Laser in situ keratomileusis (LASIK) has

Changes in Intraocular Pressure After Laser in situ Keratomileusis Khaled M. Rashad, MD; Ahmed A. Bahnassy, MSc, PhD ABSTRACT PURPOSE: To evaluate changes in intraocular pressure (IOP) measurements by Goldmann applanation tonometry after laser in situ keratomileusis (LASIK) for myopia and myopic astigmatism, and to assess the accuracy of Goldmann applanation tonometry measurements after LASIK in these eyes. METHODS: LASIK was performed on 166 eyes of 93 patients for correction of myopia and myopic astigmatism. Intraocular pressure was measured by Goldmann applanation tonometry at the central and temporal parts of the cornea before and at 1, 3, 6, and 12 months after LASIK. The amount of change in IOP was computed and its relation to different variables was evaluated by regression analysis. RESULTS: Intraocular pressure measured at the center of the cornea was reduced by a mean of 3.69 ± 1.63 mmhg after LASIK. Multiple regression analysis showed that the decrease in IOP was related to the preoperative IOP and the change in central corneal thickness after LASIK. Measurements of IOP at the temporal part of the cornea were also reduced by a mean of 2.39 ± 1.71 mmhg. There was wide variability in the amount of difference between the temporal and central measurements after LASIK (temporal measurements were higher than central by 0 to +4 mmhg). CONCLUSION: LASIK for myopia produced underestimation of IOP measured by Goldmann applanation tonometry at the central part of the cornea by a mean of 3.69 ± 1.63 mmhg. The decrease of IOP was related to preoperative IOP and the change in central corneal thickness after LASIK. Temporal, although decreased after LASIK, were less reliable. [J Refract Surg 2001;17:420-427] From the Ophthalmology Department, Faculty of Medicine, Alexandria University, Egypt and Almana General Hospital, Dammam, Saudi Arabia (Rashad) and Department of Family and Community Medicine, College of Medicine, King Faisal University, Saudi Arabia (Bahnassy). The authors have no proprietary interest in the materials presented herein. Correspondence: Khaled M. Rashad, MD, AL Mana General Hospital, P. O. Box 2366, Dammam 31451 Saudi Arabia. Tel: 966.3.8262111 ext.#1007; Fax: 966.3.8274747. Received: April 3, 2000 Accepted: February 16, 2001 Laser in situ keratomileusis (LASIK) has become an accepted method to treat many refractive errors. Recent studies have shown that it is a safe, effective, and predictable procedure for the correction of myopia and myopic astigmatism. 1,2 As the number of patients undergoing LASIK increases, there is concern about the accuracy of intraocular pressure (IOP) measurements after excimer laser refractive surgery. Goldmann applanation tonometry is considered the standard method for assessment of IOP, and is used routinely in ophthalmic practice. 3,4 Measurements of IOP by Goldmann applanation tonometry are influenced by corneal thickness and to a lesser extent by corneal curvature. 5 In a normal population, thin corneas tend to give lower readings (underestimation) by Goldmann applanation tonometry, whereas thick corneas are associated with elevated readings (overestimation). 5,6 Previous studies reported decreased corneal thickness in many cases of low-tension glaucoma, and increased corneal thickness in cases of ocular hypertension. 7,8 Studies also showed a positive correlation between IOP measurements by Goldmann applanation tonometry and increasing corneal curvature. 9 Thus, flat corneas can cause the IOP measurement to be underestimated by Goldmann applanation tonometry. 10 LASIK for myopia (and myopic astigmatism) alters corneal parameters including central corneal thickness and corneal curvature, hence LASIK can change the accuracy of IOP measurements by Goldmann applanation tonometry. 11,12 This study prospectively evaluates the changes that occur in IOP measurements by Goldmann applanation tonometry after LASIK for myopia and myopic astigmatism. PATIENTS AND METHODS LASIK was performed on 166 eyes of 93 patients for correction of myopia and myopic astigmatism. Fifty-two patients (55.9%) were males, and 41 (44.1%) were females. The age of the patients 420 Journal of Refractive Surgery Volume 17 July/August 2001

ranged from 20 to 64 years with a mean age of 30.6 ± 8.0 years. The spherical equivalent manifest refraction ranged from -1.13 to -19.13 diopters (D) with a mean of -5.77 ± 3.75 D. The amount of refractive astigmatism (as measured by manifest refraction) ranged from 0 to 6.50 D with a mean of 0.81 ± 0.99 D. Preoperative examination included manifest refraction, uncorrected and spectacle-corrected visual acuity, slit-lamp biomicroscopy, and fundus examination. Corneal topography (videokeratography) was examined using the Computed Anatomy Topographic Modeling System TMS-1 (Computed Anatomy Inc., New York, NY). The average keratometric power was calculated from the Sim K (simulated keratometry) readings of the corneal topography. 12 The intraocular pressure (IOP) was measured using a Goldmann applanation tonometer (Haag- Streit). The calibration of the tonometer was checked periodically. To measure IOP, corneal anesthesia was achieved with oxybuprocaine hydrochloride 0.4% (Novesin) eye drops. The tear film was stained by touching the lower tarsal conjunctiva with a moistened strip of fluorescein impregnated paper. The patient was asked to look straight ahead and to avoid blinking, and IOP was measured at the center of the cornea. Three measurements were done in each eye, and IOP was recorded as the mean of the three readings. Then, the patient was asked to look slightly nasally, and IOP was measured at the temporal part of the cornea close to, but still inside, the limbus. Again, three measurements were done, and the mean was recorded. The central corneal thickness was measured by ultrasonic pachymetry using a Teknar Ophthasonic pachymeter (Teknar Corporation, St. Louis, MO). The calibration of the pachymeter was checked according to the manufacturer s instructions. After topical anesthesia, the ultrasonic probe was held perpendicular to the corneal surface opposite the center of the pupil. Three measurements were recorded, and the mean was calculated. Selection criteria included an age of 20 years or older, with stable refraction over the past year, and a spectacle-corrected visual acuity of 20/50 or better. Patients with keratometric power more than 47.00 D, those with keratoconus or suspected keratoconus, or those with IOP greater than 22 mmhg by Goldmann applanation tonometry were excluded. Patients had no corneal pathology or ocular disease apart from myopia (and astigmatism). Patients using soft contact lenses were asked to discontinue contact lens use for 1 week (4 weeks for gaspermeable hard lenses) prior to preoperative evaluation. Patients with diabetes or collagen diseases that are likely to affect corneal healing were also excluded. All LASIK procedures were performed under topical anesthesia by the same surgeon (KMR) at Almana General Hospital in Dammam between March and November 1998. The refractive goal was emmetropia in all eyes. The surgical technique has been described in detail. 1,2 In the first 80 eyes, the Automated Corneal Shaper (Chiron Vision, Claremont, CA) was used to create a 160-µm, nasally hinged corneal flap, about 8.5 mm in diameter. Laser ablation was performed using the Chiron- Technolas Keracor 116 excimer laser (Chiron Technolas GMBH, Munich, Germany), with multizone ablation (usually ablation zones of 5.0 and 6.0 mm were used with an outer blend zone of 7.0 mm). In the remaining 86 eyes, the Carriazo- Barraquer microkeratome (Moria, Antony, France) was used to create a 160-µm corneal flap with a superior hinge, about 9 to 9.5 mm in diameter. Laser ablation was performed using the Keracor 117C PlanoScan excimer laser (Technolas Bausch and Lomb Surgical), with an ablation zone size of 5.5 to 6.0 mm and a transition zone of 8.1 to 9.0 mm. Postoperatively, spersadexoline eye drops (0.1% dexamethasone, 0.5% chloramphenicol, 0.025% tetrahydrozoline hydrochloride, Ciba Vision, Hettlingen, Switzerland) were prescribed four times daily for 1 week, and artificial tears were used for 6 to 8 weeks. Follow-up examinations were scheduled at 1 day, 1 week, 1, 3, 6, and 12 months after LASIK for all eyes. Postoperative examination included manifest refraction, uncorrected and spectacle-corrected visual acuity, and slit-lamp biomicroscopy. From the second postoperative examination (1 week after LASIK), the examination included corneal topography, measurement of IOP by Goldmann applanation tonometry in the central and temporal parts of the cornea, and ultrasonic pachymetry to measure central corneal thickness. The preoperative and postoperative were performed at the same time interval (between 5 and 8 PM) to minimize the possible effects of diurnal IOP variations. Statistical Analysis The data were analyzed using SPSS Statistical Package for windows, Version 7.5 (SPSS Inc., Redmond, WA). Descriptive statistics were analyzed for all variables. Paired t-test was used to compare Journal of Refractive Surgery Volume 17 July/August 2001 421

Table 1 Paired t-test Comparing Data for 166 Eyes Before and 1 Year After LASIK Preoperative Postoperative Change Paired t-test Mean ± SD (Range) Mean ± SD (Range) Mean ± SD (Range) Significance (P-value) Spherical equivalent manifest refraction (D) -5.77 ± 3.75-0.34 ± 0.63 5.43 ± 3.52 <.001* (-1.13 to -19.13) (+0.75 to -3.00) (1.13 to 18.00) Refractive astigmatism (D) 0.81 ± 0.99 0.17 ± 0.43 0.64 ± 0.79 <.001* (0.00 to 6.50) (0.00 to 2.75) (0.00 to 6.25) Average keratometric power (D) 44.01 ± 1.69 39.77 ± 2.56-4.24 ± 2.04 <.001* (40.40 to 47.00) (31.40 to 45.00) (-0.8 to -10.8) Central corneal thickness (µm) 530.8 ± 39.1 468.1 ± 45.8-62.73 ± 28.99 <.001* (405.0 to 634.0) (380.0 to 614.0) (-10 to -136) Central Goldmann applanation tonometry (mmhg) 12.63 ± 2.69 8.94 ± 2.13-3.69 ± 1.63 <.001* (7.0 to 20.0) (4.0 to 14.0) (0.0 to -9.0) Temporal Goldmann applanation tonometry (mmhg) 12.71 ± 2.74 10.32 ± 2.54-2.39 ± 1.71 <.001* (7.0 to 20.0) (5.0 to 16.0) (0.0 to -7.0) * Statistically significant (P<.001) preoperative and postoperative data. Pearson correlation coefficient was computed to find the linear relationship between IOP and different variables. Stepwise multiple linear regression analysis was used to predict the change in IOP after LASIK from the related variables. P values less than.05 were considered statistically significant. RESULTS Table 1 shows the preoperative and 1-year postoperative spherical equivalent manifest refraction, refractive astigmatism, average keratometric power, central corneal thickness, central and temporal of intraocular pressure (IOP), and the amount of change that occurred after LASIK in these parameters. The mean ablation depth, as estimated by the software of the excimer laser, was 93.04 ± 37.36 µm (range, 22 to 182 µm). The actual change in central corneal thickness after LASIK, as measured by ultrasonic pachymetry, was less than the estimated ablation depth in all eyes (Table 1). Intraocular Pressure Before surgery, there were no significant differences between IOP measured by Goldmann applanation tonometry at the central and temporal parts of the cornea (Table 2). Measurements of IOP by Goldmann applanation tonometry at the central part of the cornea ranged from 7 to 20 mmhg with a mean of 12.63 ± 2.69 mmhg. At 1 year after LASIK, IOP measured by Goldmann applanation tonometry at the center of the cornea ranged from 4 to 14 mmhg (mean, 8.94 ± 2.13 mmhg). Comparing the IOP measurements at the center of the cornea among the different postoperative examinations, there were no significant changes during the 12-month period of follow-up (P >.05). None of the operated eyes had a postoperative IOP higher than the preoperative measurements. After LASIK, IOP measured by Goldmann applanation tonometry at the center of the cornea was reduced by a mean of 3.69 ± 1.63 mmhg (range, 0 to -9 mmhg), compared to preoperative measurements. The decrease of IOP after LASIK was statistically significant (Table 1). After LASIK, measurements of IOP by Goldmann applanation tonometry at the temporal part of the cornea were generally higher than those obtained from the center of the cornea. At 1 year after LASIK, the temporal measurements of IOP by Goldmann applanation tonometry were higher than the central measurements by a mean of 1.38 ± 1.11 mmhg. The difference between the temporal and central measurements of IOP by Goldmann applanation tonometry after LASIK was statistically significant at all postoperative examinations (Table 2). However, there was wide variability in the amount of difference between the temporal and central measurements of IOP after LASIK. In 48 of the operated eyes (28.9%), the two measurements were identical; 422 Journal of Refractive Surgery Volume 17 July/August 2001

Table 2 Paired t-test Comparing IOP Measured by Goldmann Applanation Tonometry at the Central and Temporal Parts of the Cornea Before and After LASIK (166 Eyes) Central Temporal Difference Paired t-test Mean ± SD (Range) Mean ± SD (Range) Mean ± SD (Range) Significance (P-value) Preoperative 12.63 ± 2.69 12.71 ± 2.74 0.08 ± 0.28 0.357 (7.0 to 20.0) (7.0 to 20.0) (-1.0 to +1.0) Postoperative 1 month 8.77 ± 2.27 10.09 ± 2.51 1.32 ± 1.09 <.001* (5.0 to 14.0) (5.0 to 17.0) (0.0 to +4.0) 3 months 8.86 ± 2.11 10.27 ± 2.49 1.41 ± 1.17 <.001* (4.0 to 15.0) (5.0 to 16.0) (0.0 to +4.0) 6 months 8.99 ± 2.24 10.30 ± 2.43 1.31 ± 1.23 <.001* (5.0 to 14.0) (5.0 to 16.0) (0.0 to + 3.0) 12 months 8.94 ± 2.13 10.32 ± 2.54 1.38 ± 1.11 <.001* (4.0 to 14.0) (5.0 to 16.0) (0.0 to +4.0) * Statistically significant (P<.001) Table 3 Correlation Coefficient of Preoperative Central Goldmann Applanation Tonometry Measurements of IOP With Different Variables (166 Eyes) Variable Pearson Statistical Correlation Significance (r) With Pre- (P-value) operative IOP Age 0.139* 0.037 Sex -0.045 0.282 Preoperative central corneal thickness 0.125 0.055 Preoperative average keratometric power 0.103 0.093 Preoperative spherical equivalent refraction -0.141* 0.035 * Correlation is statistically significant at the.05 level in three eyes (1.8%), the temporal measurements were higher than the central measurements by 4 mmhg. In the remaining 115 eyes (69.3%), the temporal measurements were higher than the central measurements by 1 to 3 mmhg. There were no significant changes in the postoperative temporally measured IOP during the follow-up period. At 1 year after LASIK, the IOP measured by Goldmann applanation tonometry at the temporal part of the cornea was reduced by a mean of 2.39 ± 1.71 mmhg (range, 0 to -7 mmhg), compared to preoperative measurements (Table 1). Correlation of IOP With Different Parameters Before surgery, there was a significant correlation between the preoperative IOP (measured by Goldmann applanation tonometry at the center of the cornea) and the age of the patients (Table 3). The preoperative IOP also had a significant negative correlation with the spherical equivalent refraction (ie, patients with higher amounts of myopia had higher IOP). The correlation between preoperative IOP and preoperative central corneal thickness almost reached statistical significance (P =.055). After LASIK, the postoperative IOP (measured by Goldmann applanation tonometry at the center of the cornea) maintained a significant positive correlation with age. The postoperative IOP also had a statistically significant correlation with the preoperative IOP, and with the postoperative central corneal thickness (Table 4). A significant correlation was found between the amount of change in IOP after LASIK (postoperative minus preoperative IOP measured by Goldmann applanation tonometry at the center of the cornea) and the preoperative IOP. This means that eyes with higher preoperative IOP had a greater decrease in IOP after LASIK. The change in IOP also had a statistically significant correlation with the change in central corneal thickness, change in average keratometric power, and the change in spherical equivalent refraction (Table 5). Journal of Refractive Surgery Volume 17 July/August 2001 423

Table 4 Correlation Coefficient of 1-year Postoperative Central Goldmann Applanation Tonometry Measurements of IOP With Different Variables (166 Eyes) Variable Pearson Statistical Correlation Significance (r) With Pre- (P-value) operative IOP Age 0.190*.007 Sex -0.073.174 Preoperative IOP 0.794* <.001 Change in central corneal thickness 0.127.052 Change in spherical equivalent refraction 0.029.354 Change in average keratometric power 0.020.397 Postoperative central corneal thickness 0.182*.010 Postoperative average keratometric power 0.115.070 * Correlation is statistically significant at the.01 level Table 5 Correlation Coefficient of Change in Central Goldmann Applanation Tonometry Measurements of IOP After LASIK With Different Variables (166 Eyes) Variable Pearson Statistical Correlation Significance (r) With Change (P-value) in IOP Age 0.020.398 Sex -0.021.392 Preoperative IOP -0.608* <.001 Change in central corneal thickness 0.266* <.001 Change in average keratometric power 0.245*.001 Change in spherical equivalent refraction -0.212*.003 Postoperative central corneal thickness 0.126.053 Postoperative average keratometric power 0.212*.003 * Correlation is statistically significant at the.01 level Table 6 Model Derived by Stepwise Multiple Linear Regression Analysis to Predict IOP After LASIK From Related Independent Variables Variable B t Statistical Significance (P-value) Preoperative IOP 0.627 17.419 <.001 Change in central corneal thickness 0.0143 4.273 <.001 Age 0.03044 2.481.014 (Constant) 0.987 1.737.084 R 2 = 0.674 The change in average keratometric power and the change in the spherical equivalent refraction were excluded because they did not affect the predictability of postoperative IOP. Predictability of Change in IOP After LASIK The preoperative IOP measured by Goldmann applanation tonometry at the center of the cornea, the change in central corneal thickness after LASIK, and the patient s age, were significant predictors for postoperative IOP (R 2 = 0.674). This means that these variables explained 67.4% of the variability in the postoperative IOP (Table 6). The equation derived by stepwise multiple linear regression analysis to predict the postoperative IOP was: Table 7 Model Derived by Stepwise Multiple Linear Regression Analysis to Predict Change in IOP After LASIK From Related Independent Variables Variable B t Statistical Significance (P-value) Preoperative IOP -0.373-10.343 <.001 Change in central corneal thickness 0.01434 4.273 <.001 Age 0.0304 2.481.014 (Constant) 0.987 1.737.084 R 2 = 0.443 The change in average keratometric power and the change in the spherical equivalent refraction were excluded because they did not affect the predictability of the change in IOP after LASIK. [Postoperative IOP (mmhg) = 0.987 (constant) + 0.627 x preoperative IOP (mmhg) + 0.0143 x change in central corneal thickness (µm) + 0.03044 x patient s age (yr)]. Preoperative IOP, change in central corneal thickness after LASIK, and the patient s age were also significant predictors for the amount of change in IOP after LASIK (R 2 = 0.443). These variables explained 44.3% of the variability in the amount of decrease of IOP after LASIK (Table 7). The equation 424 Journal of Refractive Surgery Volume 17 July/August 2001

derived by stepwise multiple linear regression analysis to predict the change in IOP after LASIK was: [Change in IOP (mmhg) = 0.987 (constant) 0.373 x preoperative IOP (mmhg) + 0.01434 x change in central corneal thickness (µm) + 0.0304 x patient s age (yr)]. DISCUSSION Evaluation of IOP is a fundamental part of ocular examination in both health and disease. IOP measurement is not only important in the diagnosis and management of glaucoma, but its assessment is important in the management of corneal, lenticular, uveal, and retinal diseases. 5 Currently, the most common way to assess IOP is by Goldmann applanation tonometry, which is considered the standard method for measurement of IOP. 3,4 Goldmann applanation tonometry gives accurate measurements of IOP in eyes with normal corneal curvature, and with normal (average) corneal thickness of 520 µm. 5,14-17 In this study, the centrally measured IOP by Goldmann applanation tonometry was reduced by a mean of 3.69 ± 1.63 mmhg after LASIK for myopia. The reduction of IOP by Goldmann applanation tonometry after LASIK for myopia was consistent throughout the 12-month follow-up period. During the study, all preoperative and postoperative were performed at the same time interval (between 5 and 8 PM) to minimize the possible effects of diurnal IOP variations. Also, corticosteroid response could not affect the measurements because topical corticosteroids were used for 1 week only postoperatively. The cause of the decrease or underestimation of the centrally measured IOP after LASIK for myopia must be related to the change in the corneal parameters. A reduction of IOP measured by Goldmann applanation tonometry has been reported in different studies after excimer laser photorefractive keratectomy (PRK) and LASIK for myopia. 1,2,6,10-13,17-22 Rashad and Kalaway were the first to report the decrease of IOP measurement by Goldmann applanation tonometry after PRK for myopia. In that study, PRK was performed on 307 eyes with mean myopia of -5.64 D, and IOP decreased from a mean of 12.9 ± 3.1 mmhg preoperatively to a mean of 11.0 ± 2.7 mmhg after PRK. 23 Schipper and colleagues performed PRK on 35 eyes with mean myopia of -6.40 D and found a 2- to 3-mmHg decrease in centrally measured IOP after PRK; temporal measurements remained unchanged. 24 The reduction of IOP by Goldmann applanation tonometry after PRK for myopia in other reported studies varied between 2.0 and 3.1 mmhg. 10,17,20,21 Studies of LASIK for myopia also demonstrated underestimation of the IOP by Goldmann applanation tonometry after surgery. In our previous study on LASIK performed on 157 eyes with mean myopia of -5.29 ± 3.74 D, mean IOP was reduced from 11.9 ± 2.7 mmhg preoperatively to 9.0 ± 2.4 mmhg after LASIK. 1 Emara and coauthors reported that IOP decreased from 16.1 ± 2.9 mmhg before surgery to 13.6 ± 3.3 mmhg after LASIK for myopia. 13 Underestimation of the IOP measurements by Goldmann applanation tonometry after LASIK for myopia in other reported studies varied between 1.9 and 3.8 mmhg. 11,18,19 In this study, there were no significant differences in IOP measured from the central or temporal parts of the cornea before surgery. Similar observations have been reported in previous studies of PRK. 17,24 At 1 year after LASIK, the measurements of IOP by Goldmann applanation tonometry at the temporal part of the cornea were higher than the central measurements by a mean of 1.38 ± 1.11 mmhg. In LASIK for myopia (or myopic astigmatism), the deepest ablation is in the center of the cornea, with less ablation toward the edge of the ablation zone. 13,18,25 Higher temporal measurements of IOP are explained by the relatively increased thickness of the peripheral cornea. However, our results showed that there was variability in the amount of difference between temporal and central IOP measurements by Goldmann applanation tonometry after LASIK. In some eyes the two measurements were identical; in other eyes the temporal readings were higher than the central measurements by as much as 4.0 mmhg. Measurements of IOP at the temporal part of the cornea after LASIK were still lower than preoperative measurements by a mean of 2.39 ± 1.71 mmhg. Our results differ from the studies of Schipper and Abbasoglu where central measurements were lower than peripheral measurements by 2 to 3 mmhg after PRK. 17,24 In these studies, ablation of the cornea was performed only in the central 6.0-mm zone, and the peripheral portion of the cornea was not ablated. In the present study, the diameter of the ablation zone was usually 6.0 mm with a large transition zone extending to 9.0 mm in some eyes, ie, the peripheral portion of the cornea was also partially ablated, although to a lesser extent than the central zone. Therefore, in our study, the peripheral corneal thickness was also reduced after LASIK. This may explain the variability in the amount of Journal of Refractive Surgery Volume 17 July/August 2001 425

difference between the temporal and central after LASIK in our study. Before surgery, the correlation between IOP and central corneal thickness in this study almost reached statistical significance. After LASIK, central corneal thickness was reduced by a mean of 62.73 ± 28.99 µm. Regression analysis showed that there was a strong correlation between the drop in IOP and the change in central corneal thickness after LASIK. Several clinical and manometric studies have demonstrated a significant relationship between central corneal thickness and the error in IOP measurements by Goldmann applanation tonometry. 5,6,12,13,20,26-29 The Goldmann tonometer was found to give accurate readings when the central corneal thickness was 520 µm. 28 Previous studies showed that thin corneas produced underestimation of IOP by as much as 5 mmhg. 5,6,26,28 Many eyes diagnosed as having low-tension glaucoma were found to have thin corneas. 6,7,27 It is possible that central corneal thickness is related to the resistance of the cornea to applanation (corneal rigidity). 6,26 A thin cornea will be less rigid than a thick cornea and thus the force required to overcome corneal rigidity is reduced. When corneal rigidity is decreased, as a result of thinning of the central cornea after LASIK, the cornea requires less pressure to be applanated, resulting in low tonometer readings and underestimation of IOP. 5,6,12,13,20,26 The assumed thickness of corneal tissue removed, as estimated by the excimer laser software (estimated ablation depth), was always greater than the actual decrease in central corneal thickness observed after LASIK (as measured by ultrasonic pachymetry). Schipper 24 and Faucher 10 reported similar observations after PRK. Our results showed that patients with higher preoperative IOP had a larger decrease in IOP measurements after LASIK. Fournier and colleagues also reported that the decrease in IOP after myopic LASIK was greater if preoperative IOP was high. 11 The reduction of IOP in this study had a positive correlation with the reduction of average keratometric power after LASIK. Previous studies reported that Goldmann applanation tonometry readings are influenced by corneal curvature. 5,9,30,31 It is speculated that the steeper the corneal curvature, the more the cornea must be indented to produce the same area of applanation. 14 Flatter corneas are more readily applanated, and this may result in underestimation of IOP measurements by Goldmann applanation tonometry. 17,30 The flatter cornea after LASIK for myopia will require less force by the tonometer to reach the desired applanation diameter. 5,20 Also, flattening of the corneal curvature may cause an increased force of capillary attraction of the precorneal tear film, resulting in lower tonometer readings. 5 Results of multiple regression analysis in this study showed that preoperative IOP and change in central corneal thickness were important factors in determining the amount of reduction of IOP after LASIK for myopia. Fournier and colleagues also showed that reduction in IOP after LASIK was related to preoperative IOP. 11 Chatterjee and colleagues correlated the amount of IOP reduction to the amount of myopia corrected by PRK. 21 Other studies of PRK or LASIK could not demonstrate a statistically significant relationship between the amount of reduction of IOP after surgery and the changes in central corneal thickness, keratometric power, spherical equivalent refraction, or estimated ablation depth. 10,11,13,17,18,20,22,24 Myopic patients have an increased risk of developing glaucoma. 5,15,24,32 Assessment of visual fields and cup-disc ratios in these patients is more difficult than in patients without myopia. 19 Monitoring of IOP in these patients is important for detecting ocular hypertension, steroid-induced glaucoma, or primary glaucoma. Currently, Goldmann applanation tonometry is considered the standard method for measurement of IOP. 3,4 However, Goldmann applanation tonometry measurements were found to be inaccurate after LASIK. Ophthalmologists should keep in mind the underestimation of IOP by Goldmann applanation tonometry when treating myopic patients who have undergone LASIK. Postoperative of IOP at the temporal part of the cornea, which usually gave higher readings than the central measurements, were still lower than preoperative measurements. Measuring IOP by Goldmann applanation tonometry at the temporal part of the cornea did not give reliable information about the true postoperative IOP. A more practical approach to assess IOP after LASIK is to take Goldmann applanation tonometry measurements at the center of the cornea in the usual technique and then add 3 to 4 mmhg to the reading, or use a prediction formula (similar to that in this study) to calculate the approximate value of the true IOP. It is advisable that each patient who undergoes corneal refractive surgery should be given a record of his Goldmann applanation tonometry measurements before and after surgery. Such 426 Journal of Refractive Surgery Volume 17 July/August 2001

a record will be useful to compare future measurements. The results of this study indicate that Goldmann applanation tonometry measurements, obtained in the standard technique from the center of the cornea, will underestimate IOP after LASIK for myopia (by a mean of 3.69 ± 1.63 mmhg in this study). This underestimation of IOP persisted throughout the 12-month duration of this study, and was correlated with preoperative IOP and decrease in the central corneal thickness after LASIK. This may delay the diagnosis and proper management of future glaucoma in these myopic patients. With increasing numbers of refractive surgery procedures for myopia, there is an increased need for awareness of underestimation of IOP by Goldmann applanation tonometry after LASIK. Although Goldmann applanation tonometry measurements from the temporal part of the cornea usually gave higher readings than central measurements after LASIK, these temporal measurements were not reliable and also underestimated postoperative IOP. There is a need for a more accurate tonometer to assess true IOP after LASIK. Patients with ocular hypertension or glaucoma should not have LASIK until a valid method for evaluating IOP after corneal refractive surgery is available. REFERENCES 1. Rashad KM. Laser assisted in situ keratomileusis (LASIK) for correction of myopia. Egypt J Cataract Refract Surg 1996;2:17-29. 2. Rashad KM. Laser in situ keratomileusis for myopic astigmatism. J Refract Surg 1999;15:653-660. 3. Midelfart A, Wigers A. Clinical comparison of the Pro Ton and Tono-Pen tonometers with the Goldmann applanation tonometer. Br J Ophthalmol 1994;78:895-898. 4. Holladay JT, Allison ME, Prager TC. Goldmann applanation tonometry in patients with regular corneal astigmatism. Am J Ophthalmol 1983;96:90-93. 5. Kohlhaas M, Lerche RC, Draeger J, Klemm M, Ehlers N, Hjortdal J, Olsen H, Barraquer C, Barraquer JI, Flicker D, Rivera F, Carriazo C. The influence of corneal thickness and corneal curvature on tonometry readings after corneal refractive surgery. Eur J Implant Ref Surg 1995;7:84-88. 6. Shah S, Chatterjee A, Mathai M, Kelly SP, Kwartz J, Henson D, McLeod D. Relationship between corneal thickness and measured intraocular pressure in a general ophthalmology clinic. Ophthalmology 1999;106:2154-2160. 7. Ehlers N, Hansen FK. Central corneal thickness in low-tension glaucoma. Acta Ophthalmol 1974;52: 740-746. 8. Johnson M, Kass MA, Moses RA, Grodzki WJ. Increased corneal thickness simulating elevated intraocular pressure. Arch Ophthalmol 1978;96:664-665. 9. Mark HH. Corneal curvature in applanation tonometry. Am J Ophthalmol 1973;76:223-224. 10. Faucher A, Gregoire J, Blondeau P. Accuracy of Goldmann tonometry after refractive surgery. J Cataract Refract Surg 1997;23:832-838. 11. Fournier AV, Podtetenev M, Lemire J, Thompson P, Duchesne R, Perreault C, Chehade N, Blondeau P. Intraocular pressure change measured by Goldmann tonometry after laser in situ keratomileusis. J Cataract Refract Surg 1998;24:905-910. 12. Price FW, Koller DL, Price MO. Central corneal pachymetry in patients undergoing laser in situ keratomileusis. Ophthalmology 1999;106:2216-2220. 13. Emara B, Probst LE, Tingey DP, Kennedy DW, Williams LJ, Machat J. Correlation of intraocular pressure and corneal thickness in normal myopic eyes and after laser in situ keratomileusis. J Cataract Refract Surg 1998;24:1320-1325. 14. Whitacre MM, Stein R. Sources of error with the use of Goldmann-type tonometers. Surv Ophthalmol 1993;38:1-30. 15. Phelan PS, McGhee CN, Bryce IG. Excimer laser PRK and corticosteroid induced IOP elevation: the tip of an emerging iceberg. Br J Ophthalmol 1994;78:802-803. 16. Brubaker RF. Tonometry. In: Duane's Ophthalmology, CD- Rom edition. Philadelphia, PA: Lippincott-Raven Publishers;1997:Record 35210-35279. 17. Abbasoglu OE, Bowman RW, Cavanagh HD, McCulley JP. Reliability of intraocular pressure measurements after myopic excimer photorefractive keratectomy. Ophthalmology 1998;105:2193-2196. 18. Zadok D, Tran DB, Twa M, Carpenter M, Schanzlin DJ. Pneumotonometry versus Goldmann tonometry after laser in situ keratomileusis for myopia. J Cataract Refract Surg 1999;25:1344-1348. 19. Perez-Santonja JJ, Bellot J, Claramonte P, Ismail MM, Alio JL. Laser in situ keratomileusis to correct high myopia. J Cataract Refract Surg 1997;23:372-385. 20. Levy Y, Zadok D, Glovinsky Y, Krakowski D, Nemet P. Tonopen versus Goldmann tonometry after excimer laser photorefractive keratectomy. J Cataract Refract Surg 1999;25:486-491. 21. Chatterjee A, Shah S, Bessant DA, Naroo SA, Doyle SJ. Reduction in intraocular pressure after excimer laser photorefractive keratectomy. Correlation with pretreatment myopia. Ophthalmology 1997;104:355-359. 22. Mardelli PG, Piebenga LW, Whitacre MM, Siegmund KD. The effect of excimer laser photorefractive keratectomy on intraocular pressure measurements using the Goldmann applanation tonometer. Ophthalmology 1997;104:945-948. 23. Rashad K, Kalaway H. Photorefractive keratectomy (PRK) for myopia using a wide ablation zone. Bull Ophthalmol Soc Egypt 1994;87:713-722. 24. Schipper I, Senn P, Thomann U, Suppiger M. Intraocular pressure after excimer laser photorefractive keratectomy for myopia. J Refract Surg 1995;11:366-370. 25. Probst LE, Machat JJ. Mathematics of laser in situ keratomileusis for high myopia. J Cataract Refract Surg 1998;24:190-195. 26. Whitacre MM, Stein R, Hassanein K. The effect of corneal thickness on applanation tonometry. Am J Ophthalmol 1993;115:592-596. 27. Herndon LW, Choudhri SA, Cox T, Damji KF, Shields MB, Allingham RR. Central corneal thickness in normal, glaucomatous, and ocular hypertensive eyes. Arch Ophthalmol 1997;115:1137-1141. 28. Ehlers N, Bramsen T, Sperling S. Applanation tonometry and central corneal thickness. Acta Ophthalmol 1975;53: 34-43. 29. Argus WA. Ocular hypertension and central corneal thickness. Ophthalmology 1995;102:1810-1812. 30. Tomlinson A, Leighton DA. Ocular dimensions in low tension glaucoma compared with open angle glaucoma and the normal. Br J Ophthalmol 1972;56:97-105. 31. Argento C, Cosentino MJ, Moussalli MA. Intraocular pressure measurement following hyperopic LASIK. J Cataract Refract Surg 1998;24:145. 32. Dougherty PJ, Wellish KL, Maloney RK. Excimer laser ablation rate and corneal hydration. Am J Ophthalmol 1994;118:169-176. Journal of Refractive Surgery Volume 17 July/August 2001 427