0 1st month postop 11.4± ± ± ± ± ±1.8 3rd month postop 11.1± ± ± ± ± ±2.2 6th month postop 11.1± ± ± ± ± ±2.1 12th month postop 11.0± ± ± ± ± ±2.4 (r=0.117, p=0.037) was found. No such correlation was detected for PDCT (all p>0.05). Discussion In this study, we examined the IOP readings obtained with GAT and PDCT in eyes undergoing PRK for myopia or LASIK for myopia or hyperopia. To the best of our knowledge, this is the first report with a follow-up of 12 months. Additionally, it is the first study to examine the use of PDCT in PRK-treated eyes as well as LASIK-treated hyperopic eyes. With regard to patients undergoing PRK for the correction of myopia, our results indicate that their preoperative CCT values were significantly thinner in comparison to the CCT values of LASIK patients. This was expected, as eyes offered PRK are poor candidates for LASIK mainly due to their thin corneas. Our findings show that postoperative GAT readings were similar but significantly lower in comparison to preoperative ones. On the contrary, PDCT measurements were not affected by the refractive treatment. Although the exact reason remains unclear, it seems that corneal thinning and corneal flattening significantly affect Goldmann, but not Pascal tonometry readings. Laser ablation of the central cornea may induce complex biomechanical changes in addition to thinning [17, 18], accounting for the observed discrepancies. Recently published data [19] point to the importance of the layered construction of the cornea and the inter-lamellar adhesions as determinants of corneal stiffness. Thus, simply separating the corneal stroma in two layers, such as in flap creation during LASIK, significantly reduces tissue stiffness, and may account for the post-lasik IOP reduction observed with GAT [19]. In addition, merely flattening the central cornea may allow for the applanation semicircles during Goldmann tonometry to be aligned at their appropriate position at a falsely-low intraocular pressure. Similarly, concerning patients undergoing LASIK for the correction of myopia, postoperative GAT readings remained constant but were significantly reduced in comparison to preoperative ones. Again, PDCT measurements were not affected by the refractive treatment. Our pre- and postoperative observed difference in GAT-determined IOP values for patients undergoing myopic LASIK are in good agreement with the values predicted by the formula developed by Kohlhaas et al. [20]. Thus, assuming that flap creation alone accounts for a discrepancy of 0.75 mmhg, the difference in spherical equivalent of 5.1 diopters accounts for 5.1/2.7= 1.9 mmhg, and the postoperative CCT reduction of 71.7 μm causes an underestimation of approximately 1 mmhg, the final predicted GAT-determined IOP decrease is expected to be 3.6 mmhg. This value is very close to our result (3.7 mmhg). With regard to patients undergoing LASIK for the correction of hyperopia, a similar pattern of measurement discrepancies was observed: when the Goldmann tonometer was used, postoperative readings at all time-points were similar but significantly lower than preoperative ones. Nonetheless, the difference between pre- and postoperative GAT readings was less pronounced for eyes treated with LASIK for hyperopia correction (Table 3). Measurements with the Pascal tonometer were again immune to the refractive treatment. Our results about the effect of hyperopic LASIK ablation on IOP values are in agreement Table 4 Differences between Goldmann applanation (GAT) and Pascal Dynamic Contour tonometry (PDCT) readings preoperatively and at 1,3,6 and 12 months postoperatively for the three treatment groups PRK diop (mmhg) Myopic LASIK diop (mmhg) Preoperative Postoperative 1 month months months months Mean postoperative d IOP diop= IOP difference (GAT minus PDCT) Hyperopic LASIK diop (mmhg)

6 with data published by Alonso-Munoz et al. [11] who reported lower readings in patients treated with LASIK for hyperopia when the Goldmann tonometer was used. The exact reason for the weaker impact on GAT readings after hyperopic laser ablation is still to be identified. However, it may be reasonable to assume that because hyperopic treatment leaves the central cornea relatively unaffected in terms of both thickness and curvature, Goldmann tonometry can be performed at the minimally altered central cornea, thus allowing for readings closer to the preoperative ones. In addition, it can be assumed that flap creation alone for hyperopic LASIK has a similar effect on postoperative GAT readings (i.e., underestimation by 0.75 mmhg) to that of flap creation for myopic LASIK [20]. If this is the case, the postoperative difference we observed in this treatment group (i.e., 1.2 mmhg in the first postoperative month) can largely be attributed to flap creation, rather than photoablation. Moreover, when comparing IOP differences between Goldmann and Pascal tonometers (diop, GAT-PDCT) preoperatively, we found an average diop of 2.0 between the two tonometers for the PRK group, and an average difference of 0.4 for the other two groups. As patients planned for PRK treatment had thinner corneas, these results indicate that the discrepancy between GAT and PDCT values may be inversely related to central corneal thickness. It should be noted that patients in the PRK group had a significantly lower preoperative GAT-determined mean IOP value (12.9±2.9 mmhg) than the values of patients in the hyperopic LASIK (14.2±2.0 mmhg) or myopic LASIK group (14.5±2.8 mmhg). However, if the GAT readings of patients in the PRK group are corrected for the approximately 40 μm thinner CCT, all three treatment groups can be considered to have comparable IOP values [21]. Although the effect of myopic ablation on the underestimation of IOP readings with Goldmann tonometry is greater with LASIK than PRK in our study (Table 4), it is not possible to determine which of the two treatments more strongly affects postoperative GAT values for the same amount of photoablated tissue or magnitude of refractive correction. Thus, as shown in Table 2, despite the greater flattening of the central cornea in the PRK group, more tissue was ablated and a greater refractive correction was achieved in the myopic LASIK group. These differences can be explained by the fact that LASIK removes deeper layers of corneal tissue and leaves the superficial strata unaffected, hence causing less flattening. The validity of IOP measurements after refractive surgery is of crucial importance, because the vast popularity of refractive surgery has already resulted in great numbers of people who need to be assessed and managed for glaucoma. In addition, it should be kept in mind that myopia may be a risk factor for glaucoma development and progression [22]. The Pascal tonometer seems to be unaffected by the corneal biomechanical changes following refractive surgery, probably because of its property to measure IOP transcorneally, without the need to applanate the corneal surface. Besides, it has been found that PDCT is practically not influenced by CCT [23, 24]. Further efforts to develop an IOP-correcting formula for patients undergoing laser refractive correction seems warranted. In conclusion, in our study we found that Goldmann tonometry is influenced by CCT and keratometry changes after refractive surgery in PRK and LASIK-treated myopic eyes. A less pronounced IOP underestimation was found after hyperopic LASIK when Goldmann tonometry was employed. In contrast, Pascal tonometry was unaffected by laser refractive surgery. References 1. 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